Strong Lateral Velocity Variations Research Articles

Lateral Transport Controls the Tidally Averaged Gravitationally Driven Estuarine Circulation: Tidal Mixing Effects

Abstract In classic models of the tidally averaged gravitationally driven estuarine circulation, denser salty oceanic water moves up the estuary near the bottom, while less dense riverine water flows toward the ocean near the surface. Traditionally, it is assumed that the associated pressure gradient forces and salt advection are balanced by vertical mixing. This study, however, demonstrates that lateral (across the estuary width) transport processes are essential for maintaining the estuarine circulation. This is because for realistic estuarine bathymetry, the depth-integrated salt transport up the estuary is enhanced in the deeper estuary channel. A closed salt budget then requires the lateral transport of this excess salt in the deeper channel toward the estuarine flanks. To understand how such lateral transport affects the estuarine salt and momentum balances, we devise an idealized model with explicit lateral transport focusing on tidally averaged lateral mixing effects. Solutions for the along-estuary velocity and salinity are nondimensionalized to depend only on one single nondimensional parameter, referred to as the Fischer number, which describes the relative importance of lateral to vertical tidal mixing. For relatively strong lateral tidal mixing (greater Fischer number), salinity and velocity variations are predominantly vertical. For relatively weak lateral tidal mixing (smaller Fischer number), salinity and velocity variations are predominantly lateral. Overall, lateral transport greatly affects the estuarine circulation and controls the estuarine salinity intrusion length, which is demonstrated to scale inversely with the Fischer number.

Tomographic imaging of South Rewa basin, Central India: Implications of Gondwana rifting and Late Cretaceous volcanism

We have imaged thick (3.0–5.0 km) sub-trappean Gondwana sediments deposited in a typical rift-environment of the south Rewa basin in Central India masked by the Deccan basalts. The basalts intrude through the basin forming several dykes, sills, and other intrusions along with large-scale undulations of the basement showing complex geology and tectonic settings due to Late Cretaceous volcanism (∼65 Ma). To delineate complex subsurface geological structures and understand the tectonic settings, we use long-offset seismic reflection data along the N-S trending 127 km Kuvari-Shahdol profile of the basin. The conventional stack and pre-stack time migration techniques fail to image the complex subsurface structures dominated by steeply dipping faults. The steeply dipping faults with small-scale structures are influenced by strong lateral and vertical velocity variations that cause problems in imaging the basin. Hence, a robust tomographic inversion with pre-stack depth migration technique is adopted to image fine-scale subtle subsurface geological structures with smooth velocity variations. The tomographic velocity model and pre-stack depth migration seismic image could decipher basaltic trap thickness, numerous dyke intrusions, alternate horsts and grabens with deposition of thick hydrocarbon-bearing sub-trappean Gondwana rocks along with the basement configuration. The deep basinal faults with highly distorted basement structures represent the intense tectonic activity of this Gondwana rift-basin. The deep-seated hydrocarbon reservoir is discovered with an excellent trapping mechanism forming both pre-rift and syn-rift settings of the rift-basin. The seismic image is further constrained and corroborated by the inversion of residual Bouguer gravity anomaly data to obtain corresponding density model derived from tomographic velocity model. The large basement faults, horsts and grabens, several intrusives like dyke, sill, laccolith, and thick sub-trappean Gondwana formations with Deccan trap cover indicate complex geology and tectonic settings of south Rewa basin forming the Late Cretaceous Volcanic Province of India.

