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Surface wave methods have been developed as a commonly used non-invasive tool for shear-wave velocity (Vs) profiling of near-surface soil and rock formations. Inversion is a key step to back-calculate the Vs profile from dispersion curves; however, the non-linear and ill-posed nature of the problem sometimes yields non-unique Vs profiles. To overcome these challenges, a new method was developed to circumvent inversion and directly estimate Vs profiles from the fundamental mode dispersion trend for sites with Vs gradually increasing with depth. This method is developed based on the relationship between the phase velocity of surface waves at a given frequency with the time-averaged shear-wave velocity within a depth of one-half wavelength. Three field sites and three simulated sites covering shallow, medium and deep locations with gradually increasing Vs with depth are employed to verify the feasibility of the proposed methodology. Results indicate that the method can efficiently give a reasonable and realistic estimation of Vs profiles. The goal of this method is to provide an alternative to obtain Vs profiles with high certainty and low computational cost for surface wave testing, but not necessarily to find a ‘better’ profile than inversion or to locate each layer interface.

A major electrically conducting crustal structure has been imaged since the 1980 s using widely-spaced long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) sites across the Archean to Paleoproterozoic Eyre Peninsula, South Australia. The structure extends from the southern continental margin, trending SW-NE for at least 300 km. To constrain depth and lateral extent of the anomaly a transect of 39 broadband MT sites were collected with ∼ 2 km separation across lower Eyre Peninsula. Two-dimensional inversion images a low-resistivity conductor at depth 2–10 km, 20 km wide and dipping ∼ 10° to the east, spatially correlated with a topographic high. The conductor is bound on the eastern margin by the transpressional Kalinjala Shear Zone along the Paleoproterozoic Donington Suite, and on the western margin by the Archean Sleaford Complex. The low resistivity structure, with conductance of 20,000 S occurs in Paleoproterozoic carbonaceous marine sediments of the Darke Peak and Cleve groups that were thickened to amphibolite-grade metamorphic depths of at least 15 km and 550 ⁰C during the Kimban Orogeny (1730–1690 Ma), converting carbon to flake graphite in metapelites. Tight folding and exhumation of amphibolite-grade metapelites during the Kimban Orogeny, and dextral strain along the Kalinjala Shear Zone resulted in a laterally continuous electrical crustal conductor which outcrops as economic-grade graphite deposits adjacent to Kalinjala Shear Zone mylonites. In addition, a ∼ 40 km wide lower crustal conductor of integrated conductance 2000 S is imaged west from the Kalinjala Shear Zone. This lower crustal conductor may also be graphitic and represents deeply buried metapelites that have not been exhumed. We argue that the Paleoproterozoic Eyre Peninsula Anomaly results from graphitisation of sediments with significantly enhanced amount of organic carbon compared to any other time in Earth’s history, during the ∼ 2 Ga Lomagundi–Jatuli Event, which is associated with the great oxidation event.

Late Cretaceous-Cenozoic large-scale extension in the eastern South China Block (SCB) has been widely studied. While the exact timing is not well constrained and the geodynamics is inconclusive. Here, we studied the cooling and uplifting history of the southern Lower Yangtze River area (LYRA) in the northeastern SCB using the fission track chronology. The apatite fission track (AFT) single grain ages are decomposed into two best-fit age peaks during 80–50.2 Ma and 43.7–34.5 Ma, respectively. The thermal history modeling of apatite reveals that there exists a rapid cooling process during ca. 80–50 Ma, a slow cooling process during ca. 50–35 Ma and another rapid cooling process during ca. 35–12 Ma. The 80–50.2 Ma age peak corresponds to the rapid cooling process during ca. 80–50 Ma, and the 43.7–34.5 Ma age peaks roughly correspond to the slow cooling process during ca. 50–35 Ma. The tectonic analysis shows that the rapid cooling stage during ca. 80–50 Ma, linked to the rapid rifting in the southern LYRA and coeval strong rifting in the northern LYRA, has resulted from flat subduction of young oceanic crust of the Izanagi Plate. The slow cooling stage during ca. 50–35 Ma is linked to the subduction of the Izanagi-Pacific spreading ridge. The another rapid cooling stage during ca. 35–12 Ma corresponds to a rapid uplifting of the southern LYRA, resulted from the regional compression and tectonic inversion in East China.

The northeastern of the South China Sea (SCS) has undergone tectonic inversion from a Mesozoic convergent continental margin to a Cenozoic divergent continental margin. The remnant subduction-related structures, such as volcanic arc and forearc basin, likely preserve key clues to reveal the dynamic mechanism of this tectonic inversion. We applied ocean bottom seismometers (OBS) data, combining with multi-channel seismic data, gravity data, drilling data, and petrologic observations, to reveal geophysical and structural features of the Dongsha Basin, a Mesozoic basin in the northeastern SCS. The Mesozoic strata are characterized by large thickness (~1–8 km), relatively high P-wave velocities (3.5–5.5 km/s), high densities (2.55–2.65 g/cm3), alternating folds, and thrust faults, which are significantly different from the features of Cenozoic strata. The Dongsha Basin is flanked by a Mesozoic granitic arc to the northwest and an inferred Palaeo-Pacific subduction zone to the southeast, indicating that it is geographically located in the Mesozoic forearc region. Considering the similar Mesozoic strata detected in the southern margin of the SCS and the contemporaneous accretionary complex outcrops observed in the Palawan Continental Terrane, we propose that the Mesozoic strata in these two regions originated from a common Mesozoic forearc basin, which was broken by the forearc rifting and subsequently separated by the spreading of Cenozoic SCS. Accordingly, a continuous Palaeo-Pacific subduction system is reconstructed, including the “arc-forearc-accretionary wedge” tectonic sequences. Reviewing the mechanisms of forearc rifting globally and the retreat of volcanic arcs onshore South China, we speculate that rollback of the Palaeo-Pacific subduction slab might be one of the causes for the forearc rifting and the initial extension of the SCS.

An Ms 6.0 earthquake occurred in Yao'an County, Yunnan Province, on 9 June 2009. According to the results of relocated aftershocks, we find that the distribution of aftershocks has differences in the strike direction and depth in local areas. Therefore, the fault is divided into two segments. Combined with the surface deformation information obtained from InSAR Data as the constraint condition of the slip model, we inverse the three-dimensional coseismic slip distribution of the seismogenic fault of the Yao'an earthquake and discussed the influence of the Yao'an earthquake on the surrounding faults and the rupture direction of the event. The main research results are as follows: 1) we quickly obtain reasonable fault parameters, model, and slip distributions through relocated aftershocks and InSAR data, and the results obtained after inversion are consistent with the results of the geological structure and scholars' research. At the same time, the abnormal deep slip is found, which proves that there is a complex structure underground in the Yao'an area. 2) The results of static Coulomb stress change show that the aftershocks are basically distributed in the area where the static Coulomb stress increases. Faults with a relatively significant increase of Coulomb stress include the northwest section of Honghai Fault, the middle of Chenghai Fault, the middle of Hongge Fault, the middle of Nanhua-Chuxiong Fault, and the north of Yuanmou Fault. The future seismic risk of these faults should be considered. 3) The main direction of the Yao'an earthquake rupture is controlled by regional stress, and the abnormal rupture direction is affected by the complex underground structure.

