Lateral Velocity Variations Research Articles

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.

Application of a complete workflow for 2D elastic full-waveform inversion to recorded shallow-seismic Rayleigh waves

The S-wave velocity of the shallow subsurface can be inferred from shallow-seismic Rayleigh waves. Traditionally, the dispersion curves of the Rayleigh waves are inverted to obtain the (local) S-wave velocity as a function of depth. Two-dimensional elastic full-waveform inversion (FWI) has the potential to also infer lateral variations. We have developed a novel workflow for the application of 2D elastic FWI to recorded surface waves. During the preprocessing, we apply a line-source simulation (spreading correction) and perform an a priori estimation of the attenuation of waves. The iterative multiscale 2D elastic FWI workflow consists of the preconditioning of the gradients in the vicinity of the sources and a source-wavelet correction. The misfit is defined by the least-squares norm of normalized wavefields. We apply our workflow to a field data set that has been acquired on a predominantly depth-dependent velocity structure, and we compare the reconstructed S-wave velocity model with the result obtained by a 1D inversion based on wavefield spectra (Fourier-Bessel expansion coefficients). The 2D S-wave velocity model obtained by FWI shows an overall depth dependency that agrees well with the 1D inversion result. Both models can explain the main characteristics of the recorded seismograms. The small lateral variations in S-wave velocity introduced by FWI additionally explain the lateral changes of the recorded Rayleigh waves. The comparison thus verifies the applicability of our 2D FWI workflow and confirms the potential of FWI to reconstruct shallow small-scale lateral changes of S-wave velocity.

Ambient seismic noise tomography of the Colima Volcano Complex

The Colima Volcanic Complex (CVC) located in the western sector of the Trans-Mexican Volcanic Belt contains the most active Mexican volcano, Volcan Colima. The CVC is located within the Colima Rift, a regional north south striking extensional structure. We used ambient seismic noise recorded by stations deployed in western Mexico during the Mapping the Rivera Subduction Zone (MARS) and the Colima Volcano Deep Seismic Experiment (CODEX). We computed the cross-correlations of the vertical component of continuous records of ambient noise data to extract empirical Greens functions. These functions provide detailed images of Rayleigh wave group velocity for different periods. Using the arrival travel time of these waves for a given period, estimates can be obtained of the lateral variations in velocity for a given period using 2D tomography. The study aims to better understand the geometry and the seismic surface wave velocity structure of the CVC and relate it to the volcanoes' structure and the geologic setting of the region. Source of low velocity anomaly over CVC is distributed fairly continuously with depth in the subsurface, which indicates magma rising along fractures. The progressive increasing toward the south in the size of low velocity anomalies indicates migration towards the south of the melting that correlates with the trend of the stratovolcanoes that form the CVC. The zone of magma generation presently fully developed under Volcan de Fuego might be starting to shift towards south to the area NW of Armeria where a new void in the tear zone may be starting to form.

Lateral variation in seismic velocities and rheology beneath the Qinling-Dabie orogen

The Qinling-Dabie orogen is an important tectonic belt that trends east-west and divides continental China into northern and southern parts. Due to its strong deformation, complicated structure, multiphase structural superposition and the massive exposed high and ultrahigh metamorphic rocks, its tectonic formation and geodynamical evolution are hot research topics worldwide. Previous studies mainly focused on the regional geological or geochemical aspects, whereas the geophysical constraints are few and isolated, in particular on the orogenic scale. Here, we integrate the available P- and S-wave seismic and seismicity data, and construct the rheological structures along the Qinling-Dabie orogen. The results demonstrate that: (1) there are strong lateral variations in the crustal velocity between the western and eastern sections of the Qinling-Dabie orogen, indicating the different origin and tectonic evolution between these two parts; (2) the lateral variations are also manifested in the rheological structure. The rigid blocks, such as South China and Ordos basin (North China Craton), resist deformation and show low seismicity. The weak regions, such as the margin of Tibet and western Qinling-Dabie experience strong deformation and accumulated stress, thus show active seismicity; (3) in the lower crust of most of the HP/UHP terranes the values of P-wave velocity are higher than the global average ones; finally (4) low P- and S-wave velocities and low strength in the lower crust and lithospheric mantle beneath Dabie indicate lithospheric delamination, and/or high temperature, and partial melting condition.

