Prestack Depth Migration Research Articles

Common-Reflection-Point-Based Prestack Depth Migration for Imaging Lithosphere in Python: Application to the Dense Warramunga Array in Northern Australia

Abstract We exploit estimates of P-wave reflectivity from autocorrelation of transmitted teleseismic P arrivals and their coda in a common reflection point (CRP) migration technique. The approach employs the same portion of the vertical-component seismogram, as in standard Ps receiver function analysis. This CRP prestack depth migration approach has the potential to image lithospheric structures on scales as fine as 4 km or less. The P-wave autocorrelation process and migration are implemented in open-source software—the autocorrelogram calculation (ACC) package, which builds on the widely used the seismological Obspy toolbox. The ACC package is written in the open-source and free Python programming language (3.0 or newer) and has been extensively tested in an Anaconda Python environment. The package is simple and friendly to use and runs on all major operating systems (e.g., Windows, macOS, and Linux). We utilize Python multiprocessing parallelism to speed up the ACC on a personal computer system, or servers, with multiple cores and threads. The application of the ACC package is illustrated with application to the closely spaced Warramunga array in northern Australia. The results show how fine-scale structures in the lithospheric can be effectively imaged at relatively high frequencies. The Moho ties well with conventional H−κ receiver analysis and deeper structure inferred from stacked autocorrelograms for continuous data. CRP prestack depth migration provides an important complement to common conversion point receiver function stacks, since it is less affected by surface multiples at lithospheric depths.

Novel strategies for complex foothills seismic imaging — Part 1: Mega-near-surface velocity estimation

Seismic imaging in foothills areas is challenging because of the complexity of the near-surface and subsurface structures. Single seismic surveys often are not adequate in a foothill-exploration area, and multiple phases with different acquisition designs within the same block are required over time to get desired sampling in space and azimuths for optimizing noise attenuation, velocity estimation, and migration. This is partly because of economic concerns, and it is partly because technology is progressing over time, creating the need for unified criteria in processing workflows and parameters at different blocks in a study area. Each block is defined as a function of not only location but also the acquisition and processing phase. An innovative idea for complex foothills seismic imaging is presented to solve a matrix of blocks and tasks. For each task, such as near-surface velocity estimation and static corrections, signal processing, prestack time migration, velocity-model building, and prestack depth migration, one or two best service companies are selected to work on all blocks. We have implemented streamlined processing efficiently so that Task-1 to Task-n progressed with good coordination. Application of this innovative approach to a mega-project containing 16 3D surveys covering more than [Formula: see text] in the Kelasu foothills, northwestern China, has demonstrated that this innovative approach is a current best practice in complex foothills imaging. To date, this is the largest foothills imaging project in the world. The case study in Kelasu successfully has delivered near-surface velocity models using first arrivals picked up to 3500 m offset for static corrections and 9000 m offset for prestack depth migration from topography. Most importantly, the present megaproject is a merge of several 3D surveys, with the merge performed in a coordinated, systematic fashion in contrast to most land megaprojects. The benefits of this approach and the strategies used in processing data from the various subsurveys are significant. The main achievement from the case study is that the depth images, after the application of the near-surface velocity model estimated from the megasurveys, are more continuous and geologically plausible, leading to more accurate seismic interpretation.

3-D joint inversion of seismic waveform and airborne gravity gradiometry data

SUMMARYSeismic full waveform inversion (FWI) is a robust velocity model building technique for hydrocarbon exploration. However, the density reconstruction within the framework of multiparameter FWI leads to more degrees of freedom in the parametrization, and the sensitivity of the inversion change significantly from velocity to density, thereby increasing the ill-posedness of the inverse problem. Gravity gradiometry data inversion is an effective method for resolving density distribution. Combining gravity gradiometry data in FWI could alleviate the non-linearity of the inversion by contributing additional density information for the velocity model building. I develop a 3-D joint seismic waveform and gravity gradiometry inversion method for estimating the velocity and density distribution in the subsurface. The method alternatingly minimizes the waveform and gravity gradiometry misfit. The cross-gradient constraint is applied to enhance the structural similarity between the density and velocity models. The effectiveness of the joint inversion algorithm is demonstrated by a 3-D checkerboard model and 3-D SEAM model. Synthetic examples demonstrate that the joint inversion can improve the image quality in geologically complex areas. A case study from the South China Sea shows that the joint inversion improves the velocity and density solutions compared to a standalone seismic FWI. The joint inversion results are consistent with the pre-stack depth migration section and the shape of the salt body is well resolved.