With the aim to investigate the rupture complexity and the radiated wave field of 2017, Mw 3.9, Ischia earthquake, south-west of Naples (Italy), we used finite-fault modeling to invert the near-source (and searched for the best-fit kinematic rupture parameters. This analysis showed that the rupture nucleated at about 600 m west of IOCA and 1.1-km depth, along a 1 km, NW-SE striking fault (i.e., thrust with right-lateral component), with a rupture velocity of about 0.7 km/s. The retrieved rupture model coupled with multipath reverberations effects related to a thin, low-velocity near-surface volcanic sedimentary layer, well explains the observed long ground motion duration and the large amplitudes recorded all over the island. Finally, the apparent source time function (STF), obtained from inverse modeling using a theoretical Green’ function (GF), is validated by implementing an empirical GF (EGF) analysis.

A new three-dimensional (3D) inversion strategy is applied to new compilations of gravity and magnetic data to reassess the role of crustal lineaments in the development of the western Laurentian margin, Selwyn Basin, and associated sediment-hosted zinc–lead (Zn–Pb) deposits. The region’s history is obscured by multiple tectonic overprints, including terrane accretion, plutonism, and thrust faulting. Regionally continuous, broadly northeast-trending crustal lineaments, including the Liard line, Fort Norman structure, and Leith Ridge fault, were interpreted as having had long-standing influence on craton, margin, and sedimentary basin development. An east–northeast-trending lineament, Mackenzie River, traced from the Misty Creek Embayment to Great Bear Lake, is interpreted as the southern edge of a cratonic promontory. The location of the Liard line, associated with a transfer fault that bounds the Macdonald Platform promontory, is refined. New geophysical results support the continuity of the Fort Norman structure below the Selwyn Basin, but limited evidence exists for the Leith Ridge fault in this area. A northwest-trending lineament that bounds the craton is interpreted as a crustal manifestation of lithospheric thinning of the Laurentian margin, as echoed by a change in the depth of the lithosphere–asthenosphere boundary. The structure delimits the eastern extent of mid- to Late Cretaceous granitic intrusions and is straddled by Mississippi Valley-type Zn–Pb occurrences, following their palinspastic restoration. Clastic-dominated Zn–Pb occurrences are aligned along another northwest-trending lineament interpreted to be associated with a shallowing of lower crustal rocks.

The Qena Bend of the Nile River, named after Qena town in Upper Egypt, is considered a remarkable geomorphological feature in southern Egypt. It is unique and its geodynamic formation is still not fully understood. This study addresses the relationship between the bend geometry and the structural setting of the underlying basement complex using remotely sensed and aeromagnetic data. The hillshade and drainage extraction algorithms were applied to the digital elevation model (DEM) of the Shuttle Radar Topography Mission (SRTM) to define the lineaments dissecting the limestone plateaus and control the Nile Valley cliffs. Moreover, the Interferometry Synthetic Aperture Radar (InSAR) coherence image was derived from two Sentinel-1 images to delineate and/or confirm the structures, which underlie and control the sand dunes movement. In addition, various edge-detection derivatives were applied on the reduced-to-pole (RTP) aeromagnetic anomaly to define the basement structures. The results showed the derived surface and subsurface structures are controlled by the NW–SE and NE–SW trends of the Suez–Red Sea, Aqaba–Dead Sea, and Qena–Safaga Shear Zone. The RTP anomaly reveals a strong NE–SW positive anomaly zone coinciding with the bend. The magnetic 2D forward modelling and 3D depth inversion suggest the basement consists of granitic rocks (0.02 – 0.033 cgs) and the positive anomaly below the bend probably attributes to a major uplift at a depth of 750 m. This uplift broke the plateau forming NE–SW-oriented weak structural zones along which the bend was developed.

The study aims to understand geometry and evolution of accommodation zones along restraining and releasing bends in strike slip systems which is often complex and characterized by sharp change in polarity, resulting in a difficult characterization of traps location. A batch of sandbox modelling experiments was performed using a layered scaled sand material. The setup parameters of the wooden baseplates used in the first two experiments (SS1 and SS2) are basement offset of 6.5 cm/3 cm, 90°/90° stepovers in releasing and restraining bends of total 96 cm length and 25 cm width. The third experiment (SS3) was performed with basement offset of 6.5 cm/3 cm, 156°/126° stepovers in releasing and restraining bends of total 96 cm length and 25 cm width. The experiments were performed with special attention to the role of syn-kinematic sedimentation and the pre-existing structure of the basement. A sequence of pop-up and pull-apart structures was produced. Along the main structures, complex fault trend was compatible with a sinistral riedel distribution and a counterclockwise rotation of pre-existent elements. Change of fault's polarity at depth and dip direction flipping of high angle faults in accommodation zones were clearly observed. Even more, progressive propagation of the pop-up structures produces a spectacular inversion of adjacent extensional structures. Comparing results of the model with the Seagap fault zone, it is possible to put in evidence similar fault orientation and distribution. Internal structures developed were strongly influenced by syn-kinematic sedimentation as observed by characteristic helicoidal shape of growth faults. The analogue models clearly describe how boundary faults of the pull-apart basin along a paired bend, initially characterized by a normal kinematic can be subsequently inverted in the later stages, which can potentially have a strong impact on the petroleum system.

We investigate regional variations in the Lg-wave quality factor (Q) in the Indian Shield with a tomographic method using the LSMR (an iterative solver of least squares algorithm) iterative solver. Vertical broadband seismograms of 223 crustal earthquakes recorded by 113 stations deployed on the Indian Shield are processed to extract the Lg amplitude spectra. The relatively large dataset with good ray coverage allows us to image the lateral variation of the crustal attenuation over the region. The tomographic inversion reveals that the Lg Q0 in the Indian Shield varies from 50 to 650, while the frequency-dependent parameter (η) varies between 0.4 and 1.1 with an average value of 0.7. The attenuation image at 1 Hz suggests the heterogeneities in the Indian crust and clearly demarcates the diverse geological regimes that were hitherto not known. Structural features such as rift, suture zones, sedimentary, and active regions are characterized by high attenuation (Q0

We re-examined the slip distribution on faults of the 2004 Sumatra–Andaman (M 9.1 according to USGS) earthquake by the inversion of tsunami data with phase-corrected Green’s functions applied to linear long waves. The correction accounts for the effects of compressibility of seawater, elasticity of solid earth, and gravitational potential variation associated with the motion of mass to reproduce the delayed arrivals and the reversed phase of the first tsunami waves. We used sea surface height (SSH) data from satellite altimetry (SA) measurements along five tracks, and the tsunami waveforms recorded at tide gauges (TGs) and ocean bottom pressure gauges (OBPGs) in and around the Indian Ocean. The inversion results for both data sets for different rupture velocities (Vr) show that the reproducibility of the spatiotemporal SSHs and tsunami waveforms is improved by the phase corrections, although the effects are not so significant within the Indian Ocean. The best slip distribution model from joint inversion of SA, TG and OBPG data with Vr of 1.3 km/s shows the largest slips of 16–25 m off Sumatra Island, large slips of 2–11 m off the Nicobar Islands, and moderate slips of 2–6 m in the Andaman Islands. The inversion results reproduce the far-field tsunami waveforms well at distant stations even more than 13,000–25,000 km from the epicenter. The total source length is about 1400 km and the seismic moment is Mw 9.2, longer and larger than that of our previous estimates based on TG records.

