Search

In seismotectonic studies, seismic reflection data are a powerful tool to unravel the complex deep architecture of active faults. Such tectonic structures are usually mapped at the surface through traditional geological surveying, whilst seismic reflection data may help to trace their continuation from the near surface down to hypocentral depths. On seismic reflection data, seismic attributes are commonly used by the oil and gas industry to aid exploration. In this study, we propose using seismic attributes in seismotectonic research for the first time. The study area is a geologically complex region of central Italy, struck during 2016–2017 by a long-lasting seismic sequence, including a Mw 6.5 main shock. Three vintage seismic reflection profiles are currently the only ones available at the regional scale across the epicentral zone. These represent a singular opportunity to attempt a seismic attribute analysis by running attributes like the “energy” and the “pseudo-relief”. Our results are critical, as they provide information on the relatively deep structural setting, mapping a prominent, high-amplitude regional reflector interpreted as the top of basement, which is an important rheological boundary. Complex patterns of high-angle discontinuities crossing the reflectors have also been identified by seismic attributes. These steeply dipping fabrics are interpreted as the expression of fault zones belonging to the active normal fault systems responsible for the seismicity of the region. Such peculiar seismic signatures of faulting are consistent with the principal geological and tectonic structures exposed at surface. In addition, we also provide convincing evidence of an important primary tectonic structure currently debated in the literature (the Norcia antithetic fault) as well as several buried secondary fault splays. This work demonstrates that seismic attribute analysis, even if used on low-quality vintage 2D data, may contribute to improving the subsurface geological interpretation in areas characterized by limited and/or low-quality subsurface data but with potentially high seismic hazard.

The Zinkgruvan mining area is located on the south-eastern part of the Bergslagen district, one of the three major mineral producing regions in Sweden. In this study, we present the results from three (P1, P2 and P8) reflection seismic profiles each approximately 3000 m-long crossing the Zinkgruvan Zn-Pb-Ag-(Cu) mining area. P1 was acquired using cabled geophones with 10 m receiver and source interval and crossed major geological features. The other two profiles (P2 and P8) were acquired by wireless recorders with 20 m receiver and 10 m source interval and ran perpendicular to P1. Through a special data processing workflow adapted to this dataset, good quality seismic sections were obtained along these profiles, although a high noise level due to high voltage electric power lines was present. The interpretations were constrained by (1) seismic P-wave velocity and density data from a series of downhole logging measurements, (2) 3D forward reflection traveltime modelling in both pre- and post-stack domains, and (3) other geophysical and geological observations available from the site. Despite the notably complex geology, the processed seismic sections clearly reveal a series of horizontal to gently dipping reflections associated with known geological formations. Results indicate that most structures and lithological contacts dip or plunge to the northeast, including the targeted Zinkgruvan Formation. The results from this seismic survey are encouraging regarding the potential of the seismic method for base-metal exploration in Sweden and in particular in the Bergslagen district. It shows the high resolving power of the reflection seismic methods for imaging complex geological structures in a cost-effective and environmentally-friendly way., EIT-RawMaterials is gratefully thanked for funding this up-scaling project 17024.

In Spring 2011 (11th of May), the vicinity of Lorca city (Murcia, SE Iberian Peninsula) was hit by two main seismic shocks that reach a maximum magnitude of 5.2 Mw. The earthquake caused serious widespread damage in the city and its surroundings. Similar events have affected the area regularly in the past (for example: on May 6,1977, 4.2 mg). These events are distributed along a relatively broad band (roughly NE-SW oriented) parallel to the coast, associated to the activation of the Alhama de Murcia Fault (AMF), an oblique-slip (reverse-strike-slip) fault system located in the Eastern Betics Shear Zone. The current study aims to characterize the shallow subsurface across some of the surface outcrop of a few of the main faults that lie within this seismogenic strike-slip fault system. Six normal-incidence seismic reflection profiles were acquired in the area crossing the AMF and the Carrascoy fault, among others). This study focuses on the determination of the shear-wave velocity depth model by applying Multichannel Analysis of Surface Waves (MASW), using the shot records of the seismic reflection profiles. The 1D velocity-depth functions acquired were pasted together to obtain the final 2D velocity models. The hand-picked dispersion curves were inverted using two different approaches to address the consistency of the inversion schemes. The final models reveal relevant differences across the different fault zones, reflecting the heterogeneity and lateral variability that characterizes a complex seismogenic zone, a most probably, diffuse plate boundary.This research is supported by: Generalitat de Catalunya (AGAUR) grant 2017SGR1022 (GREG); EU (H2020) 871121 (EPOS-SP); EIT-RawMaterials 17024 (SIT4ME), CGL2013-47412-C2-1-P.

Hardware and software innovations taking place since the commercial development of seismic reflection imaging in the 60¿s and early 70¿s have resulted in various improved powerful seismic imaging solutions. Overall, these have been very successful in contrasting geological environments pursuing a wide variety of different targets. The innovative advances in seismic processing may constitute critical tools when analyzing seismic data acquired in highly heterogeneous geologic environments as they can efficiently increase the resolution power. In addition, they can become relevant when using modern acquisition instrumentation and strategies. Furthermore, these new developments significantly increase the value of legacy seismic reflection data. Currently, reassessing controlled source seismic data is becoming a critical issue mostly due to the increasing difficulties for acquiring new profiles posed by environmental regulations and high prices. However, the knowledge of the subsurface is an asset for our society, for example: land-use planning and management; natural risk assessments; or exploration and exploitation for geo-resources. Here we present examples of analysis schemes such as seismic attribute analysis and Common Reflection Surface stacking applied on a number of old seismic reflection profiles (Deep lithospheric transects as well as high resolution profiles) in an effort to bring up their validity. Results indicate how these leading edge methods contribute to significantly improve the quality of vintage seismic data, significantly reducing reflector uncertainties and easing their interpretation., This research is supported by: Generalitat de Catalunya (AGAUR) grant 2017SGR1022 (GREG); EU (H2020) 871121 (EPOS-SP); EIT-RaewMaterias 17024 (SIT4ME).

The Iberian Central System represents an outstanding topographic feature in the central Iberian Peninsula. It is an intraplate mountain range formed by igneous and metasedimentary rocks of the Variscan Iberian Massif that has been exhumed since the Eocene in the context of the Alpine orogeny. The Iberian Central System has been conventionally interpreted as a thick-skinned pop-up mountain range thrust over the Duero and Tajo foreland basins. However, its lithospheric structure and the P-wave velocity distribution are not yet fully resolved. In order to place geophysical constraints on this relevant topographic feature, to identify lithospheric discontinuities, and to unravel the crustal deformation mechanisms, a wide-angle seismic reflection and refraction experiment, CIMDEF (Central Iberian Mechanism of DEFormation), was acquired in 2017 and 2019. It is a NNW-SSE oriented 360-km long profile that runs through the Duero basin, the Iberian Central System and the Tajo basin. First results based on forward modeling by raytracing show an irregularly layered lithosphere and allow to infer the depth extent of the northern Iberian Central System batholith. The crust is ~ 31 km thick under the Duero and Tajo basins and thickens to ~ 39 km under the Iberian Central System. A conspicuous thinning of the lower crust towards the south of the Iberian Central System is also modeled. Along this transect, a continuous and high amplitude upper mantle feature is observed and modeled as the reflection of an interface dipping from 58 to 62 km depth featuring a P-wave velocity contrast of 8.2 to 8.3 km/s. Our preliminary results complement previous models based on global-phase seismic and noise interferometry and gravity data, provide new constraints to validate the accuracy of passive seismic methods at lithospheric scale, and contribute with a resolute P-wave velocity model of the study area to unravel the effect of the Alpine reactivation on the central Iberian Massif.This project has been funded by the EIT-RawMaterials 17024 (SIT4ME) and the MINECO projects: CGL2016-81964-REDE, CGL2014-56548-P.

Near Surface Geoscience Conference & Exhibition Online 2020, This work explores the first results of the seismic data acquired in the Sotiel-Coronada mine as part of the SIT4ME (Seismic Imaging Techniques for Mineral Exploration) project. In this experiment, a multi-source seismic data-set was acquired at the end of 2018 in the Sotiel- Coronada area of the Iberian Pyrite Belt (IPB) (southwest Spain). In the seismic experiment presented, 653 seismic receivers were deployed distributed in a pseudo 3D grid and six crooked lines across the study area. The sensors recorded c. 900 vibration points produced by a 32 Tn vibrotruck. Here, we present the stack results of the 2D seismic sections. Data from over 100 wells have been incorporating to aid in the structural interpretation. The preliminary interpretation reveals the complexity of this highly faulted ore-bearing area. Correlations between well log data information and 2D seismic profiles, suggest the location of a potentially mineralized area. The SIT4ME project has been funded by EIT Raw Materials (17024).

3rd Conference on Geophysics for Mineral Exploration and Mining, 7-8 december, 2020 - online, The Zinkgruvan mining area is located in the south-eastern part of the Bergslagen district, one of the three most mineral prospective regions in Sweden. In this study, we present the results from two recently acquired approximately 6000 m-long reflection seismic profiles crossing the Zinkgruvan mining area. Profile P1 was acquired using cabled geophones with 10 m receiver and source interval and crossed a major NE-plunging fold. Profile P2 was acquired perpendicular to P1, used wireless recorders with 20 m receiver and 10 m source intervals. Despite the notably complex geology, the processed seismic sections clearly reveal a series of moderately-to-steeply dipping reflections associated with the mineral-bearing Zinkgruvan formation. The results from this seismic survey are encouraging regarding the potential of the seismic method for mineral exploration purposes and its high resolving power on complex geological structures in a cost-effective and environmentally friendly way.

