Shot Domain Research Articles

Separating prestack diffractions with SVMF in the flattened shot domain

Abstract Separating weak diffractions in the prestack domain plays a vital role in high-resolution imaging small-scale discontinuities or inhomogeneities, which are associated with the migration paths and storage spaces for carbonate fracture-cave reservoirs. A shot-domain diffraction separation method is proposed with a SVMF (space-varying media filter) in the flattened shot domain. The diffractions with fast dynamic changings, including damping amplitudes and phase reversal, are filtered out by the spacing-varying median filter along the prediction-step directions of the flattened shot domain. The proposed method opens a possibility for separating diffractions in the shot domain and has an advantage in the traditional common-offset diffraction separation method. A numerical experiment illustrates the good performance of the proposed method in eliminating reflections and separating diffractions in the shot domain, and the field application further verifies its potential value in resolving the hidden small-scale cavities and tiny faults in carbonate reservoirs.

Reconstruction of the near‐offset gap in marine seismic data using seismic interferometric interpolation

ABSTRACTIn conventional seismic exploration, especially in marine seismic exploration, shot gathers with missing near‐offset traces are common. Interferometric interpolation methods are one of a range of different methods that have been developed to solve this problem. Interferometric interpolation methods differ from conventional interpolation methods as they utilise information from multiples in the interpolation process. In this study, we apply both conventional interferometric interpolation (shot domain) and multi‐domain interferometric interpolation (shot and receiver domain) to a synthetic and a real‐towed marine dataset from the Baltic Sea with the primary aim of improving the image of the seabed by extrapolation of a near‐offset gap. We utilise a matching filter after interferometric interpolation to partially mitigate artefacts and coherent noise associated with the far‐field approximation and a limited recording aperture size. The results show that an improved image of the seabed is obtained after performing interferometric interpolation. In most cases, the results from multi‐domain interferometric interpolation are similar to those from conventional interferometric interpolation. However, when the source–receiver aperture is limited, the multi‐domain method performs better. A quantitative analysis for assessing the performance of interferometric interpolation shows that multi‐domain interferometric interpolation typically performs better than conventional interferometric interpolation. We also benchmark the interpolated results generated by interferometric interpolation against those obtained using sparse recovery interpolation.

Two field trials for deblending of simultaneous source surveys: Why we failed and why we succeeded?

Currently, deblending is the main strategy for dealing with the intense interference problem of simultaneous source data. Most deblending methods are based on the property that useful signal is coherent while the interference is incoherent in some domains other than common shot domain. In this paper, two simultaneous source field trials were studied in detail. In the first trial, the simultaneous source survey was not optimal, as the dithering code had strong coherency and the minimum distance between the two vessels was also small. The chosen marine shot scheduling and vessel deployment made it difficult to deblend the simultaneous source data, and result was an unexpected failure. Next, we tested different parameters (the dithering code and the minimum distance between vessels) of the simultaneous source survey using the simulated blended data and got some useful insights. Then, we carried out the second field trial with a carefully designed survey that was much different from the first trial. The deblended results in common receiver gather, common shot gather or the final stacked profile were encouraging. We obtained a complete success in the second field trial, which gave us confidence in the further test (such as a full three dimensional acquisition test or a high-resolution acquisition test with denser spatial sampling). Remembering that failures with simultaneous sourcing seldom reported, in this paper, our contribution is the discussion in detail about both our failed and successful field experiments and the lessons we have learned from them with the hope that the experience gained from this study can be very useful to other researchers in the same field.

Shot-domain deblending using least-squares inversion

Simultaneous source acquisition, which allows a temporal overlap between shot records, has significant advantages to improve data quality (e.g., denser shooting) and reduce acquisition cost (e.g., efficient wide-azimuth shooting). We have developed a novel shot-domain deblending approach for a wide-azimuth simultaneous shooting survey based on an inversion scheme. The inverse problem is formulated by introducing two convolution operators that can respectively destruct the signal and interference and is solved by a conjugate-gradient algorithm in the least-squares sense. Our approach breaks down the limit that source separation can only be implemented in some domains other than the shot domain based on the incoherency principle. Our shot-domain approach does not require the random dithering time; thus, it is very flexible to use in a wide-azimuth simultaneous shooting survey. Three numerically blended synthetic examples were developed to demonstrate the excellent performance of our method. The feasibility has been further validated via a field-data example that is numerically blended from two realistic marine towed streamers.

