Pre-stack time migration and velocity analysis methods with common scatter-point gathers

A prestack time migration is presented that is simple, efficient, and provides detailed velocity information. It is based on Kirchhoff prestack time migration and can be applied to both 2-D and 3-D data. The method is divided into two steps: the first is a gathering process that forms common scatterpoint (CSP) gathers; the second is a focusing process that applies a simplified Kirchhoff migration on the CSP gathers, and consists of scaling, filtering, normal moveout (NMO) correction, and stacking. A key concept of the method is a reformulation of the double square‐root equation (of source‐scatterpoint‐receiver traveltimes) into a single square root. The single square root uses an equivalent offset that is the surface distance from the scatterpoint to a colocated source and receiver. Input samples are mapped into offset bins of a CSP gather, without time shifting, to an offset defined by the equivalent offset. The single square‐root reformulation gathers scattered energy to hyperbolic paths on the appropriate CSP gathers. A CSP gather is similar to a common midpoint (CMP) gather as both are focused by NMO and stacking. However, the CSP stack is a complete Kirchhoff prestack migrated section, whereas the CMP stack still requires poststack migration. In addition, the CSP gather has higher fold in the offset bins and a much larger offset range due to the gathering of all input traces within the migration aperture. The new method gains computational efficiency by delaying the Kirchhoff computations until after the CSP gather has been formed. The high fold and large offsets of the CSP gather enables precise focusing of the velocity semblance and accurate velocity analysis. Our algorithm is formulated in the space‐time domain, which enables prestack migration velocity analysis to be performed at selected locations and permits prestack migration of a 3-D volume into an arbitrarily located 2-D line.

The seismic method is one of the major geophysical tools to study the structure of the earth. The extraction of the common scatter point (CSP) gather is a critical step to accomplish the seismic imaging with a scattered wave. Conventionally, the CSP gather is obtained with the assumption that the earth surface is horizontal. However, errors are introduced to the final imaging result if the seismic traces obtained at the rugged surface are processed using the conventional method. Hence, we propose the method of the extraction of the CSP gather for the seismic data collected at the rugged surface. The proposed method is validated by two numerical examples and expected to reduce the effect of the topography on the scattered wave imaging.

The Guando field is in an area of rough topography and complex geology. In order to better site wells in such fields it is of value to have a sound understanding of this geological complexity. Obtaining a satisfactory seismic image to enable such interpretation requires pre-stack imaging. The geological complexity is such that post stack imaging fails due to the failure of the common mid point assumption. Is it sufficient to perform pre-stack time migration or is it necessary to perform pre-stack depth migration? In this instance both have been performed on this field and results for each of them will be shown. The basic theory of both pre-stack time and pre-stack depth migration will be described without using mathematics. Model data will then be used to illustrate the shortcomings of pre-stack time migration using a model of similar geological complexity to the Guando field. Both pre-stack migrations require a velocity field. These differ and the manner of their derivation also differs. Velocity model building for both techniques will be described. Particular emphasis will be placed on the use of tomographic and scanning techniques for pre-stack depth migration. Examples will be shown of the velocity models from both migrations. It will be shown that the pre-stack depth migration velocity model is also a valuable interpretation asset. Data from both migrations will be shown. These will demonstrate the superiority of the pre-stack depth migrated data and the reasons why it is superior.