The deep structure of the Carrascoy Fault (CAF) is characterized by utilizing the analysis of surface waves identified in shallow high-resolution seismic reflection data. The Multichannel analysis of surface waves (MASW) is used to unravel the 2-D Vs wave velocity model and image the depth geometry of the fault system. The study area includes segments of the fault located in the deformation zone in southeastern Iberian Peninsula. Two approaches have been used to achieve a well-constrained velocity model in the CAF fault zone. Conventional seismic reflection processing workflow has been used to clean the seismic data and increase its S/N ratio. And Occam's approach has been used to invert the digitized surface wave dispersion curves. 1D shear wave velocity-depth profiles were estimated in shot and CDP/CMP domains. Relatively well-resolved 2D velocity-depth models were obtained by the composite of the 1D Velocity-depth functions. These composite results of the 2D-velocity models are able to constrain the depth geometry of the fault zone down to 200 m depth. The fault zone is indicated by a relatively broad low-velocity anomaly that correlates with the fault¿s surface expression in CMP number 1201-1281. The low velocities would then be related to fracturing and associated local weathering that would reduce the shear modulus value., We would thank for the data permission to project of InterGEO (CGL2013-47412- C2-1-P) ICTJA-CSIC•The Ministry of Education and Culture of the Republic of Indonesia is thanked for main author’s PhD scholarship •JA is funded by MICINN (IJC2018-026335-I). We thank the GIPP-GFZ, (Germany) and•Lisbon University (Portugal) for the instrumentation provided

EGU2020: Sharing Geoscience Online, 4-8 may 2020, Sub-Moho reflectors have been identified in seismic refraction and wide-angle reflection recordings in western Iberia since the late `80s. These control source seismic wide-angle shot records have energy large enough to illuminate the uppermost mantle showing strong sub-Moho arrivals at distant offsets (>180 km) with amplitudes significantly higher than the Pn and a relatively long coda. The kinematics and wavelet characteristics of these features are probably produced by an increase in P-wave velocity, and forward modeling indicates that these arrivals reflect off an interface in the 60-80 km depth range beneath the Iberian Massif. The waveform and time length of this arrival suggests that it can result from the interaction of the seismic energy with a ~10 km thick heterogeneous layer. To test this hypothesis, we used a 2D second-order finite-difference acoustic and elastic full wave-field scheme with a layer consisting of randomly distributed bodies smaller than ¿ of the wavelength of the seismic waves in thickness and ¿Vp=±0.2 km/s at the considered depth range. Resulting synthetic shot gathers reproduce well the observed amplitudes and codas as a result of the constructive interference caused by the tuning effect produced by this gradient heterogeneous zone. The contrast in physical properties and depth level of this feature are consistent with the top of the phase transition from spinel to garnet lherzolite, the so-called Hales discontinuity. Some of the available gathers show a second and deeper reflection. Detailed analysis of the reflected wave-forms suggests that the reflected wavelet has reversed polarity, a feature suggesting. a velocity decrease with depth. Finite difference acoustic and elastic full wave-field modeling places this discontinuity around 90 km depth beneath the Ossa-Morena Zone (south Iberian Massif). A lateral change is observed beneath the Centro-Iberian Zone (central Iberian Massif) where it is imaged at 103-110 km depth on the southeast and shallows up to 80 km depth on the northeast. The indicated depth would be consistent with the depth location of the LAB, which is relatively well constrained for the target area by other geophysical observations., Funding resources: EU EIT-RawMaterials Ref: 17024_20170331_92304; MINECO: CGL2016-81964-REDE CGL2014-56548-P: JCYL: SA065P17).

EGU2020: Sharing Geoscience Online, 4-8 may 2020, Controlled source seismic data acquisition experiments have produced a vast amount of Deep Seismic Sounding (DSS) data since its development in the late 50¿s. These datasets provide critical information on the structure and nature of the crust and the lithosphere, which constitutes a fundamental research tool within Solid Earth Sciences. The DSS datasets are unique and constitute the output of an expensive (in time, effort and cost) scientific process, which evidences the need for their preservation, both the recently acquired and the legacy data. Furthermore, the new developments in processing and imaging techniques generate new possibilities for re-use of the vintage datasets. The availability and accessibility of these datasets, therefore, is of foremost importance for scientists, decision-makers and the general public. The research community, aware of the value of these data, has pushed forward Open Data policies based on the FAIR principles of data management (Findable, Accessible, Interoperable and Reusable). In this respect, a long-term plan has been launched by the European Plate Observation System (EPOS, https://www.epos-ip.org/) e-infrastructure. The focus is to streamline the integrated use of scientific data, data products and services. In close link with EPOS, the Seismology and Earthquake Engineering Research Infrastructure Alliance for Europe (SERA, http://www.sera-eu.org/home, a Horizon 2020 project) includes a working package to set up a network on DSS data and products management. This initiative ensures the traceability of the data allowing that third parties can freely access, exploit and disseminate the data by means of permanent, international identifiers: a Digital Object Identifier (DOI) and a Uniform Resource Identifier (URI) or handle. Furthermore, the current aim is to go beyond the FAIR principles by linking the data with its related peer-reviewed publications, other scientific contributions and technical reports, facilitating its re-use. A prototype DSS data exchange system has been developed jointly between the DIGITAL.CSIC (the Spanish National Research Council) services and the Institute of Earth Sciences Jaume Almera-CSIC (https://digital.csic.es/handle/10261/101879, last access January 2020). Within the platform, each dataset includes the acquired raw data and a metadata file. The metadata provides information of the nature of the data itself, list of authors, the context of the data (time and location of the experiments), funding agencies and other relevant legal aspects. The technical information includes the acquisition parameters, data processing and format of the data (SEGY standard in this case - www.seg.org-, broadly used in the geophysics community). In the developed storage protocol, a permanent identifier is assigned once it has been checked that the data meets all the described requirements. This permanent identifier ensures that any visit or download is accounted for. This information is entered into a statistics referencing database and can also be used as a measure of the impact of the data and/or data product., This work is funded by the European Commission (Grant Agreement no: 676564-EPOS IP, Call H2020-INFRADEV-2014-2015/H2020-INFRADEV-1-2015-1, SERA 730900).

Reprocessing of multi-channel seismic reflection data acquired over the northern margin of the Gulf of Cádiz (SW Iberian margin) places new constraints on the upper crustal structure of the Guadalquivir-Portimão Bank. The data presented have been processed with optimized stacking and interval velocity models, a better approach to multiple attenuation, preserved amplitude information to derive the nature of seismic reflectivity, and accurate time-to-depth conversion after migration. The reprocessed data reveal a bright upper crustal reflector just underneath the Paleozoic basement that spatially coincides with the local positive free-air gravity high called the Gulf of Cádiz Gravity High. To investigate the nature of this reflector and to decipher whether it could be associated with pieces of mantle material emplaced at upper crustal levels, we calculated its reflection coefficient and compared it to a buried high-density ultramafic body (serpentinized peridotite) at the Gorringe Bank. Its reflection coefficient ratio with respect to the sea floor differs by only 4.6% with that calculated for the high-density ultramafic body of the Gorringe Bank, while it differs by 35.8% compared to a drilled Miocene limestone unconformity. This means that the Gulf of Cádiz reflector has a velocity and/or density contrast similar to the peridotite at the Gorringe Bank. However, considering the depth at which it is found (between 2.0 and 4.0 km) and the available geological information, it seems unlikely that the estimated shortening from the Oligocene to present is sufficient to emplace pieces of mantle material at these shallow levels. Therefore, and despite the similarity in its reflection coefficient with the peridotites of the Gorringe Bank, our preferred interpretation is that the upper crustal Gulf of Cádiz reflector represents the seismic response of high-density intracrustal magmatic intrusions that may partially contribute to the Gulf of Cádiz Gravity High., This research was funded by projects WE-ME (PIE-CSIC-201330E111) and MITE (CGL2014-59516-P).

A multidisciplinary study has been carried out in Hontomín (Spain) to determine the basement structural setting, its geometry and the geometry of the sedimentary succession of an area aimed to be the first Spanish pilot plant for CO2 storage. An integration of coincident 3-D seismic results, borehole data and unpublished microgravity data were used to reproduce the deep structure and topography of the basement and to quantify the thickness of the Triassic Keuper evaporites. The subsurface structure is characterized by a half-graben setting filled with Keuper evaporites (up to 2000m thick), forming an extensional forced fold. All data sets clearly identify two main fault systems, compartmentalizing the main structural domain into three differentiated blocks. These faults have been interpreted to be reactivated normal faults that have led to the formation of the Hontomín dome.