Gravity inversion, as a static potential field inversion, has inherent ambiguity with low vertical resolution. To reduce the nonuniqueness of the inversion, it is necessary to impose the a priori constraints derived by other geophysical inversions, drilling, or geologic modeling. Based on the a priori normalized gradients derived from seismic imaging or reference models, a structure-guided gravity inversion method with a few known point constraints is developed for mapping density with multiple layers. The cubic B-spline interpolation is used to parameterize the forward modeling calculation of the gravity response to smooth density fields. A recently adopted summative gradient is used to maximize the structural similarity between the a priori and inverted models. First, we have determined the methodology, followed by a synthetic fault model example to confirm its validity. Monte Carlo tests and uncertainty tests further examine the stability and practicality of the method. This method is easy to implement; consequently, it produces an interpretable density model with geologic consistency. Finally, we apply this method to the density modeling of the Chezhen Depression in the Bohai Bay Basin. Our work determines the distribution of deep Lower Paleozoic carbonate rocks and Archean buried hills with high-density characteristics. Our results are consistent with the existing formation mechanism of the “upper source-lower reservoir” type oil-gas targets.

In this study, the crustal shear velocity structure beneath the Indo-Burma Ranges (IBR) is estimated using inversion modeling of the receiver functions computed at seismic stations operated during 2011–2018. The inversion results describe significant variations in the shear-wave velocity (Vs), velocity ratio (Vp/Vs) and crustal thickness in and around the IBR. The Moho deepens from 41 km in the west to 70 km in the east of the IBR. It is also observed that most of the earthquakes are confined to the crust in the west of the Indo-Burma arc, whilst the east of the IBR is associated with sub-crustal earthquakes, supporting the fact that the Indian plate is dipping beneath the Burma Plate. The dip angle of the Moho in the north of the IBR is smaller (7.8o) than that in its south (9.5o). We found that an average crustal Vs of 3.5 km/s for the highest value of the Vp/Vs ratio (> 1.8) was prevalent at all the stations in the IBR, associated with mafic prehnite-pumpellyite facies basalt and hydroxyl zeolite facies basalt, which suggests that the subducting Indian slab beneath the Myanmar microplate in and around the IBR is under a state of dehydration, where intra-slab earthquakes at varying depths (> 50 km) occur. This observation is highly consistent with the earlier 3-D tomographic results obtained independently.

Fault block reservoirs are one of the main types of hydrocarbon reservoirs found in offshore basins, and they are widely distributed within the Mesozoic and Cenozoic basins of the northern South China Sea. Conventional seismic profiles of complex fault areas often contain obvious fragmentation and distortion of seismic events that is corresponding to geological structure under the fault. This phenomenon is known as a fault shadow; it occurs in relation to rapid changes in velocity near the fault that deviate the ray path of waves, and it seriously affects the ability to determine the geological structure and subsequently evaluate the reserves of fault reservoirs. In the current conventional tomography inversion method, the velocity model is over-smoothed, which results in distortion of the reflection layer under the fault. Based on the velocity tomography inversion of imaging gathers method and the concept of regularization, this paper first introduces the anisotropy Gauss regularization operator. A high-resolution tomography inversion method is then developed, and the fault-controlled geological guidance is constrained. This technology is then applied to a complex fault block reservoir basin in the South China Sea, and the results show that it can significantly solve the problem of fault shadow imaging and determine the geological structures in the target area. The newly developed method thus has very good application prospects.

The upper Assam shelf is a self-slope basin in north-east India, filled with nearly 7 km of sedimentary rocks of tertiary period with the granite basement and various oil fields along the border of the Naga thrust. The major producing fields are structural and strati-structural. The study area is placed in between the Mikir hills and Naga thrust. The objective of the study is to identify potential hydrocarbon reservoir zones in the geologically complex south upper of the Assam shelf using estimates of acoustic impedance and porosity derived by 3D post-stack seismic inversion. Well data, such as sonic velocity and density logs, from two wells (namely, KA and TE) are used in the inversion and validation of results. Inversion results are used to build a geological model in the form of acoustic impedance from which we derive 3D porosity cube which are used for hydrocarbon potential in the Paleocene to lower Oligocene sands, and the Precambrian basement. Although the amplitude maps provide an indication of potential reservoirs, the extent of these zones are much better identified in the inverted impedance maps and the corresponding estimated high-porosity zones. The analysis predicted the potential reservoir rocks in the Sylhet, Kopili and Barail formations, in which the Sylhet and Kopili appear to have good potential zones. Near the vicinity of the Naga thrust belt, the proximity of potential reservoir is predicted in the Kopili, Sylhet formation and in the fractured basement, respectively.

The estimation of petrophysical and fluid-filling properties of subsurface reservoirs from seismic data is a crucial component of reservoir characterization. Seismic amplitude-variation-with-offset (AVO) inversion driven by rock physics is an effective approach to characterize reservoir properties. In general, PP-wave reflection coefficients, elastic moduli, and petrophysical parameters are nonlinearly coupled, especially in multiple-type pore-space reservoirs, which makes seismic AVO petrophysics inversion ill-posed. We have developed a new approach that combines Biot-Gassmann’s poroelasticity theory with Russell’s linear AVO approximation, to estimate the reservoir properties including elastic moduli and petrophysical parameters based on multitrace probabilistic AVO inversion algorithm. We first derive a novel PP-wave reflection coefficient formulation in terms of porosity, stiff-pore volume fraction, rock-matrix shear modulus, and fluid bulk modulus to incorporate the effect of pore structures on elastic moduli by considering the soft and stiff pores with different aspect ratios in sandstone reservoirs. Through the analysis of the four types of PP-wave reflection coefficients, the approximation accuracy and inversion feasibility of the derived formulation are verified. Our stochastic inversion method aims to predict the posterior probability density function in a Bayesian setting according to a prior Laplace distribution with vertical correlation and prior Gaussian distribution with lateral correlation of model parameters. A Metropolis-Hastings stochastic sampling algorithm with multiple Markov chains is developed to simulate the posterior models of porosity, stiff-pore volume fraction, rock-matrix shear modulus, and fluid bulk modulus from seismic AVO gathers. The applicability and validity of our inversion method is illustrated with synthetic examples and a real data application.

The oil-sand reservoirs in the Athabasca region of Canada are estuarine deposits affected by tides. The strata are inclined, and the interlayers are well-developed. Accurate spatial characterization of reservoirs and interlayers is the key for efficient oil-sand development. In this paper, we have used prestack Bayesian lithofacies classification technology to predict the spatial distribution characteristics of reservoirs and interlayers of oil-sand reservoirs. We first use log lithofacies data as a label, select lithofacies sensitive elastic parameters to make a lithofacies classification probability distribution crossplot, and then project the lithofacies-sensitive elastic parameter volumes into the lithofacies classification probability distribution crossplot. Finally, we predict the spatial probability distribution of different lithofacies. Probabilistic characterization can enhance the recognition of transitional lithology and thin layers in the inversion results, reduce the uncertainty in the prediction of reservoirs and interlayers, and significantly improve the prediction accuracy of reservoirs and interlayers. The field application results in the Kinosis study area indicate that the probability volume predicted by this technology can distinguish interlayers greater than 1 m thick and identify interlayers greater than 2 m thick, which meets the technical requirements of oil-sand steam-assisted gravity drainage development.