Three-dimensional angle-domain double-square-root migration in VTI media for the large-scale wide-azimuth seismic data

With the development of oil and gas exploration, the conventional seismic migration imaging technology based on the isotropic assumption no longer meets our current requirements for high-resolution images. Migration in anisotropic media has become an essential requirement for oil and gas exploration. Marine seismic exploration has gradually entered the wide-azimuth and high-density seismic data acquisition stage. However, even for current large high-performance computer clusters, it is still very difficult to implement pre-stack depth migration based on shot gathers. Thus, we present a double-square-root (DSR) equation based on three-dimensional (3D) pre-stack depth migration in midpoint-offset domain for a wide-azimuth dataset in transversely isotropic media with a vertical symmetry axis (VTI media). Considering VTI media, the DSR migration requires extensive memory and computation; we adopted the phase-shift plus interpolation approach to improve the computational efficiency. Then, we extract the angle-domain common-image gathers (ADCIGs) during DSR migration. For real large-scale seismic data, we designed an effective parallel implementation of 3D DSR migration with ADCIGs outputs. Finally, we applied the proposed angle-domain VTI DSR migration on wide-azimuth SEG/EAGE salt dome-based data and real data from the China South Sea. Numerical and practical data illustrate the effectiveness of the proposed method.

Thick, strong sediment subduction along south-central Chile and its role in great earthquakes

The south-central Chile margin experienced the largest and sixth largest earthquakes ever recorded - the 1960 Mw 9.5 Valdivia and 2010 Mw 8.8 Maule megathrust earthquakes, respectively. In early 2017, we conducted a seismic survey along 1,000 km of south-central Chile to image these rupture zones using a 15.15-km-long multi-channel seismic streamer. We processed these data using pre-stack depth migration, which provides the best look at the shallow part of the south-central Chile margin to date. Relative to other sediment-dominated subduction zones, where sediment is typically accreted at the toe, an unusually large percentage of the thick trench sediments are consistently subducted beneath the slope with little thrust faulting or deformation. Analysis of the sediment P-wave velocities and structure in the trench and outer wedge leads us to conclude that most of south-central Chile contains well-drained, strong sediments. An exception in the vicinity of the subducting Mocha Fracture Zone (MFZ) has trench sediments that appear to experience localized delayed compaction, thus lowering their strength and allowing the development of protothrusts, similar to what is seen in other accretionary subduction zones. The very shallow décollement along the south-central Chile allows more sediment to pass beneath the lower slope than almost all other subduction zone, many of which have much thicker trench sections. We conclude that subduction of the strong, well-drained, thick sediment layer beneath the lower slope is typical for nearly all of the south-central Chile. Comparison to other thick-sedimented subduction zones worldwide reveals that the subduction of such a large fraction of the trench sediment is particularly unusual. This strong, thick, subducting sediment is likely a determining factor for developing a smooth plate interface located well above the subducted crust topography that ultimately becomes a broad megathrust with high, homogeneous frictional properties, and generates particularly large earthquakes along the south central Chile margin.