The difficulties of seismic imaging beneath high velocity structures are widely recognised. In this setting, theoretical analysis of synthetic wide-angle seismic reflection data indicates that velocity models are not well constrained. A two-dimensional velocity model was built to simulate a simplified structural geometry given by a basaltic wedge placed within a sedimentary sequence. This model reproduces the geological setting in areas of special interest for the oil industry as the Faroe-Shetland Basin. A wide-angle synthetic dataset was calculated on this model using an elastic finite difference scheme. This dataset provided travel times for tomographic inversions. Results show that the original model can not be completely resolved without considering additional information. The resolution of nonlinear inversions lacks a functional mathematical relationship, therefore, statistical approaches are required. Stochastic tests based on Metropolis techniques support the need of additional information to properly resolve sub-basalt structures.

Publicado en la revista como Special Issue:Abstracts of the International Symposium on Deep Earth Exploration and Practices, 24–26 Oct 2018, Beijing, China, The Iberian Peninsula and its margins have been extensively studied by multi-seismic data acquisition experiments since the early 70’s. Relatively old seismic refraction (form the 70’s to the late 80’s) and more modern (early 90’s to present day) spatially dense wide-angle seismic reflection transects have constrained the distribution of P and S wave velocity and the depth and geometry of the main crustal and lithospheric interfaces. Deep normal incidence high resolution seismic reflection surveys imaged: the crustal architecture; the distribution of deformation and help determine the evolution of the most relevant geologic features within Iberia and its margins. Deep seismic imaging started with the Spanish-French collaboration across the Pyrenees (ECORS-Pyrenees), and it was soon after followed by the ESCI program in the mid-80’s. Similar to other Deep Continental Seismic Reflection Programs (ECORS, DEKORP, BIRPS in EU and LITHOPROBE, COCORP in Canada and USA), ESCI acquired a series of on-land and marine seismic transects that revealed the internal architecture of key orogenic belts within Iberia, namely the Betic Cordillera and the northern part of the Iberian Massif (Iberian part of the European Variscan orogen). Later, the EUROPROBE initiative fostered detailed studies of the central and southern part of the Iberian Massif. Two 300 km long transects (IBERSEIS and ALCUDIA) imaged this orogen in SW Iberia. The resulting seismic profiles sampled three major tectonic blocks which collided in the late Paleozoic: form south to north these are: the South Portuguese, the Ossa-Morena and, the Central Iberian blocks. These terranes and their sutures show characteristic seismic fabrics. These seismic signatures reveal a distribution of the deformation that gives valuable insights into the evolution of the orogen, e.g. thickening and thinning mechanisms, igenous activity and strike slip tectonics. The upper and lower crusts appear to be decoupled along an interface that runs through the entire seismic transects. Deformation seems to have been accommodated in different ways at upper and crustal levels in the three tectonic blocks. Furthermore, the northern part of the southernmost transect (the IBERSEIS transect) images a 1-2 s thick high amplitude reflective sill-like structure located at mid crustal levels. It has been interpreted as a mafic sill intrusion along a mid-crustal decollement. The Central Iberian Zone, imaged by the ALCUDIA transect, reveals thick and highly reflective lower crust, the reflection fabric consists of relatively continuous, sub-horizontal to arcuate events, suggesting that deformation is mostly associated with ductile deformation. The Moho features a laterally variable seismic signature (highly reflective beneath the Central Iberian Zone, discontinuous and diffuse below the Ossa-Morena Zone and, sharp and well defined to the south of the South Portuguese Zone). Along the entire transect, the Moho is subhorizontal and located at an average depth of 31-35., Research supports: CGL2014-56548-P, CGL2016-81964-REDE; SIT4ME EIT-KIC-RawMaterials; 2017-SGR-1022

The main objective of this study is to improve the geophysical characterization resulting from a shallow 3D high resolution travel-time tomography survey (500 x 500 m). This survey was acquired in Villar de Cañas (Cuenca, Spain) in late 2013 and early 2014. Lithology down to 150 m depth at this site is characterized by endorheic sediments, mainly siltstone and gypsum. After processing the tomography data, the velocity model showed a good correlation with geology models and borehole data except for the siltstone-gypsum transition. The model involves two lithological limits: the “transition layer - massive gypsum layer” (well resolved by a relatively high velocity contrast) and the “siltstone layer - transition layer” (constrained only in the central part of the model by a relatively low velocity contrast). As electrical resistivity is able to characterize shale-gypsum transitions, we complemented the seismic data with results from a collection of 2D ERT surveys, for which we build a new 3D grid with 2 parameters by node: velocity and resistivity. In order to derive a geological interpretation, we applied a statistical classification method (Linear Discriminant Analysis) to the new bi-parametric grid, using reference classes from well logs. This process resulted in a final 3D lithological model with less ambiguity and thus with a better definition of the two limits under discussion. Our study shows that the integration of seismic and electric methods significantly improves geological characterization in a gypsiferous context. Seismics, electrics and well logging have been shown as a good set for shallow subsoil exploration. However, the integration of the results of these techniques in order to derive a geological interpretation is complicated beyond a qualitative (subjective) correlation. In this study, we propose a quantitative joint interpretation of three separate geophysical datasets (a velocity model, a resistivity model and well logs) to characterize geological transitions in a shallow gypsiferous context in Villar de Cañas (Cuenca, Spain). We integrated the two models in a new 3D grid, and in order to derive a geological joint interpretation, we applied a supervised statistical classification method, LDA. The algorithm was fitted using a training set compiled from the well logs. Thus, every node is lithologically classified according to its velocity-resistivity relationship, resulting in a new 3D lithological model. This new model integrates both seismics and electrics resolution capacities, showing better agreement with geology profiles and topwells than those techniques separately., Research supports: CGL2014-56548-P, CGL2016-81964-REDE; SIT4ME EIT-KIC-RawMaterials; 2017-SGR-1022

Science has grown from our need to understand the world around us. Seismology is no different, with earthquakes and their destructive effect on society providing the motivation to understand the Earth's seismic wavefield. The question of when seismology as a science really began is an interesting one, but it is unlikely that there will ever be a universally agreed-upon date, partly because of the incompleteness of the historical record, and partly because the definition of what constitutes science varies from person to person. For instance, one could regard 1889 as the true birth of seismology, because that is when the first distant earthquake was detected by an instrument; in this case Ernst von Rebeur-Paschwitz detected an earthquake in Japan using a pendulum in Potsdam, Germany (Ben-Menahem, 1995). However, even the birth of instrumental seismology could be contested; the so-called Zhang Heng directional “seismoscope” (detects ground motion but not as a function of time) was invented in 132 CE (Rui and Yan-xiang, 2006), and is said to have detected a four-hundred mile distant earthquake which was not felt at the location of the instrument Needham, 1959, Dewey and Byerly, 1969. Prior to instrumental seismology, observations of earthquakes were not uncommon; for instance, Aristotle provided a classification of earthquakes based on the nature of observed ground motion (Ben-Menahem, 1995).

The elevation of the intracontinental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco. Our imaging resolves the location and shape of lithospheric cavities and of delaminated lithosphere ∼400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle sourced from regional upwellings in northern Africa or the Canary Islands enhanced the instability of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, which we infer to be the site of the most recent delamination. The Atlas Mountains of Morocco stand as an example of large-scale lithospheric loss in a mildly contractional orogen. Key Points We image lithospheric cavities beneath the Middle Atlas and central HighAtlas We image the delaminated lithosphere of the Middle Atlas at ∼400 km depth We propose piecewise delamination of Atlas lithosphere © 2014. American Geophysical Union. All Rights Reserved., This work is funded by the PICASSO project (NSF grant EAR-0808939). The deployment and data processing for Spanish stations was funded by Consolider-Ingenio 2010 project TOPO-IBERIA (CSD2006-00041) as well as ALERT-ES (CGL2010-19803-C03-02).

[1] We present a 3-D shear wave velocity model for the crust and upper mantle of the western Mediterranean from Rayleigh wave tomography. We analyzed the fundamental mode in the 20–167 s period band (6.0–50.0 mHz) from earthquakes recorded by a number of temporary and permanent seismograph arrays. Using the two-plane wave method, we obtained phase velocity dispersion curves that were inverted for an isotropic Vs model that extends from the southern Iberian Massif, across the Gibraltar Arc and the Atlas mountains to the Saharan Craton. The area of the western Mediterranean that we have studied has been the site of complex subduction, slab rollback, and simultaneous compression and extension during African-European convergence since the Oligocene. The shear velocity model shows high velocities beneath the Rif from 65 km depth and beneath the Granada Basin from ∼70 km depth that extend beneath the Alboran Domain to more than 250 km depth, which we interpret as a near-vertical slab dangling from beneath the western Alboran Sea. The slab appears to be attached to the crust beneath the Rif and possibly beneath the Granada Basin and Sierra Nevada where low shear velocities (3.8 km/s) are mapped to >55 km depth. The attached slab is pulling down the Gibraltar Arc crust, thickening it, and removing the continental margin lithospheric mantle beneath both Iberia and Morocco as it descends into the deeper mantle. Thin lithosphere is indicated by very low upper mantle velocities beneath the Alboran Sea, above and east of the dangling slab and beneath the Cenozoic volcanics.