Semi-2D modeling of river morphological changes caused by exceptional flooding

This paper presents the application of the model, MHYSER 1.0 (Model for the HYdraulics of SEdiments in Rivers, version 1.0), to study river morphological changes due to exceptional flooding. MHYSER 1.0 is a semi-two-dimensional model using the stream tubes concept to achieve lateral variations of velocity, flow stresses and sediment transport rate. Each stream tube has the same conveyance as the other ones. In MHYSER 1.0, the uncoupled approach is used to solve the set of conservation equations’. After the backwater calculation, the river is divided in a finite number of stream tubes of equal conveyances. The sediment routing and bed adjustments calculations are accomplished separately along each stream tube taking into account lateral mass exchanges. MHYSER 1.0 is applied to the Ha! Ha! River (Quebec, Canada), a tributary of the Saguenay River, for the 1996 “downpour“. The results show good correlation of with collected data. This application shows the capabilities of this model and predicts its promising role in solving complex real engineering cases.

Seismic velocity variations beneath central Mongolia: Evidence for upper mantle plumes?

Central Mongolia is marked by wide spread recent volcanism as well as significant topographic relief even though it is far from any plate tectonic boundaries. The cause of the recent magmatism and topography remains uncertain partially because little is known of the underlying mantle seismic structure due to the lack of seismic instrumentation in the region. From August 2011 through August 2013, 69 broadband seismic stations were deployed in central Mongolia. Teleseismic traveltime residuals were measured using waveform correlation and were inverted to image upper mantle P and S velocity variations. Significant lateral variations in seismic velocity are imaged in the deep upper mantle (100 to 800 km depth). Most significant are two continuous slow anomalies from the deep upper mantle to near the surface. One slow feature has been imaged previously and may be a zone of deep upwelling bringing warm mantle to beneath the Hangay Dome resulting in uplift and magmatism including the active Khanuy Gol and Middle Gobi volcanoes. The second, deep low velocity anomaly is seen in the east from 800 to 150 km depth. The anomaly ends beneath the Gobi Desert that is found to have fast shallow mantle indicating a relatively thick lithosphere. We interpret the second deep slow anomaly as a mantle upwelling that is deflected by the thick Gobi Desert lithosphere to surrounding regions such as the Hentay Mountains to the north. The upwellings are a means of feeding warmer than normal asthenospheric mantle over a widely distributed region beneath Mongolia resulting in distributed volcanic activity and uplift. There is no indication that the upwellings are rooted in the deep lower mantle i.e. classic plumes. We speculate the upwellings may be related to deep subduction of the Pacific and Indian plates and are thus plumes anchored in the upper mantle.

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.

Wave equation dispersion inversion using a difference approximation to the dispersion-curve misfit gradient

We present a surface-wave inversion method that inverts for the S-wave velocity from the Rayleigh wave dispersion curve using a difference approximation to the gradient of the misfit function. We call this wave equation inversion of skeletonized surface waves because the skeletonized dispersion curve for the fundamental-mode Rayleigh wave is inverted using finite-difference solutions to the multi-dimensional elastic wave equation. The best match between the predicted and observed dispersion curves provides the optimal S-wave velocity model. Our method can invert for lateral velocity variations and also can mitigate the local minimum problem in full waveform inversion with a reasonable computation cost for simple models. Results with synthetic and field data illustrate the benefits and limitations of this method.