Due to high metal values and raised difficulties in finding superficial deposits, the exploration for mineral resources needs to reach greater depths. To achieve this objective, integrating ASTER and 3D inversion of aeromagnetic can be a useful tool in imaging deep magnetic mineralization hosting-structures. In this study, we present such integrated approach for deep mineral exploration in some promising zones of Gabal (G) Semna region, Eastern Desert (ED) of Egypt. Aeromagnetic data was used to delineate the structures and their relation to ore deposits and ASTER data was utilized for mapping lithological units and the hydrothermal zones. The 3D inversion was employed to imagine the depth and geometry of the magnetized ore bodies in the zones that identified by processing ASTER data. The 3D magnetic inversion revealed that mafic rocks and metavolcanics exhibit high-amplitude magnetic anomalies. High amplitude (positive) magnetic anomalies are joined with the large susceptibilities metavolcanics. Alteration zones in the mafic to basic bedrock and alongside major fault systems are high potential zones that revealed by ASTER image analysis. Aster and aeromagnetic data were used to map and alteration zones surface geology that can host mineralization and the 3D inversion was applied to outline the subsurface extent of these promising areas. This approach can be broadly used for deep mineral exploration in the Egyptian ED and identical areas around the world.

Highlights • The crustal thickness of the Zhongsha Block ranging from ~6 to ~25 km. • Rapid transition from the Zhongsha Block to the adjacent oceanic basins was revealed. • Different oceanic structures were observed in the adjacent oceanic basins. • The pre-rift lithospheric configuration may affect the process of rifting and seafloor spreading. Abstract Continental rifting, break-up, and onset of seafloor spreading are inherently controlled by the segmentation and structure of the continental domain suffering from extension. Today, the Zhongsha Atoll (ZS) is wedged between the Northwest Sub-basin (NWSB) and the Southwest Sub-basin (SWSB), two oceanic abyssal basins of the South China Sea (SCS). The nature of the crust and the structure of the transition from continental to oceanic domain are key to revealing the processes and dynamics during the rifting and break-up of the Zhongsha block. In this paper, we present a P-wave velocity model obtained from both forward modeling and tomographic inversion of wide-angle seismic line OBS2017-2. The results support the continental nature of the Zhongsha Block with a thickness of up to ~25 km. However, the transition from the thick continental domain of the ZS into both adjacent abyssal basins shows clear differences. To the north, a ~120 km wide domain of extended continental crust was observed. Farther north, the NWSB is characterized as a narrow basin with typical oceanic crust. The transitional domain between the continental and oceanic crust shows a ~30–40 km wide region with a high-velocity lower crust reflecting excessive magmatism. In contrast, the SWSB is characterized by a sharp transition from the thick continental crust of the ZS to thin oceanic crust which is probably underlain by serpentinized mantle. The strong rheological properties of the pre-rift crust in the western part of the SCS margin may be the reason that rifting concentrated on narrow rifts and thinning focused on necking domains, while the ZS avoided any intense extension. The configuration of rigid blocks thereafter affected the break-up position and the style of oceanic crust.

The 14 January 2021 Mw 6.2 Mamuju-Majene earthquake occurred in West Sulawesi, Indonesia, preceded by three earthquakes with Mw 5.7, Mw 5.2 and Mw 4.3. The fault responsible for the mainshock remains an enigma. Using static Global Positioning System (GPS) data surrounding the epicentre, we estimated the coseismic displacements of the mainshock. Significant coseismic displacements were estimated at stations located northward and southward from the epicentre. Inversion of GPS data was performed along with two possible fault sources; they are the Makassar Strait Central fault and the Mamuju fault. While the Makassar Strait Central fault is an offshore sloping fault where the location of the most southern segment of the fault is in off the western coast of the Mamuju region, the Mamuju fault is a nearshore fault that runs northward from off the western coast of Majene to Mamuju. During our investigation, we found a larger misfit between GPS data and the resulting model on the coseismic slip modelling along the Makassar Strait Central fault. We also found that the Coulomb stress change using the coseismic slip along the Mamuju fault model explains aftershocks better than the other models. This study suggests that the 2021 Mamuju-Majene earthquake occurred along the bending fault plane of Mamuju fault with the cumulative seismic moment of four earthquakes occurred on 14 January 2021 is 6.6 × 1018 N·m, or equivalent to Mw 6.4.

The 2019–2020 Southwest Puerto Rico earthquake sequence ruptured multiple faults with several moderate magnitude earthquakes. Here, we investigate the seismotectonics of this fault system using high-precision hypocenter relocation and inversion of the near-field strong motions of the five largest events in the sequence (5.6≤Mw≤6.4) for kinematic rupture models. The Mw 6.4 mainshock occurred on a northeast-striking, southeast-dipping normal fault. The rupture nucleated offshore ∼15 km southeast of Indios at the depth of 8.6 km and extended southwest–northeast and up-dip with an average speed of 1.55 km/s, reaching the seafloor and shoreline after about 8 s. The 6 January 2020 (10:32:23) Mw 5.7 and the 7 January 2020 (11:18:46) Mw 5.8 events occurred on two east–southeast-striking, near-vertical, left-lateral strike-slip faults. However, the 7 January 2020 (08:34:05) Mw 5.6 normal-faulting aftershock, which occurred only 10 min after the Mw 6.4 normal-faulting mainshock, ruptured on a fault with almost the same strike as the mainshock but situated ∼8 km farther east, forming a set of parallel faults in the fault system. On 11 January 2020, an Mw 6.0 earthquake occurred on a north–northeast-striking, westing-dipping fault, orthogonal to the faults hosting the strike-slip earthquakes. We apply template matching for the detection of missed, small-magnitude earthquakes to study the spatial evolution of the main part of the sequence. Using the template-matching results along with Global Positioning System analysis, we image the temporal evolution of a foreshock sequence (Caja swarm). We propose that the swarm and the main sequence were a response to a tectonic transient that most affected the whole Puerto Rico Island.

Summary Lufeng oil field was discovered in the 1980s in the Pearl River Mouth Basin (PRMB) of the South China Sea and has entered the stage of secondary oil recovery. One major problem that has restricted the subsequent exploration of the oil field is the unclear regional reservoir and caprock distribution, because practitioners have been using post-stack attributes and acoustic impedance inversion to analyze the distribution of sandstone (reservoir) and mudstone (caprock). In current geophysical research on reservoirs and caprocks, prestack inversion has been widely used because of its advantage over post-stack inversion. However, the accuracy of using single P/S-wave velocity ratio (VP/VS) or density (Vden) inverted by prestack to predict lithology is still insufficient. In this study, we created a new attribute VRDEN, the sum of VP/VS and the weighted Vden, to capture the lithology variation of the reservoir. We integrated 3D seismic data and well log data and applied simultaneous prestack inversion and multiattribute regression analysis to determine reservoir properties, such as sand thickness, effective porosity, and distribution of sandstone and mudstone of the Lufeng oil field. Then, we calculated the new attribute VRDEN from VP/VS and Vden obtained from simultaneous prestack inversion to determine the lithology variation. The multiattribute regression analysis, combining prestack attributes and post-stack attributes, indicates the effective porosity and sand volume in the Enping Formation, which contains the main oil-bearing reservoirs in the Lufeng oil field. Results show that when the sandstone thickness is greater than 12.5 m, the prediction error of VRDEN is the lowest compared with VP/VS and Vden. In En-2 member of the Lower Enping Formation, medium- to high-porosity (14 to 17%) sandstone (19.5 m thickness) is widely distributed in the west and middle of the study area, with an area of nearly 300 km2. The high-porosity sand zones stretch from the east of the Lower Enping Formation to the west of the Upper Enping Formation, which is the result of westward progradation of the braided delta. Our workflow used a novel seismic attribute VRDEN in the simultaneous prestack inversion and multiattribute regression process to provide a more predictive spatial distribution of reservoir-nonreservoir features. The improved reservoir understanding will allow more efficient exploitation of the Lufeng oil field, and the improved workflow will facilitate exploration of other oil fields in the world.

