Prestack Depth Migration Research Articles

Traveltime tomography and prestack depth migration for vertical seismic profiling of an angle-domain walkaway on a complex surface

Walkaway VSP cannot obtain accurate velocity field, as it asymmetrically reflects ray path and provides uneven coverage to underground target, thereby presenting issues related to imaging quality. In this study, we propose combining traveltime tomography and prestack depth migration for VSP of an angle-domain walkaway, in a bid to establish accurate two-dimensional and three-dimensional (3D) velocity models. First, residual curvature was defined to update velocity, and an accurate velocity field was established. To establish a high-precision velocity model, we deduced the relationship between the residual depth and traveltime of common imaging gathers (CIGs) in walkaway VSP. Solving renewal velocity using the least squares method, a four-parameter tomographic inversion equation was derived comprising formation dip angle, incidence angle, residual depth, and sensitivity matrix. In the angle domain, the reflected wave was divided into up- and down-transmitted waves and their traveltimes were calculated. The systematic cumulative method was employed in prestack depth migration of a complex surface. Through prestack depth migration, the offset-domain CIGs were obtained, and dip angle was established by defining the stack section horizon. Runge—Kutta ray tracing was employed to calculate the ray path from the reflection point to the detection point, to determine the incident angle, and to subsequently calculate the ray path from the reflection point to the irregular surface. The offset-domain residual depths were mapped to the angle domain, and a new tomographic equation was established and solved. Application in the double complex area of the Tarim Basin showed the four-parameter tomographic inversion equation derived in this paper to be both correct and practical and that the migration algorithm was able to adapt to the complex surface.

Applying integrated seismic technology to complex foothill areas of foreland basins in China

Due to complicated near-surface conditions, including large elevation changes and complex geologic structures, accurate imaging of subsurface structures for hydrocarbon exploration in the foreland basins of western China has been challenging for many years. After decades of research and fieldwork, we developed an effective seismic exploration workflow that uses the latest technologies from acquisition to imaging. They include 3D high-density and wide-azimuth (WAZ) acquisition, 3D true-surface tilted transverse isotropy (TTI) anisotropic prestack depth migration, and dual-detachment structural modeling and interpretation. To further reduce uncertainty in velocity model building and improve imaging quality, our geologists, geophysicists, and reservoir engineers worked closely through the exploration cycle (seismic acquisition, processing, and interpretation). This exploration model has been used successfully in hydrocarbon exploration of many complex foothill areas in western China. Three-dimensional WAZ high-density seismic surveys have been conducted over 40,000 km2of the foreland basins, greatly improving the field seismic data quality. After application of 3D true-surface TTI anisotropic depth model building and imaging with integrated structural interpretation, new discoveries of hydrocarbon reservoirs have increased. The application of new technologies not only increased drilling success but also reduced depth well-tie errors between seismic data and wells.

Angle-Domain Differential Reflection Traveltime Tomography

Seismic reflection tomography based on prestack depth migration has been widely used for seismic imaging in the presence of complex geology. However, it still has some well-known and unfavorable features, such as slow convergence and requirements of many rounds of migration and residual moveout picking. To avoid the dependence on the unknown true depths of the reflection interfaces and to mitigate the ill-condition of the inverse problem, we propose a differential reflection tomography approach based on the angle-domain common-image gathers. The objective of our method is to iteratively update the velocity model so that the depth deviations between pairs of adjacent incident angles are minimized. Eigenvalue analysis of the tomographic matrices indicates that the conditioning is improved by our double-difference measurement instead of the commonly used float-datum criterion. Aiming at a stable and efficient velocity update, we introduce a structure-oriented model regularization and multi-level grid refinement in each round of residual moveout picking. Synthetic and field data examples show that the proposed approach can successfully retrieve the velocity structures for seismic imaging using fewer rounds of prestack depth migration and residual moveout picking.