Least-Squares Migration Technique Improves Imaging and Inversion Offshore China

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 19285, “Improving Imaging and Inversion Through Least-Squares Q-Kirchhoff APSDM: A Case Study From Offshore China,” by Chenghai Jiao, Jianfeng Yao, and Keat Huat Teng, CGG, et al., prepared for the 2019 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2019 International Petroleum Technology Conference. Reproduced by permission. Least-squares migration (LSM) has become an increasingly important imaging tool. Recently, several case studies have shown that LSM provides greatly improved seismic imaging. However, only a few examples reveal its advantages in both imaging and amplitude-vs.-offset (AVO) inversion. In this paper, the authors propose a least-squares Q migration (LSQM) method that combines the benefits of both LSM and Q prestack depth migration (QPSDM) to improve the amplitude fidelity and image resolution of seismic data. Introduction Seismic data can be considered an outcome of a forward modeling experiment with an unknown Earth velocity model and absorption Q model. Several major factors can affect the resolution and amplitude fidelity of the resulting seismic images, namely the acquisition con-figuration, absorption of the anelastic Earth, and the complexity of the subsurface velocity model. With regard to the first factor, seismic images can suffer from limited migration aperture and nonuniform illumination. This illumination issue can be addressed through image-domain single-iteration LSM. With regard to the second and third factors, it is important to include Q absorption in the velocity model-building process and the migration algorithm, and to use least-squares means to control the compensation of energy loss through LSQM. In the South China Sea, complex faulting geology poses a significant challenge to imaging with narrow-azimuth towed streamer data. Over the past decade, Q tomography and Q migration have been developed to tackle the issue with Q absorption. However, because both the Q compensation level and the noise level of seismic data usually increase with travel time and frequency, high-frequency noise and migration swings are often overamplified in Q migration. The issues with amplitude distortion associated with fault shadows and smeared fault images with a limited usable aperture can be coped with better through the use of LSM, and seismic image resolution can be increased by compensating Q effect with travel time and frequency. The high-frequency noise generated from QPSDM can be attenuated better through LSM if Q can be brought into the process of LSM. To fully exploit the merits of LSM and QPSDM, seismic imaging can be improved through LSQM. The authors apply LSQM to a narrow-azimuth marine-towed streamer survey acquired over an area with complicated faulting structures and significant absorption in the South China Sea. Using image-domain single-iteration LSQM and reservoir-oriented processing, the authors demonstrate that the previously mentioned problems can be addressed, with the result of improved AVO inversion at the wells.

Spatiotemporal Variations in Upper Crustal Extension Across the Different Basement Terranes of the Lake Tanganyika Rift, East Africa

AbstractPreexisting crustal heterogeneities are shown to influence rift process at a variety of scales. However, our understanding of how crustal inheritance influences rift‐scale spatiotemporal kinematics of faulting in magma‐poor rift environments is still very limited. Studies of active continental rifts can provide high‐fidelity assessments of extensional processes and structures that are not possible through examination of ancient rifts that have undergone subsequent deformation events or are buried deeply beneath passive margins. We examine the influence of crustal inheritance on active rifting through balancing and restoring a series of regional cross sections across the Lake Tanganyika Rift in the Western Branch of the East African Rift System. The cross sections are produced using legacy seismic reflection data, reprocessed through prestack depth migration. This type example of a young, magma‐poor continental rift transects several different basement terranes, including an Archean/Paleoproterozoic craton, and Proterozoic mobile belts. The Lake Tanganyika Rift exhibits two classic bell‐shaped profiles of extension along strike, reaching a maximum of 7.15 km. A spatiotemporal integration of the extension data, and comparison with the various basement terranes the rift transects, reveals that extension in cratonic blocks is more widely distributed compared to mobile belt terranes, where strain rapidly localizes onto border faults by later rift stages. These results reveal how crustal inheritance exerts a fundamental control on the evolution of extension localization, ultimately impacting the geometry and structural architecture of rift basins.

Seismic modeling and expression of common fold-thrust structures

We have conducted seismic modeling of common fold-thrust structures to understand the common geologic parameters influencing seismic data and to understand the common pitfalls associated with interpreting prestack time migration (PSTM) and prestack depth migration (PSDM) data. Mode 1 fault-bend folds are generally well-imaged in PSTM data, provided the correct migration velocities are used for the dipping back and front limbs. Seismic pull-ups of the footwall related to lateral velocity variations can result in problems in interpreting the fault geometry and the subthrust area underlying the crest. Fault-tip fault-propagation folds also show significant footwall pull-ups and show poor to no imaging of the steep front limbs. The geometry of trishear fault-propagation folds is dependent on the maximum slip on the fault (S) and the fault propagation to slip ratio (P/S ratio). We found that the slip has a strong influence on the dip of the front limb and therefore the quality of imaging whereas the P/S ratio, which controls the degree of folding versus thrust faulting, has only a secondary effect. For the front limb, only the area near the synclinal axial plan is well-imaged, so that the fault geometry and extent of propagation are typically difficult to interpret. The front limb dips are also sensitive to the accuracy of the rms velocity model used for migration. Lower velocities result in steeper dipping reflectors, whereas higher velocities result in shallower dips. In general, PSDM provides better imaging of the structures; however, the accuracy and quality of the image are dependent on the velocity models and interpretation derived from the PSTM data.

