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The lithosphere–asthenosphere boundary is the most extensive boundary on Earth, separating the mobile plate above from the convecting mantle below, but its nature remains a matter of debate. Using an ultra-deep seismic reflection technique, here we show a systematic seismic image of two deep reflectors that we interpret as the upper and lower limits of the lithosphere–asthenosphere boundary beneath a 40–70-million-year-old oceanic lithosphere in the Atlantic Ocean. These two reflections correspond to 1,260 °C and 1,355 °C isotherms and bound a low-velocity channel, suggesting that the lithosphere–asthenosphere boundary is thermally controlled. We observe a clear age dependency of this sublithospheric channel: its depth increases with age from 72 km where it is 40-Myr-old to 88 km where it is 70-Myr-old, whereas its thickness decreases with age from 18 km to 12 km. We suggest that partial melting, facilitated by water, is the main mechanism responsible for the low-velocity channel. The required water concentration for melting increases with age; nevertheless, its corresponding total mass remains relatively constant, suggesting that most of the volatiles in the oceanic sublithospheric channel originate from a horizontal flux near the ridge axis.

An unusual pattern of two bottom simulating reflections (BSRs) has been observed on seismic profiles from the continental margin offshore Western Norway. One of these reflections (BSR1) extends over large areas and has the characteristics of the classical BSR, that is a phase-reversed reflection from the base of the gas-hydrate stability zone. The second BSR (BSR0) occurs at approximately 70 ms two-way travel time beneath BSR1 and is here called a double BSR. The distribution of BSR0 is more local than that of BSR1 and it does not show the phase-reversal relative to the sea floor reflection that is characteristic for a BSR at the gas hydrate-free gas boundary. Results from an industrial borehole, from full waveform inversion of multichannel seismic data, from high-frequency ocean bottom hydrophones, and interpretation of seismic profiles, clearly indicate that BSR1 is reflected from the base of the methane hydrate equilibrium field. Results from full waveform inversion indicate that BSR0 corresponds to a 16–20 m zone where the velocity drops from about 1.8 km/s to a minimum of 1.4 km/s and then increases again. The low velocity of 1.4 km/s suggests the presence of free gas. The results support the hypothesis that BSR0 is a reflection from the base of gas hydrates containing hydrocarbons with a heavier molecular weight in addition to methane gas. Interference of reflections from the top and base of the low-velocity zone associated with BSR0 explain why BSR0 is not phase-reversed.

Quantifying the melt distribution and crustal structure across ridge-axis discontinuities is essential for understanding the relationship between magmatic, tectonic and petrologic segmentation of mid-ocean-ridge spreading centres. The geometry and continuity of magma bodies beneath features such as overlapping spreading centres can strongly influence the composition of erupted lavas and may give insight into the underlying pattern of mantle flow. Here we present three-dimensional images of seismic reflectivity beneath a mid-ocean ridge to investigate the nature of melt distribution across a ridge-axis discontinuity. Reflectivity slices through the 9 degrees 03' N overlapping spreading centre on East Pacific Rise suggest that it has a robust magma supply, with melt bodies underlying both limbs and ponding of melt beneath large areas of the overlap basin. The geometry of melt distribution beneath this offset is inconsistent with large-scale, crustal redistribution of melt away from centres of upwelling. The complex distribution of melt seems instead to be caused by a combination of vertical melt transport from the underlying mantle and subsequent focusing of melt beneath a magma freezing boundary in the mid-crust.

The determination of along-axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle, eruption history along the ridge crest, and the process of crustal accretion. Seismic reflection images1,2,3,4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths (250–4,500?m), but variations in its seismic properties and thickness have remained elusive, despite several attempts to constrain these properties5,6,7. Here we report that the melt sill along the southern East Pacific Rise, which is about 50?m thick, undergoes abrupt changes in its internal properties, ranging from pure melt to mush. The 60-km-long ridge-crest segment near 14°?00′?S is characterized by three 2–4-km sections containing pure melt embedded within a magma chamber rich in mush. These small pure melt pockets may represent a fresh supply of magma from the mantle, capable of erupting and forming the upper crust. Conversely, the 80–90% of the magma chamber which is mushy is unlikely to erupt and may influence the lower-crustal accretion.

Summary Bottom simulating reflectors (BSRs) at the base of the hydrate stability zone are often observed in marine seismic reflection data. The compressional (P-)wave velocity structure of a BSR offshore Costa Rica was analysed by applying a full-waveform inversion technique. The resulting velocity profile indicates that the BSR is caused by a thin low-velocity layer, suggesting the presence of at least a small amount of free gas in the sediment pore space. In undisturbed sediment sections, BSRs offshore Costa Rica are observed over much of the continental margin. They frequently occur at shallow depths beneath the seafloor, usually ~ 100–400 m; at some locations BSRs appear to intersect the seafloor. This was interpreted as an indication that hydrates in the study area form partly from methane which was produced beneath the hydrate stability zone and migrated upwards. In the study area, the vertical movement of the hydrate stability zone relative to a given point in the sediment column, one of the potential factors leading to BSR formation or suppression, is controlled by both sedimentation and vertical tectonism. Both processes may hence play a role in controlling BSR distribution. BSRs are absent in areas which have been affected by slumping, except where the sedimentary section above the BSR remained intact during slumping. This indicates that slope failure can cause the destruction of BSRs.