Abstract. The Kalahari Manganese Field (KMF) in the Northern Cape Province of South Africa hosts some of the world's richest manganese deposits, largely concealed beneath thick Cretaceous to Cenozoic Kalahari Group and Karoo Supergroup sediments. To improve imaging of the concealed Transvaal Supergroup strata, a high-resolution 2D reflection seismic survey was conducted in November 2023 across the Severn farm area. The survey comprised five profiles totalling 18.9 km, acquired using 5 Hz 1C geophones connected to wireless nodes, enabling effective burial beneath loose aeolian sand for improved coupling. A compact 500 kg drop hammer, mounted on a Bobcat, served as the seismic source, offering excellent manoeuvrability across challenging sandy terrain. Shot spacing was 10 m, with four vertical stacks per shot to enhance signal-to-noise ratio. Refraction tomography using first-break travel times provided near-surface P-wave velocity models, revealing variable Kalahari sediment thicknesses ranging from 20 to 70 m and bedrock velocities of ∼ 5500 m s−1 associated with Karoo Supergroup strata. Despite the challenges posed by the thick sand cover, lithified calcrete horizons within the Kalahari sediments significantly aided seismic energy propagation. The data were processed using a conventional pre-stack imaging workflow. We tested both Kirchhoff pre-stack time migration (KPreSTM) and Kirchhoff pre-stack depth migration (KPreSDM) and compared the results. Both migration approaches revealed a high degree of similarity in reflector geometries and structural patterns, suggesting minimal lateral velocity variation across the study area. KPreSTM results were then used in the final seismic interpretation. Pre-stack time migrated sections exhibit nine laterally continuous high-amplitude reflectors between 0.05 and 3.42 km depth, corresponding to major stratigraphic boundaries from the Kalahari Group down to the Ghaap Group. Of particular interest is the moderate-amplitude reflection pair at 1.05–1.35 km depth, interpreted as the Hotazel Formation, the primary host for manganese mineralization. This study demonstrates that, when appropriately designed, reflection seismic imaging can be a powerful tool for delineating deep mineralized strata beneath thick sedimentary cover in arid environments.

Summary VTI anisotropic media effects travel times of seismic wave at long offsets and HTI anisotropic media reflects azimuth character of seismic wave velocity. Simultaneously considering VTI and HTI anisotropic parameters as orthorhombic symmetry anisotropic media in Kirchhoff pre-stack time migration (PSTM) for PP-wave or PS-wave data is an important data processing technology at present in improving the quality of migration. We propose a PSTM technology for PS-wave data considering both VTI and HTI anisotropic parameters in equations of travel times and state workflow of anisotropic velocity modeling for data processing to verify the method practicality. The feasibility and effectiveness in improving the continuity and resolution of imaging section are demonstrate with field 3D PS-wave data from West China.

In this study, we propose an example of body wave retrieval using ambient noise correlation. The correlated data are migrated using a Kirchhoff pre-stack time migration (PSTM) and then with the migration obtained with conventional active data. It allows to considerably broaden the final image spectrum toward the low frequencies.

Prestack time migration (PreSTM) still represents the majority of seismic imaging activities in the industry. The reason for this is the speed and robustness of time imaging and its ability to focus seismic events for most geological settings. However, well defined migration velocities are necessary. When we speak about migration velocities and migration operators we implicitly assume time migration velocities and time migration operators. Usually, the migration velocities are not known and, therefore, approximated by stacking velocities. The stacking velocity is attached to the stationary point of the migration operator, however the migration velocity is defined at the apex of the migration operator. Due to this discrepancy, the stacking velocity may deviate from the migration velocity, e.g., in the case of dipping reflectors, and the final time image may be unfocused. In order to correct for the velocity difference and to enhance the time imaging, the velocity model needs to be refined. There are several techniques to provide an updated velocity model. However, all the techniques are based on an iterative approach requiring a strong manual interaction. We propose a new workflow for the time imaging which includes an automatic update of time migration velocities. The workflow is based on an automatic Common‐Reflection‐Surface (CRS) stack of Common Scatter Point (CSP) gathers and provides an improved time migration velocity model and, hence, a highly focused time image.

Summary High-resolution seismic reflection profiles were acquired in the Kylylahti polymetallic mine area in the Outokumpu region, Finland, in the framework of the COGITO-MIN project. Acquisition was performed along two segmentally straight lines using both vibroseis and dynamite sources. We present three different versions of the data imaging, one utilizing post-stack time migration, second pre-stack time migration and third Fresnel Volume pre-stack depth migration algorithms. Seismic images reveal steep reflectors at less than 500 m depth that correlate with the black schist intercalations in the mica schist as well as strong reflectors at 5–8 km depth attributed to mafic dykes on top of Archean basement rocks. The rock assemblage hosting the mineralisation is imaged as prominent but discontinuous reflectivity due to its complex internal structure caused by folding and faulting.