Improved seismic interpretation of a salt diapir by utilization of diffractions, exemplified by 2D reflection seismics, Danish sector of the North Sea

Development of salt diapirs affects the hydrocarbon trapping systems in the Danish sector of the North Sea, where the reservoirs mainly consist of chalk. Seismic imaging and interpretation of the salt structures are challenging, primarily due to the complex geometry of the salt bodies and typically strong velocity contrast with the neighboring sediment layers. The quality of seismic imaging in the North Sea is highly dependent on the quality of the estimated velocity model. We have studied diffracted arrivals originating from the salt flanks and adjacent sedimentary structures using a diffraction imaging technique. The diffracted waves carry valuable information regarding seismic velocity and the location of geologic discontinuities, such as faults, fractures, and salt delimitations. We apply a plane-wave destruction method to separate diffractions from our stacked data. We optimize imaging based on diffraction analysis by using a velocity continuation migration technique, which leads to an estimation of the optimum focusing velocity model. We determine that the diffraction-based approach significantly improves the seismic imaging adjacent to the salt diapirs and the neighboring layers when compared with a standard approach in which we mostly ignore the diffractions. The new poststack time-migrated results provide detailed information that optimizes our interpretation of the salt diapir itself (e.g., the width of the salt neck) as well as the sediment layers related to the rim synclines. Processing schemes such as prestack depth migration and full-waveform inversion may potentially provide high-resolution images of the salt structures. We only account for diffractions in nonmigrated stacked data to better constrain seismic velocity and improve imaging around the salt diapir. The obtained results are critical for reservoir characterization.

2D anisotropic multicomponent Gaussian-beam migration under complex surface conditions

Elastic-wave migration in anisotropic media is a vital challenge, particularly for areas with irregular topography. Gaussian-beam migration (GBM) is an accurate and flexible depth migration technique, which is adaptable for imaging complex surface areas. It retains the dynamic features of the wavefield and overcomes the multivalued traveltimes and caustic problems of Kirchhoff migration. We have extended the GBM method to work for 2D anisotropic multicomponent migration under complex surface conditions. We use Gaussian beams to calculate the wavefield from irregular topography, and we use two schemes to derive the down-continued recorded wavefields. One is based on the local slant stack as in classic GBM, in which the PP- and PS-wave seismic records within the local region are directly decomposed into local plane-wave components from irregular topography. The other scheme does not perform the local slant stack. The Green’s function is calculated with a Gaussian beam summation emitted from the receiver point at the irregular surface. Using the crosscorrelation imaging condition and combining with the 2D anisotropic ray-tracing algorithm, we develop two 2D anisotropic multicomponent Gaussian-beam prestack depth migration (GB-PSDM) methods, i.e., using the slant stack and nonslant stack, for irregular topography. Numerical tests demonstrate that our anisotropic multicomponent GB-PSDM can accurately image subsurface structures under complex topography conditions.

PRE STACK DEPTH MIGRATION UNTUK KOREKSI EFEK PULL UP DENGAN MENGGUNAKAN METODE HORIZON BASED DEPTH TOMOGRAPHY PADA LAPANGAN ‘A1 DAN A2’

In the case of seismic data processing with sandstone lithology such as shale and carbonate often get the result of data processing which have pull up effect especially on the time domain migration result. Pre stack depth migration is a processing based on focusing the amplitude according to the actual depth by using the input interval velocity. Migration is performed using kirchhoff pre stack depth migration algorithm. Pre stack depth migration is done with modeling of horizontal based depth tomography method. This method uses residual moveout correction applied along the horizon-picking line. This research uses two field data that is A1 and A2 Field. A1field has characteristics of carbonate rock that produce pull up shaped similar to carbonate layer. A2 field has a pull-up effect that is not very clear but has build up because of the layer above it. Stages performed starting from the processing of pre stack time migration in the form of velocity picking, generate rms velocity and migration time domain. The pre stack depth migration process begins with a velocity transformation with the dix transformation equation to generate interval velocity, migrate Pre stack depth migration, perform horizon interpretations and perform velocity modeling using the horizon based depth tomography method. The iteration is done 4 times and resulted in the final section of pre stack depth migration which has been corrected by pull up effect.