Diffraction imaging method by Mahalanobis-based amplitude damping

Clarifying and locating small-scale discontinuities or inhomogeneities in the subsurface, such as faults and collapsed columns, plays a vital role in safe coal mining because these discontinuities or inhomogeneities may destroy the continuity of layers and result in dangerous mining accidents. Diffractions carry key information from these objects and therefore can be used for high-resolution imaging. However, diffracted/scattered waves are much weaker than reflected waves and consequently require separation before being imaged. We have developed a Mahalanobis-based diffraction imaging method by modifying the classic Kirchhoff formula with an exponential function to account for the dynamic differences between reflections and diffractions in the shot domain. The imaging method can automatically account for destroying of reflected waves, constructive stacking of diffracted waves, and strengthening of scattered waves. The method can overcome the difficulties in handling Fresnel apertures, and it is suitable for high-resolution imaging because of the consistency of the waveforms in the shot domain. Although the proposed method in principle requires a good migration velocity model for calculating elementary diffraction traveltimes, it is robust to an inaccurate migration velocity model. Two numerical experiments demonstrate the feasibility of the proposed method in removing reflections and highlighting diffractions, and one field application further confirms its efficiency in resolving masked faults and collapsed columns.

Seismic denoising using the redundant lifting scheme

Separating linear coherent noise, such as ground roll from reflections, remains a key challenge in seismic processing. By adapting the redundant lifting scheme (RLS), a wavelet transform method, to seismic data, we have determined how the wavelet domain can be used to suppress coherent and random noise. The RLS operates on a trace-by-trace basis decomposing each time series into wavelet-coefficient (WC) time series and consequently a single gather (in a shot, receiver, or common depth point domain) into a series of WC subgathers (SGs). The decomposition changes the relative magnitude of WCs of various events (reflection, head wave, ground roll, etc.) from one SG to another without affecting their moveout. In SG(s) in which the WCs of undesired events were significantly stronger than the desired events, the WCs can be surgically muted. Selective muting in carefully chosen SGs attenuates undesired events while having minimal effects on frequency spectra of the desired events. In addition, random noise can be suppressed in the individual SGs by designing a local thresholding mechanism (we have used modified Otsu thresholding) in combination with adaptive Wiener filtering. We have developed this approach of suppressing coherent and random noise in a step-by-step manner first using a synthetic shot gather, followed by demonstration on two real gathers. Our RLS-based denoising method has minimal effects on the lower end of signal frequency spectra, and it could be a valuable tool in a processor’s toolbox when data preconditioning for advanced processing such as waveform inversion, which benefits from low frequencies, is desired.

Double-weave 3D seismic acquisition — Part 1: Sampling and sparse Fourier reconstruction

A stochastic simulation technique analyzes the effectiveness of sparse Fourier reconstruction methods for multidimensional sampling functions. A regular 2D sampling pattern, called a weave pattern, is a suitable sampling scenario for sparse Fourier recovery. The simplicity of a weave sampling pattern provides a good alternative to replace random sampling patterns in compressive sensing applications. A double-weave 3D seismic acquisition design has been developed by using the weave sampling pattern for the shot and receiver distributions. Besides its homogeneous spatial coverage in the shot-receiver and offset-azimuth coordinates, double-weave 3D acquisition is a compatible survey design for alias-free 5D sparse Fourier interpolation in the shot and receiver domains. Zigzag and orthogonal double-weave acquisition layouts were proposed for land seismic surveys considering the economic and environmental parameters. The effectiveness of 5D interpolation for double-weave 3D acquisition were tested using synthetic planar 5D seismic events.