The seismic signature of the Moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image. Seismic data of different types, passive source (earthquake) broad-band recordings, and controlled source seismic refraction, densely recorded wide-angle deep seismic reflection, and normal incidence reflection (using VibroseisTM, explosives, or airguns), have contributed to the description of the Moho as a relatively complex transition zone. Of critical importance for the quality and resolution of the seismic image are the acquisition parameters, used in the imaging experiments. A variety of signatures have been obtained for the Moho at different scales generally dependent upon bandwidth of the seismic source. This variety prevents the development of a single universally applicable interpretation. In this way source frequency content, and source and sensor spacing determine the vertical and lateral resolution of the images, respectively. In most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the Moho, and its relationship to a petrologic crust-mantle transition. In regional seismic studies carried out using passive source recordings the Moho is a relatively well defined structure with marked lateral continuity. The characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area. Refraction and wide-angle studies suggest the Moho to be often a relatively sharp velocity contrast, whereas the Moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity. The origin of the Moho and its relation to the crust-mantle boundary is probably better constrained by careful analysis of its internal details, which can be complex and geographically varied. Unlike the oceanic Moho which is formed in a relatively simple, well understood process, the continental Moho can be subject to an extensive variety of tectonic processes, making overarching conclusions about the continental Moho difficult. Speaking very broadly: 1) In orogenic belts still undergoing compression and active continental volcanic arcs, the Moho evolves with the mountain belt, 2) In collapsed Phanerozoic orogenic belts the Moho under the collapse structure was formed during the collapse, often by a combination of processes. 3) In regions having experienced widespread basaltic volcanism, the Moho can result from underplated basalt and basaltic residuum. In Precambrian terranes the Moho may be as ancient as the formation of the crust, in others Precambrian tectonic and magmatic processes have reset it. We note that seismic reflection data in Phanerosoic orogens as well as from Precambrian cratonic terranes often show thrust type structures extending as deep as the Moho, and suggest that even where crust and mantle xenoliths provide similar age of formation dates, the crust may be semi-allochothonous. © 2013 Elsevier B.V., This contribution was developed while R. Carbonell was at the Earthquake Research Institute of the Tokyo University. R. C. acknowledges funding from the Spanish Ministry of Science and Innovation (grants: CSD2006-00041; CGL2011-24101) and the Generalitat de Catalunya (grant: 2009SGR006).

Hontomín (N of Spain) hosts the first Spanish CO2 storage pilot plant. The subsurface characterization of the site included the acquisition of a 3-D seismic reflection and a circumscribed 3-D magnetotelluric (MT) survey. This paper addresses the combination of the seismic and MT results, together with the available well-log data, in order to achieve a better characterization of the Hontomín subsurface. We compare the structural model obtained from the interpretation of the seismic data with the geoelectrical model resulting from the MT data. The models correlate well in the surroundings of the CO2 injection area with the major structural differences observed related to the presence of faults. The combination of the two methods allowed a more detailed characterization of the faults, defining their geometry, and fluid flow characteristics, which are key for the risk assessment of the storage site. Moreover, we use the well-log data of the existing wells to derive resistivity-velocity relationships for the subsurface and compute a 3-D velocity model of the site using the 3-D resistivity model as a reference. The derived velocity model is compared to both the predicted and logged velocity in the injection and monitoring wells, for an overall assessment of the computed resistivity-velocity relationships. The major differences observed are explained by the different resolution of the compared geophysical methods. Finally, the derived velocity model for the near surface is compared with the velocity model used for the static corrections in the seismic data. The results allowed extracting information about the characteristics of the shallow unconsolidated sediments, suggesting possible clay and water content variations. The good correlation of the velocity models derived from the resistivity-velocity relationships and the well-log data demonstrate the potential of the combination of the two methods for characterizing the subsurface, in terms of its physical properties (velocity, resistivity) and structural/reservoir characteristics. This work explores the compatibility of the seismic and magnetotelluric methods across scales highlighting the importance of joint interpretation in near surface and reservoir characterization., Xènia Ogaya is currently supported in the Dublin Institute for Advanced Studies by a Science Foundation Ireland grant IRECCSEM (SFI grant 12/IP/1313). Juan Alcalde is funded by NERC grant NE/M007251/1, on interpretational uncertainty. Juanjo Ledo, Pilar Queralt and Alex Marcuello thank Ministerio de Economía y Competitividad and EU Feder Funds through grant CGL2014-54118-C2-1-R. Funding for this Project has been partially provided by the Spanish Ministry of Industry, Tourism and Trade, through the CIUDEN-CSIC-Inst. Jaume Almera agreement (ALM-09-027: Characterization, Development and Validation of Seismic Techniques applied to CO2 Geological Storage Sites), the CIUDEN-Fundació Bosch i Gimpera agreement (ALM-09-009 Development and Adaptation of Electromagnetic techniques: Characterisation of Storage Sites) and the project PIERCO2 (Progress In Electromagnetic Research for CO2 geological reservoirs CGL2009-07604). The CIUDEN project is co-financed by the European Union through the Technological Development Plant of Compostilla OXYCFB300 Project (European Energy Programme for Recovery).

The complex tectonic interaction processes between the European and African plates at the Western Mediterranean have left marked imprints in the crustal architecture of this area, particularly concerning the lateral variations in crustal thicknesses. The detailed mapping of such variations is hence of large interest, as it provides a major constraint to geophysical and geodynamic modeling at different scales. Controlled-source seismic profiling and receiver functions from natural seismicity are widely considered as major tools to constrain Moho topography. We compile here the Moho depths determined from a comprehensive number of both types of seismic surveys, to end up with a new 3D Moho depth map of the Iberian Peninsula, its continental margins and North Morocco. Since the 1970s, the lithospheric structure beneath this study area has been extensively investigated using multichannel normal incidence seismic reflection and refraction/wide-angle reflection profiling. In the last few years some high-resolution surveys at sea and inland have been acquired, the latter ones involving ~. 1000 land stations. On the other hand, the TopoIberia-IberArray experiment has triggered the investigations on crustal and lithospheric structure using natural seismicity, providing a homogeneous spatial resolution never achieved before. The availability of good quality results from both methodologies in a common area provides an excellent opportunity to check the consistency of the Moho depth estimations. The integration of both datasets has resulted in a new, high resolution map of the crustal thickness variations. The final grid evidences large Moho topography variations, including crustal imbrication in the Pyrenean range, a large and relatively undisturbed Variscan Massif in the center of Iberia and a probable delamination process beneath the Gibraltar Arc. The crustal thicknesses vary from ~. 15. km in continental margins up to values exceeding 50. km beneath the Pyrenees or the Rif Cordillera. © 2016., Funding for this project has been available from Spanish Ministry of Science and Innovation under grants: CGL 2014-09727, CGL2014-54582-REDC (RedTopoIberia), CGL2009-09727 (RIFSIS), CSD 2006-00041 (TopoIberia), CGL2007-63889 (SIMA), CGL2008-3474 (TopoMed), and by Generalitat de Catalunya grant 2009SGR1595. We have also used data from the PICASSO project, funded by the U.S. NSF grant EAR0808939, the Morocco Münster project funded under grant DFGTH1530/5-1, the WILAS project funded by the Portuguese Fundação para a Ciência e Tecnologia (PTDC/CTE-GIX/097946/2008) and the PYROPE experiment supported by the French Research Agency ANR blanc program (project PYROPE, ANR-09-BLAN-0229).

A multidisciplinary geophysical study along a large seismic transect in the SW-Iberian Peninsula has been carried out. This study integrates the crustal structure, geometry and composition obtained from normal incidence and wide-angle seismic reflection data with other observables (geoid, gravity and topography). The internal architecture of the lithosphere across the Variscan Orogen of SW-Iberia is constrained by the 300 km long high resolution deep normal seismic reflection IBERSEIS Transect. The most prominent feature imaged by the seismic survey is the Iberseis Reflective Body (IRB), a 140 km long high amplitude reflective body located in the middle crust of the northern half of the transect. The seismic velocity (Vp) distribution within the crust and the upper mantle is constrained by two wide-angle seismic transects acquired in the same area. The velocity models show a complex crust, with a specially complex middle crust, which features higher velocities than the average continental crust. Also the wide-angle data revealed that the IRB is characterized by high velocities. This feature was then interpreted as sill-like structure built up by a series of mafic intrusions. Therefore, a key issue is to study if this relatively mafic crust is consistent with other geophysical observables. Based on the velocity models, two lithospheric density models have been derived along the IBERSEIS wide-angle transects. The geoid, gravity and topography response of these models have been calculated using a finite elements code that solves, simultaneously, the geopotential, lithostatic, and heat flow equations. The resulting values are then compared with the measured observables and the crustal and lithospheric mantle geometry and density is modified until the best fit is obtained. The initial density models calculated from the seismic data adjust quite well to the real potential field data. However, minor modifications have been required in order to properly fit the observables. The final density models are consistent with the existence of relatively high density bodies in the mid-crust providing further support to the seismic interpretation. In addition, they place new constraints on the location of the lithosphere–asthenosphere boundary and on the tectonic evolution of SW-Iberia.