In 1992, British Institutions' Reflection Profiling Syndicate conducted a two-ship coincident near-vertical and wide-angle experiment to determine the detailed velocity structure of the continental crust in the Central North Sea. The results show a near-surface high velocity gradient layer (P-wave velocity reaches 6.0 km/s at ∼5 km depth), a high-velocity zone (6.5 km/s) at 10–15 km depth, a 19-km-thick upper crust and the Moho at a depth of 32–34 km along the 135-km profile. The travel-time and amplitude of wide-angle data require the presence of a dipping reflector originating at the Moho (32 km depth) and continuing down to 38 km in the mantle. The velocity between 19 and 24 km depth is slightly lower (6.1 km/s) than that in the upper crust. The waveform modelling of the combined normal-incidence and wide-angle data requires a number of alternating high- and low-velocity layers in the lower crust (19–33 km). The velocity contrast in the lower part of the lower crust (29–33 km) is large, of the order of 1 km/s (6.5–7.5 km/s). There are a few alternating high- (8.2 km/s) and low-velocity (7.5 km/s) layers in the upper part of the mantle (33–36 km). The high velocity (6.5 km/s) in the upper crust may be due to a high grade of metamorphism, and it may have been formed during the subduction process. The low-velocity (6.1 km/s) in the upper part of the lower crust might be caused by the presence of meta-sediments. The large velocity variations (6.5–7.5 km/s) in the lower part of the lower crust suggest the presence of mafic materials, and may result from under-plating of mafic materials from the mantle. The fractionation of melt can also produce the layering in the upper part of the mantle that is observed from seismic analyses.

SUMMARY The reliability of information obtained from surface seismic measurements is an important factor to be considered before one attempts to depth migrate or invert seismic data. The uniqueness of the retrieved model can only be achieved if the observed seismic data contain information on all the wavelengths of the Earth structure. In this study we investigated the sensitivity of model parameters obtainable from elastic multi-offset seismic data. We perturbed a background model, which was defined in terms of longitudinal and shear velocities and density, with quasi-sinusoidal perturbations. The reference and perturbed synthetic seismograms were calculated in the intercept time and slowness (τ-p) domain. The seismic waveform misfit between these two synthetic seismograms computed in a least-squares sense was used as a quantitative measure of the sensitivity of model parameters. We found that middle-wavelength variations of model parameters are sensitive to critical-angle seismic data. This was further confirmed by using a 1-D waveform inversion. These results suggest that all wavelengths of model parameters can be retrieved from waveform analysis of seismic reflection/refraction data: the long wavelengths from traveltime data and the short to medium wavelengths from waveform analysis. The non-uniqueness associated with the information gap for middle wavelengths, as previously proposed for seismic reflection data, can therefore be avoided.

SUMMARY Deep seismic-reflection profiles have shown that the lower part of the continental crust often contains many strong, sub-horizontal reflectors. Various suggestions have been made about the origin of such reflections. One explanation, which does not require the invocation of any complex tectonic process, is layering of mafic/ultramafic intrusions of partial melts derived from the upper mantle. We show that the internal layering within large mafic and ultramafic igneous intrusions can form the necessary impedance contrasts with appropriate length scales to produce seismograms similar to those observed from the lower crust, and suggest that such intrusions could be responsible for the character of the observed lower crustal reflections. Such layering could be formed at any stage of crustal formation. This suggestion is consistent with earlier geochemical proposals and does not impose any new constraints on the origin and structure of the continental crust.

Geological sequestration of CO2 is considered to be an important greenhouse gas mitigation technology (Washington et al. 2009). Current public resistance to onshore sequestration sites makes offshore sites an attractive alternative. The Sleipner gas field in the North Sea is such a site and has been the worlds first industrial scale CO2 storage project. An effective CO2 storage requires monitoring and verification over large areas. Time lapse seismic monitoring is an integral tool to do so. Here we use time lapse elastic 2D full wave form inversion (FWI) to monitor CO2 in the Sleipner aquifer reservoir and retrieve the amount of free CO2. Inverted P-wave velocities (Vp) are related to gas saturations using a fluid substitution model employing Gassmann theory assuming patchy saturation. We find that a considerable amount of the injected CO2 could be in dissolved form by 2006 and hence saline aquifers could be a natural reservoir for CO2 storage.