An effective scheme of joint migration inversion in the pseudo‐time domain

AbstractTraditional full‐waveform inversion is a non‐linear and ill‐posed inversion problem. To reduce the non‐linearity of it, joint migration inversion (joint migration inversion) was proposed as an alternative. Joint migration inversion tries to minimize the mismatch between measured and modelled reflection data. One key feature of joint migration inversion is its parameterization: two separate parameters, reflectivity (for the amplitudes of reflected events) and propagation velocity (for the phase effects). This separation helps to reduce the non‐linearity of the inversion. During joint migration inversion, with the velocity being updated, the reflectors in the updated image are also shifting in depth accordingly, this phenomenon is called depth–velocity ambiguity. This interaction between the two parameters during inversion is desired to keep the image time consistent with the measured data but may lead to non‐robustness of joint migration inversion due to the presence of local minima. Therefore, we propose a more robust joint migration inversion scheme, which parameterizes the models with vertical time, termed pseudo‐time joint migration inversion. In pseudo‐time, the updates of velocity will not result in the associated vertical location changes of reflectors in the estimated image. Instead, the reflectors are mainly getting more focused. One limitation is that the depth‐pseudo‐time conversion process assumes a simple linear relationship between depth and pseudo‐time, which might cause some artefacts in the converted models when there exist strong lateral velocity variations. One subsequent round of depth joint migration inversion is recommended to resolve this issue. We demonstrate the effectiveness of our proposed method with a two‐dimensional synthetic example in an extreme scenario, where the initial velocity model is homogeneous, a realistic offshore two‐dimensional synthetic example, a two‐dimensional field example from the Vøring basin in Norway and a simple three‐dimensional synthetic example. In all examples, pseudo‐time joint migration inversion manages to recover more reasonable updates in the inverted velocity and invert more focused reflectors in the inverted image, compared to depth joint migration inversion.

Deep-learning for accelerating prestack correlative least-squares reverse time migration

Prestack least-squares reverse time migration based on a correlative objective function denoted as PCLSRTM can retrieve a higher quality image of the subsurface than standard least-squares reverse time migration based on a waveform-matching objective function. However, to invert the optimal migration images of different shots via a gradient-based iteration scheme, PCLSRTM tends to require a huge computational overhead for a large number of reverse time migration (RTM) and reverse time de-migration operators. We introduce a convolutional neural network (CNN) framework to improve the computational efficiency in PCLSRTM. Specifically, the CNN architecture aims to reconstruct the optimal reflection image for a single-shot recording from the standard RTM image. Besides, the migration model can be used as an additional input of CNN architecture. Limited by the absence of true reflectivity of the subsurface as the label data in seismic imaging, we employ the standard PCLSRTM images of a small part of total shot recordings as the labels to train the constructed CNN model. There are two benefits for the well-trained CNN model. On the one hand, it can directly invert a comparable image quality with the PCLSRTM image at a low computational cost, when the migration model does not contain a strong lateral velocity variation. On the other hand, one can use the inverted image from the well-trained CNN model as the initial image of the PCLSRTM approach to accelerate convergence. Through synthetic experiments with the Marmousi-2 and SEG/EAGE salt models and marine field data, it can determine that our approach can build a reflection image with balanced amplitude and good continuity and merely requires about one-ninth to one-fourth of the computational costs of PCLSRTM.

Reverse time migration with Gaussian beams using optimized ray tracing systems in transversely isotropic media

ABSTRACTPrestack depth migration is a key technology for imaging complex reservoirs in media with strong lateral velocity variations. Prestack migrations are broadly separated into ray‐based and wave‐equation‐based methods. Because of its efficiency and flexibility, ray‐based Kirchhoff migration is popular in the industry. However, it has difficulties in dealing with the multi‐arrivals, caustics and shadow zones. On the other hand, wave‐equation‐based methods produce images superior to that of the ray‐based methods, but they are expensive numerically, especially methods based on two‐way propagators in imaging large regions. Therefore, reverse time migration algorithms with Gaussian beams have recently been proposed to reduce the cost, as they combine the high computational efficiency of Gaussian beam migration and the high accuracy of reverse time migration. However, this method was based on the assumption that the subsurface is isotropic. As the acquired azimuth and maximum offsets increase, taking into account the influence of anisotropy on seismic migration is becoming more and more crucial. Using anisotropic ray tracing systems in terms of phase velocity, we proposed an anisotropic reverse time migration using the Gaussian beams method. We consider the influence of anisotropy on the propagation direction and calculate the amplitude of Gaussian beams with optimized correlation coefficients in dynamic ray tracing, which simplifies the calculations and improves the applicability of the proposed method. Numerical tests on anisotropic models demonstrate the efficiency and accuracy of the proposed method, which can be used to image complex structures in the presence of anisotropy in the overburden.