Deep and fast wide-angle reflection arrivals observed at offsets over 180 km, and over a reduced time interval of 1–1.5 s, have been observed in a seismic experiment shot across SW Iberia as part of the IBERSEIS project. Using different modelling approaches, these reflections have been found to be consistent with a heterogeneous gradient zone located at 61–72 km depth that features an absolute P-wave velocity contrast from 8.2 to 8.3 km/s. Paradoxically, this interface has not been observed in coincident vertical incidence data, probably due to the change in the reflection coefficient with decreasing incidence angles, the lack of energy at high recording times for the near-vertical (vibroseis) data, and/or the different location of the CDPs in both experiments. Although the mantle is acknowledged to be highly heterogeneous and mantle lithologies are capable of giving impedance contrasts high enough as to be observed in seismic data, it is often seen as transparent from a seismic point of view. The short wavelength of mantle compositional heterogeneities is probably what hinders their identification with active source seismic data and only big and sharp discontinuities are imaged in vertical incidence experiments whereas regional transitional boundaries may be also observed at high incidence angles. Accordingly, we propose that deep reflectivity observed in SW Iberia must correspond to a regional–continental scale feature, not sharp enough as to be seen with vertical incidence energy. This feature, already identified in previous DSS experiments carried out in Iberia, has a depth, a P-wave velocity contrast and a transitional nature that match the characteristics proposed for the spinel-lherzolite to garnet-lherzolite phase transition, i.e. the Hales interface or gradient zone. This boundary is relatively narrow (at least 2–3 kb) in enriched mantle and appears deeper and along wider intervals when the mantle is depleted. In addition, it is a worldwide scale boundary already identified over large areas with different types of datasets. The variability in depth and sharpness of this interface, which is related to mantle chemistry, constrains the type of seismic techniques that should be used to identify it.

Crooked-line 2D seismic reflection survey geometries violate underlying assumptions of 2D imaging routines, affecting our ability to resolve the subsurface reliably. We compare three crooked-line imaging schemes involving prestack and poststack time migration using the 2D IBERSEIS deep seismic reflection profile running over the South Portuguese Zone thrust-and-fold belt to obtain crisp high-resolution images of the shallow crust. The crust is characterized by a complex subsurface geometry with conflicting dips of up to [Formula: see text]. In summary, the three schemes are (1) normal-moveout (NMO) corrections, dip-moveout (DMO) corrections, common-midpoint (CMP) stacking, CMP projection, and poststack time migration; (2) NMO corrections, DMO corrections, CMP projection, zero-offset time migration of the common-offset gathers, and CMP stacking; (3) CMP projection, prestack time migration in the common-offset domain, and CMP stacking. An essential element of all three schemes is a CMP projection routine, projecting the CMPs first binned along individual segments for preprocessing onto one straight line, which is parallel to the general dip direction of the subsurface structures. After CMP projection, the data satisfy the straight-line assumption of 2D imaging routines more closely. We observe that the prestack time-migration scheme yields comparable or more coherent synthetic and field-data images than the other two DMO-based schemes along the parts of the profile where the acquisition overall follows a straight line. However, the schemes involving DMO corrections are less plagued by migration artifacts than the prestack time-migration scheme along profile parts where the acquisition line is crooked. In particular, prominent migration artifacts on the prestack migrated synthetic data can be related to significant variations in source-receiver azimuths for which 2D prestack migration cannot account. Thus, the processing scheme including DMO corrections, CMP projection, and zero-offset migration of common-offset gathers offers a reliable and effective alternative to prestack migration for crooked-line 2D seismic reflection processing.

The Moho is the most conspicuous feature in the ALCUDIA deep normal incidence and wide-angle seismic reflection datasets acquired across the Central Iberia Zone. This discontinuity appears to be sub-horizontal with a slight dip toward the center of the Iberian Peninsula, beneath the Tajo Basin. Densely spaced wide-angle seismic reflection data reveal conspicuous PmP and SmS reflections in the shot records. These reflections, arriving over a 1.5. s time window and characterized by a relatively low frequency content, are used to infer the internal structure and characteristics of this crustal boundary in an effort to place constraints on its nature beneath the studied area. The wide-angle shot records were processed in order to generate low-fold stacks that reveal gradual deepening of Moho PmP phases from 10.2. s TWT (~. 31. km) in the Central Iberian Zone to 11.8. s TWT (~. 36. km) beneath the Tajo Basin. The Moho seismic signature was the main objective of a series of 1-D and 2-D synthetic modeling simulations. Reflectivity modeling was used as the 1-D modeling approach and an elastic finite difference scheme was used for the 2-D modeling. The Moho can be considered to be a self-similar/fractal heterogeneous media with a bimodal velocity distribution, alternating crustal and mantle values, with horizontal correlation lengths that range between 1 and 4. km and vertical correlation lengths that range from 0.2 to 1. km. The depth of this boundary is most probably the result of late to post-variscan re-equilibration processes affecting a previously deformed and laminated lower crust. The mechanisms that took part in this process might have differed to the north and south of the sampled Central Iberian Zone, being more thermally induced to the north, near the Central System. Processes leading to the layered structure of the Moho boundary, either tectonic or magmatic, should be further investigated., Seismic data were collected in 2012 with funding provided by the Spanish Ministry of Science and Innovation (grants: CGL2004-04623/BTE, CGL2007-63101/BTE, CGL2011-24101, CSD2006-00041) and the Junta de Castilla-La Mancha. S.A. (JCA-UG003) We thank an academic crew for data acquisition. Ehsan is funded by the European Commission grant Marie Curie Actions (264517-TOPOMOD-P7-PEOPLE-2010-ITN).

Consistent constraints of an over-thickened crust beneath the Rif Cordillera (N. Morocco) are inferred from analyses of recently acquired seismic datasets including controlled source wide-angle reflections and receiver functions from teleseismic events. Offline arrivals of Moho-reflected phases recorded in RIFSIS project provide estimations of the crustal thicknesses in 3D. Additional constraints on the onshore-offshore transition are inferred from shots in a coeval experiment in the Alboran Sea recorded at land stations in northern Morocco. A regional crustal thickness map is computed from all these results. In parallel, we use natural seismicity data collected throughout TopoIberia and PICASSO experiments, and from a new RIFSIS deployment, to obtain receiver functions and explore the crustal thickness variations with a H-κ grid-search approach. This larger dataset provides better resolution constraints and reveals a number of abrupt crustal changes. A gridded surface is built up by interpolating the Moho depths inferred for each seismic station, then compared with the map from controlled source experiments. A remarkably consistent image is observed in both maps, derived from completely independent data and methods. Both approaches document a large crustal root, exceeding 50. km depth in the central part of the Rif, in contrast with the rather small topographic elevations. This large crustal thickness, consistent with the available Bouguer anomaly data, favors models proposing that the high velocity slab imaged by seismic tomography beneath the Alboran Sea is still attached to the lithosphere beneath the Rif, hence pulling down the lithosphere and thickening the crust. The thickened area corresponds to a quiet seismic zone located between the western Morocco arcuate seismic zone, the deep seismicity area beneath western Alboran Sea and the superficial seismicity in Alhoceima area. Therefore, the presence of a crustal root seems to play also a major role in the seismicity distribution in northern Morocco., Funding for this project has been available from the Spanish Ministry of Science and Innovation under grants: CGL2009-09727 (RIFSIS), CSD 2006-00041 (TopoIberia), CGL2007-63889 (SIMA), and CGL2008-3474 (TopoMed), and by Generalitat de Catalunya grant: 2009SGR996. We have also used data from the PICASSO project, founded by the U.S. NSF grant EAR0808939.

The Iberian Chain is a Cenozoic intraplate thrust belt located within the Iberian plate. Unlike other belts in the Iberia Peninsula, the scarcity of geophysical studies in this area results in a number of unknowns about its crustal structure. The Iberian Chain crust was investigated by means of a NE-SW refraction/wide-angle reflection seismic transect and two magnetotelluric profiles across the chain, oriented along the same direction. The seismic profile was designed to sample the crust by means of three shots designed to obtain a reversed profile. The resulting velocity-depth model shows a moderate thickening of the crust toward the central part of the profile, where crustal thickness reaches values above 40. km, thinning toward de SW Tajo and NE Ebro foreland basins. The crustal thickening is concentrated in the upper crust. The seismic results are in overall agreement with regional trends of Bouguer gravity anomaly and the main features of the seismic model were reproduced by gravity modeling. The magnetotelluric data consist of 39 sites grouped into two profiles, with periods ranging from 0.01 s to 1000 s. Dimensionality analyses show significant 3D effects in the resistivity structure and therefore we carried out a joint 3D inversion of the full impedance tensor and magnetic transfer functions. The Mesozoic and Cenozoic basins along the Chain are well characterized by shallow high conductive zones and low velocities. Elongated conductors reaching mid-crustal depths evidence the presence of major faults dominating the crustal structure. The results from the interpretation of these complementary geophysical data sets provided the first images of the crustal structure of the Iberian Chain. They are consistent with a Cenozoic shortening responsible of the upper crust thickening as well as of the uplift of the Iberian Chain and the generation of its present day topography., Funding for the research was provided by the I + D + I research projects: BTE-2001-4993-E, CGL2008-04916, and by the Consolider-Ingenio 2010 programme, under CSD 2006-00041 “Topo-Iberia.”