Modelling subsurface geologic structures at the Ikogosi geothermal field, southwestern Nigeria, using gravity, magnetics and seismic interferometry techniques

Abstract We present results and a technique for imaging the subsurface structures of a geothermal field with particular focus on the Ikogosi geothermal field in Nigeria. The intent was to provide an understanding of the subsurface structural setup in the region and assess its viability for further geothermal resources exploitation. High-resolution aeromagnetic and gravity data were used for the study. A constrained 2D forward modelling technique was applied to these datasets to map the shape and corresponding depths of geologic structures in the region. This study has gone deeper to ascertain the basement structure and configurations and how it influences the heat source of the Ikogosi Warm Spring (IWS) region. The dominant host quartzite rock unit at the IWS location reaches average depths of 2.5–3.0 km and is located directly on an intruded high-density geologic formation in the subsurface. Fault structures traversing the IWS source have also been uncovered. We infer that these structural setups are central to the geothermal system of the IWS. Valid reflection responses from the profile model have been retrieved from randomly induced noise sources, using a passive seismic interferometry technique. Pre-stack depth migration of the reflected responses suitably imaged the reflectors within the subsurface of the IWS region, tracing fault boundaries and delineating intruded geological structures. This has provided pre-survey insights into the subsurface seismic imagery of the region. Results derived from this study could assist informed decision making regarding geothermal exploration and exploitation in the region.

Pre-stack Depth Migration based on VTI Media in Common Shot Domain

With the increasing complexity of seismic exploration targets, the basic imaging theory based on the basic assumption of simple isotropic media cannot meet the actual needs of oil and gas exploration at this stage. Therefore, the study of imaging methods in anisotropic media is of great significance to the exploration of various underground structures and mineral exploitation. In this paper, the SSF continuation operator for VTI anisotropic medium is derived and the one-way prestack depth migration with VTI media in common shot domain is realized. The VTI-HESS model is verified by calculation. Compared with prestack depth migration of various isotropic media, it can be seen that prestack depth migration of VTI media has better imaging effect because of considering anisotropic parameters, which is closer to the actual imaging effect of exploration data.

Anisotropic Kirchhoff pre-stack depth migration at the COSC-1 borehole, central Sweden

SUMMARYA remarkably well preserved representation of a deeply eroded Palaeozoic orogen is found in the Scandinavian Caledonides, formed by the collision of the two palaeocontinents Baltica and Laurentia. Today, after 400 Ma of erosion along with uplift and extension during the opening of the North Atlantic Ocean, the geological structures in central western Sweden comprise far transported allochthonous units, the underlying Precambrian crystalline basement, and a shallow west-dipping décollement that separates the two and is associated with a thin layer of Cambrian black shales. These structures, in particular the Seve Nappes (upper part of the Middle Allochthons), the Lower Allochthons and the highly reflective basement are the target of the two approximately 2.5 km deep fully cored scientific boreholes in central Sweden that are part of the project COSC (Collisional Orogeny in the Scandinavian Caledonides). Thus, a continuous 5 km tectonostratigraphic profile through the Caledonian nappes into Baltica’s basement will be recovered. The first borehole, COSC-1, was successfully drilled in 2014 and revealed a thick section of the seismically highly reflective Lower Seve Nappe. The Seve Nappe Complex, mainly consisting of felsic gneisses and mafic amphibolites, appears to be highly anisotropic. To allow for extrapolation of findings from core analysis and downhole logging to the structures around the borehole, several surface and borehole seismic experiments were conducted. Here, we use three long offset surface seismic profiles that are centred on the borehole COSC-1 to image the structures in the vicinity of the borehole and below it. We applied Kirchhoff pre-stack depth migration, taking into account the seismic anisotropy in the Seve Nappe Complex. We calculated Green’s functions using an anisotropic eikonal solver for a VTI (transversely isotropic with vertical axis of symmetry) velocity model, which was previously derived by the analysis of VSP (Vertical Seismic Profile) and surface seismic data. We show, that the anisotropic results are superior to the corresponding isotropic depth migration. The reflections appear significantly more continuous and better focused. The depth imaging of the long offset profiles provides a link between a high-resolution 3-D data set and the regional scale 2-D COSC Seismic Profile and complements these data sets, especially in the deeper parts below the borehole. However, many of the reflective structures can be observed in the different data sets. Most of the dominant reflections imaged originate below the bottom of the borehole and are situated within the Precambrian basement or at the transition zones between Middle and Lower Allochthons and the basement. The origin of the deeper reflections remains enigmatic, possibly representing dolerite intrusions or deformation zones of Caledonian or pre-Caledonian age.