Case study: joint seismic reflection and CSAMT data interpretation for mineral explorations in Fujian, China

The Southwestern area of Fujian Province in China contains a major metallogenic belt. There have been three major tectonic movements since the Late Paleozoic, and the thrusts here are the main mineral deposits. These occurred between the Early Carboniferous and Late Permian eras. Due to dramatic undulations in the surface and associated complex underground structures, a single geophysical method cannot provide reliable imaging results. This is largely due to difficulties in data acquisition and processing. Seismic exploration provides one example. Using this method, raw shot gathers have a low signal-to-noise ratio because of the undulations in the topography. Furthermore, strong lateral velocity variation makes the migration process exceedingly difficult. This means that interpretations of the resulting reflection seismic profile give rise to uncertainty. For this reason, two-dimensional reflection seismology and controlled source audio-frequency magnetotellurics (CSAMT) electromagnetic sounding techniques were performed. After initially completing conventional processing on the seismic reflection data, subsequent pre-stack depth migration (PSDM) over the rugged topography yielded a much better image. It was able to identify thrust faults and magma intrusions. Then, using CSAMT, inversion was conducted with the same topography as the seismic reflection. By combining the migrated seismic profile, inverted resistivity profile, and borehole data, a vertical geological model was constructed. This proved the existence of an overlapped thrust, and it also indicated the possible presence of deeper mineralization.

Source‐Receiver Interferometric Redatuming Using Sparse Buried Receivers to Address Complex Near‐Surface Environments: A Case Study of Seismic Imaging Quality and Time‐Lapse Repeatability

AbstractDeploying sparse sources and receivers in a complex near‐surface environment for seismic imaging and monitoring remains a challenge. Complicated near‐surface structures with strong lateral velocity variations and thick weathering layers distort seismic wave paths and produce strong reverberations and scattering. Consequently, acquisition with a buried receiver system below the weathering layer is preferred for land surveys, but the cost of this approach is excessively high in real applications. To mitigate this problem, we propose a source‐receiver interferometry‐based redatuming method that can generate redatumed data beneath the near surface with a reduced cost. The new acquisition geometry consists of dense surface and sparse buried receivers. The workflow based on source‐receiver redatuming combines surface and buried systems by cross correlating direct waves recorded by buried receivers with the corresponding reflected waves recorded by surface receivers. The approach transforms surface source‐receiver records into virtually buried source‐receiver records. We effectively suppress the acquisition noises associated with the near‐surface structures because the proposed technique generates redatumed data below the weathering layer. We apply dip‐guided interpolation to the redatumed data to make the geometry consistent with the original surface geometry. The resulting records not only improve the seismic image quality and repeatability compared with the original records but also have a lower cost than a denseburied acquisition system. We use 13 seismic surveys carried out at different times to demonstrate the feasibility and advantage of the proposed method.

One-way propagators based on matrix multiplication in arbitrarily lateral varying media with GPU implementation

Conventional one-way migration methods have some shortcomings, such as their limitations in imaging large angles. A novel strategy is presented here to solve these issues. The Helmholtz operator of the two-way wave equation is a power of the square root operator of the one-way wave equation. Based on this connectivity, the computation of the square root of the Helmholtz operator based on matrix multiplication and the calculation of a one-way propagator with a Taylor series expansion, is studied. The one-way propagators used in this paper involve complete matrix multiplication, which is a distinct feature of this method and is suitable for parallel implementation. Analysing the values of matrix square roots generated by the classical eigenvalues decomposition method and an algorithm of computing matrix square roots with matrix multiplication, we prove that our algorithm can calculate a stable result, which lays a solid foundation for further study. The impulse response test proves the advantage of our proposed method over conventional one-way migration in describing the wavefield propagation of large angles in a medium with a strong lateral velocity variation. Moreover, we implement GPU and CPU versions of the algorithm and compared their efficiencies. The post-stack migration result of the Salt model and the pre-stack migration result of the Marmousi model further illustrate the superiority of our proposed migration method in imaging the complex and fine-scale structures compared to the conventional one-way migration method. This provides a promising practical application in seismic exploration.

