Seismic Depth Migration Research Articles

Subsalt imaging in northern Germany using multiphysics (magnetotellurics, gravity, and seismic)

Imaging subsalt is still a challenging task in oil and gas exploration. We have used magnetotellurics (MT) to improve the integration of seismic and gravity data to image the Wedehof salt dome, located in the Northern German Basin. High-density natural field source broadband MT data were acquired and enhanced the definition of the top and overhanging salt structures in addition to imaging the salt dome root. Salt boundaries show strong resistivity contrasts with the surrounding sediments and thus represent a good target for electromagnetic measurements, especially for top salt and salt flanks imaging. With integrated 3D gravity modeling focusing on the salt dome’s flanks at intermediate depths, an improved model was achieved. The new model provided sound input to a follow-up seismic depth migration that led to an improved imaging of the subsalt target proven by subsequent exploration drilling. The integrated interpretation of MT, gravity, and seismic combines the strengths of the different physics, thus increasing imaging reliability and reducing exploration drilling risks. Using a conservative workflow that included a feasibility study with field noise evaluation and careful acquisition parameter testing prior to survey start, a broadband array data acquisition, and advanced processing, the survey area's severe cultural noise issues could be overcome.

LIQUEFACTION RISK ASSESSMENT BY THE USE OF GEOPHYSICAL TECHNIQUES: THE TEST AREA OF NAFPLION CITY, GREECE

An efficient and cost effective site characterization, with regard to the seismic hazard and liquefaction risk assessment, was accomplished with the aid of geophysics in the area, where the Nafplion city of Greece is expanding. The methodology adopted includes the recognition of the possible earthquake sources of the wider region, their modelling, in order to stochastically simulate the strong ground motion at the investigation area, and finally the calculation of the liquefaction risk. The investigation area was suspected of high liquefaction potential since the foundation ground consists of loose sandy silt with very shallow aquifer. The geophysical techniques considerably contributed to the detection and characterization of possible local seismic faults with the implementation of gravity and seismic methods. Special emphasis was given to the seismic depth migration and particularly to the construction of valid velocity models, in order to precisely calculate the dips of the possible faults. Additionally the geophysical techniques provided the near surface velocity structure for the calculation of the amplification of the seismic motion up to the surface, also required for the final estimation of the liquefaction risk. The seismic methods (seismic reflection, seismic refraction, seismic modelling, MASW, multichannel analysis of microtremors and crosshole investigations), if combined with geo-technical borehole testing, enhance their reliability and cover large areas in a cost-effective way in comparison with the standard borehole tests. In Nafplion area, evidence was found for a low factor of safety against liquefaction at specific sites within the study area. The results show that liquefaction probability can reach 80% at some sites depending on selected earthquake scenario, mainly at depths between 5 and 10 meters. This should be considered as highly important information for making risk-based design decision in this region.

Reducing seismic positioning uncertainty with “simple” tilted transverse isotropy earth models — Gulf of Mexico Lobster field case study

The remaining potential in many mature fields in the Gulf of Mexico consists of smaller long-reach targets that require ever-more-accurate vertical and horizontal location for drilling precision. This is particularly true at the Lobster field in Ewing Bank, where, during the previous platform drilling campaigns, some midreach prospects were interpreted to have been missed due to mispositioning. Potential future drilling at the Lobster field would target primarily long-reach prospects in areas of significant geologic dip — increasing the risk of missing the prospects. This mispositioning risk could sometimes exceed the geologic risk, and it could have significant impact on the risked reserves and on the economics. Achieving the desired positioning accuracy requires the use of 3D seismic and anisotropic prestack depth migration with accurate velocity and anisotropy models. We have developed a case study of using a long offset walkaway checkshot and a geologic frame of reference to build a tilted transverse isotropy velocity model sufficiently accurate to support potential future drilling campaigns at the Lobster field.

Near-Surface Multi-Point Vibration Location Method Based on Seismic Depth Migration

This paper presents a method for multi-point vibration location near-surface. Unlike traditional source location technologies, it is not using travel times of seismic waves for positioning calculation, but according to the entire seismic data record, using the wave equation migration method to calculate the source location. Similar to exploration seismic, the records from a survey line within a certain period of time are data volumes with dimensions of time and ground coordinates. Based on the data, combined with surface seismic wave propagation characteristics, by using the improved seismic depth migration algorithm, the vibration energy will return to the starting position where exactly the source location is. The method can solve the problem of location calculation error by using traditional method when several vibration at the same time or continuous vibration occurs at some point.