Simplify the variable-depth streamer data processing through pre- migration deghosting: a case study from NWS Australia Data

Variable-depth streamer acquisition, as one of the key marine broadband solutions, has shown great advantages in providing high resolution seismic imaging and better low frequency penetration than conventional data from examples around the world. By utilizing the notch diversity, the receiver ghost was fully removed through proprietary joint deconvolution method in the imaging domain which produces broadband imaging with bandwidth up to six octaves. However, this post- migration deghosting method requires the receiver ghost to be well preserved prior to final migration thus introduces complexity in key steps like multiple attenuation, velocity analysis and migration than the conventional processing flow.The objective of this work is to simplify the processing flow by applying bootstrap deghosting method right after shot domain de-noise. After this step, processing flow will be very similar to the conventional flow. This new flow has been tested on the variable-depth streamer data from NWS Australia. Compared to the post-migration deghosting flow, the new results not only provide similar benefit as to broader bandwidth and rich images, but also show improvement on multiple attenuation and primary preservation as well as seismic inversion.

Automation in geophysical data acquisition: From ocean-bottom nodes to RoboNodes

The future of seismic data acquisition clearly dictates increased sampling in both receiver and shot domains. A significant amount, almost 80%, of the total cost of an exploration and development seismic project is spent on data acquisition. It is, therefore, extremely important that we reduce acquisition-related costs and at the same time we strive to improve seismic data quality. Indeed, the number of seismic channels follows a form of Moore's law; that is, the number of channels laid out in the field is steadily increasing by an order of magnitude every 10 years (Figure 1) while the recent introduction of simultaneous sources provides improved spatial sampling in the shot domain. Figure 2 shows three time slices that clearly demonstrate the link between increased number of channels recorded and improved resolution and image quality.

Evaluation of 3C microelectromechanical system data on a 2D line: Direct comparison with conventional vertical-component geophone arrays and PS-wave analysis

A 2D vibroseis line was acquired in the Vienna Basin (Austria) for the purpose of comparing the data of digital multicomponent single sensors based on microelectromechanical system (MEMS) sensors alongside conventional vertical-component geophone arrays. For efficient removal of coherent noise during processing, all source points were recorded in single-sweep mode, i.e., no vertical stacking was performed in the field. On this densely sampled data set, several noise-reduction techniques, such as digital array forming, frequency-wavenumber (f-k) filtering in shot and receiver domains, and polarization filters, proved to be valuable in reducing source-generated noise. The results showed that, with the use of single-sweep recording and polarization filter techniques, it is possible to produce seismic sections for a single-receiver three-component (3C) MEMS line that are comparable to a conventional geophone array line in signal-to-noise ratio. However, the higher number of single geophones and hence the stronger attenuation of random noise in the conventional array resulted in an advantage for the analog geophone data set. The second goal for this survey was to evaluate additional information contained in the horizontal components of the MEMS data. The multicomponent data allowed for the processing of mode-converted shear-wave data, performed for the first time in the Vienna Basin. Azimuthal anisotropy related to horizontal stresses was observed in the Neogene section of the shear-wave data set. A PP-PS event correlation allowed the identification of major shallow horizons. Interpretation of the final sections confirmed that the PS data are useful to distinguish between gas reservoirs and high-porosity water sands, which can cause similar P-wave amplitude variation with offset (AVO) effects.

Simultaneous time imaging, velocity estimation, and multiple suppression using local event slopes

Starting with the double-square-root equation we derive expressions for a velocity-independent prestack time migration and for the associated migration velocity. We then use that velocity to identify multiples and suppress them as part of the imaging step. To describe our algorithm, workflow, and products, we use the terms velocity-independent and oriented. While velocity-independent imaging does not require an input migration velocity, it does require input [Formula: see text]-values (also called local event slopes) measured in both the shot and receiver domains. There are many possible methods of calculating these required input [Formula: see text]-values, perhaps the simplest is to compute the ratio of instantaneous spatial frequency to instantaneous temporal frequency. Using a synthetic data set rich in multiples, we test the oriented algorithm and generate migrated prestack gathers, the oriented migration velocity field, and stacked migrations. We use oriented migration velocities for prestack multiple suppression. Without this multiple suppression step, the velocity-independent migration is inferior to a conventional Kirchhoff migration because the oriented migration will flatten primaries and multiples alike in the common image domain. With this multiple suppression step, the velocity-independent are very similar to a Kirchhoff migration generated using the known migration velocity of this test data set.