3D Prestack Fourier Mixed-Domain (FMD) depth migration for VTI media with large lateral contrasts

Although many 3D One-Way Wave-equation Migration (OWEM) methods exist for VTI media, most of them struggle either with the stability, the anisotropic noise or the computational cost. In this paper we present a new method based on a mixed space- and wavenumber-propagator that overcome these issues very effectively as demonstrated by the examples. The pioneering methods of phase-shift (PS) and Stolt migration in the frequency-wavenumber domain designed for laterally homogeneous media have been followed by several extensions for laterally inhomogeneous media. Referred many times to as phase-screen or generalized phase-screen methods, such extensions include as main examples of the Split-step Fourier (SSF) and the phase-shift plus interpolation (PSPI). To further refine such phase-screen techniques, we introduce a higher-order extension to SSF valid for a 3D VTI medium with large lateral contrasts in vertical velocity and anisotropy parameters. The method is denoted Fourier Mixed-Domain (FMD) prestack depth migration and can be regarded as a stable explicit algorithm. The FMD technique was tested using the 3D SEG/EAGE salt model and the 2D anisotropic Hess model with good results. Finally, FMD was applied with success to a 3D field data set from the Barents Sea including anisotropy.

Seismic velocity structure of seaward-dipping reflectors on the South American continental margin

Seaward dipping reflectors (SDRs) are a key feature within the continent to ocean transition zone of volcanic passive margins. Here we conduct an automated pre-stack depth-migration imaging analysis of commercial seismic data from the volcanic margins of South America. The method used an isotropic, ray-based approach of iterative velocity model building based on the travel time inversion of residual pre-stack depth migration move-out. We find two distinct seismic velocity patterns within the SDRs. While both types show a general increase in velocity with depth consistent with expected compaction and alteration/metamorphic trends, those SDRs that lie within faulted half grabens also have high velocity zones at their down-dip ends. The velocity anomalies are generally concordant with the reflectivity and so we attribute them to the presence of dolerite sills that were injected into the lava pile. The sills therefore result from late-stage melt delivery along the large landward-dipping faults that bound them. In contrast the more outboard SDRs show no velocity anomalies, are more uniform spatially and have unfaulted basal contacts. Our observations imply that the SDRs document a major change in rift architecture, with magmatism linked with early extension and faulting of the upper brittle crust transitioning into more organised, dike-fed eruptions similar to seafloor spreading.

Crustal-scale depth imaging via joint full-waveform inversion of ocean-bottom seismometer data and pre-stack depth migration of multichannel seismic data: a case study from the eastern Nankai Trough