Well penetrations on the UK East Shetland Platform (ESP) prove 1–8 km-thick Devonian post-orogenic extensional collapse-related successions. Conversely, extremely thick (1–6 km) Permo-Triassic basin-fills without Devono-Carboniferous units were in the past interpreted west of the Utsira High, on the Norwegian Horda Platform and Stord Basin, albeit Pre-Triassic well penetrations are here very rare. In this work, the nature and age of Paleozoic–Triassic strata and structures in these underexplored platform regions are tentatively constrained by performing cross-border regional seismic interpretations east and west of the Viking Graben. We highlight cross-border analogies in structural style and seismic facies, with a similar evolution dominated by polyphase inversion tectonics and structural grain preservation. In the Norwegian study areas, much of the half-graben sedimentary fills may be interpreted as Devonian–?Carboniferous in age as in the ESP, rather than overly thick Permo-Triassic successions. Major graben-bounding extensional faults are low angle ( c. 25°–33°), approximately north-striking and are likely to be rooting downwards into reactivated Caledonian shear zones. Rifting development occurred in multiple episodes, possibly creating different traps. Prior to Permian–Jurassic rifting, many low-angle Caledonian thrusts were subject to extensional inversion in the Devonian and then to Variscan compressional reactivation, causing vertical extrusion and deformation of Devonian synrift wedges.

Subsurface rocks contain pores and cracks of various sizes. The cracked porous medium elastic wave theory that describes wave propagation characteristics due to the pore-crack interaction is extended to include cracks of different aspect ratios. The extended theory is applied to model P- and S-wave velocity data of dry and fluid-saturated rock under pressure loading conditions, so as to determine the pore-aspect-ratio spectrum through an inversion procedure. The inversion result is consistent with that from the scanning electron microscope analysis, showing significant improvement versus previous inversion. The inverted pore-aspect-ratio spectrum is input into the wave theory to predict the velocity dispersion of the rock in the full frequency range. The predicted dispersion and its variation trend with pressure agree with the data measured in the (2–200, 106) Hz range at various differential pressures, whereas the modeling using a single-aspect-ratio theory has difficulty matching the data. This research work provides not only a method for analyzing the pore structure characteristics of rocks from the laboratory ultrasonic velocity data, but also a way to predict the seismic wave dispersion from the data.

Earthquakes of the 2015 Teresva series have been relocated using differential arrivals only of their P-waves at the same set of seismic stations and source-specific station terms. At least six distinct groups had been identified in the series as a result of single linkage clustering analysis of cross-correlations between their waveforms. Differential arrivals were estimated separately in each group, and not directly relative to the master event, but through the chains of events with the largest cross-correlations. Time drift at some Ukrainian stations had been detected by comparing intervals between the first P-waves from the same earthquakes at pairs of stations and taken into account, assuming a linear drift rate. The relocated epicenter of the main MSH3.5 earthquake was only ~ 2.3 km to the east of the macroseismic one and almost exactly at the intersection of the two major local faults, perpendicular and parallel to the Carpathians arc. The almost linear alignment of the other earthquakes in azimuth ~ 320° almost coincided with the parallel fault and with the nodal plane of the almost purely strike-slip focal mechanism estimated for the strongest earthquake from its first polarities at 26 stations, and by moment tensor inversion of the ground displacement amplitudes and duration of the first P-wave pulses at 18 stations. A very interesting oscillatory (cyclic) pattern of the epicenter migration along the SE–NW axis, obtained as a result of relocation, was also confirmed by variations in S-relative to P-wave delays: During the cycle, the epicenters gradually shifted to NW and at the beginning of the new cycle returned.

Water depth is important information for lake research. However, it is difficult to obtain the depth distribution of a whole lake. The method of remote-sensing inversion by combining bathymetric da...

In this paper, we elucidated deformational and kinematic characteristics of the NW-SE trending Vigodi Fault (VF) and its associated subsidiary faults in the western Kachchh basin. We applied paleostress inversion technique to understand the past stress regime from the acquired fault slip data. The subsidiary faults strike NW-SE and NE-SW and slickensided surfaces are well developed on the Fault planes. We present slip analysis and changes in the stress reorientation using the plane solutions to identify the principal stress axis (σ1) position regionally. To obtain Pressure Tension and Null axis (PBT), stress ratio, and principal (σ1) axis we used the right Dihedron Method using the Win-Tensor and T-Tecto software's respectively. Geomorphologically, the VF zone comprises of bedrock strath terraces towards the upthrown block and fill terraces on the downthrown block of the fault. Development of fault scarps, warped Quaternary sediments, slit canyons within Quaternary sediments, river nick-points, and deflected rivers are the major striking features observed along the strike length of the Vigodi Fault. The analyses indicate that NE-SW compressional stress dominates the regional area and the local area is affected by transpressional stress exhibiting oblique slip motion. The combined observations made from the present investigation can be used to estimate seismic hazard and mitigation planning of the study area.

The Abiz fault is the longest surface rupture in Iran, which constructed the northeastern boundary of the Lut Block in Eastern Iran. The relationship between geometrical complexities and the stress heterogeneities along the segmented and curved Abiz fault has been examined to get insight into the cause of deflection of the fault segments and also the degree of fault maturity. The northern half of the Abiz fault rupture, based on the position of the splay fault, the surface and the depth slip variations, the width of step-overs, the ground motion and damages, and the aftershock concentration, has more maturity than the southern half. The Abiz fault has nine step-overs along the length (i.e., four releasing step-overs and five restraining step-overs) in which from the north of Ardekul toward the south, with the decrease in the degree of maturity, the width of the step-overs and the size of overlaps are larger than those in the northern segments. The Abiz fault has been divided into seven segments based on the step-overs, bends, the previous earthquake ruptures, the slip deficits, and the dip direction deviation. These segments have become favorably oriented with respect to the stress field. The stress heterogeneities along the fault segments represent the local stress pattern around the curved slip surfaces. It is expected that this pattern would be compatible with the stress orientations due to the aftershock focal mechanism inversion.

Quantitative characterization of gas hydrate systems on continental margins from seismic data is challenging, especially in regions where no well logs are available. However, probabilistic seismic inversion provides an effective means for constraining the physical properties of subsurface strata in such settings and analyzing the variability related to the results. We have applied a workflow for the characterization of two deepwater gas hydrate reservoirs east of New Zealand, where high concentrations of gas hydrate have been inferred previously. We estimate porosity and gas hydrate saturation in the reservoirs from multichannel seismic data through a two-step procedure based on geostatistical seismic and Bayesian petrophysical inversion built on a rock-physics model for gas hydrate-bearing marine sediments. We find that the two reservoirs together host between [Formula: see text] and [Formula: see text] of gas hydrate, with the best estimate at [Formula: see text]. This estimate provides a first-order assessment for further gas hydrate evaluations in the region. The two-step statistically based seismic inversion method is an effective approach for characterizing gas hydrate systems from long-offset seismic reflection data.