Illumination and Resolution Analyses on Marine Seismic Data Acquisitions by the Adjoint Wavefield Method

We applied a wave-equation based adjoint wavefield method for seismic illumination and resolution analyses. A two-way wave-equation is used to calculate directional and diffracted energy fluxes for waves propagating between sources and receivers to the subsurface target. The first-order staggered-grid pressure-velocity formulation, which lacks the characteristic of being self-adjoint is further validated and corrected to render the modeling operator before its practical application. Despite most published papers on synthetic kernel research, realistic applications to two field experiments are demonstrated and emphasize its practical needs. The Frechet sensitivity kernels are used to quantify the target illumination conditions. For realistic illumination measurements and resolution analyses, two completely different survey geometries and nontrivial pre-conditioning strategies based on seismic data type are demonstrated and compared. From illumination studies, particle velocity responses are more sensitive to lateral velocity variations than pressure records. For waveform inversion, the more accurately estimated velocity model obtained the deeper the depth of investigation would be reached. To achieve better resolution and illumination, closely spaced OBS receiver interval is preferred. Full waveform approach potentially provides better depth resolution than ray approach. The quantitative analyses, a by-product of full waveform inversion, are useful for quantifying seismic processing and depth migration strategies.

Reducing two-way splitting error of FFD method in dual domains

The Fourier finite-difference (FFD) method is very popular in seismic depth migration. But its straightforward 3D extension creates two-way splitting error due to ignoring the cross terms of spatial partial derivatives. Traditional correction schemes, either in the spatial domain by the implicit finite-difference method or in the wavenumber domain by phase compensation, lead to substantially increased computational costs or numerical difficulties for strong velocity contrasts. We propose compensating the two-way splitting error in dual domains, alternately in the spatial and wavenumber domains via Fourier transform. First, we organize the expanded square-root operator in terms of two-way splitting FFD plus the usually ignored cross terms. Second, we select a group of optimized coefficients to maximize the accuracy of propagation in both inline and crossline directions without yet considering the diagonal directions. Finally, we further optimize the constant coefficient of the compensation part to further improve the overall accuracy of the operator. In implementation, the compensation terms are similar to the high-order corrections of the generalized-screen method, but their functions are to compensate the two-way splitting error rather than the expansion error. Numerical experiments show that optimized one-term compensation can achieve nearly perfect circular impulse responses and the propagation angle with less than 1% error for all azimuths is improved up to 60° from 35°. Compared with traditional single-domain methods, our scheme can handle lateral velocity variations (even for strong velocity contrasts) much more easily with only one additional Fourier transform based on the two-way splitting FFD method, which helps retain the computational efficiency.

Comparison of the Hill’s method with the seismic depth migration by the Gaussian beam summation method

The mathematical justification of the Hill’s method of prestack depth migration is analyzed. It is shown that this method should be classified as the ray based Kirchhoff migration with the so-called “fat” rays, that is, where rays are replaced by isolated Gaussian beams centered on the rays and associated somehow with the input data on the seismic surface. That makes essential difference with our approach to seismic depth migration based on the Gaussian beam summation method. The results of implementation of the basic Hill’s idea in computations of the wave field are shown with numerical examples. Bibliography: 10 titles.

The contribution of geophysical techniques to site characterisation and liquefaction risk assessment: Case study of Nafplion City, Greece