The Babbage Field, Block 48/2a, UK North Sea

Abstract The Babbage gas field was discovered in 1988 by exploration well 48/2-2 which drilled into the Permian-age lower Leman Sandstone Formation below a salt wall. Seismic imaging is compromised by the presence of this salt wall, which runs east–west across the southern part of the structure, creating uncertainties in depth conversion and in the in-place volumes. Pre-stack depth migration with beam and reverse time migrations appropriate for the complex salt geometry provided an uplift in subsalt seismic imaging, enabling the development of the field, which is located at the northern edge of the main reservoir fairway in a mixed aeolian–fluvial setting. Advances in artificial fracturing technology were also critical to the development: in this area, deep burial is associated with the presence of pore-occluding clays, which reduce the reservoir permeability to sub-millidarcy levels. The Babbage Field was sanctioned in 2008, based on an in-place volume range of 248–582 bcf; first production was in 2010. It produces from five horizontal development wells that were artificially fracced to improve deliverability of gas from the tight matrix. None of the wells has drilled the gas–water contact, which remains a key uncertainty to the in-place volumes, along with depth-conversion uncertainty below the salt wall.

CarbonNet progress towards a declaration of identified storage formation at the Pelican site with a new high-resolution 3D seismic dataset

The CarbonNet project is making the first ever application for a ‘declaration of an identified greenhouse gas storage formation’ (similar to a petroleum location) under the Offshore Petroleum and Greenhouse Gas Storage Act. Unlike a petroleum location, however, there is no ‘discovery’ involved in the application. Instead, a detailed technical assessment is required of the geological suitability for successful long-term storage of CO2. The key challenges to achieving a successful application relate to addressing ‘fundamental suitability determinants’ under the act and regulations. At Pelican (Gippsland Basin), a new high-resolution 3D seismic survey and over 10 nearby petroleum wells (and over 1500 basinal wells) supplement a crestal well drilled in 1967 that proved the seal and reservoir stratigraphy. The GCN18A 3D marine seismic survey has the highest spatial and frequency resolution to date in the Gippsland Basin. The survey was acquired in water depths from 15 to 35 m with a conventional eight-streamer seismic vessel, aided by LiDAR bathymetry. The 12.5 m bin size and pre-stack depth migration with multiple tomographic velocity iterations have produced an unprecedented high-quality image of the Latrobe Group reservoirs and sealing units. The 3D seismic data provides excellent structural definition of the Pelican Anticline, and the overlying Golden Beach-1A gas pool is excellent. Depositional detail of reservoir-seal pairs within the Latrobe Group has been resolved, allowing a confident assessment of petroleum gas in place and CO2 storage opportunities. The CarbonNet project is progressing with a low-risk storage concept at intra-formational level, as proven by trapped pools at nearby oil and gas fields. Laterally extensive intra-formational shales provide seals across the entire structure, providing pressure and fluid separation between the overlying shallow hydrocarbon gas pool and the deeper CO2 storage opportunity. CarbonNet is assessing this storage opportunity and progressing towards a ‘declaration of an identified greenhouse gas storage formation’.

Seismic fine imaging and its significance for natural gas hydrate exploration in the Shenhu Test Area, South China Sea

ABSTRACT Shenhu area in South China Sea includes extensive collapse and diapir structures, forming high-angle faults and vertical fracture system, which functions as a fluid migration channel for gas hydrate formation. In order to improve the imaging precision of natural gas hydrate in this area, especially for fault and fracture structures, the present work propose a velocity stitching technique that accelerates effectively the convergence of the shallow seafloor, indicating seafloor horizon interpretation and the initial interval velocity for model building. In the depth domain, pre-stack depth migration and residual curvature are built into the model based on high-precision grid-tomography velocity inversion, after several rounds of tomographic iterations, as the residual velocity field converges gradually. Test results of the Shenhu area show that the imaging precision of the fault zone is obviously improved, the fracture structures appear more clearly, the wave group characteristics significantly change for the better and the signal-to-noise ratio and resolution are improved. These improvements provide the necessary basis for the new reservoir model and field drilling risk tips, help optimize the favorable drilling target, and are crucial for the natural gas resource potential evaluation.