Reliability of data-driven wavefront attributes in laterally heterogeneous media

3D wavefront attributes play a major role in many processing steps, such as prestack data enhancement, diffraction separation, and wavefront tomography. For the determination of the 3D wavefront attributes, various stacking operators can be used by adopting semblance optimization. These operators are derived for laterally homogeneous media. In praxis, however, they are applied in real geologic environments with even strong lateral velocity variations such as salt structures. This leads to the question of the quality of the 3D wavefront attributes using these operators when determined in the presence of strong lateral velocity changes. We compared the 3D wavefront attributes determined by 3D common-reflection-surface (CRS) operator (called data-driven wavefront attributes) with the 3D wavefront attributes computed by 3D kinematic and dynamic ray tracing (called model-driven wavefront attributes). For the determination of the 3D CRS wavefront attributes, we have developed a global optimization scheme based on differential evolution. Reflection seismic data of the laterally heterogeneous 3D SEG C3WA salt model are considered, and the model-driven wavefront attributes are computed for a smoothed version of the 3D SEG salt model. The comparison reveals that the wavefront attributes for the normal-incidence-point ray indicate a very good match not only in areas of mild lateral velocity variation but even in regions with strong lateral velocity variations. Approximately 80%–90% of the total picks indicate the good match with a relative error of less than 10% when a semblance threshold of 0.1 is considered in the automatic picking process. This confirms the validity of the determined wavefront attributes even in the presence of strong lateral velocity changes. Using a higher semblance threshold in the automatic picking leads to fewer picks but with an even better match between model- and data-driven wavefront attributes.

Upper Permian Zechstein Supergroup carbonate-evaporite platform palaeomorphology in the UK Southern North Sea

Abstract Seismic interpretation, geological mapping and depth conversion of the Zechstein Supergroup (Z2 cycle), using high-quality well-calibrated three-dimensional (3-D) seismic, has revealed the complex palaeomorphology of a deeply-buried ancient carbonate shelf-margin on the southwestern margin of the Southern Permian Basin. The new mapping shows that the margin comprises a series of corrugated embayments and promontories which extend up to 5 km (3 mi) into the basin and locally have a palaeobathymetric relief of >200 m (650 ft). Well calibration across the margin demonstrates a strongly bipartite lateral thickness distribution between a high velocity anhydrite-dominated shelf and a lower velocity halite-dominated basin. Strong lateral and vertical velocity variations in the Upper Permian Zechstein Supergroup are known to have major impacts upon seismic imaging and depth conversion in the SPB. The resulting uncertainty remains one of the major challenges when interpreting and assessing the prospectivity of the underlying Upper Permian, Rotliegend Group (Leman Sandstone Formation) reservoirs in the UK Southern North Sea. An understanding of the Zechstein shelf edge's 3-D physiography and its velocity variation has implications for the delineation of traps containing the prospective reservoirs that lie below. Recognition of the complexity and effects of this shelf-margin contributed to the recent Juliet Discovery whose position outside of the presently defined Rotliegend Group play fairway suggests that prospectivity is more extensive along the basin margin than previously thought. As such, the work provides a means by which to identify and delineate new structures and extend the life of this mature yet prolific gas province.

Gas Hydrate and Free Gas Concentrations in Two Sites inside the Chilean Margin (Itata and Valdivia Offshores)

Two sectors, Itata and Valdivia, which are located in the Chilean margin were analysed by using seismic data with the main purpose to characterize the gas hydrate concentration. Strong lateral velocity variations are recognised, showing a maximum value in Valdivia offshore (2380 ms−1 above the BSR) and a minimum value in the Itata offshore (1380 m·s−1 below the BSR). In both of the sectors, the maximum hydrate concentration reaches 17% of total volume, while the maximum free gas concentration is located Valdivia offshore (0.6% of total volume) in correspondence of an uplift sector. In the Itata offshore, the geothermal gradient that is estimated is variable and ranges from 32 °C·km−1 to 87 °C·km−1, while in Valdivia offshore it is uniform and about 35 °C·km−1. When considering both sites, the highest hydrate concentration is located in the accretionary prism (Valdivia offshore) and highest free gas concentration is distributed upwards, which may be considered as a natural pathway for lateral fluid migration. The results that are presented here contribute to the global knowledge of the relationship between hydrate/free gas presence and tectonic features, such as faults and folds, and furnishes a piece of the regional hydrate potentiality Chile offshore.