High lateral resolution exploration using surface waves from noise records

Determination of the shear-wave velocity structure at shallow depths is a constant necessity in engineering or environmental projects. Given the sensitivity of Rayleigh waves to shear-wave velocity, subsoil structure exploration using surface waves is frequently used. Methods such as the spectral analysis of surface waves (SASW) or multi-channel analysis of surface waves (MASW) determine phase velocity dispersion from surface waves generated by an active source recorded on a line of geophones. Using MASW, it is important that the receiver array be as long as possible to increase the precision at low frequencies. However, this implies that possible lateral variations are discarded. Hayashi and Suzuki (2004) proposed a different way of stacking shot gathers to increase lateral resolution. They combined strategies used in MASW with the common mid-point (CMP) summation currently used in reflection seismology. In their common mid-point with cross-correlation method (CMPCC), they cross-correlate traces sharing CMP locations before determining phase velocity dispersion. Another recent approach to subsoil structure exploration is based on seismic interferometry. It has been shown that cross-correlation of a diffuse field, such as seismic noise, allows the estimation of the Green’s Function between two receivers. Thus, a virtual-source seismic section may be constructed from the cross-correlation of seismic noise records obtained in a line of receivers.In this paper, we use the seismic interferometry method to process seismic noise records obtained in seismic refraction lines of 24 geophones, and analyse the results using CMPCC to increase the lateral resolution of the results. Cross-correlation of the noise records allows reconstructing seismic sections with virtual sources at each receiver location. The Rayleigh wave component of the Green’s Functions is obtained with a high signal-to-noise ratio. Using CMPCC analysis of the virtual-source seismic lines, we are able to identify lateral variations of phase velocity inside the seismic line, and increase the lateral resolution compared with results of conventional analysis.

Full-matrix capture with phased shift migration for flaw detection in layered objects with complex geometry

This paper introduces a method for an ultrasonic imaging with a phased array based on a wave migration algorithm. The method allows for imaging layered objects with lateral velocity variations such as objects with a complex geometry or layers that are not perpendicular to the array’s axis. The full-matrix capture ensures that there is enough information to reconstruct an image even when the wave indication angle is large. The method is implemented in a omega-k domain.The proposed algorithm is first tested in a single simulation of a concave object with side drilled holes under the concave surface. For evaluating the algorithm’s performance three experiments are presented: one with a tilted object (surface not perpendicular with respect to the array axis) with side drilled holes and two experiments of an object with concave surface and two artificial defects under it. The results presented in the paper verify that the proposed method reconstructs images from the data gathered with the phased array.

Shear-wave velocity structure of the shallow sediments in the Bohai Sea from an ocean-bottom-seismometer survey

To investigate the seismic velocity structure of the shallow sediments in the Bohai Sea of China, we conducted a shear-wave velocity inversion of the surface-wave dispersion data from a survey of 12 ocean-bottom seismometers (OBSs) and 377 shots of a [Formula: see text] air gun. With OBS station spacing of approximately 5 km and air-gun shot spacing of approximately 190 m, high-quality Scholte-wave data were recorded by the OBSs within 0.4–5 km offset. We retrieved the Scholte-wave phase-velocity dispersion for the fundamental mode and first overtone in the frequency band of 0.9–3.0 Hz with the phase-shift method and inverted for the shear-wave velocity structure of the shallow sediments with a damped iterative least-squares algorithm. Pseudo-2D shear-wave velocity profiles with a depth of approximately 400 m revealed coherent features of relatively weak lateral velocity variation. We also estimated the uncertainty in shear-wave velocity structure based on the pseudo-2D profiles from six trial inversions with different initial models, which suggested a velocity uncertainty less than [Formula: see text] for most parts of the 2D profiles. The layered structure with little lateral variation may be attributable to the continuous sedimentary environment in the Cenozoic sedimentary basin of the Bohai Bay basin. The shear-wave velocity of 200–300 [Formula: see text] in the top 100 m of the Bohai seafloor may provide important information for offshore site response studies in earthquake engineering. Furthermore, the very low shear-wave velocity structure (150–600 [Formula: see text]) down to 400 m depth could produce a significant traveltime delay of approximately 1 s in the shear-wave arrivals, which needs to be considered to avoid serious bias in shear-wave traveltime tomographic models.

Phase-Shifted Based Numerical Method for Modeling Frequency-Dependent Effects on Seismic Reflections