Full waveform inversion is increasing being used in oil industry to quantify P and S-wave velocities of the sub-surface. So far, waveform inversion methods have been either 3D acoustic or 2D elastic. Furthermore, only low frequencies have been used because of the high computation cost. The application of elastic full waveform for 3D reservoir monitoring is still beyond the reach of present day computing technology. Here, we demonstrate the application an injected grid method, where the forward modelling for iterative elastic waveform inversion is performed in a small volume surrounding the reservoir, reducing the cost significant for full waveform. The application of algorithm is demonstrated on Marmousi II data set in a time-lapse mode.

Since the 1980’s a significant amount of research in the field of full waveform has been conducted, and we are now at the stage where more accurate descriptions of the subsurface are being sought in addition to the elastic parameters. Elasticity accounts for materials that have a capacity to store mechanical energy with no dissipation of energy. A more accurate description of real earth media takes into account the time-dependent nature of previous stress states. In other words, the material possess a characteristic referred to as a memory effect (Robertsson et al. 1994).

Elastic full waveform inversion of seismic reflection data represents an alternative to conventional stack based processing. Both travel-time and amplitude information within shot domain data are used to invert for the subsurface velocity, from a starting model containing the long wavelength features.

OBC technology provides information on both pressure and 3 components particle velocity recorded at the seafloor level, allowing complementary PP and PS imaging. In the following paper, we describe a new elastic decomposition scheme in the tau-p domain that separates upgoing-downgoing P and S wavefields and eliminates all water layer related multiples. We introduce frequency dependant operators to calibrate the geophones and to characterize the P wave reflection coefficient at the seafloor. The process does not require any a priori knowledge of seafloor parameters. We start by describing theoretical aspects for complete decomposition. Then, we detail a multi-step procedure for estimating the shear wave velocity at the seafloor, for extracting the upgoing S wavefield and finally for removing peg-leg arrivals. We show successful tests on a real dataset. The results suggest promising avenues for further processing.

In prestack migration or imaging processing, estimation of a background velocity structure is a critical step. There are two main approaches to estimate seismic velocity using reflection traveltimes: one approach is traveltime tomography and the other is inversion using the coherency measure semblance.

Usual tomography methods require ray tracing to numerically compote the traveltime in order to estimate, at each iteration of the inversion, the error between the data, i.e. the recorded times, and the computed times out of the current velocity model. Furthermore pre-stack migration also requires intensive traveltime computation to calculate the Green's function, which constitutes a time consuming step for both roversion and migration processes.

According to ray theory, the traveltime T(R,S) between any source S and receiver R is related to the slowness distribution ŋ at any point X by the integral: T(R,S) = ∫ г(ŋ) ŋ(X)dl, where г(ŋ) is the ray path which yields the minimum traveltime and dl is the arc length. Because the ray path г(ŋ) depends upon the actual slowness ŋ, the inverse problem consisting of recovering the slowness distribution from the traveltime T(R,S) is non linear. A common approach is to linearize the traveltime with respect to a known slowness and iteratively update the current estimate. This requires ray tracing to compute the traveltime misfit.

Seismic reflection profiles across many continent al margins have imaged "bottom simulating reflectors" (BSRs), which are interpreted as marking the base of a methane hydrate stability field. BSRs often have very high amplitude and can be continuous for many kilometres, so they provide an ideal target for a waveform inversion scheme. Knowledge of the detailed velocity structure in the vicinity of the BSR allows us to determine whether it is underlain by a free gas zone, and estimate the thickness of such a zone where present.

Deep seismic reflection profiles show that the lower part of the continental crust often contains many strong sub-horizontal reflections. These reflections are interpreted as a sequence of finely layered structuren. To confirm or invalidate this interpretation, we can use a seismic waveform inversion to estimate the reflectivity in the lower crust. However, it is very difficult to invert for the whole data set, partly because of large computation time in modelling and partly because the reflected energy in the upper crost dominates the misfit function.

Seismic waveform inversion consists of minimizing misfit between observed and calculated wavefields. Since this misfit function could be highly non-linear, the only way to be sure of arriving at the global solution is to use a Monte-Carlo search. This is feasible if the number of unknown parameters is not too large. If it is large, as is the case with the seismic inverse problems, it is extremely expensive to use the Monte-Carlo search to optimize for a single misfit function over the whole model space. However, if one uses a number of misfit functions depending up on the sensitivity of the model parameters on the data, one can implement the Monte-Carlo method efficiently. Fortunately, this is possible as the different wavelengths of the model parameters influence the different parts of the data.

A full-waveform inversion of two-ship, wide-aperture, seismic reflection data from a ridge-crest seismic line at the southern East Pacific Rise indicates that the axial magma chamber here is about 50 m thick, is embedded within a solid roof, and has a solid floor. The 50--60-m-thick roof is overlain by a 150--200-m-thick low-velocity zone that may correspond to a fracture zone that hosts the hydrothermal circulation, and the roof itself may be the transition zone separating the magma chamber from circulating fluids. Furthermore, enhanced hydrothermal activity at the sea floor seems to be associated with a fresh supply of magma in the crust from the mantle. The presence of the solid floor indicates that at least the upper gabbros of the oceanic lower crust are formed by cooling and crystallization of melt in magma chambers.