The Paleozoic prospectivity of the Browse Basin, Australia

Most of the discovered hydrocarbons in the Browse Basin occurred within the Mesozoic intervals, while deeper Paleozoic sequences have been seldom explored. Lack of Paleozoic exploration in the Browse Basin has been attributed to the lack of well penetrations, poor understanding of the petroleum systems and paucity of seismic data. The onshore Canning Basin with several commercial fields and discoveries is the most appropriate analogue for understanding the Paleozoic sequences in the region. With the integration of geophysical data (i.e. gravity, magnetic and seismic), well data and geology, the Paleozoic prospectivity of the Browse Basin can be further enlightened. Modern long offset (8 m) Vampire 2D seismic data were acquired by Searcher to address some of the complex challenges in the Browse Basin. Reservoir quality of the Brewster Formation, volcanic discrimination within the Plover Formation and identification of deeper Triassic and Paleozoic plays are some examples of these challenges in the Browse Basin. Recently Searcher reprocessed this regionally important Vampire 2D seismic dataset that ties to 60 wells. The broadband pre-stack depth migration reprocessed data were inverted to extract three petro-elastic properties of acoustic impedance, Vp/Vs and density by three-term amplitude versus offset inversion algorithm to improve imaging of deeper plays and delineate reservoir properties. This paper discusses how several potential Paleozoic reservoir-seal pairs can be identified in the Browse Basin by utilising the integration of Vampire 2D seismic data, quantitative interpretation products, regional geology and knowledge of the Canning Basin’s fields and discoveries. Previously there was little exploration of Paleozoic plays because they could not be imaged on seismic data. The potential Paleozoic reservoirs identified in this study include Permo-Carboniferous subcrop, Carboniferous-Devonian anticline and Carboniferous-Devonian rollover plays.

Impedance inversion of pre-stack seismic data in the depth domain

The extensive application of pre-stack depth migration has produced huge volumes of seismic data, which allows for the possibility of developing seismic inversions of reservoir properties from seismic data in the depth domain. It is difficult to estimate seismic wavelets directly from seismic data due to the nonstationarity of the data in the depth domain. We conduct a velocity transformation of seismic data to make the seismic data stationary and then apply the ridge regression method to estimate a constant seismic wavelet. The estimated constant seismic wavelet is constructed as a set of space-variant seismic wavelets dominated by velocities at different spatial locations. Incorporating the weighted superposition principle, a synthetic seismogram is generated by directly employing the space-variant seismic wavelets in the depth domain. An inversion workflow based on the model-driven method is developed in the depth domain by incorporating the nonlinear conjugate gradient algorithm, which avoids additional data conversions between the time and depth domains. The impedance inversions of the synthetic and field seismic data in the depth domain show good results, which demonstrates that seismic inversion in the depth domain is feasible. The approach provides an alternative for forward numerical analyses and elastic property inversions of depth-domain seismic data. It is advantageous for further studies concerning the stability, accuracy, and efficiency of seismic inversions in the depth domain.

Tutorial — time conversion of depth migrated data: part II, TTI preSDM

Contemporary depth imaging projects often require final pre-Stack Depth-Migrations (preSDM) to be converted to time for comparison to vintage pre-Stack Time-Migrations (preSTM), or to facilitate conversion to ‘geological’ depth through calibration to well and check-shot data. Here, I consider the situation of having performed an anisotropic Tilted Transverse Isotropy (TTI) preSDM and wanting to convert it to time via vertical stretch in order to compare it, say, to an anisotropic preSTM. Such a comparison is inherently invalid, as time-migration will explicitly treat any anisotropy as if it were Vertical Transverse Isotropy (VTI), and, in addition, the lateral positioning error inherent in preSTM will render such comparisons questionable on steeply dipping structures. I show here that the most appropriate type of ‘velocity’ to use for conversion to time of TTI preSDM reflection events should be the vertical component of the phase velocity. Conversely, if we are considering point-to-point measurements, such as the direct arrival travel time, a down-hole or check-shot measurement, then the group velocity should be used, as it is with this speed that energy travels. In addition, subsequent depth conversion of any time product for interpretational purposes would best be accomplished using a velocity calibrated to well check-shots.