Although amplitude versus angle (AVA) analysis is currently performed on unmigrated prestack data, there is increasing evidence that more reliable and better resolved AVA attributes can be obtained by inversion after prestack migration, either in time (Mosher et al., 1996) or in depth (Beydoun et al., 1993)—even when considering flat structures. In principle at least, the use of prestack depth migration before inversion appears a better option because it involves a less approximate description of wave propagation and provides better results in terms of location, resolution, and accuracy of AVA targets. For successful AVA inversion after migration, amplitude preservation naturally is of utmost importance. Two approaches may be used. The classical sequence consists of “preserved amplitude preprocessing” in which trace amplitudes are corrected by estimating the decay due to wave propagation and then so-called “kinematic” processing (DMO, prestack time or depth migration), without amplitude weights. In that case, final amplitude values simply come from the amplitudes of the preprocessed prestack data and this can obviously affect the AVA analysis adversely. As an alternate solution, traces are preprocessed with as little alteration as possible of their original amplitudes and then fed to a “preserved-amplitude” PsDM scheme (PAPsDM), in which migration weights are designed to fully account for dynamic propagation effects and acquisition effects (e.g., geometrical spreading and irregular illumination corrections). The latter solution is being recognized as a valuable and efficient tool to improve quantitative discrimination of reservoirs. In this article, we compare results obtained with these two approaches on calibrated synthetic data in order to underline that reliable amplitudes of subsurface parameters can be computed in heterogeneous media. Because real-sized applications require accurate and efficient tools, a second example is presented that uses real data. This is included to establish that such applications of ray-based quantitative Kirchhoff migration and subsequent …

In this paper we present the improved algorithm for angle domain common imaging gathers (ADCIG) during Kirchhoff pre‐stack time migration (PSTM). The algorithm can calculate relatively true incident angle under three‐dimensional circumstance, while the traditional ray‐parameter method could not handle the incident angle for the dipping events. Compared with straight‐ray Kirchhhoff angle domain gathers method presented in recent years, our method is able to extract wide‐angle information. Increased angular illumination is beneficial for AVA inversion. The results of synthetic model data have verified that: for V(z) (layered) media, the traveltime calculation taking ray‐bending effect into account, is much more accurate than straight ray for large angle. Thus the wide angles of ADCIG are flatter than the straight assumption. Moreover, the range of ADCIG is enlarged. The ADCIG and its AVA inversion of 3‐D field data have also demonstrated the improvement at wide angle.

Amplitude versus offset (AVO) studies are easiest to implement if the raw data is free of noise and forward modeling can be used to make the interpretation. In general these ideal conditions do not exist and a number of processing approaches have been suggested in order to prepare the data for AVO studies. Unfortunately most of the noise reduction techniques affect the signal as well as the noise. A better estimate of AVO on noisy data can be accomplished using an amplitude-preserving prestack time or depth migration on constant offset gathers. The clear advantage to this approach is the ability to emphasize the desired signal along a migrated wavefront at the expense of other coherent and incoherent signals while performing the migration. Different types of noise can be handled with this technique as long as the moveout of the desired signal is distinquishable from the noise. One undesirable side-effect of viewing prestack migrated data is the wavelet stretch associated with the imaging principle used in most migration algorithms. The wavelet stretch is same type of effect observed in RMO stretch. In this presentation an intuitive discussion of migration stretch effects will be discussed along with amplitude preservation during migration. The prestack migration method is applied to an example dataset with strong multiples from the Gulf of Mexico.

In an ever more challenging context for the acquisition of seismic data in the Mediterranean Sea, reprocessing to improve the quality of legacy data has become increasingly important. This work presents the newly reprocessed, open access dataset SALTFLU acquired in the Algerian basin by the National Institute of Oceanography and Applied Geophysics (OGS) in 2012. We apply a ‘broadband’ reprocessing strategy adapted for short-offset, deep water airgun reflection seismic data and assess if the reprocessed images provide new geological insights on the Mediterranean sub-surface. The workflow relies on an integrated approach combining geophysics and geological interpretation to iteratively build the velocity model. In this way we aim to tackle some of the challenges linked to imaging complex geological structures containing high velocity contrasts with 2-D, short-offset seismic data. We first broaden the bandwidth of the data through multi-domain de-noising, deghosting and a source designature using an operator derived from the seabed reflection. We then perform iterative migration velocity analysis, pre-stack time migration and multiple attenuation in the Radon domain to obtain time-migrated images. The initial velocity model is derived from the resulting time migration velocities, and geologically driven model updates are generated using a combination of travel-time tomography, seismic interpretation of the major salt horizons and velocity gradient flooding. The gradient flooding aims to reproduce the large scale first-order velocity variations, while the travel-time tomography aims to resolve the smaller second-order velocity variations. The results improve our deep geological knowledge of the under-explored Algerian basin down to the base salt and the pre-salt. Fluid indicators are imaged within the Plio-Quaternary of the Algerian basin, which we interpret as thermogenic or biogenic gas sourced from either the Messinian Upper Unit or from the pre-salt, migrating through a hydro-fractured salt. The reprocessed data image lateral and vertical seismic facies variation within the Messinian units that could shed new light on the tectono-stratigraphic processes acting during the Messinian Salinity Crisis. It also reveals numerous previously unresolved volcanic structures within the Formentera basin.