Preserved amplitude migration based on the one way wave equation in the angle domain

Traditional pre-stack depth migration can only provide subsurface structural information. However, simple structure information is insufficient for petroleum exploration which also needs amplitude information proportional to reflection coefficients. In recent years, pre-stack depth migration algorithms which preserve amplitudes and based on the one-way wave equation have been developed. Using the method in the shot domain requires a deconvolution imaging condition which produces some instability in areas with complicated structure and dramatic lateral variation in velocity. Depth migration with preserved amplitude based on the angle domain can overcome the instability of the one-way wave migration imaging condition with preserved amplitude. It can also offer provide velocity analysis in the angle domain of common imaging point gathers. In this paper, based on the foundation of the one-way wave continuation operator with preserved amplitude, we realized the preserved amplitude prestack depth migration in the angle domain. Models and real data validate the accuracy of the method.

Migração PSPI baseada nas equações da onda unidirecionais com amplitude verdadeira

As equações da onda unidirecionais com amplitude verdadeira, propostas nos trabalhos mais recentes, levam em conta de forma correta a parte cinemática e dinâmica da equação completa da onda equivalentemente às obtidas a partir da aproximação de altas freqüências. Isto é, as equações iconal e de transporte obtidas para estes operadores se correspondem com as obtidas com a equação completa da onda. Num meio com variação lateral de velocidade, a implementação das equações unidirecionais com amplitude verdadeira envolve o cálculo numérico de operadores integro-diferenciais que podem ser aproximados por somatórios e resolvidos por técnicas de diferenças finitas. Para um meio que varia somente com a profundidade, as novas equações unidirecionais com amplitude verdadeira podem ser solucionadas através do tradicional método phase-shift , e as novas condições de fronteira e de imagem se reduzem a multiplicações no domínio de Fourier. Neste trabalho estamos propondo estender a migração phase-shift com amplitude verdadeira para meios com variação lateral v(x,z), através da metodologia Phase shift plus interpolation (PSPI). Experimentos numéricos no domínio de tiro comum com o novo método PSPI com amplitude verdadeira e sua extensão split-step (PSPI-SS), produzem imagens com amplitudes mais corretas em relação aos operadores unidirecionais convencionais, enquanto as características cinemáticas da migração PSPI e PSPI-SS são preservadas.

Arrival time analysis for seismic airgun data classification

Survey data were collected in order to characterize the output of a marine seismic airgun array for various propagation geometries. Near uniform sound speed in the survey area allowed for classification of first-break and secondary arrivals as direct arrival and either reflected or refracted path arrival, respectively, using simple propagation geometries. Plots of initial arrival versus horizontal offset between the airgun and receiver allow classification of the arrival as via direct, reflected, or refracted path in a manner similar to that employed in the analysis of seismic reflection data in the shot domain. Beyond a certain horizontal range (approximately 4.5 km), the critically refracted arrival overtakes the reflected arrival, affording an estimate of the propagation speed in the seafloor layer. Analysis of this sort allows separation of the direct arrival for analysis purposes.

The revolution in seismic resolution: High density 3D spatial sampling developments and results

Recent marine case studies have demonstrated that a significant component of the seismic "noise" contaminating 3D images actually arises during processing, as an unfortunate, and inescapable artefact from poor 3D spatial sampling. When the cross-line acquisition dimension is sampled at an equally small interval as the inline dimension, a much larger frequency bandwidth than typical of standard 3D acquisition can preserved throughout all stages of processing, free of aliasing, and free of artefacts. Hence, it is observed that once the random noise component is suppressed below a certain threshold, other factors than mere fold are clearly contributing to the quality of a seismic image. It is quite poorly established how more complicated acquisition parameters, such as multi-streamer spread dimensions and shooting templates, influence the "S/N ratio" of seismic data - particularly after the application of multichannel pre-stack processing algorithms, notably pre-stack migration.Historically, efforts at towing the source and streamer at shallower depths rather fruitlessly delivered higher dominant signal frequencies, at the cost of degraded lower frequency amplitudes, increased survey noise, and with minimal perceivable improvements in target resolution. Even if means can be found to reduce the inherent noise incurred, resolution remains frustratingly restricted, and the emphasis upon higher frequencies during acquisition was largely wasted. The solution is to sample densely in both the shot and receiver domains, particularly in the cross-line direction.Several case study examples demonstrate significant improvements in resolution and signal-to-noise content are routinely achieved by high-density seismic acquisition. Depending upon local geological conditions, high frequency amplitudes can be increased by up to 15 dB, frequency bandwidth can be doubled, 3D steep dip imaging can be significantly improved, and overall signal-to-noise ratio is improved, further contributing to better resolution. Hence, a powerful demonstration is made that tight 3D spatial sampling must be the foundation for all high resolution seismic acquisition.