Abstract. Imaging via pre-stack depth migration (PSDM) of reflection towed-streamer multichannel seismic (MCS) data at the scale of the whole crust is inherently difficult. This is because the depth penetration of the seismic wavefield is controlled, firstly, by the acquisition design, such as streamer length and air-gun source configuration, and secondly by the complexity of the crustal structure. Indeed, the limited length of the streamer makes the estimation of velocities from deep targets challenging due to the velocity–depth ambiguity. This problem is even more pronounced when processing 2-D seismic data due to the lack of multi-azimuthal coverage. Therefore, in order to broaden our knowledge about the deep crust using seismic methods, we present the development of specific imaging workflows that integrate different seismic data. Here we propose the combination of velocity model building using (i) first-arrival tomography (FAT) and full-waveform inversion (FWI) of wide-angle, long-offset data collected by stationary ocean-bottom seismometers (OBSs) and (ii) PSDM of short-spread towed-streamer MCS data for reflectivity imaging, with the former velocity model as a background model. We present an application of such a workflow to seismic data collected by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER) in the eastern Nankai Trough (Tokai area) during the 2000–2001 Seize France Japan (SFJ) experiment. We show that the FWI model, although derived from OBS data, provides an acceptable background velocity field for the PSDM of the MCS data. From the initial PSDM, we refine the FWI background velocity model by minimizing the residual move-outs (RMOs) picked in the pre-stack-migrated volume through slope tomography (ST), from which we generate a better-focused migrated image. Such integration of different seismic datasets and leading-edge imaging techniques led to greatly improved imaging at different scales. That is, large to intermediate crustal units identified in the high-resolution FWI velocity model extensively complement the short-wavelength reflectivity inferred from the MCS data to better constrain the structural factors controlling the geodynamics of the Nankai Trough.

Sparse 3D Seismic Imaging in the Kylylahti Mine Area, Eastern Finland: Comparison of Time Versus Depth Approach

A 10.5 km2 3D seismic survey was acquired over the Kylylahti mine area (Outokumpu mineral district, eastern Finland) as a part of the COGITO-MIN (COst-effective Geophysical Imaging Techniques for supporting Ongoing MINeral exploration in Europe) project, which aimed at the development of cost-effective geophysical imaging methods for mineral exploration. The cost-effectiveness in our case was related to the fact that an active-source 3D seismic survey was accomplished by using the receiver spread originally designed for a 3D passive survey. The 3D array recorded Vibroseis and dynamite shots from an active-source 2D seismic survey, from a vertical seismic profiling experiment survey, as well as some additional “random” Vibroseis and dynamite shots made to complement the 3D source distribution. The resulting 3D survey was characterized by irregular shooting geometry and relatively large receiver intervals (50 m). Using this dataset, we evaluate the effectiveness of the standard time-imaging approach (post-stack and pre-stack time migration) compared to depth imaging (standard and specialized Kirchhoff pre-stack depth migration, KPreSDM). Standard time-domain processing and imaging failed to convincingly portray the first ~1500 m of the subsurface, which was the primary interest of the survey. With a standard KPreSDM, we managed to obtain a good image of the base of the Kylylahti formation bordering the extent of the mineralization-hosting Outokumpu assemblage rocks, but otherwise the image was very noisy in the shallower section. The specialized KPreSDM approach (i.e., coherency-based Fresnel volume migration) resulted in a much cleaner image of the shallow, steeply dipping events, as well as some additional deeper reflectors, possibly representing repetition of the contact between the Outokumpu assemblage and the surrounding Kalevian metasediments at depth.

Seismogenic Zone Structures Revealed by Improved 3‐D Seismic Images in the Nankai Trough off Kumano

AbstractTo reveal the detailed deformation structures related to plate subduction in the Nankai Trough, we applied up‐to‐date technologies to improve our 3‐D seismic images. This region is dominated by a megasplay fault system that consists of a coseismic out‐of‐sequence thrust branching from the plate interface and separating the inner and outer accretionary prism. The 3‐D seismic volume was acquired off Kumano in 2006 as a preliminary site survey as part of the Nankai Trough Seismogenic Zone Experiment. The preprocessed data quality was improved by recovering the broadband responses and by better attenuating multiple reflections and noise. New reflection images were then produced using prestack time migration and prestack depth migration with updated velocity models. The new velocity model suggests the possible existence of a high‐velocity zone just above the megasplay fault, which might indicate petrophysical alteration in the seismogenic zone. The fault geometry with spatial dip angle variation and the overburden velocities are important factors for further estimating the force distribution along the coseismic fault. Deformation structures newly imaged beneath the Kumano Basin and dipping layers above the megasplay fault imply complex thrusting and possible fluid flow paths within the inner prism. Fine‐scale deformation features are clarified in the shallow areas from the outer prism to the transition zone, which are useful for reconstructing the accretionary prism development. A low‐reflectivity zone, including an isolated layered block, may be originally underthrusted sediments that have been remobilized during later strike‐slip faults along the edge of Kumano Basin.