© 2015 Shagun Mudgil, Harika Nambula and B. Bharathi. The upper crust beneath the Moroccan Atlas has been modeled using the travel time of wide-angle arrivals from the SIMA (Seismic Imaging of the Moroccan Atlas) dataset. The detailed knowledge of the internal structure of this orogen allows understanding its uncommon characteristics, featuring high topography, moderate tectonic shortening and moderate crustal thickening. The -700 km long SIMA wide-angle reflection seismic profile has provided a high resolution geophysical data set to obtain a shallow P-wave velocity model along the transect. The seismic data processing has enabled us to accurately pick the first arrivals of the seismic records. Subsequently, the development of a numerical code to mathematically model the hodochrones defined by the first arrivals, has provided us the P-wave velocity structure of the crust down to 10 km. The resulting model shows a detailed image of the Atlas upper crust and reveals several relevant features that help to understand the structure of the orogen and its composition., SIMA was funded by a grant from the Spanish Science Foundation (FECYT), involves several institutions like ICTJA-CSIC, Salamanca University and Autonomous University of Barcelona, the Scientific Institute of Rabat, University Cadi Ayyad in Marrakech, FST of Errachidia, University Sidi Mohammed ben Abdellah in Fes, and was supported as part of PICASSO by grant EAR 0808939 from the NSF Continental Dynamics Program

A detailed characterization of the physical properties of a granitic pluton, including a three dimensional reconstruction of the fractures network and its surrounding are obtained by high resolution seismic tomography of P and S-waves. The input P- and S-wave travel times were picked from three-component offset and azimuth variable seismic profile (OVSP) acquired in a 500 m deep vertical borehole. The travel time inversion resulted in two three-dimensional P- and S-wave velocity models, and the uncertainty, robustness and sensitivity are estimated by performing a Monte Carlo-type analysis and checkerboard test. This study provides an indicator of the uniqueness of the determined model. The 500 m deep borehole was continuously cored, and extensively surveyed by geophysical techniques. Thus, the detailed surface geological mapping, fracture index logs, borehole televiewer images and P- and S-wave velocities derived from sonic logs, provide solid constraints for the interpretation of the tomographic images. The three-dimensional tomographic images reveal fracture zones mapped at surface that intersect the borehole at different depths and they can be identified in the core descriptions and in the borehole televiewer images. The fractures identified on the cores feature relatively low values for the sonic P- and S-wave velocities, and high values for Poisson's ratio (σ). For single type lithologies, the distribution of Poisson's ratio differentiates fractured and altered granite from fresh unfractured domains within the rock. Poisson's ratios close to 0.3 correlate with fractured volumes consistent with fracture index and sonic logs, otherwise Poisson's ratio values close to 0.25 are related to unaltered and unfractured rock.

The parameters used for the acquisition of the IBERSEIS deep seismic reflection profile in the SW Iberian Peninsula provide seismic images of the deep crust as well as a high resolution section of the shallow subsurface. A very dense array of sources and receivers allowed high resolution tomographic studies in zones of special interest (granitic plutons). The three dimensional tomographic inversion produced velocity models along a 500 m wide and 1000 m deep strip along the IBERSEIS transect in the areas of La Bazana, La Dehesilla, Feria and Villafranca. In these high resolution velocity models (sampled by 50 × 50 × 50 m cells), high velocity anomalies indicate the geology and extension of the granitic plutons at depth. This directly correlates with the surface outcrops. Moreover, tomographic models provided valuable information for the geometry and characterization of fractured and fresh domains in a rock volume. Furthermore, a piggy back seismic acquisition experiment using additional seismic instrumentation from the University of Paris Sud (40-channel DMT) provided perpendicular, offline recordings of the Vibroseis sources. This additional recording system was deployed perpendicularly to the main IBERSEIS seismic reflection line and provided additional 3D control.

A seismic survey with a receiver spacing of 50 m provided one of the most densely sampled wide-angle seismic reflection images of the lithosphere. This unique data set, recorded by an 18-km-long spread, reveals that at wide-angles the shallow subcrustal mantle features high amplitude reflectivity which contrasts with a lack of reflectivity at latter travel times. This change in the seismic signature is located at approximately 120–150 km depth, which correlates with the depth estimates of the lithosphere–asthenosphere boundary (LAB) of previous DSS studies. This seismic signature can be simulated by two-layer mantle model. Both layers with similar average velocities differ in their degree of heterogeneity. The shallow heterogeneous layer and the deeper and more homogeneous one correlate with the lithosphere and the asthenosphere, respectively. Studies involving surface outcrops of ultramafic massifs and mantle xenoliths infer that the upper mantle is a heterogeneous mixture of ultramafic rocks (lherzolites, harzburgites, pyroxenites, peridotites, dunites, and small amounts of eclogites). Laboratory measurements of physical properties of these mantle rocks indicate that compositional variations alone can account for the wide-angle reflectivity. A temperature increase would homogenize the mixture, decreasing the seismic reflection properties due to melting processes. It is proposed that this would take place below 120–150 km (1200 °C, the LAB).

Theoretical considerations (Snell's law) suggest that low-velocity fanomalies are undersampled and therefore should be poorly resolved by inversion schemes based on ray-tracing methods. A synthetic study considering a 40×20 m low-velocity anomaly (300 m/s) placed at the center of a 400×50 m block with gradient background velocity model (from 3000 m/s at the surface to 4000 m/s at the base) indicates that the low ray density in ray-tracing coverage diagrams of tomographic inversion studies can be used as evidence for the existence of low-velocity anomalies. Combined normal incidence seismic reflection images and the velocity models obtained by tomographic inversions of first-arrival travel times form an efficient scheme to resolve low-velocity anomalies such as fracture zones. Furthermore, the velocity models derived from tomographic inversions are used in a wave equation datuming algorithm to account for statics caused by a strongly laterally variable shallow surface (weathering) layer and provide seismic reflection images of fracture zones (low-velocity anomaly) within a granitic pluton.

SUMMARY Fractured and altered zones within a granitic pluton are mapped in three dimensions by means of high resolution seismic traveltime tomography. The input traveltimes were picked from offset and azimuth variable vertical seismic profiles (OVSP) acquired in three boreholes and from seismic shot gathers of four CDP high resolution seismic reflection profiles recorded on the surface. For the OVSP data a hydrophone streamer placed in the boreholes recorded the acoustic energy generated (a signal with a frequency content between 15 to 150 Hz) by a Vibroseis truck at source points distributed every 30 m in a rectangular grid of 620 m by 150 m. The combination of borehole and surface seismic data resulted in an increase in the ray density of the shallow subsurface. The tomographic algorithm uses a variable model grid, with a finer grid spacing close to the surface were ray density is highest and the velocity variations are strongest. Therefore the resulting velocity models feature more detail at shallow levels. A simple and smooth starting velocity model was derived from P-wave velocity logs. Careful surface geological mapping, and borehole geophysical data, P- and S-wave velocity logs and Poisson’s ratio depth functions, provided key constraints for a physically reasonable 3-D interpretation of the tomograms. The low velocity anomalies constrained by the tomographic images were interpreted as unconsolidated rock, fractures and altered zones which correlate with structures mapped at the surface or velocity anomalies identified in the logs. Subsequent resolution analysis revealed that the derived velocity model is well constrained to depths of 60 m.

The first Spanish Technological Development plant for CO2 storage is currently under development in Hontomín (Spain), in a fractured carbonate reservoir. The subsurface 3D geological structures of the Hontomín site were interpreted using well-log and 3D seismic reflection data. A shallow low velocity zone affects the wave propagation and decreases the coherency of the underlying seismic reflections, deteriorating the quality of the seismic data, and thus preventing a straightforward seismic interpretation. In order to provide a fully constrained model, a geologically supervised interpretation was carried out. In particular, a conceptual geological model was derived from an exhaustive well-logging analysis. This conceptual model was then improved throughout a detailed seismic facies analysis on selected seismic sections crossing the seismic wells and in consistency with the regional geology, leading to the interpretation of the entire 3D seismic volume. This procedure allowed characterizing nine main geological levels and four main fault sets. Thus, the stratigraphic sequence of the area and the geometries of the subsurface structures were defined. The resulting depth-converted 3D geological model allowed us to estimate a maximum CO2 storage capacity of 5.85 Mt. This work provides a 3D geological model of the Hontomín subsurface, which is a challenging case study of CO2 storage in a complex fractured carbonate reservoir., Jaume Almera agreement (ALM-09-027: Characterization, Development and Validation of Seismic Techniques applied to CO2 Geological Storage Sites) and by the European Union through the Technology Demonstration Plant of Compostilla OXYCFB300 Project (European Energy Programme for Recovery). Additional support has been provided by Spanish Ministry of Education Science (CSD2006-00041), Generalitat de Catalunya (2009SGR006) and CSIC JAE-Doc postdoctoral research contract (E.S.).