Must magmatic intrusion in the lower crust produce reflectivity?

The Færoe–Iceland Ridge (FIR) provides a laboratory in which to investigate the reflectivity and velocity structure of thick crust generated above a mantle plume in order to constrain models of underplating and the origins of lower-crustal layering in an environment dominated by young igneous processes. Over 600 km of common midpoint (cmp) data were collected along and across the FIR using a large airgun array with a 240-channel streamer. The interpretation of these data has been integrated with a velocity model of the crust and upper mantle along the FIR obtained from wide-angle seismic arrivals into ocean bottom and land seismometers. Due to the intermediate water depths and the presence of basalt near the water bottom, specialized processing steps were required for the cmp data. A wave equation-based multiple attenuation scheme was applied to the prestack data, which used a forward model of the multiple series to predict and attenuate multiple energy. Array simulations were applied in the shot and receiver domains in order to minimize spatial aliasing and reduce low apparent-velocity noise. Most of the sections over the central (oceanic) portion of the FIR show no pronounced reflectivity, although occasional Moho and/or lower-crustal reflections are observed. We believe that the poor reflectivity results largely from a lack of physical property contrasts rather than being an effect of acquisition or processing, although we also conclude that residual energy from strong multiple reflection remains in the final sections. Amplitude decay and reflection strength vary along the FIR, but there is good signal-to-noise ratio to travel times of at least 9 s (i.e., into the lower crust), implying that the reduced reflectivity beneath the main part of the FIR is not an artifact of signal penetration loss. We conclude that the addition of melt to the lower crust along the trace of the plume apparently did not produce strong physical property contrasts in the lower crust, where little reflectivity is apparent. Perhaps this was because the entire crust was hot at the time of formation. In contrast, igneous intrusion into preexisting continental crust (at the Færoe Islands end of the FIR) and into older igneous crust (at the Iceland end of the FIR) produces significant lower-crustal reflectivity. Strong lower-crustal reflectivity elsewhere beneath the northwestern European continental margins may have a similar intrusive origin.

Tau-p velocity imaging of regolith structure

The tau-p velocity imaging method, first developed for obtaining the velocity field from marine multichannel seismic data, has been applied to refracted waves from the regolith in regional seismic reflection surveys on land. The technique converts travel time picks from the refracted wavefield into two-dimensional velocity models, by transforming from time-offset into the tau-p domain. Each arrival is mapped individually, and the ?true? velocity and position of the ray turning point is obtained by considering reversed raypaths. Thus the data are transformed directly into a depth or two-way time image of the subsurface displayed in seismic velocity. The method is extremely fast and involves no interpretive steps or iteration. Ideal datasets contain the refracted wavefield sampled densely and equally in the shot and receiver domains. It was therefore decided to test the application of the method for mapping the velocity structure of a portion of regolith and to compare the results with those obtained using more conventional methods. The area chosen for study was part of a regional seismic reflection line across the Lachlan River palaeo-valley in central NSW. The data set consisted of the first break picks for 240 channels with receivers spaced every 40 m and vibration points every 40 m. The velocity images were produced as both time and depth sections and compared with one and two layer refractor models from a refraction tomographic approach. A low velocity region on the image corresponds to the deepest part of the refractor model, interpreted as the thickest part of the palaeo-valley. Bedrock velocity variations are also mapped and agree with the changes along the (lowest) refractor velocity profile. While further tuning may be required for land work, the technique has the advantage that velocities can be directly imaged and potentially related to regolith physical properties.