Abstract We present an example of how geophysical methodologies can considerably contribute to seismic and liquefaction risk assessment in an area where urban development is planned. The inspection for possible hidden faults by geophysical methods is particularly critical, since such a possibility could practically hinder the town planning in this area. However, even if no primary threats exist within the area, the response of the foundation soil to various scenarios of historical earthquakes, which have affected the place in the past, must be examined. The geophysical methodologies could also assist this analysis, contributing with the calculation of the amplification of the seismic motion from the bedrock up to the surface. The investigation area of Nafplion, Greece, was suspected to have high liquefaction potential since the foundation soil consists of loose sandy silt with a very shallow aquifer. The implementation of gravity and seismic methods considerably aided the investigation for possible seismic faults. Special emphasis was given to seismic depth migration and particularly to the construction of valid velocity models, in order to precisely calculate the dip characteristics of the structures. Shallow seismic techniques were also applied to provide the near-surface velocity structure, which is a prerequisite for assessing the liquefaction risk. In particular, our case study provides an example of how seismic methods (seismic reflection, seismic refraction, seismic modelling, MASW, multichannel analysis of microtremors and crosshole investigations) when combined with geotechnical borehole testing, enhance the reliability of their output and allow the coverage of wide areas in a cost-effective way in comparison to standard borehole tests. Data and information provided by the application of the geophysical methods were subsequently incorporated in the liquefaction risk assessment at several selected sites within the study area. Factors of safety against liquefaction and liquefaction potential values were computed for three scenario earthquakes that were selected on the basis of the known seismic impact of past earthquakes in the town of Nafplion. We found that liquefaction probability can reach values as high as 80% at some sites depending on the selected earthquake scenario. The formations most prone to liquefaction are detected at depths between 5 and 10 m. This information can be helpful for making risk-based design decision in this region.

Depth migration by the Gaussian beam summation method

Seismic depth migration aims to produce an image of seismic reflection interfaces. Ray methods are suitable for subsurface target-oriented imaging and are less costly compared to two-way wave-equation-based migration, but break down in cases when a complex velocity structure gives rise to the appearance of caustics. Ray methods also have difficulties in correctly handling the different branches of the wavefront that result from wave propagation through a caustic. On the other hand, migration methods based on the two-way wave equation, referred to as reverse-time migration, are known to be capable of dealing with these problems. However, they are very expensive, especially in the 3D case. It can be prohibitive if many iterations are needed, such as for velocity-model building. Our method relies on the calculation of the Green functions for the classical wave equation by per-forming a summation of Gaussian beams for the direct and back-propagated wavefields. The subsurface image is obtained by cal-culating the coherence between the direct and backpropagated wavefields. To a large extent, our method combines the advantages of the high computational speed of ray-based migration with the high accuracy of reverse-time wave-equation migration because it can overcome problems with caustics, handle all arrivals, yield good images of steep flanks, and is readily extendible to target-oriented implementation. We have demonstrated the quality of our method with several state-of-the-art benchmark subsurface models, which have velocity variations up to a high degree of complexity. Our algorithm is especially suited for efficient imaging of selected subsurface subdomains, which is a large advantage particularly for 3D imaging and velocity-model refinement applications such as subsalt velocity-model improvement. Because our method is also capable of providing highly accurate migration results in structurally complex subsurface settings, we have also included the concept of true-amplitude imaging in our migration technique.

Improving explicit seismic depth migration with a stabilizing Wiener filter and spatial resampling

We present a new approach to the design and implementation of explicit wavefield extrapolation for seismic depth migration in the space-frequency domain. Instability of the wavefield extrapolation operator is addressed by splitting the operator into two parts, one to control phase accuracy and a second to improve stability. The first partial operator is simply a windowed version of the exact operator for a half step. The second partial operator is designed, using the Wiener filter method, as a band-limited, least-squares inverse of the first. The final wavefield extrapolation operator for a full step is formed as a convolution of the first partial operator with the complex conjugate of the second. This resulting wavefield extrapolation operator can be designed to have any desired length and is generally more stable and more accurate than a simple windowed operator of similar length. Additional stability is gained by reducing the amount of evanescent filtering and by spatially downsampling the lower temporal frequencies. The amount of evanescent filtering is controlled by building two operator tables, one corresponding to significant evanescent filtering and the other to very little evanescent filtering. During the wavefield extrapolation process, most steps are taken with the second table while the first is invoked only for roughly every tenth step. Also, the data are divided into frequency partitions that are optimally resampled in the spatial coordinates to further enhance the performance of the extrapolation operator. Lower frequencies are downsampled to a larger spatial sample size. Testing of the algorithm shows accurate, high-angle impulse responses and run times comparable to the phase shift method of time migration. Images from trial depth migrations of the Marmousi model show very high resolution.