The significant velocity dispersion and attenuation has often been observed when seismic waves propagate in fluid-saturated porous rocks. Both the magnitude and variation features of the velocity dispersion and attenuation are frequency-dependent and related closely to the physical properties of the fluid-saturated porous rocks. To explore the effects of frequency-dependent dispersion and attenuation on the seismic responses, in this work, we present a numerical method for seismic data modeling based on the diffusive and viscous wave equation (DVWE), which introduces the poroelastic theory and takes into account diffusive and viscous attenuation in diffusive-viscous-theory. We derive a phase-shift wave extrapolation algorithm in frequencywavenumber domain for implementing the DVWE-based simulation method that can handle the simultaneous lateral variations in velocity, diffusive coefficient and viscosity. Then, we design a distributary channels model in which a hydrocarbon-saturated sand reservoir is embedded in one of the channels. Next, we calculated the synthetic seismic data to analytically and comparatively illustrate the seismic frequency-dependent behaviors related to the hydrocarbon-saturated reservoir, by employing DVWE-based and conventional acoustic wave equation (AWE) based method, respectively. The results of the synthetic seismic data delineate the intrinsic energy loss, phase delay, lower instantaneous dominant frequency and narrower bandwidth due to the frequency-dependent dispersion and attenuation when seismic wave travels through the hydrocarbon-saturated reservoir. The numerical modeling method is expected to contribute to improve the understanding of the features and mechanism of the seismic frequency-dependent effects resulted from the hydrocarbon-saturated porous rocks.

Time-Lapse Seismic for Reservoir Management: Case Studies From Offshore Niger Delta, Nigeria

Summary This paper presents case studies focused on the interpretation and integration of seismic reservoir monitoring from several fields in conventional offshore and deepwater Niger Delta. The fields are characterized by different geological settings and development-maturity stages. We show different applications varying from qualitative to quantitative use of time-lapse (4D) seismic information. In the first case study, which is in shallow water, the field has specific reservoir-development challenges, simple geology, and is in phased development. On this field, 4D seismic, which was acquired several years ago, is characterized by poor seismic repeatability. Nevertheless, we show that because of improvements from seismic reprocessing, 4D seismic makes qualitative contributions to the ongoing field development. In the second case study, the field is characterized by complex geological settings. The 4D seismic is affected by overburden with strong lateral variations in velocity and steeply dipping structure (up to 40°). Prestack-depth-imaging (PSDM) 4D seismic is used in a more-qualitative manner to monitor gas injection, validate the geologic/reservoir models, optimize infill injector placement, and consequently, enhance field-development economics. The third case study presents a deep offshore field characterized by a complex depositional system for some reservoirs. In this example, good 4D-seismic repeatability (sum of source- and receiver-placement differences between surveys, dS+dR) is achieved, leading to an increased quantitative use of 4D monitoring for the assessment of sand/sand communication, mapping of oil/water (OWC) front, pressure evolution, and dynamic calibration of petro-elastic model (PEM), and also as a seismic-based production-logging tool. In addition, 4D seismic is used to update seismic interpretation, provide a better understanding of internal architecture of the reservoirs units, and, thereby, yield a more-robust reservoir model. The 4D seismic in this field is a key tool for field-development optimization and reservoir management. The last case study illustrates the need for seismic-feasibility studies to detect 4D responses related to production. In addition to assessing the impact of the field environment on the 4D- seismic signal, these studies also help in choosing the optimum seismic-survey type, design, and acquisition parameters. These studies would possibly lead to the adoption of new technologies such as broad-band streamer or nodes acquisition in the near future.

POLENET/LAPNET teleseismic <i>P</i> wave travel time tomography model of the upper mantle beneath northern Fennoscandia

Abstract. The POLENET/LAPNET (Polar Earth Observing Network) broadband seismic network was deployed in northern Fennoscandia (Finland, Sweden, Norway, and Russia) during the third International Polar Year 2007–2009. The array consisted of roughly 60 seismic stations. In our study, we estimate the 3-D architecture of the upper mantle beneath the northern Fennoscandian Shield using high-resolution teleseismic P wave tomography. The P wave tomography method can complement previous studies in the area by efficiently mapping lateral velocity variations in the mantle. For this purpose 111 clearly recorded teleseismic events were selected and the data from the stations hand-picked and analysed. Our study reveals a highly heterogeneous lithospheric mantle beneath the northern Fennoscandian Shield though without any large high P wave velocity area that may indicate the presence of thick depleted lithospheric “keel”. The most significant feature seen in the velocity model is a large elongated negative velocity anomaly (up to −3.5 %) in depth range 100–150 km in the central part of our study area that can be followed down to a depth of 200 km in some local areas. This low-velocity area separates three high-velocity regions corresponding to the cratonic units forming the area.