In an ever more challenging context for the acquisition of seismic data in the Mediterranean Sea, reprocessing to improve the quality of legacy data has become increasingly important. This work presents the newly reprocessed, open access dataset SALTFLU acquired in the Algerian basin by the National Institute of Oceanography and Applied Geophysics (OGS) in 2012. We apply a ‘broadband’ reprocessing strategy adapted for short-offset, deep water airgun reflection seismic data and assess if the reprocessed images provide new geological insights on the Mediterranean sub-surface. The workflow relies on an integrated approach combining geophysics and geological interpretation to iteratively build the velocity model. In this way we aim to tackle some of the challenges linked to imaging complex geological structures containing high velocity contrasts with 2-D, short-offset seismic data. We first broaden the bandwidth of the data through multi-domain de-noising, deghosting and a source designature using an operator derived from the seabed reflection. We then perform iterative migration velocity analysis, pre-stack time migration and multiple attenuation in the Radon domain to obtain time-migrated images. The initial velocity model is derived from the resulting time migration velocities, and geologically driven model updates are generated using a combination of travel-time tomography, seismic interpretation of the major salt horizons and velocity gradient flooding. The gradient flooding aims to reproduce the large scale first-order velocity variations, while the travel-time tomography aims to resolve the smaller second-order velocity variations. The results improve our deep geological knowledge of the under-explored Algerian basin down to the base salt and the pre-salt. Fluid indicators are imaged within the Plio-Quaternary of the Algerian basin, which we interpret as thermogenic or biogenic gas sourced from either the Messinian Upper Unit or from the pre-salt, migrating through a hydro-fractured salt. The reprocessed data image lateral and vertical seismic facies variation within the Messinian units that could shed new light on the tectono-stratigraphic processes acting during the Messinian Salinity Crisis. It also reveals numerous previously unresolved volcanic structures within the Formentera basin.

We have improved the so-called deabsorption prestack time migration (PSTM) by introducing a dip-angle domain stationary-phase implementation. Deabsorption PSTM compensates absorption and dispersion via an actual wave propagation path using effective Q parameters that are obtained during migration. However, noises induced by the compensation degrade the resolution gained and deabsorption PSTM requires more computational effort than conventional PSTM. Our stationary-phase implementation improves deabsorption PSTM through the determination of an optimal migration aperture based on an estimate of the Fresnel zone. This significantly attenuates the noises and reduces the computational cost of 3D deabsorption PSTM. We have estimated the 2D Fresnel zone in terms of two dip angles through building a pair of 1D migrated dip-angle gathers using PSTM. Our stationary-phase QPSTM (deabsorption PSTM) was implemented as a two-stage process. First, we used conventional PSTM to obtain the Fresnel zones. Then, we performed deabsorption PSTM with the Fresnel-zone-based optimized migration aperture. We applied stationary-phase QPSTM to a 3D field data. Comparison with synthetic seismogram generated from well log data validates the resolution enhancements.

Converted–wave pre-stack time migration is an important step in multicomponent seismic data processing not only for its ability to produce a better PS image, but also as a preparation of the data for further analysis such as joint PP, PS inversion. Technically, its implementation is similar to, but not the same as, P-wave migration because the down-going P-wave and up-going S-wave travel at different speeds and are affected by anisotropy differently. Some technical issues such as velocity and anisotropy parameterization, anti-aliasing, reflection amplitude preservation, random noise attenuation and 5D interpolation prior to converted wave (P-S) pre-stack time migration need to be handled appropriately in order to optimize the migrated image.