Seismic reservoir characterization is a subfield of geophysics that combines seismic and rock-physics modeling with mathematical inverse theory to predict reservoir variables from the measured seismic data. An open-source comprehensive modeling library that includes the main concepts and tools is still missing. We have developed a Python library called SeReMpy with state-of-the-art of seismic reservoir modeling for reservoir properties characterization using seismic and rock-physics models and Bayesian inverse theory. The most innovative component of the library is the Bayesian seismic and rock-physics inversion to predict the spatial distribution of petrophysical and elastic properties from seismic data. The inversion algorithms include Bayesian analytical solutions of the linear-Gaussian inverse problem and Markov chain Monte Carlo (MCMC) numerical methods for nonlinear problems. The library includes four modules: geostatistics, rock physics, facies, and inversion, as well as several scripts with illustrative examples and applications. We illustrate the use of the functions of the module and develop codes for practical inversion problems using synthetic and real data. The applications include a rock-physics model for the prediction of elastic properties and facies using well-log data, a geostatistical simulation of continuous and discrete properties using well logs, a geostatistical interpolation and simulation of 2D maps of temperature, an elastic inversion of partial stacked seismograms with Bayesian linearized amplitude-variation-with-offset inversion, a rock-physics inversion of partial stacked seismograms with MCMC methods, and a 2D seismic inversion.

2D electrical resistivity tomography (ERT) data inversion and ground penetrating radar (GPR) techniques were used to investigate a cave in New Netim area, southeastern Nigeria. The aim of the research was to apply the non-destructive geophysical techniques in delineating a cave in New Netim. The Wenner electrode configuration was used during the electrical resistivity tomography data acquisition while the common-offset mode was used in the ground penetrating radar scanning. The ERT inversion and GPR results quantitatively show strong contrast in electrical resistivity and dielectric permittivity between the host rock (i.e., limestone) and the void. The differences in the electrical properties and dielectric permittivities of the top loamy soils, weathered limestone and the host rock (i.e., Mfamosing limestone) observed, in contrast to the response from the void were explored. The two combined geophysical techniques with their inversion results gave a reliable near surface geologic model, where both methods show relatively good correlation with regards to delineating the depth to top of the cave (~2.7 m). The joint interpretation of the ERT and GPR was effective in reducing the uncertainty associated with geophysical methods used in locating cavities on a standalone basis. The low cost, fast, environmental friendly and high resolving abilities though at shallow depths of the ERT and GPR techniques have continued to favour their usage in resolving near surface environmental and engineering problems. Generally, caves provide important geotourism attraction sites, hence certain recommendations were put forward that can guide further studies, like the application of Steiner’s geostatistical inversion techniques and other post-exploration development strategies.

We have developed 3D inversion models derived from airborne gravity and magnetic data, which are constrained by seismic and well data, in eastern Somaliland. The density model reveals a northwest–southeast-trending basin, 125 km long and 25 km wide and called the Dood Arale Basin. The basin comprises two subbasins separated by a basement high and is infilled by up to 2500–3200 m of sediments. Smaller and shallower subbasins are also identified to the west of Lafaweyne and northeast of Dararweyne. The density model shows that the top basement in the platform areas is at approximately 1500–1700 m in depth and shallows to approximately 300 m at the Bur Anod, Hagraajin, and Hagrin Ranges and northwest of Eil Afwein. The basement depths in these areas are more uncertain and could be deeper because they occur in areas of high gravity anomalies caused by a combination of near-surface high-density sediments and high-density plutonic bodies within the basement. The susceptibility model indicates that the basement consists of very weakly magnetized metasediments of the Inda Ad Complex intruded by three northeast–southwest-trending magnetic bodies with upper surfaces at depths of approximately 300–3000 m. These magnetic bodies are interpreted as plutonic complexes of similar age and composition to the Lower Cretaceous syenite intrusions outcropping at Gorei in the Shilah Madu Range. Seismic reflection profiles image the sedimentary sequences, but they do not clearly map the top basement or detect any of the plutonic bodies. The plutonic bodies could have controlled the location of the basin’s border faults and contributed to the high geothermal gradient recorded at the Faro Hills-1 well. The Upper Cretaceous Gumburo and Jesomma Formations in the basin could potentially have reached maturation close to and above the plutonic bodies within the center of the basin.

Block F3 North Sea is a block with pore pressure values that vary over time due to complex geological conditions such as burial and various sedimentation zones. Pore pressure is one of the important aspects that need to be analyzed as a basis for the identification of zones and overpressure mechanisms. Overpressure is a greater pore pressure condition than normal pressure and may cause drilling problems, such as kicks, blowouts, etc. This study calculated pore pressure values using the eaton method approach with well data and seismic data. Both data are integrated for generating pore pressure values in 1D and 3D. 1D Modelling uses Interactive Petrophysics 3.5, while 3D modeling uses Petrel software. In 3D modeling, the variables used are interval velocity and inversion velocity obtained by acoustic impedance inversion. The sub-variables used are the inversion density and the regression density obtained from well density acoustic impedance inversion. The existence of a 1D overpressure zone at a depth of 1,100 – 1,800 m with an overpressure value of 3,836 – 18,975 kPa. In addition, the overpressure value based on the 3D model is 8,000 – 18,000 kPa. The overpressure zone is validated using an acoustic impedance inversion model with a high value of 5,200 – 5,380 (m/s)*(gr/cc). Overpressure in Block F3 is predicted to occur from disequilibrium compaction..

On January 27, 2021, a small-scale loess landslide occurred in Dangchuan Village, Yongjing County, Gansu Province, China. Timely monitoring and warning helped to prevent any casualties. The sliding mass of the Dangchuan landslide fell in two stages with an interval of 24 s and lasted for a total of 31 s. Eventually, the Dangchuan landslide had a descent height of approximately 120 m and a sliding portion stretching approximately 350 m. The landslide volume was approximately 3 × 104 m3. Based on the investigation of different stages of the Dangchuan landslide and the characteristics of the deposition area, the induction of this landslide was attributed to the effect of frozen stagnant water. Following field investigation of the landslide source and crack areas, a potential sliding mass model with a volume of 5 × 104 m3 was obtained via calculation. To predict the disaster scope of potential landslides, we first used DAN3D software to invert this Dangchuan landslide. Then, based on the numerical model parameters obtained from the inversion, the sliding accumulation process of the potential slide mass was simulated. The results showed that the maximum disaster scope of the potential slide mass would be larger than that of the Dangchuan landslide. The new deposit would cover an area of approximately 3.5 × 104 m2, with a thickness in the range of 0–6 m. The new deposition area would expand by approximately 50 m in the long-axis direction compared to the Dangchuan landslide, impacting the houses and roads. The DAN3D prediction results of the disaster scope will help residents in the potential danger zone evacuate in advance, reducing unnecessary damage.

– Applicability of seismic FWI to archaeological targets with strong topography. – Reconstruction of building phases and state of preservation using seismic FWI.

The 8 August 2017 Mw 6.5 Jiuzhaigou earthquake occurred in a tectonically fractured region in southwest China. We investigate the multifault coseismic rupture process by jointly analyzing teleseismic, strong-motion, high-rate Global Positioning System, and Interferometric Synthetic Aperture Radar (InSAR) datasets. We clearly identify two right-stepping fault segments and a compressional stepover based on variations in focal mechanisms constrained by coseismic InSAR deformation data. The average geometric parameters of the northwest and southeast segments are strike = 130°/dip = 57° and strike = 151°/dip = 70°, respectively. The rupture model estimated from a joint inversion of the seismic and geodetic datasets indicates that the rupture initiated on the southeastern segment and jumped to the northwestern segment, resulting in distinctive slip patches on the two segments. A 4-km-long coseismic slip gap was identified around the stepover, consistent with the aftershock locations and mechanisms. The right-stepping segmentation and coseismic rupture across the compressional stepover exhibited by the 2017 Jiuzhaigou earthquake are reminiscent of the multifault rupture pattern during the 1976 Songpan earthquake sequence farther south along the Huya fault system in three successive Ms∼7 events. Although the common features of fault geometry and stepover may control the similarity in event locations and focal mechanisms of the 2017 and 1976 sequences, the significantly wider (~15 km) stepover in the 1976 sequence likely prohibited coseismic rupture jumping and hence reduced seismic hazard.