The multichannel normal incidence (230 km long) deep seismic reflection profile ALCUDIA was acquired in summer 2007. This transect samples an intracontinental Variscan orogenic crust going across, from north to south, the major crustal domain (the Central Iberian Zone) and its suture zone with the Ossa–Morena Zone (the Central Unit) both build up most of the southwestern part of the Iberian Peninsula basement. This high resolution (60–90 fold) profile images about 70 km depth (20 s TWTT) of the continental lithosphere. A new data processing flow provides better structural constraints on the shallow and deep structures resulting in an image that reveals indentation features which strongly suggest horizontal tectonics. The ALCUDIA seismic image shows an upper crust c. 13 km thick decoupled from the comparatively reflective lower crust. The shallow reflectivity of the upper crust can be correlated with surface geological features mapped in the field whereas the deep reflectivity represents inferred imbricate thrust systems and listric extensional faults. The reflectivity of the mid-lower crust is continuous, high amplitude, and horizontal to arcuate though evidences of deformation are present as ductile boudinage structures, thrusting and an upper mantle wedge, suggesting a transpressional flower structure. The image reveals a laminated c. 1.5 km thick, subhorizontal to flat Moho indicating an average crustal thickness of 31–33 km. The Moho shows laterally variable signature, being highly reflective beneath the Central Iberian Zone, but discontinuous and diffuse below the Ossa–Morena Zone. The gravity response suggests relatively high density bodies in the mid-lower crust of the southern half of the transect. The seismic results suggest two major horizontal limits, a horizontal discontinuity at c. 13–15 km (corresponding to the brittle–ductile transition) and the Moho boundary both suggested to act as decoupling surfaces., Seismic data were collected in 2007 with funding provided by the Spanish Ministry of Science and Innovation (grants: CGL2004-04623/BTE, CGL2007-63101/BTE, CGL2011-24101, CSD2006-00041) and the Junta de Castilla-La Mancha. S.A. Ehsan is funded by the European Commission grant Marie Curie Actions (264517-TOPOMOD-FP7-PEOPLE-2010-ITN).

The crustal structure and topography of the Moho boundary beneath the Atlas Mountains of Morocco has been constrained by a controlled source, wide-angle seismic reflection transect: the SIMA experiment. This paper presents the first results of this project, consisting of an almost 700 km long, high-resolution seismic profile acquired from the Sahara craton across the High and the Middle Atlas and the Rif Mountains. The interpretation of this seismic data set is based on forward modeling by raytracing, and has resulted in a detailed crustal structure and velocity model for the Atlas Mountains. Results indicate that the High Atlas features a moderate crustal thickness, with the Moho located at a minimum depth of 35 km to the S and at around 31 km to the N, in the Middle Atlas. Upper crustal shortening is resolved at depth through a crustal root where the Saharan crust underthrusts the northern Moroccan crust. This feature defines a lower crust imbrication that, locally, places the Moho boundary at 40-41 km depth in the northern part of the High Atlas. The P-wave velocity model is characterized by relatively low velocities, mostly in the lower crust and upper mantle, when compared to other active orogens and continental regions. These low deep crustal velocities together with other geophysical observables such as conductivity estimates derived from MT measurements, moderate Bouguer gravity anomaly, high heat flow, and surface exposures of recent alkaline volcanism lead to a model where partial melts are currently emplaced at deep crustal levels and in the upper mantle. The resulting model supports the existence of a mantle upwelling as mechanism that would contribute significantly to sustain the High Atlas topography. However, the detailed Moho geometry deduced in this work should lead to a revision of the exact geometry and position of this mantle feature and will require new modeling efforts, This work has been primarily funded by the Spanish MEC project CGL2007–63889. Additional funding was provided by projects CGL2010–15416, CSD2006-00041, and GL2009–09727 (Spain), CGL2008–03474-E, 07-TOPO_EUROPE_FP-006 (ESF Eurocores) and EAR-0808939 (US, NSF).

The western Mediterranean tectonic system consists of the Betic Mountains in southern Spain and the Rif Mountains in northern Morocco curved around the back-arc extensional Alboran basin. Multiple tectonic models have been developed to explain the coeval compressional and extensional tectonic processes that have affected the western Mediterranean since the Oligocene. In order to provide constraints on these evolutionary models, we use Ps teleseismic receiver functions (RF), thermobarometric analyses of post-Oligocene basalts, and previous teleseismic tomography images to investigate the lithospheric structure of the region. Ps RFs were calculated using seismic data from 239 broadband seismic stations in southern Iberia and northern Morocco and thermobarometric analysis was performed on 19 volcanic samples distributed throughout the region. The RF images reveal a highly variable Moho depth (∼25 to ∼55 km), as well as a strong positive, sub-Moho horizon between ∼45 and ∼80 km depth beneath the central Betic and Rif Mountains, which we interpret to be the top of the previously imaged Alboran Sea slab. Thermobarometric constraints from magmas in the eastern Betics and Rif indicate mantle melting depths between 40 and 60 km, typical of melting depths beneath mid-oceanic ridges where little to no lithosphere exists. Together, the RF and thermobarometric data suggest ongoing and recent slab detachment resulting from delamination of the continental lithosphere. Key Points There is a highly variable lithospheric structure throughout the Gibraltar Arc Slab is delaminating continental lithosphere in Gibraltar Arc Slab pull force is exerted on base of lithosphere causing depression of Moho © 2014. American Geophysical Union. All Rights Reserved., This research was funded by the U.S. National Science Foundation EAR-0808939. The deployment of the IberArray broadband seismic network is part of the CONSOLIDER CSD2006-00041 (Geosciences in Iberia: Integrated studies on Topography and 4D Evolution) grant from the Spanish Ministry of Science and Innovation. Additional funding was provided by the Spanish ministry under grants CGL2010–17280 and by Generalitat de Catalunya under grant 2009 SGR 6.

Deep seismic profiling aims to obtain the structure of crust and upper mantle using techniques such as seismic reflection, and seismic reflection/refraction as well as a series of broad-band seismic techniques including receiver functions and seismic tomography. Data are collected both on land and at sea, and in many cases across crystalline basement areas or basin–basement boundary zones. Over the years, deep seismic profiling has evolved both in methodology and understanding and has been integrated with other earth imaging techniques. Modern deep seismic transects provide clear images of the crust and mantle across various tectonic environments on the globe which constrain geodynamic evolution within key geological provenances.

The different geodynamic models proposed since the late 1990s to account for the complex evolution of the Gibraltar Arc System lack definite constraints on the crustal structure of the Rif orogen. Here we present the first well-resolved P-wave velocity crustal models of the Rif Cordillera and its southern continuation toward the Atlas made using controlled-source seismic data. Two 3001 km-long wide-angle reflection profiles crossed the Rif along NS and EW trends. The profiles recorded simultaneously five land explosions of 1Tn each using 850 high frequency seismometers. The crustal structure revealed from 2-D forward modeling delineates a complex, laterally varying crustal structure below the Rif domains. The most surprising feature, seen on both profiles, is a 50 km deep crustal root localized beneath the External Rif. To the east, the crust thins rapidly by 20 km across the Nekkor fault, indicating that the fault is a crustal scale feature. On the NS profile the crust thins more gradually to 40 km thickness beneath Middle Atlas and 42 km beneath the Betics. These new seismic results are in overall agreement with regional trends of Bouguer gravity and are consistent with recent receiver function estimates of crustal thickness. The complex crustal structure of the Rif orogen in the Gibraltar Arc is a consequence of the Miocene collision between the Iberian and African plates. Both the abrupt change in crustal thickness at the Nekkor fault and the unexpectedly deep Rif crustal root can be attributed to interaction of the subducting Alboran slab with the North African passive margin at late Oligocene-early Miocene times., This research was funded by Spanish RIFSIS project CGL2009–09727. AG benefited from a PhD grant associated to this project. Support was provided by Spanish projects CSD2006-00041 ‘‘Topo-Iberia’’ and CGL2008–03474 ‘‘Topo-Med,’’ as well as the NSF Continental Dynamics Program Grant EAR0808939.

The Basque–Cantabrian Basin of the northern Iberia Peninsula constitutes a unique example of a major deformation system, featuring a dome structure developed by extensional tectonics followed by compressional reactivation. The occurrence of natural resources in the area and the possibility of establishing a geological storage site for carbon dioxide motivated the acquisition of a 3-D seismic reflection survey in 2010, centered on the Jurassic Hontomín dome. The objectives of this survey were to obtain a geological model of the overall structure and to establish a baseline model for a possible geological CO2 storage site. The 36 km 2 survey included approximately 5000 mixed (Vibroseis and explosives) source points recorded with a 25 m inline source and receiver spacing. The target reservoir is a saline aquifer, at approximately 1450 m depth, encased and sealed by carbonate formations. Acquisition and processing parameters were influenced by the rough topography and relatively complex geology. A strong near-surface velocity inversion is evident in the data, affecting the quality of the data. The resulting 3-D image provides constraints on the key features of the geologic model. The Hontomín structure is interpreted to consist of an approximately 107 m2 large elongated dome with two major (W–E and NW–SE) striking faults bounding it. Preliminary capacity estimates indicate that about 1.2 Gt of CO2 can be stored in the target reservoir, Funding for this Project has been partially provided by the Spanish Ministry of Industry, Tourism and Trade, through the CIUDEN-CSIC-Inst. Jaume Almera agreement (Characterization, Development and Validation of Seismic Techniques applied to CO2 Geological Storage Sites) and by the European Union through the Technology Demonstration Plant of Compostilla OXYCFB300 project (European Energy Programme for 534 Recovery). Additional support has been provided by Spanish Ministry of Education Science (CSD2006-00041), Generalitat de Catalunya (2009SGR006) and CSIC JAE-Doc postdoctoral research contract (E.S.).