Accelerating Kirchhoff Pre-stack depth migration on a GPU by overlapping ray tracing and imaging

Kirchhoff-based depth migration is in great demand for imaging at large scales and high-density seismic data exploration. A Graphic Processing Unit, or GPU, can compose a heterogeneous hardware that contains a traditional Central Processing Unit (CPU), and it can greatly accelerate migration calculations. An improved acceleration method is proposed here that uses overlapped ray tracing, which provides two-way travel times, and an imaging kernel. In our method, ray tracing is calculated by the CPU due to its complicated divergences. The imaging kernel is passed to the GPU by unlooping the imaging volume. Streams and asynchronous calculation functions are used to overlap the imaging kernel of a previous trace to the ray tracing of the next trace. The nvprof profiling tool, along with the elapsed time, are used to measure the efficiency of the accelerated work, and it is found that the GPU code results in a speedup factor of 60 relative to single thread CPU code, and when using the overlapped computation, the GPU efficiency is improved by 30%. This method shows great potential for further application in large scale seismic data imaging with GPU.

Using orthorhombic depth imaging to image fractures in tight reservoirs of the RDG Field, Barmer Basin, India

Raageshwari Deep Gas (RDG) Field is situated in the southern part of the Barmer Basin in Rajasthan, India, at a depth of 3000 m. With both clastic and volcanic lithologies, the main reservoirs are tight, and hydraulic fracturing is required to enhance productivity, especially to improve permeability through interaction of induced fractures with natural fractures. Therefore, optimal development of the RDG Field reservoirs requires characterization of faults and natural fractures. To address this challenge, a wide-azimuth 3D seismic data set over the RDG Field was processed to sharply define faults and capture anisotropy related to open natural fractures. Anisotropy was indicated by the characteristic sinusoidal nature of gather reflection events processed using conventional tilted transverse imaging (TTI); accordingly, we used orthorhombic imaging to correct for these, to quantify fracture-related anisotropy, and to yield a more correct subsurface image. During prestack depth migration (PSDM) processing of the RDG data, TTI and orthorhombic velocity modeling was done with azimuthal sectoring of reflection arrivals. The resultant PSDM data using this velocity model show substantial improvement in image quality and vertical resolution at the reservoir level compared to vintage seismic data. The improved data quality enabled analysis of specialized seismic attributes like curvature and thinned fault likelihood for more reliable characterization of faults and fractures. These attributes delineate the location and distribution of probable fracture networks within the volcanic reservoirs. Interpreted subtle faults, associated with fracture zones, were validated with microseismic, production, and image log data.

Linked thick- to thin-skinned inversion in the central Kirthar Fold Belt of Pakistan