To investigate the crustal structure of the Bitlis–Poturge Suture Zone and its vicinity, gravity complete Bouguer anomaly data obtained from the World Gravity Map (WGM2012) were analyzed using Spectral Depth Estimation, Frequency Filtering, Total Horizontal Derivative (THD) and Parker-Oldenburg inversion techniques. The THD method was applied to the data after band-pass filtering of Bouguer gravity data in order to image the discontinuities in the basement levels which are determined by the critical wave numbers determined from the amplitude spectrum of gravity complete Bouguer anomaly data. The maximum amplitude values of the THD were used to reveal the discontinuities caused by the density difference in the BPSZ and its vicinity. In addition, the basement upper surface topography of the region was calculated and mapped with the inverse solution. The presence of an uplift area that is an antiroot in the south of the study area, as well as the subsidence that is a root of the BPSZ was determined. As a result of the inverse solution, it was determined that the depth of the sedimentary basement in the south approximately reaches 6 km and was detected the presence of approximately 7 km thick sedimentary deposit on the BPSZ. Basement upper surface depth is calculated as 8 km in average under the Eastern Anatolian High Plateau.

To study the crustal structure of the Eastern Pontides, gravity data obtained from the World Gravity Map (WGM2012) were analyzed using total horizontal derivative (THD), Parker–Oldenburg (PO) inversion, and normalized full gradient (NFG) techniques. The THD method was applied to the data after bandpass filtering of the Bouguer gravity data in order to image the discontinuities in the basement levels. The maximum amplitude values of the THD were used to reveal the discontinuities caused by the density difference in the Eastern Pontides and its vicinity. In addition, the basement upper surface topography of the region was calculated and mapped with the inverse solution, and the presence of uplift areas in the Eastern Pontides was determined. As a result of the inverse solution, it was found that the depth of the basement in the Eastern Pontides belt reached approximately 5.5 km. An average basement upper surface depth of 3 km was calculated for the area under the Black Sea coastline. Finally, depth values were calculated using the NFG method under the determined profiles, and the depth values found by the inverse solution were tested. For the basement upper surface topography, the results obtained from the two depth calculations were found to be compatible, within error limits of ±0.2 km.

The near surface in the Middle East, particularly in the Sultanate of Oman, is characterized by very shallow high-velocity carbonates and anhydrites interleaved by slow-velocity clastic layers, resulting in sharp velocity inversions in the first few hundred meters below the surface. In addition, the surface is characterized by features such as unconsolidated materials within dry riverbeds (known as “wadis”), small jebels, and sand dunes, which cause distortions in the underlying shallow and deeper seismic images. This work presents the building of a near-surface model by using multiwave inversion that jointly inverts information from P-wave first breaks and surface-wave dispersion curves. The use of surface waves in combination with first breaks captures the lateral and vertical velocity variations, especially in the shallowest parts of the near surface. This paper focuses on the analysis of two drawbacks of this technology: the picking of the input data information, which can be cumbersome and time consuming, and the limited penetration depth of surface waves at the typical frequencies of active data. To overcome these issues, an innovative workflow is proposed that combines the use of an unsupervised machine learning technique to guide the pick extraction phase and the reconstruction of ultra-low-frequency surface waves (0.5 to 1.5 Hz) through an interferometry process using information from natural and ambient noise. Deeper near-surface P- and S-wave velocity models can be obtained with multiwave inversion using these ultra-low frequencies. The integration of a near-surface model into the velocity model building workflow brings a major improvement in depth imaging from shallow to deep structures, as demonstrated on two data sets from the Sultanate of Oman.

Gas hydrates are ice-like crystalline materials that form under submarine environments of moderate pressure and low temperature. Another key factor to their formation is the abundance in gas supply from depth in addition to local biogenic gas. Detailed imaging and velocity analysis of the plumbing system of gas hydrates can provide confidence that amplitude anomalies in seismic data are related to gas hydrate accumulations. We have conducted 2D elastic full-waveform inversion (FWI) along a 14 km long segment of a 2D multichannel seismic profile to obtain a high-resolution velocity model of a hydrate system on the southern Hikurangi margin. We compare the FWI velocity model to previously published semblance- and tomography-based velocity models from the same data to explore how much more can be gained from the FWI. The FWI yielded a structurally more accurate velocity model that better delineated the low-velocity zone associated with free gas beneath the bottom simulating reflector (BSR) compared to the semblance- and tomography-based velocity models. Our results also find a lateral velocity inversion, that is, a narrow low-velocity zone surrounded by bands of higher velocities at a seaward-verging protothrust fault, which the two other methodologies failed to resolve. The FWI provides an improved lateral resolution making it an important tool when imaging the “plumbing” systems of gas hydrate reservoirs. In the southeastern limb of the anticline, our results find that the closely spaced landward-vergent protothrusts provide gas-charged fluids for hydrate formation above the BSR. Moreover, at the center of the anticline, our results find that a seaward-vergent protothrust fault appears to be acting as a conduit for gas-rich fluids into strata, although there is no accumulation of any significant hydrate above the BSR at the apex of the anticline. Our finding emphasizes the significance of densely spaced faults and fractures for providing gas for hydrate formation in the hydrate stability zone.

Orogenic gold deposits provide a significant source of the world’s gold and form along faults over a wide range of crustal depths spanning sub-greenschist to granulite grade faces, but the source depths of the gold remains poorly understood. In this paper we compiled thirty years of long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) data across western Victoria and south-eastern South Australia that have sensitivity to the electrical resistivity of the crust and mantle, which in turn depend on past thermal and fluid processes. This region contains one of the world’s foremost and largest Phanerozoic (440 Ma) orogenic gold provinces that has produced 2% of historic worldwide gold production. Three-dimensional inversion of the long-period MT and GDS data shows a remarkable correlation between orogenic gold deposits with > 1 t production and a 20 km. This low-resistivity region is consistent with seismically-imaged tectonically thickened marine sediments in the Lachlan Orogen that contain organic carbon (C), sulphides such as pyrite (FeS2) and colloidal gold (Au). Additional heat sources at 440 Ma due to slab break-off after subduction have been suggested to rapidly increase the temperature of the marine sediments at mid to lower crustal depth, releasing HS− ligands for Au, and CO2. We argue that the low electrical resistivity signature of the lower crust we see today is from a combination of flake graphite produced in situ from the amphibolite grade metamorphism of organic-carbon in the marine sediments, and precipitated graphite through retrograde hydration reactions of CO2 released during the rapid heating of the sediments. Thus, these geophysical data image a fossil source and pathway zone for one of the world’s richest orogenic gold provinces.

The seismic responses of the shale properties are critical for shale gas reservoir evaluation and production. It has been widely reported that the clay minerals have substantial influences on the seismic wave anisotropy and brittleness. Hence, knowing the seismic responses of the clay-rich shales and estimation of shale elastic properties are significant for the shale gas industry. A physical model containing two groups of shale blocks as the target formations is constructed in laboratory. The group S contains six shale blocks with different clay contents, and the group N contains six shale blocks with different porosity. The acquired 2D seismic data is used to analyze the seismic responses of two corresponding seismic lines. An anisotropic three-term inversion method is applied to one of the seismic inline to estimate the elastic properties the target shale blocks. The inversed attributes can be used to reveal the effects of shale clay contents. This study shows the substantial significance of using a physical model to observe the seismic responses of shale properties. The inversion results indicate that the anisotropic three-term inversion method could provide accurate results of elastic properties as well as the P- wave anisotropy parameter for shale formations.