Prediction of lateral variations in reservoir properties throughout an interpreted seismic horizon using an artificial neural network

Successful use of an artificial neural network is shown to predict lateral variations of seismic velocity, density, thickness, and gamma rays associated with sand dune reservoirs identified on a previously interpreted seismic horizon. The work is presented in two main sections. Section one is a feasibility analysis based on synthetic data. A known geologic model is used, performed by pseudowells, in which lateral variations in seismic velocity, density, and gamma ray values are related to the dunes. The synthetic seismic model and the attributes derived are used as training input in the neural network. Section two is a real case example where the methodology is applied to a real seismic data set. Results indicate that using a set of data and attributes restricted to a time interval corresponding to a previously interpreted seismic horizon is more efficient than using a whole data cube, involving a very large volume of data.

Near-Surface Velocity Model To Enhance PSDM Seismic Imaging of Dukhan Field

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IPTC 18293, “Near-Surface Velocity Model of Dukhan Field From Multiphysics Survey To Enhance PSDM Seismic Imaging,” by K. Setiyono, Qatar Petroleum; S. Gallo, C. Boulanger, F. Bruere, F. Moreau, and B. Rondeleux, CGG; and J. Snow, Qatar Petroleum, prepared for the 2015 International Petroleum Technology Conference, Doha, Qatar, 7-9 December. The paper has not been peer reviewed. Dukhan field affords an opportunity to assess alternative near-surface velocity-modeling work flows that specifically integrate microgravity (MGR) and vertical electrical sounding (VES). A pilot study of the Dukhan seismic data demonstrates that integration of MGR/VES into prestack depth migration (PSDM) improves the fidelity of seismic images and results in an improved depth match. The resultant MGR/VES PSDM pilot model improved the velocity model between the surface and top Simsima formation on the basis of visible impact on the deeper structure obtained from imaging and better fit between the seismic depth horizons and well measured depths. Introduction The Dukhan field (Fig. 1) along the southwest coast of Qatar is an elongated anticline that has significant hydrocarbon reservoirs in the Jurassic Arab and deeper formations. In 2007, Qatar Petroleum invested in a new full-field, land and shallow-water 3D-seismic survey of the area. Despite considerable and continuous progress in seismic technology, obtaining an accurate, quantitative, well-focused seismic depth image at reservoir level remains challenging. In land seismic, a poor characterization of the near-surface geology is detrimental to the depth imaging of underlying structures. The proper knowledge of the near surface to correct for its perturbations onto the seismic wavefield remains one of the main challenges to be addressed. The near surface influences the kinematics as well as the dynamics of the source-generated seismic waves. Knowledge of the near surface to correct for its induced distortions is critical for a reliable use of seismic images, structures, and amplitude at reservoir level. The near-surface lateral and vertical velocity variations often are not accessible with seismic methods, even to the seismic refraction survey, because their depth of investigation is limited to the top of the shallowest hard layer. In addition, rapid lateral changes and large velocity contrasts are unfavorable to seismic refraction.

Seismic image enhancement in post stack depth migration by finite offset CDS stack method

Seismic imaging in complex geological structures such as thrust belts and areas with complex geological structures is affected by several factors that often lead to poor-quality final result. Usually such structures produce locally very steep dips, strong lateral variations in velocity and abrupt truncation of the reflectors. Common reflection surface stack is a macro velocity model independent method that is introduced for seismic imaging in complex media. However, this method has some drawbacks in imaging of low-quality data from complex structures. Many improvements to this method have been introduced in several researches to overcome this drawback. However, the problem of conflicting dips situation is still a problematic issue in this method. In this study, a new method, called finite offset common diffraction surface (FO-CDS) stack, is introduced to overcome this problem and remove some geological interpretation ambiguities in seismic sections. This method is based on improving the CDS stack operator, with the idea of partial common reflection surface stack. This modification will enhance the quality of the final seismic image, where it suffers from conflicting dips problem and low signal to noise ratio. The new idea is to change the operator of the CDS equation into a finite offset mode in different steps for each sample. Subsequently, a time-variant linear function is designed for each sample to define the offset range using FO-CDS operator. The width of this function is designed according to the Fresnel zone. The new operator was applied on a synthetic and a real low fold land data. Results show the ability of the new method in enhancing the quality of the stacked section in the presence of faults and conflicting dips.