Detailed study of the reflected phases from the Moho, identified in the seismic reflection recordings of the 1986 Nevada PASSCAL lithospheric seismic experiment, place important constraints on the fine structure and geological nature of the Mohorovicic discontinuity. Two different spectral ratio techniques are used to estimate the reflectivity of the Moho. We evaluate the ratio of the reflected PmP phase to the incident source amplitude. We also calculated the spectral ratio of the PmP phase with a reference reflection. Reflectivity estimates, calculated using both spectral ratio techniques, are very consistent. High reflectivity within the range 0.28–0.39 was obtained. The analysis of the spectral characteristics suggests a layered structure for the Moho with an average thickness between 85 and 250 m. These results require a layered Moho structure with high impedance contrasts. Amplitude modeling using a reflectivity algorithm demonstrates that constructive interference from a 1.0–1.5-km-thick layered structure produces amplitudes and waveforms comparable to the data. To achieve such high reflectivities, 20–25% of the Moho consists of layers with velocities below 6.5 km/s. The remaining layers have velocities between 7.7 and 8.1 km/s. Geologically, the low velocities are consistent with velocities characteristic of basaltic magmas (partial melts). The high velocities are characteristic of mantle material. The partial melts may represent an expression of present-day underplating.

ALCUDIA is a 230 km long, vertical incidence deep seismic reflection transect acquired in spring 2007 across the southern Central Iberian Zone (part of the pre-Mesozoic Gondwana paleocontinent) of the Variscan Orogen of Spain. The carefully designed acquisition parameters resulted in a 20 s TWTT deep, 60–90 fold, high-resolution seismic reflection transect. The processed image shows a weakly reflective upper crust (the scarce reflectivity matching structures identified at surface), a thick, highly reflective and laminated lower crust, and a flat Moho located at 10 s TWTT (30 km depth). The transect can be divided into three segments with different structural styles in the lower crust. In the central segment, the lower crust is imaged by regular, horizontal and parallel reflectors, whereas in the northern and southern segments it displays oblique reflectors interpreted as an important thrust (north) and tectonic wedging involving the mantle (south). The ALCUDIA seismic image shows that in an intracontinental orogenic crust, far from the suture zones, the upper and lower crust may react differently to shortening in different sectors, which is taken as evidence for decoupling. The interpreted structures, as deduced from surface geology and the seismic image, show that deformation was distributed homogeneously in the upper crust, whereas it was concentrated in wedge/thrust structures at specific sectors in the lower crust. The seismic image also shows the location of late Variscan faults in spatial association with the lower crustal thickened areas.

Slant stacks of seismic data from rifted ancient Archean crust along the southwest coast of Greenland indicate that the lower crust is strongly seismically anisotropic. The slant stack data show that a lower-crustal, wide-angle S-wave reflection has a different intercept time and ray parameter on the radial and transverse components from a receiver gather recorded on Passcal Reftek three-component seismometers. From the S-wave analysis, the continental crust is clearly seismically anisotropic above a high-velocity wedge in the lower crust. Possibly, magmatic underplating during Late Cretaceous Labrador Sea rifting heated the pre-existing lower crust promoting plastic flow and enabling alignment of anisotropic minerals to produce the seismic anisotropy.

In 2003 two wide-angle reflection/refraction seismic transects were acquired in the Variscan Belt of SW Iberia. The approximately 250 and 300 km long, dense trace spacing transects revealed clear S wave arrivals in the shot gathers recorded by vertical component sensors in both transects. First S wave arrivals (Sg) and Moho reflections (SmS) are the most prominent phases that can be correlated from shot to shot. Sg is observed up to relatively large offsets and constrains the upper and middle crust S wave velocities. The SmS is seen from offset 0 (18 s twtt) to 150 km offset, where it intercepts first S wave arrivals (Sg). The upper mantle refracted phase (Sn) is difficult to recognize, although PmS/SmP converted phases can be identified. Using a 2-D ray tracing approach, two S wave velocity models for the crust of SW Iberia were obtained. These S wave velocity models complement the previous P wave velocity models and provide us with relatively well resolved Poisson's ratio crustal sections for SW Iberia. The resulting Poisson's ratio models present differences between tectonic zones at upper and middle crustal depths, thus supporting the existence of different tectonic zones prior to the Variscan collision. The most noteworthy feature is the high Poisson's ratio value (over 0.28) coincident with high P wave velocity areas (over 6.8 km/s) at midcrustal depths. In order to constrain the possible crustal composition, P wave velocities and Poisson's ratios have been compared with published laboratory measurements on different crustal rock types. This comparison indicates that the high P wave velocity and Poisson's ratios are compatible with a mixture of mafic to ultramafic rock types alternating with felsic ones. This result is consistent with the existence of mafic layered bodies in the middle crust, in the same way that has been suggested by previous works in this area.

Layering at the Moho as determined from seismic reflection data is a commonly observed phenomenon in the Basin and Range for which we propose a porous-flow melt model. Analysis of seismic reflection data from the 1986 PASSCAL Nevada experiment indicates that the Moho in the Nevada Basin and Range is layered and has unusually high reflectivity. This reflectivity is difficult to explain in terms of a velocity contrast caused by different rock types, and, in fact, a lower velocity of less than 6.5 km/s suggests the presence of partially molten material at the Moho in a zone containing 20% to 25% of low-velocity layers. To explain the seismic observations, we have developed a viscous, porous-flow model for basaltic melt ascent. Such a model produces systematic variations in volumetric melt flow with depth and close correspondence between seismic interpretation and modeling of partial melt ascent. This suggests that such layering involving viscous, porous flow of partial melt is a plausible explanation of the layering of the Moho in the Basin and Range and may have more general application.

In the absence of multiple scattering the lateral correlation function of the reflected wavefield can be related to the lateral correlation function of the medium. We have analyzed the lateral correlation function of a series of synthetic seismograms from stochastic models of reflective lower crust with the structural and petrophysical statistics of the Ivrea Zone. Our results show that in the presence of Ivrea-type lower crustal heterogeneity the lateral correlation function of the backscattered wavefield is insensitive to the lateral correlation function of the scattering medium, which suggests that multiple scattering is important in the reflective lower crust. This interpretation is consistent with lower crustal reflections commonly being shorter than a Fresnel zone, the absence of clear diffractions and the poor performance of standard migration algorithms on deep seismic reflection data.

13 pages, 10 figures, 4 tables, Seismic reflection profiling is applied to the study of large scale physical oceanographic processes in the Gulf of Cádiz and western Iberian coast, coinciding with the path of the Mediterranean Undercurrent. The multi-channel seismic reflection method provides clear images of thermohaline fine structure with a horizontal resolution approximately two orders of magnitude higher than CTD casting. The seismic data are compared with co-located historical oceanographic data. Three seismic reflectivity zones are identified: North Atlantic Central Water, Mediterranean Water and North Atlantic Deep Water. Seismic evidence for the path of the Mediterranean Undercurrent is found in the near-slope reflectivity patterns, with rising reflectors between about 500 and 1500 m. However, the core of the undercurrent is largely transparent. Seismic images show that central and, particularly, intermediate Mediterranean Waters have fine structure coherent over horizontal distances of several tens of kilometers. However, the intensity of the reflectors, and their horizontal coherence, decreases downstream. This change in seismic reflectivity is probably the result of diminished vertical thermohaline contrasts between adjacent water masses, so that double-diffusion processes become unable to sustain temperature and salinity staircases. Comparison of root-mean-square seismic amplitudes with temperature and salinity differences between the Mediterranean Undercurrent and the overlying central waters suggests a causal relationship between observed thermohaline fine structure and true seismic amplitudes. We estimate that, within this intermediate water stratum, impedance contrasts are mainly controlled by sound speed contrasts (a factor between 3.5 and 10 times larger than density contrasts), which are mainly controlled by temperature contrasts (a factor between 1.5 and 5 times larger than salinity contrasts), The authors would like to extend thanks to the New and Emerging Science and Technology (NEST) initiative of the European Union—The GO Project (NEST-2003-1 adventure), the Spanish Ministry of Education and Science (CGL200404623), GEOCEAN, TOPOIBERIA (CSD2006-00041) and the Generalitat de Catalunya (2005SGR00874) for financial support

[1] We reprocessed an ∼35-km-long part of the IBESREIS seismic reflection profile which runs over the Iberian Pyrite Belt section of the South Portuguese Zone, SW Iberia, with the goal to image the upper crust (

Traveltime seismic tomography used to aid subway tunnel drilling in Barcelona, Spain, provides a detailed characterization of the shallow subsurface, including a complex network of faults and dikes. We encountered many difficulties while working in this urban setting, such as cultural noise and the inability to deploy instruments along much of the tunnel trace because of the street layout. Furthermore, the shallow subsurface is highly heterogeneous (because of gas and water lines, building foundations, and the sewage system), obscuring deeper structures. We acquired seismic data using a 10-s vibroseis sweep that provided relatively good signal-to-noise (S/N) ratio. First-arrival traveltime tomography proved to be a key tool to differentiate rock vol-umes characterized by different physical properties — in particular, the contact between the weathered upper layer and the more competent rock below. We interpreted the seismic results using surface geologic observations and borehole cores. Low seismic velocities (600–1200 m/s) characterize the shallow part of the subsurface, representing quaternary sediments and weathered granite. Several high-velocity anomalies (up to 5500 m/s) are interpreted as porphyritic dikes and are surrounded by low-velocity anomalies that correspond with fault systems that cut and displace the dikes. The tomographic velocity models also provide a detailed image of an important Miocene fault that intersects the tunnel trace. These results provide insights important for the drilling process., This work was funded by UTELinia9. Additional funding was provided by Generalitat de Catalunya under contract 2005SGR00874.