Subvolcanic hydrothermal systems can lead to hydrothermal eruptions as well as unrest phenomena without an eruptive event. Historical eruptions and recent unrest events, including ground inflation, demagnetization, and a gradual decrease in the plume height, at Mt. Tokachidake, central Hokkaido, Japan, are related to such a subvolcanic hydrothermal system. This study investigates the three-dimensional (3-D) resistivity structure of Mt. Tokachidake to image its subvolcanic hydrothermal system. A 3-D inversion of the magnetotelluric data, acquired at 22 sites around the crater area, was performed while accounting for the topography. Our resistivity model was characterized by a high-resistivity layer at a shallow depth (50–100 m) and two conductors near the active crater and dormant crater. The high-resistivity layer was interpreted to be composed of dense lava, which acts as a caprock surrounding the conductor. The high conductivity beneath the active crater can be explained by the presence of hydrothermal fluid in fractured or leached zones within the low-permeability lava layer, as the sources of ground inflation and demagnetization were identified within the conductive zone immediately beneath the resistive layer. The resistivity structure was used to estimate the volume of hydrothermal fluid within the pore space. The minimum volume of hydrothermal fluid beneath the active crater that can explain the resistivity structure was estimated to be 3 × 106 m3. This estimate is comparable to the water volume that was associated with the long runout and highly fluidized lahar in 1926. The resistivity structure and volume of hydrothermal fluid presented in this study can be used as a reference for further numerical simulations, which aim to reveal the mechanisms of recent unrest events and assess the risk of hazards, such as lahar.

The origins of overpressure in the central Xihu depression have been accepted to result from disequilibrium compaction and hydrocarbon generation. This study uses organic matter correction of the log data in source rocks and five empirical methods (the multilogging combination method, Bowers’ method, the sonic velocity–density cross-plot method, the porosity method, and the pressure inversion method) to understand the origins of overpressure in the central Xihu depression. Overpressure strata are mainly distributed in the following two areas: 1) the Pinghu Formation on the western slope of the central Xihu depression; 2) the Pinghu Formation and lower part of the Huagang Formation in the western sag and the central Inversion anticline belt. The areas contain two types of pressure profiles: normal pressure–overpressure and normal pressure–overpressure–normal pressure. At the western slope and the western sag, overpressure in source rock and nonsource rock was caused by hydrocarbon generation and pressure transfer, respectively. At the central inversion anticline belt, however, overpressure was caused by hydrocarbon generation and tectonic compression in source rock and by pressure transfer and tectonic compression in nonsource rock. We also propose criteria for confirming the combination of tectonic compression and hydrocarbon generation as the origin of overpressure, where the porosity distribution in the overpressured mudstone is consistent with the normal compaction trend or conforms to a higher level of normal compaction as the depth increases. In the sonic velocity–effective stress diagram, sonic velocity increases with decreasing effective stress, whereas density and sonic velocity both increase with increasing normal compaction in the density–sonic velocity diagram.

SUMMARY We have developed a joint tomographic inversion method with seismic surface wave dispersion and gravity data for obtaining more reliable crustal 3-D shear wave structures. We take the eikonal-based direct surface wave tomographic method and adaptive gravity modelling method in spherical coordinates in the inverse problem. Based on the empirical relations between seismic velocity and density parameters, our method combines surface wave dispersion curves (i.e. surface wave traveltimes at different periods) and Bouguer gravity anomaly data together to invert for 3-D shear wave velocity structures. In our method, off-great-circle propagation of the surface wave and the earth's curvature is considered in the forward modelling. Synthetic tests suggest that the joint tomographic method could improve the reliability and obtain more convincing results than individual seismic surface wave tomography. The gravity data can provide more constraints into the model resolution and help restore the crustal anomalies better. The inversion results in the eastern Tibetan Plateau and Sichuan basin indicate complex distributions of low-velocity zone in the mid-crust of the eastern Tibetan Plateau and a craton-like basement of the Sichuan basin, which supports the crust channel flow model. Although both the 3-D shear wave velocity model from joint inversion and the individual seismic surface wave inversion can fit the surface wave data almost equally well, the joint inversion model can better match the gravity data We also found that the 3-D model from joint inversion in this study shows similar structural characteristics with the surface wave tomographic model, which indicates the icing on the cake effects of gravity data.

We present an extensive dataset of highly accurate absolute travel times and travel-time residuals of teleseismic P waves recorded by the AlpArray Seismic Network and complementary field experiments in the years from 2015 to 2019. The dataset is intended to serve as the basis for teleseismic travel-time tomography of the upper mantle below the greater Alpine region. In addition, the data may be used as constraints in full-waveform inversion of AlpArray recordings. The dataset comprises about 170 000 onsets derived from records filtered to an upper-corner frequency of 0.5 Hz and 214 000 onsets from records filtered to an upper-corner frequency of 0.1 Hz. The high accuracy of absolute and residual travel times was obtained by applying a specially designed combination of automatic picking, waveform cross-correlation and beamforming. Taking travel-time data for individual events, we are able to visualise in detail the wave fronts of teleseismic P waves as they propagate across AlpArray. Variations of distances between isochrons indicate structural perturbations in the mantle below. Travel-time residuals for individual events exhibit spatially coherent patterns that prove to be stable if events of similar epicentral distance and azimuth are considered. When residuals for all available events are stacked, conspicuous areas of negative residuals emerge that indicate the lateral location of subducting slabs beneath the Apennines and the western, central and eastern Alps. Stacking residuals for events from 90∘ wide azimuthal sectors results in lateral distributions of negative and positive residuals that are generally consistent but differ in detail due to the differing direction of illumination of mantle structures by the incident P waves. Uncertainties of travel-time residuals are estimated from the peak width of the cross-correlation function and its maximum value. The median uncertainty is 0.15 s at 0.5 Hz and 0.18 s at 0.1 Hz, which is more than 10 times lower than the typical travel-time residuals of up to ±2 s. Uncertainties display a regional dependence caused by quality differences between temporary and permanent stations as well as site-specific noise conditions.

Seismic post-stack inversion is one of the best techniques for effective reservoir characterization. This studyintends to articulate the application of Model-Based Inversion (MBI) and Probabilistic Neural Networks (PNN) for theidentification of reservoir properties i.e. porosity estimation. MBI technique is applied to observe the low impedancezone at the porous reservoir formation. PNN is a geostatistical technique that transforms the impedance volume intoporosity volume. Inverted porosity is estimated to observe the spatial distribution of porosity in the Lower Goru sandreservoir beyond the well data control. The result of inverted porosity is compared with that of well-computed porosity.The estimated inverted porosity ranges from 13-13.5% which shows a correlation of 99.63% with the computed porosityof the Rehmat-02 well. The observed low impedance and high porosity cube at the targeted horizon suggest that it couldbe a probable potential sand channel. Furthermore, the results of seismic post-stack inversion and geostatistical analysisindicate a very good agreement with each other. Hence, the seismic post-stack inversion technique can effectively beapplied to estimate the reservoir properties for further prospective zones identification, volumetric estimation and futureexploration.