Wave-equation migration of common-receiver gathers for AVO analysis

During the 1980s and 1990s, we developed a number of recursive wave-equation algorithms for prestack depth migration in the shot domain. Much of that work was driven by development of the vertical-cable recording technique. However, we applied the algorithms to image a variety of 2D and 3D land and marine surveys. Virtually all of those projects were oriented toward obtaining better structural images and without concern for preserving relative amplitude information. Increasing interest in prestack amplitude analysis of migrated data led us to revisit that decision and to evaluate the possibility of analyzing AVO (or AVA) within image gathers. Developers of other wave-equation-based algorithms have presented similar evaluations and have achieved useful results. However, our production algorithms are sufficiently different that the applicability of their results to our project work is not clear. Consequently, we decided to conduct our own study, starting with a modeling-based approach—rather than a theoretical one—to analyze application of one of our 3D shot-record depth migration algorithms to synthetic and real data. The immediate value of this would be as a measure of the “amplitude-friendliness” of an existing production algorithm under specific conditions. Beyond that, it would help us gauge the potential value of allocating additional theoretical and/or development work to enhance its usefulness as a tool for amplitude studies in projects involving wave-equation shot-record migration. The real data set we selected for testing is an OBC survey recorded by Texaco in 1997 at Teal South Field (Eugene Island Block 354) in the Gulf of Mexico. This survey was used 24 4C receiver groups fixed in a sparse pattern on the seafloor, about 82 m deep (Figure 1). The source was a small air-gun array towed at a depth of 3 m and was deployed at locations on a finer (nominally 25 × 25 m) grid …

Simple versus complicated seismic processing in the Exmouth Sub-basin

Seismic data quality in the deeper water regions of the Exmouth Sub-basin, Western Australia is generally excellent. Both the temporal and lateral resolution are very high, allowing the identification of fine geological detail. These data contrast with the data over much of Australia’s offshore exploration and producing regions, where multiples, velocity inversions, poor reflectivity contrasts, and sea floor reefs and channels seriously degrade data quality.A quite different approach to seismic data processing is required for processing seismic data from the deeper waters of the Ex mouth Sub-basin than is normally used for the more noisy areas of offshore Australia. Rather than concentrating on processes that improve the signal-to-noise ratio of the data, it is more important to concentrate on processes that do not distort the seismic wavefield.While processing seismic data recorded in 1994 in the Exmouth Sub-basin, I found that most processes typically applied to seismic data did not lead to any improvement in quality. In fact, it was quite difficult to find processes that actually improved the quality of the data. I found that the results of processing were very susceptible to the type of deconvolution applied to the data. Conventional trace-by-trace prestack deconvolution was found to introduce reverberations, degrading the quality of the final migrated section. Shot averaged deconvolution produced vastly superior results than trace-by-trace deconvolution. Other processes such as shot domain f-k filtering, trace summation and f-k demultiple had little effect on the quality of the final section. These processes, however, can introduce subtle distortions of the wavelet, and hence should only be used if it can be demonstrated that they produce noticeable improvements.The final sequence used to process the data was extremely simple, and consisted of: (1) gain recovery; (2) large gate trace equalisation; (3) shot averaged deconvolution; (4) one pass velocity analysis; (5) NMO, mute and stack; and (6) migration followed by bandpass filtering. Probably many of the deeper water regions of Australia could benefit from the use of the simple processing sequence presented in this paper.

Attenuation of near-surface diffracted energy in deep seismic data by DMO correction

Seismic data are often contaminated by scattered waves from shallow diffractors such as offshore installations and structural irregularities. As the waves travel in the water layer and shallow sub-bottom they are damped considerably less than near-vertical reflected waves. Far from the diffractors the stacking velocity of the noise will be nearly identical to the stacking velocities of the primary reflections. This implies that CMP stacking of normal-moveout corrected data does not suppress the noise, and the necessary attenuation will typically be done separately by prestack 2D velocity-filtering or array simulation in the shot and receiver domains. However, by changing the moveout of the diffraction curves through DMO correction, the stacking velocity of the noise will be close to the true velocity of the diffracted waves, and suppression through CMP stacking is possible. As DMO is related to CDP smearing, which is a marginal problem for deep seismic data, it is normally not used as a part of the standard processing schemes. The noise suppression features of the DMO processor are demonstrated with a data example from the North Sea.