Método híbrido de migração pré-empilhamento em profundidade no domínio da freqüência em duas etapas com interpolação

This paper shows the application of Phase Shift Plus Interpolation Split-Step (PSPI-SS) wavefield extrapolation operator coupled with the excitation-time imaging condition to perform prestack seismic depth migration of common shot profiles. The migration methods which are characterized by the use of different propagators for the source and receiver wavefields are called hybrid migration methods. Normally, the source wavefield is determined by the computation of the traveltimes from the solution of the eikonal equation. Here the traveltimes were calculated based on a finite difference solution of the eikonal equation (TGRID), a simple geometrical approach (FAST) and also by the paraxial ray tracing method (PARAX). For the extrapolation of the receiver data we applied the PSPI Split-Step operator, that is derived from the conventional Split-Step wave equation method using multiple references velocity. As the source traveltime computation is based on the ray assintotic theory the velocity field needs to be smoothed and the migration results obtained with the Kirchhof migration method, that is a very fast migration procedure, was used to choose the best level of smoothing of the velocity field. In this work the synthetic Marmousi data and a real dataset from Gulf of Mexico were used to test the migration methods proposed here. The migration results with the hybrid methods shown better images in quality then the kirchhoff's results and comparable to those obtained with the full PSPI-SS migration method, but a low computation cost.

Three-dimensional seismic-while-drilling (SWD) migration in the angular frequency domain

Seismic depth migration back-propagates seismic data in the correct depth position using information about the velocity of the medium. Usually, Kirchhoff summation is the preferred migration procedure for seismic-while-drilling (SWD) data because it can handle virtually any configuration of sources and receivers and one can compensate for irregular spatial sampling of the array elements (receivers and sources). Under the assumption of a depth-varying velocity model, with receivers arranged along a horizontal circumference and sources placed along the central vertical axis, we reformulate the Kirchhoff summation in the angular frequency domain. In this way, the migration procedure becomes very efficient because the migrated volume is obtained by an inverse Fourier transform of the weighted data. The algorithm is suitable for 3D SWD acquisitions when the aforementioned hypothesis holds. We show migration tests on SWD synthetic data, and we derive solutions to reduce the migration artifacts and to control aliasing. The procedure is also applied on a real 3D SWD data set. The result compares satisfactorily with the seismic stack section obtained from surface reflection data and with the results from traditional Kirchhoff migration.

A workshop on enhanced seismic imaging

One of the hottest topics in seismic depth imaging these days is gravity modeling. What? That's right, most major oil companies and many smaller ones routinely integrate rigorous gravity models with their seismic depth migration workflow. If you think about it, it makes sense. Gravity data have always been effective at predicting the distribution of mass in the subsurface. Although once thought limited in resolution, gravity has gained new respect in the last five years due to careful modeling and thoughtful integration into the workflow of seismic depth migration.

A workshop on enhanced seismic imaging

One of the hottest topics in seismic depth imaging these days is gravity modeling. What? That's right, most major oil companies and many smaller ones routinely integrate rigorous gravity models with their seismic depth migration workflow. If you think about it, it makes sense. Gravity data have always been effective at predicting the distribution of mass in the subsurface. Although once thought limited in resolution, gravity has gained new respect in the last five years due to careful modeling and thoughtful integration into the workflow of seismic depth migration.

ABSTRACT: Use of Gravity Data to Enhance Seismic Depth Migration

Abstract Seismic depth migration continues to be an important, but expensive, tool for reducing exploration risks. Newly acquired high-resolution gravity data provide a low-cost method to enhance the seismic depth migration process by providing an independently measured constraint to the seismic velocity model. State-of-the-art commercial software tools enable rapid integration of the gravity and seismic data. In many geologic settings, integrated density/velocity modeling yields a more accurate velocity model than can be derived from the seismic data alone. A case study is presented to illustrate both steps employed in the data integration process and the resulting improvement in depth-migrated seismic data. End_of_Record - Last_Page 241-------

Seismic depth migration with the Dirac equation

We demonstrate the applicability of the Dirac equation in seismic wavefield extrapolation by presenting a new explicit one‐way prestack depth migration scheme. The method is in principle accurate up to 90° from the vertical, and it tolerates lateral velocity variations. This is achieved by performing the extrapolation step of migration with the Dirac equation, implemented in the space‐frequency domain. The Dirac equation is an exact linearization of the square‐root wave equation and is equivalent to keeping infinitely many terms in a Taylor series or continued‐fraction expansion of the square‐root operator. An important property of the new method is that the local velocity and the spatial derivatives decouple in separate terms within the extrapolation operator. Therefore, we do not need to precompute and store large tables of convolutional extrapolator coefficients depending on velocity. The main drawback of the explicit scheme is that evanescent energy must be removed at each depth step to obtain numerical stability. We have tested two numerical implementations of the migration scheme. In the first implementation, we perform depth stepping using the Taylor series approximation and compute spatial derivatives with high‐order finite difference operators. In the second implementation, we perform depth stepping with the Rapid expansion method and numerical differentiation with the pseudospectral method. The imaging condition is a generalization of Claerbout’s U / D principle. For both implementations, the impulse response is accurate up to 80° from the vertical. Using synthetic data from a simple fault model, we test the depth migration scheme in the presence of lateral velocity variations. The results show that the proposed migration scheme images dipping reflectors and the fault plane in the correct positions.