Tomographic velocity analysis and wave-equation depth migration in an overthrust terrain: A case study from the Tuha Basin, China

Although the structures associated with overthrust terrains form important targets in many basins, accurately imaging remains challenging. Steep dips and strong lateral velocity variations associated with these complex structures require prestack depth migration instead of simpler time migration. The associated rough topography, coupled with older, more indurated, and thus high-velocity rocks near or outcropping at the surface often lead to seismic data that suffer from severe statics problems, strong head waves, and backscattered energy from the shallow section, giving rise to a low signal-to-noise ratio that increases the difficulties in building an accurate velocity model for subsequent depth migration. We applied a multidomain cascaded noise attenuation workflow to suppress much of the linear noise. Strong lateral velocity variations occur not only at depth but near the surface as well, distorting the reflections and degrading all deeper images. Conventional elevation corrections followed by refraction statics methods fail in these areas due to poor data quality and the absence of a continuous refracting surface. Although a seismically derived tomographic solution provides an improved image, constraining the solution to the near-surface depth-domain interval velocities measured along the surface outcrop data provides further improvement. Although a one-way wave-equation migration algorithm accounts for the strong lateral velocity variations and complicated structures at depth, modifying the algorithm to account for lateral variation in illumination caused by the irregular topography significantly improves the image, preserving the subsurface amplitude variations. We believe that our step-by-step workflow of addressing the data quality, velocity model building, and seismic imaging developed for the Tuha Basin of China can be applied to other overthrust plays in other parts of the world.

3-D crustal velocity structure of western Turkey: Constraints from full-waveform tomography

The Sea of Marmara and western Turkey are characterized by intense seismicity and crustal deformation due to transition tectonics between the North Anatolian Fault Zone (NAFZ) and the extensional Aegean. Seismic imaging of the crust and uppermost mantle in W-NW Turkey is crucial to obtain a better understanding of its seismotectonics and geodynamics. So far, the Sea of Marmara and surroundings were considered in various active and passive seismic experiments providing significant information on crustal properties. Here, we further investigate the 3-D seismic velocity structure in this rapidly deforming region using non-linear full-waveform tomography based on the adjoint method. Our model is constrained by complete waveforms of 62 regional earthquakes (epicentral distance<10°) with magnitudes Mw≥3.7, which occurred between 2007 and 2015. Validation tests show that our final 3-D Earth model is able to explain seismic waveforms from earthquakes not used in the inversion at periods from 8-100s to within the data uncertainties. Furthermore, quantitative resolution analyses yield 15 to 35km horizontal resolution lengths in the shallow and deep crust beneath well-covered areas of W-NW Turkey. Our full-waveform tomography results indicate the presence of strong lateral and vertical velocity variations (2.55≤VS≤4.0km/s) down to depths of ∼35km. The seismic velocity distribution is characteristic of highly deformed and distributed crustal features along major fault zones (e.g. NAFZ and its branches), historic and recent regional volcanism (e.g. Kula volcanic province), and metamorphic core complex developments (e.g. Menderes and Kazdağ massifs). Radial anisotropy is very strong (around 20%) throughout the crust, further attesting to strong deformation and heterogeneity. Generally, our 3-D model is overall consistent with the active tectonics of western Turkey.