Abstract. The Kirthar Fold Belt is part of the transpressive transfer zone in Pakistan linking the Makran accretionary wedge with the Himalaya orogeny. The region is deforming very obliquely, nearly parallel to the regional S–N plate motion vector, indicating strong strain partitioning. In the central Kirthar Fold Belt, folds trend roughly N–S and their structural control is poorly understood. In this study, we use newly acquired 2-D seismic data with pre-stack depth migration, published focal mechanisms, surface and subsurface geological data, and structural modelling with restoration and balancing to constrain the structural architecture and kinematics of the Kirthar Fold Belt. The central Kirthar Fold Belt is controlled by Pliocene to recent linked thick-skinned to thin-skinned deformation. The thick-skinned faults are most likely partially inverting rift-related normal faults. Focal mechanisms indicate dip-slip faulting on roughly N–S-trending faults with some dip angles exceeding 40∘, which are considered too steep for newly initiated thrust faults. The hinterland of the study area is primarily dominated by strike-slip faulting. The inverting faults do not break straight through the thick sedimentary column of the post-rift and flexural foreland; rather, the inversion movements link with a series of detachment horizons in the sedimentary cover. Large-scale folding and layer-parallel shortening has been observed in the northern study area. In the southern study area progressive imbrication of the former footwall of the normal fault is inferred. Due to the presence of a thick incompetent upper unit (Eocene Ghazij shales) these imbricates develop as passive roof duplexes. In both sectors the youngest footwall shortcut links with a major detachment and the deformation propagates to the deformation front, forming a large fault-propagation fold. Shortening within the studied sections is calculated to be 18 %–20 %. The central Kirthar Fold Belt is a genuine example of a hybrid thick- and thin-skinned system in which the paleogeography controls the deformation. The locations and sizes of the former rift faults control the location and orientation of the major folds. The complex tectonostratigraphy (rift, post-rift, flexural foreland) and strong E–W gradients define the mechanical stratigraphy, which in turn controls the complex thin-skinned deformation.

2D Seismic Imaging Improvement through Prestack Depth Migration (PSDM) Method

The quality of seismic is important for interpretation. Prestack Depth Migration produce better quality of seismic imaging. The seismic generated through PSDM method has better seismic reflector and geological structure appearance compared to Prestack Time Migration (PSTM) method. Accurate interval velocity modeling is a key in PSDM process, involving dix transformation, coherency inversion, and tomography. Comparison between PSTM and PSDM show that PSDM offer better imaging for interpretation because PSDM has better seismic reflector continuity and good geological appearance.

Unravelling the shape and stratigraphy of a glacially-overdeepened valley with reflection seismic: the Lienz Basin (Austria)

We reveal the subsurface bedrock topography and sedimentary succession of one of the deepest glacially-formed basins in the Eastern Alps: the Lienz Basin in the Upper Drau Valley (Tyrol), by means of seismic reflection. A dense source-receiver spacing, supplied by autonomous receivers, and a prestack depth-migration processing scheme were essential to distinguish the various deposits in fine detail, such as slumping, fan delta deposits, and a modified monocline on the basin flank. These details support our interpretation of the seismic stratigraphy that consists of, e.g., subglacial till of last glacial maximum (LGM) age and possibly older, laminated basin fines, and gravel/coarse sand. The maximum depth of the basin is 622 m, at the junction of two major basement faults that are not clearly visible in the seismic reflections. We regard the overdeepening in this longitudinal valley as the result of glacier confluence during the LGM. Subglacial meltwaters utilized the higher erodibility of faulted rocks, as indicated by channel structures. The adverse slope (2.6%) along the valley axis exceeds the gradient ice-surface slope (0.4–0.5%) during the LGM by more than fivefold. We thus suggest this feature is a product of a pre-LGM phase, since adverse slopes greater than ~ 1.2 times the ice surface slope promote the freezing of water in subglacial channels and prevent efficient water flushing of sediments. Integrating other studies allows us to estimate the local overdeepening of the Lienz Basin and that of the whole Upper Drau Valley to be 146 m and 530 m, respectively. At the beginning of lacustrine sedimentation, we estimate the paleo-water depth to be at least 216 m.

High-resolution depth imaging of long-offset 2D data using full-waveform inversion: The Malvinas Basin, Argentina

The Argentina Austral Malvinas survey comprises 13,784 km of 2D data extending from the shelf to the border with the Falkland Islands. The survey was acquired using a 12,000 m streamer and continuous recording technology and was processed through a comprehensive broadband prestack depth migration workflow focused on producing a high-resolution, high-fidelity data set. Source- and receiver-side deghosting to maximize the bandwidth of the data was an essential ingredient in the preprocessing. Following the broadband processing sequence, a depth-imaging workflow was implemented, with the initial model built using a time tomography approach. Several passes of anisotropic reflection tomography provided a significant improvement in the velocity model prior to full-waveform inversion (FWI). Using long offsets, FWI made use of additional information contained in the recorded wavefield, including the refracted and diving wave energy. FWI resolved more detailed velocity variations both in the shallow and deeper section and culminated in an improved seismic image.