Three‐dimensional seismic velocity structure in the Sichuan basin, China

AbstractWe present a new three‐dimensional velocity model of the crust in the eastern margin of the Tibetan Plateau. The model describes the velocity structure of the Sichuan basin and surrounding thrust belts. The model consists of 3‐D surfaces representing major geologic unit contacts and faults and is parameterized with Vp velocity‐depth functions calibrated using sonic logs. The model incorporates data from 1166 oil wells, industry isopach maps, geological maps, and a digital elevation model. The geological surfaces were modeled based on structure contour maps for various units from oil wells and seismic reflection profiles. These surfaces include base Quaternary, Mesozoic, Paleozoic, and Proterozoic horizons. The horizons locally exhibit major offsets that are compatible with the locations and displacements of important faults systems. This layered, upper crustal 3‐D model extends down to 10–15 km depth and illustrates lateral and vertical variations of velocity that reflect the complex evolution of tectonics and sedimentation in the basin. The model also incorporates 3‐D descriptions of Vs and density for sediments that are obtained from empirical relationships with Vp using direct measurements of these properties in borehole logs. To illustrate the impact of our basin model on earthquake hazards assessment, we use it to calculate ground motions and compare these with observations for the 2013 Lushan earthquake. The result demonstrates the effects of basin amplification in the western Sichuan basin. The Sichuan CVM model is intended to facilitate fault systems analysis, strong ground motion prediction, and earthquake hazards assessment for the densely populated Sichuan region.

Prestack elastic generalized-screen migration for multicomponent data

An efficient prestack depth migration method based on the elastic one-way wave equation was developed using an improved elastic generalized-screen propagator, which effectively describes the behavior of elastic waves with mode conversion at the interfaces and efficiently computes wave propagation in media with lateral velocity variations. The elastic propagator presented in this study was improved from the elastic generalized-screen propagator. Several terms of the vertical slowness right symbol with orders are corrected from the original formulation, and the vertical slowness operator in the propagator was expanded up to the 2nd order which yields a more accurate approximation. In each screen propagation step, the multicomponent wavefields are automatically separated into the P and S wavefields by the P–S decomposition operator included in the elastic generalized-screen propagator. This process facilitates the imaging of the P and S waves separately without any additional P and S separation process after the wavefield extrapolation. Impulse response tests of the improved elastic generalized-screen propagator in a uniform-property medium proved that propagation accuracy increases with order, even when large medium perturbations are assigned. Migration tests using the developed algorithm on a simple layered model and two complex models (the SEG/EAGE salt and elastic Marmousi-2 model) demonstrated the functional advantages and capabilities of the algorithm compared with elastic migration using the scalar wave equation.

Subcrustal structure of the black sea basin from seismological data

The P-wave travel time data from the earthquakes offshore and onshore around the Black Sea are used for the tomographic reconstruction of the three-dimensional (3D) velocity distribution in the lithosphere of the region. The preliminary refinement of the foci parameters (the coordinates and origin time) has reduced the random errors in the travel-time data. The earthquake data were supplemented by the previous deep seismic sounding (DSS) data on the profiles in Crimea and offshore off the Black Sea. The dataset included more than 4000 travel times overall. In order to eliminate the crustal effect, the travel times were reduced to a surface at a depth of 35 km corresponding to the mean Moho depth in the region. The improved crustal model was used for removing the contribution of the crust from the initial data. The new tomography method, which was recently developed by one of the authors and which relies on the assumption of smoothness of the lateral velocity variations, was applied for reconstructing the velocity structure of the upper mantle beneath the Black Sea up to a depth of 95 km. The lateral velocity variation maps at different depths and the vertical velocity distributions along the meridional and sublatitudinal cross sections across the Black Sea were constructed. High velocities were revealed in the subcrustal lithosphere, and the structural difference below two subbasins—the West Black Sea (WBS) and the East Black Sea (EBS) ones—was established. It shows that the high-velocity body below the WBS is located deeper than below the EBS and is distinguished by higher velocities. Based on these results, it is concluded that the lithosphere beneath the Black Sea has a continental origin.