The onset of Walvis Ridge: Plume influence at the continental margin

The opening of the South Atlantic is a classical example for a plume related continental breakup. Flood basalts are present on both conjugate margins as well as aseismic ridges connecting them with the current plume location at Tristan da Cunha. To determine the effect of the proposed plume head on the continental crust, we acquired wide-angle seismic data at the junction of the Walvis Ridge with the African continent and modelled the P-wave velocity structure in a forward approach. The profile extends 430km along the ridge and continues onshore to a length of 720km. Crustal velocities beneath the Walvis Ridge vary between 5.5km/s and 7.0km/s, a typical range for oceanic crust. The crustal thickness of 22km, however, is approximately three times larger than of normal oceanic crust. The continent-ocean transition is characterized by 30km thick crust with strong lateral velocity variations in the upper crust and a high-velocity lower crust (HVLC), where velocities reach up to 7.5km/s. The HVLC is 100 to 130km wider at the Walvis Ridge than it is farther south, and impinges onto the continental crust of the Kaoko fold belt. Such high seismic velocities indicate Mg-rich igneous material intruded into the continental crust during the initial rifting stage. However, the remaining continental crust seems unaffected by intrusions and the root of the 40km-thick crust of the Kaoko belt is not thermally abraded. We conclude that the plume head did not modify the continental crust on a large scale, but caused rather local effects. Thus, it seems unlikely that a plume drove or initiated the breakup process. We further propose that the plume already existed underneath the continent prior to the breakup, and ponded melt erupted at emerging rift structures providing the magma for continental flood basalts.

Evidence for strong lateral seismic velocity variation in the lower crust and upper mantle beneath the California margin

Regional seismograms from earthquakes in Northern California show a systematic difference in arrival times across Southern California where long period (30–50 s) SH waves arrive up to 15 s earlier at stations near the coast compared with sites towards the east at similar epicentral distances. We attribute this time difference to heterogeneity of the velocity structure at the crust–mantle interface beneath the California margin. To model these observations, we propose a fast seismic layer, with thickness growing westward from the San Andreas along with a thicker and slower continental crust to the east. Synthetics generated from such a model are able to match the observed timing of SH waveforms better than existing 3D models. The presence of a strong upper mantle buttressed against a weaker crust has a major influence in how the boundary between the Pacific plate and North American plate deforms and may explain the observed asymmetric strain rate across the boundary.

High-order generalized screen propagator migration based on particle swarm optimization

Various migration methods have been proposed to image high-angle geological structures and media with strong lateral velocity variations; however, the problems of low precision and high computational cost remain unresolved. To describe the seismic wave propagation in media with lateral velocity variations and to image high-angle structures, we propose the generalized screen propagator based on particle swarm optimization (PSO-GSP), for the precise fitting of the single-square-root operator. We use the 2D SEG/EAGE salt model to test the proposed PSO-GSP migration method to image the faults beneath the salt dome and compare the results to those of the conventional high-order generalized screen propagator (GSP) migration and split-step Fourier (SSF) migration. Moreover, we use 2D marine data from the South China Sea to show that the PSO-GSP migration can better image strong reflectors than conventional imaging methods.

Applying full-azimuth angle domain imaging to study carbonate reefs at great depths

The main goal of the seismic surveys conducted in the target area in Kazakhstan was to image and detect a major Devonian carbonate barrier reef and to characterize the density and orientation of its main fracture zones. This area is unique in that the carbonate layers occur at great depths, from 5700 to 8500 m, with a complex structure of overburden layers of interbedded clay, sandstone, salt and others (Table 1). These different lithology plays and morphology rocks create strong vertical and lateral velocity variations, resulting in a complex seismic wave phenomenon. In addition, the target carbonate structures contain heterogeneous objects and require high-quality processing of the recorded data. Under such conditions, it is crucial to use full-azimuth, long-offset and dense (high fold) acquisition patterns. Advanced processing sequence tools and high-end depth migrations are required to handle both strong heterogeneity and azimuthal anisotropy effects. Traditional Kirchhoff migrations, even the most accurate ones (wavefront reconstruction, beam), have not been able to provide the required image quality and level of detail required at the target zones (Figure 1). Conventional Kirchhoff migrations generate surface offset-azimuth/offset domain common image gathers (CIG). In this particularly complex area, the correlation between the surface offset-azimuths and the actual, in situ, subsurface slowness-azimuths (azimuth of the incidence/reflected ray pairs at the image points) is relatively poor, leading to significant errors in the estimation of fracture orientation. Additionally, Kirchhoff migrations do not account for multi-pathing (multi-wave path solutions between image points and surface source/receiver locations), which is essential when imaging in areas involving complex wave phenomena. Kirchhoff beam migrations map surface beams with given directions backward to the subsurface. Since they account for the multi-pathing, they provide better results. However, they normally generate the same type of surface offset-azimuth/offset CIGs and therefore cannot be accurately used for azimuthal studies. They also cannot ensure sufficient subsurface illumination, especially at the complex target subsurface regions.