Prestack Depth Migration Using Seismic Virtual Source Gathers

Prestack depth migration is used to image for complex geological structure. In this case, the surface reflection data are used as an input data in general. However, the surface reflection data have some problems in imaging the subsalt and the salt flank due to the complex wavefields and multiples which come from overburden. To overcome the defect of the surface reflection data, we used the virtual source in terms of seismic interferometry to image the subsurface. Inhomogeneous velocity models were developed and virtual source gathers were generated ocean bottom seismic numerical modelling. Cross-correlation gathers were generated by the crosscorrelation between the reference trace and others. The virtual source gathers were made by the integration at the stationary phase interval. Numerical test showed that the virtual source gathers integrated at the stationary interval are superior to that of all sources. To verify the possibility of subsurface imaging, prestack depth migration was applied for the virtual source gathers. This prestack depth migrated section re-produced the velocity model below receivers.. Especially artificial interface by multiples were suppressed without any further data processing. The results of imaging obtained from inhomogeneous velocity model below receivers also showed that the artificial geological interfaces were significantly reduced comparing to the homogeneous velocity models case.

Experience with the issue of seismic surveying over basalts

In some of Australia?s underground coal mining areas, near-surface basalt layers create problems for seismic reflection surveying. Affected mines have less confidence in their geological conditions and mine safety and productivity may be compromised. In order to gain insights and obtain potential solutions to this important problem, we have investigated seismic reflection and borehole vertical seismic profiling data from surveys at the North Goonyella Mine and the Moranbah South coal mine lease, which are both located in the Bowen Basin of Queensland. At North Goonyella, seismic velocities determined from the VSP survey indicate that the basalt is not fresh and up-going reflections from layers below the basalt are clearly seen. For the seismic reflection survey, advanced seismic data processing techniques such as pre-stack depth migration were able to improve the continuity of the coal seam reflections across the zone affected by the basalt. No special acquisition parameters were required. At Moranbah South, the problem with the basalts proved to be intractable. The basalts are hard and fresh, their total thickness is up 40 m, their width reaches 360 m and unconsolidated sediments lie between individual flows. The target coal seams are at depths less than 310 m and neither long-offset data acquisition nor prestack depth migration were able to produce satisfactory results. In general, the main issues for seismic surveying in basalt covered areas are (i) the generation of complex downgoing and up-going wavefields which are due to the strong impedance contrasts between the basalt and the surrounding strata, and (ii) the generation of incoherent scattered waves from inhomogeneities within the basalts and their rough margins. We have studied and confirmed these seismic phenomena through computer modelling.

Anisotropic Depth Imaging Case Studies from Australia's North West Shelf

This paper demonstrates the increase in confidence of depth prediction and structural imaging achieved using anisotropic pre-stack depth migration of 3D surface seismic data. Using examples of anisotropic travel time derived models updated with hybrid grid-based reflection tomography, this paper provides evidence of improved imaging from two data examples in Australia?s North West Shelf. The data examples described contain various imaging challenges, including short scale lateral velocity heterogeneity, carbonates, gas affected overburden zones, HRDZ?s (Hydrocarbon Related Diagenetic Zones) and reservoir level fault shadows. This paper will describe the work flow approach used and the geological considerations given to derive an appropriate earth model for each dataset. Evidence is presented to show how the velocity models converge to minimal residual moveout with each iteration of reflection tomography. Comparisons between data formed with time domain imaging and anisotropic depth imaging are made for the two surveys considered.