A preconditioned technique for SH- and Love-wave full-waveform inversion in time domain and crosstalk analysis

AbstractFor the problem of low horizontal resolution in the method of surface wave dispersion analysis, we apply the SH- and Love-wave full-waveforms to achieve 2D and 3D imaging of the subsurface. The gradient obtained by classical full-waveform inversion (FWI) is not scaled with increasing depth. The limited source frequency band, the non-uniform coverage between shot and geophone positions and double scattering are the main reasons for the phenomenon. The Hessian operator of the misfit function can clearly predict the inherent defocusing phenomenon and the artefacts generated by the double scattering in the gradient vector, and the inverse Hessian operator is used as a deconvolution operator to realise gradient preconditioning. We present an expression of the quasi-Hessian operator in SH- and Love-wave FWI based on inverse scattering theory in prestack depth migration, and apply a new preconditioned technique to the fault and large contrast (LC) model reconstruction test. The inverted results show that the new preconditioned technique can greatly improve the imaging accuracy compared with the gradient-based methods of FWI. From single-parameter tests, we can conclude that the quasi-Hessian operator plays roles in illumination compensation and parameter complement estimation in FWI. From the final reconstructed results of a dual-parameter model, we find that the inverted result of the density is slightly closer to the true model than S-wave velocity under the condition of the same comparative distance between the initial and true model, but parametric crosstalk of density is more serious.

Subducted sediments, upper-plate deformation and dewatering at New Zealand's southern Hikurangi subduction margin

The southern end of New Zealand's Hikurangi subduction margin accommodates highly oblique convergence between the Pacific and Australian plates. We carry out two-dimensional (2D) seismic reflection tomography and pre-stack depth migrations on two seismic lines to gain insight into the nature of subducted sediments and upper plate faulting and dewatering at the toe of the wedge. We also investigate the NE to SW evolution of emergent upper plate thrust faulting using 47 seismic lines spanning an along-strike distance of ∼270 km. The upper sequence of sediments that ultimately gets subducted (the MES sequence) has an anomalously-low seismic velocity character. At the southwestern end of the margin, ∼150 km east of Kaikōura, the MES sequence has experienced greater compaction (for an equivalent effective vertical stress) than it has some 200 km further to the northeast. This difference is likely attributable to greater horizontal compression in the southwest caused by impingement of the Chatham Rise on the deformation front. Relationships between velocity and effective vertical stress suggest that the MES sequence is well-drained in the vicinity of frontal thrusts, corroborated by evidence for upper plate dewatering along those thrusts. Effective drainage of the MES sequence likely promotes interplate coupling on the southern Hikurangi margin. The décollement is generally hosted near a seismic reflector known as “Reflector 7”. East of Kaikōura, however, Reflector 7 becomes accreted, indicating that subduction slip at the southwestern end of the margin is no longer hosted at (or above) this reflector. Instead, the décollement steps down to a deeper stratigraphic level further inboard. Further to the SW, approximately in line with the lower Kaikōura Canyon, the offshore manifestation of subduction-driven compression ceases.

Ultra-deepwater seismic plays offshore Brazil — Future drilling off Santos and Campos Basins

Contemporaneous seismic data acquisition in the Santos and Campos Basins offshore Brazil have focused on image characterization of deepwater and ultra-deepwater reservoirs and their relationship with hydrocarbons originating from synrift source rocks. Our interpretation has mapped the stratigraphy of postsalt turbidite reservoirs, and, on the presalt lithology, it has uncovered the underlying synrift sequences that embrace oil-bearing source rocks and the prolific, recently discovered, microbialite carbonate reservoirs. The new phase in geophysical data acquisition and offshore drilling that started in 1999 bolstered the Brazilian offshore petroleum production to record levels. The new, massive, nonexclusive, speculative 2D and 3D data acquisition surveys conducted offshore the Brazilian coast far exceed the amount of all existing cumulative vintage data. Deepwater drilling programs probed the interpreted new prospects. As whole, the modern geophysics data libraries offshore Brazil brought in the technology era to seismic interpretation, reservoir characterization, and geosteering operations in deepwater development drilling. Still, regional interpretation mapping of the outer shelf, slope, deepwater and ultra-deepwater provinces of the Santos and Campos Basins indicates plenty of prospective future drilling in the salt locked minibasins of the ultra-deepwater provinces. Salt tectonics shapes the architecture of these basins; hence, postsalt deepwater turbidite plays were readily interpreted from seismic amplitudes of the modern data that also allow for resolution images of the synrift source rocks, salt architecture, migration paths through faulting and salt windows, reservoir characterization, and regional seal mapping. The new techniques of prestack depth migration have enabled uncovering the imaging structure of the synrift that led to characterization of the presalt carbonate reservoirs and discovery of giant accumulations. Future offshore exploration will continue aiming at postsalt deepwater and ultra-deepwater minibasins plus presalt plays under the massive salt walls, still an underexplored frontier.