Near-surface velocity modeling using a combined inversion of surface and refracted P-waves

We propose an innovative workflow based on the complementary use of Rayleigh waves alongside standard P-wave refraction tomography, which better depicts the shallow part of the near-surface P-wave velocity model. Our surface-wave processing sequence led to an S-wave near-surface velocity field that can be used as a constraint for P-wave tomography and can improve P-wave statics determination. Rayleigh waves are processed in three steps. The first step consists of an accurate frequency-dependent traveltime measurement for each selected source-receiver pair in which the phase difference between two adjacent traces is used to derive the phase velocity. Then, a frequency-dependent surface-wave velocity tomography is performed from the picked traveltimes. Finally, after surface-wave tomography, the frequency-dependent phase velocity volume output by the tomography is inverted to deliver an S-wave near-surface velocity model. This model is used to constrain the first-arrival P-wave tomography. To illustrate our method, we use a 3D narrow-azimuth land data set, acquired along a river valley. Strong lateral velocity variations exist in the shallow part, with slow velocities around the unconsolidated sediments of the riverbed and faster velocities in the consolidated sediments of the surrounding hills. A combined first-arrival tomography using the S-wave velocity model, the initial unconstrained refracted P-wave velocity model, and the original first arrivals is used to obtain a more accurate near-surface P-wave velocity model. This new approach led to a constrained P-wave velocity model from which primary statics are derived and then applied, leading to an improved image with better focusing and continuity of thin layers in the shallowest part.

Rifting to drifting transition of the Southwest Subbasin of the South China Sea

Continental rupturing process and related dynamics on the onset of seafloor spreading remain poorly understood in the opening of the South China Sea. To constrain the timing and cause of major tectonic events, we focus on the rifting-to-drifting transition of the Southwest Subbasin, which has very wide extended continental margins. By carefully interpreting rifting structures and carbonate platforms and reefs, we distinguished two major unconformities, i.e., the breakup unconformity (BRU) and the mid-Miocene unconformity, in the two conjugate margins of the Southwest Subbasin. The age of the BRU in our study area is near the Oligocene/Miocene boundary (~23 Ma). Pre-stack depth migration of a recently acquired multichannel reflection seismic profile reveals complex structures and strong lateral velocity variations associated with a 3.5 km thick syn-rifting sequence developed right at the continent-ocean boundary (COB) of the Southwest Subbasin. This syn-rifting sequence is bounded landwards by a large seaward dipping fault, and tapers out seawards. An erosional truncation, which represents the mid-Miocene unconformity landwards but the older breakup unconformity on the seaward side, occurred at the top of this sequence. The overall transitional deformation style from the rifting to drifting suggests a successive episode of rifting, faulting, compression, tilting, and erosion at the COB during the crustal thinning and mantle upwelling. Localized thick syn-rifting deposition and early deposition beneath the BRU in the oceanic domain exist only at the seaward concave part of the COB, indicating discrete rifting and seafloor spreading prior to the buildup of a unified spreading center for the entire subbasin.

Reverse time migration from topography

Migration of seismic data from topography using methods based on finite-difference (FD) approximation to acoustic wave propagation commonly suffers from a number of imaging drawbacks due to the difficulty of applying FD stencils to irregular computational meshes. Altering the computational geometry from Cartesian to a topographic coordinate system conformal to the data acquisition surface can circumvent many of these issues. The coordinate transformation approach allows for acoustic wave propagation and the crosscorrelation and inverse-scattering imaging conditions to be posed and computed directly in topographic coordinates. Resulting reverse time migration (RTM) images may then be interpolated back to the Cartesian domain using the known inverse mapping. Orthogonal 2D topographic coordinates can be developed using known conformal mapping transforms and serve as the computational mesh for performing migration from topography. Impulse response tests demonstrate the accuracy of the 2D generalized acoustic wave propagation. RTM imaging examples show the efficacy of performing migration from topography directly from the data acquisition surface on topographic meshes and the ability to image complex near-surface structure even in the presence of strong lateral velocity variation.