PVM based 3-D Kirchhoff depth migration using dynamically computed travel-times: An application in seismic data processing

Seismic depth migration is an image reconstruction technique used to generate realistic images of the Earth's interior from surface recordings of sound waves reflected by buried geological structures. Migration algorithms that have been developed for digitally recorded seismic data are computationally intensive and are typically implemented on massively parallel architecture. In this paper we describe a highly accurate method of computing seismic travel-times and study the feasibility of a PVM-based SPMD parallelization of a migration algorithm based on this method. The parallel algorithm developed in this study is coarse grained and utilizes a simple geometrical decomposition of the input problem. We show that near linear speedup can be achieved on a homogeneous cluster and the algorithm can be ported to a variety of platforms, i.e. clusters of Sun workstations, Cray Y-MP, nCUBE-2, Cray T3E and SGI Origin 2000. The performance of the scheme on different computational platforms ultimately depends on a number of different factors, e.g., machine architecture, machine speed, network configuration etc. For a simple test case that was studied in this research, the performances of modest PVM clusters were found to be encouraging. This underscores the advantages of adopting a strategy based on parallelizing computationally intensive tasks and also demonstrates the suitability of the travel-time scheme for developing coarsely parallel applications.

Seismic depth migration with pseudo-screen propagator

The generalized screen propagator (GSP) is a fast and accurate method for computing wave propagation in laterally varying velocity media. The method solves the acoustic one-way wave equation by shuffling FFTs between frequency-space and frequency-wavenumber domain. The velocity model is decomposed into a vertically varying reference velocity and a laterally varying perturbation. Downward continuation of the wavefield is performed with a phase shift in a background medium followed by a correction that accommodates the lateral velocity variations. All frequencies of interest are migrated and summed to produce an image at each depth level. The screen propagators are applicable to both poststack and prestack depth migrations. We illustrate the method with the SEG-EAGE salt model and Marmousi data set. The image results show that the pseudo-screen propagator is a big improvement over the phase-screen propagator for steeply dipping events in strong-contrast heterogeneous media.

Anisotropic migration of coincident VSP and cross‐hole seismic reflection surveys

Seismic depth migration may result in false reflector positioning and destructive interference when an incorrect velocity field is used to convert from time to depth. The assumption of isotropy to describe anisotropic rocks is one major source of error in the velocity model, although individual survey images may not be impaired by such an approximation. When different survey types such as VSP and cross‐hole reflection seismics have coincident illumination of the subsurface, it is important not only to produce consistent images upon depth migration, but also to determine a consistent velocity model. Using real data sets as examples, both objectives are successfully achieved when anisotropy is incorporated into the velocity model.

A few case histories of subsurface imaging with EMAP as an aid to seismic processing and interpretation1

AbstractThe electromagnetic array profiling (EMAP) exploration method can be combined with a direct one‐dimensional inversion process for conversion to depth to produce a subsurface resistivity cross‐section. This cross‐section may then be interpreted in parallel with a seismic cross‐section to enhance the prediction of rock type and structure. In complex thrust environments and areas of shallow carbonate rocks, the EMAP method is often used to provide additional data either to help the seismic processor and/or to aid the seismic interpretation. In particular, the electromagnetic (EM) data can be used to build an independent seismic velocity file for depth migration.Three EMAP test areas in the western United States are used to demonstrate such a use of EMAP as an expioration tool. The first shows how a velocity file is estimated from resistivity data for seismic depth migration processing in a complex thrust environment. In the second example, the method is applied in layer‐cake geology with high seismic velocity rocks at the earth's surface. The third example is another complex thrust environment, but in this case the velocity file derived from the resistivity data is used for stacking the seismic data.