Salt Flank Research Articles

Nonlinear least-squares reverse time migration of prismatic waves for delineating steeply dipping structures

Prismatic reflections in seismic data carry abundant information about subsurface steeply dipping structures, such as salt flanks or near-vertical faults, playing an important role in delineating these structures when effectively used. Conventional linear least-squares reverse time migration (L-LSRTM) fails to use prismatic waves due to the first-order Born approximation, resulting in a blurry image of steep interfaces. We develop a nonlinear LSRTM (NL-LSRTM) method to take advantage of prismatic waves for the detailed characterization of subsurface steeply dipping structures. Compared with current least-squares migration methods of prismatic waves, our NL-LSRTM is nonlinear and thus avoids the challenging extraction of prismatic waves or the prior knowledge of L-LSRTM results. The gradient of NL-LSRTM consists of the primary and prismatic imaging terms, which can accurately project observed primary and prismatic waves into the image domain for the simultaneous depiction of near-horizontal and near-vertical structures. However, we find that the full Hessian-based Newton normal equation has two similar terms, which prompts us to make further comparison between the Newton normal equation and our NL-LSRTM. We determine that the Newton normal equation is problematic when applied to the migration problem because the primary reflections in the seismic records will be incorrectly projected into the image along the prismatic wavepath, resulting in an artifact-contaminated image. In contrast, the nonlinear data-fitting process included in the NL-LSRTM contributes to balancing the amplitudes of primary and prismatic imaging results, thus making NL-LSRTM produce superior images compared with the Newton normal equation. Several numerical tests validate the applicability and robustness of NL-LSRTM for the delineation of steeply dipping structures and illustrate that the imaging results are much better than the conventional L-LSRTM.

Enhancing salt model resolution and subsalt imaging with elastic FWI

Full-waveform inversion (FWI) has become the centerpiece of velocity model building (VMB) in seismic data processing in recent years. It has proven capable of significantly improving the velocity model and, thus, the migration image for different acquisition types and geologic settings, including complex environments such as salt. With the advent of FWI imaging, the scope of FWI applications has extended further from VMB into the imaging landscape. However, current FWI applications in the industry prevalently employ the acoustic approximation. One common problem of acoustic FWI (A-FWI) is the apparent salt halos at the salt-sediment interface in the resulting FWI velocity and FWI image, the presence of which hinders direct interpretation and imaging focusing around salt bodies. With synthetic and field data examples, we demonstrate that this salt halo is caused mainly by the large mismatch between the elastic recorded data and the acoustic modeled data, particularly at middle to long offsets. To overcome limitations imposed by acoustic assumptions, we developed an elastic FWI (E-FWI) algorithm that combines an elastic modeling engine with the time-lag cost function, which we call elastic time-lag FWI (E-TLFWI). With a more accurate modeling engine, E-TLFWI significantly reduces the salt halo observed in its acoustic counterpart. However, we also observe that the images migrated using the A-FWI and E-FWI velocity models remain similar overall, with some slight improvements around and beneath salt boundaries, particularly near steep salt flanks, as a result of the reduced salt halo. By contrast, FWI images derived from E-TLFWI show considerable benefits over those from acoustic time-lag FWI, such as improved event focusing, better structural continuity, and higher signal-to-noise ratio. The sharpened salt boundaries and enhanced quality of the FWI images reveal the significant value of E-FWI and provide the justification for its greatly increased cost.

Direct expansion of Fourier extrapolator for one-way wave equation using Chebyshev polynomials of the second kind

The Fourier method for one-way wave propagation is efficient but potentially inaccurate in complex media. The implicit finite-difference (FD) method can handle arbitrarily complex media, but it can be inefficient in 3D and has limited dip bandwidth. We have adopted a new Fourier method based on the Chebyshev expansion of the second kind. Theoretical analyses and numerical experiments indicate that our method is comprehensively superior to a similar method based on the Chebyshev expansion of the first kind in terms of balanced amplitude and error tolerance. Within the dip bandwidth from 0° to 65°, the fourth-order form of our method has an error tolerance of 2%, which is approximately one-third that of the Chebyshev expansion of the first kind. Our method also is superior to the implicit FD method in several important aspects: effective bandwidth, computational efficiency, numerical dispersion, and two-way splitting error. It can be easily extended from 2D to 3D compared with the FD method and from low orders to high orders compared with the optimized Chebyshev-Fourier method. Our method finds better imaging results of the SEG/EAGE model by providing a well-focused salt dome, flank, and bottom as well as the detailed structures beneath the salt body, compared with the implicit FD method and the Chebyshev expansion of the first kind; meanwhile, our method has fewer imaging artifacts because it can better position the reflectors.

Nordkapp Topseis/Node Acquisition — Lessons from a Modelling Study

Based on an extensive 3D modelling study utilising full-wavefield Finite-Difference modelling and Full-Waveform Inversion (FWI) we demonstrate that the TopSeis/OBN hybrid acquisition acquired from May to August 2021 in the Nordkapp basin in the Barents Sea has the potential to image salt flanks and sedimentary details, given an accurate initial model in the shallow and a carefully designed deblending and FWI workflow. As a part of this we demonstrate that the large offsets and multi-azimuth recorded by the ocean bottom nodes are crucial to map the complex salt diapirism in the area including the steeply dipping flanks.

Imaging complex three‐dimensional structures, applying three‐component vector migration to synthetic vertical seismic profiling data

ABSTRACTBy applying a ray‐based three‐dimensional vector migration process to fully elastic three‐component synthetic Vertical Seismic Profiling data, we are able to demonstrate that the process gives an accurate compressional reflection image of the complex formation without much visible artefacts from spatial aliasing, or from ‘migration smiles’ caused by the limited aperture of the receiver array. We demonstrate that the process reconstructs images of both steeply dipping salt flanks, and the sediments that tend to be dragged along by the moving salt and truncated against the sides of the salt body. The process uses raw unseparated data as input, and the wavefield separation of compressional and shear waves are done by projection operators inside the kernel of the migration process. The compressional reflection image appear to have no visible artefacts from the abundance of converted shear.

Velocity model building for single-offset VSP data via deformable-layer tomography: A Texas salt dome example

We have applied multiscale deformable-layer tomography (DLT) to build a laterally varying velocity model, using a single-offset vertical seismic profile (VSP) data set acquired for a salt proximity survey in southern Texas. The purpose of the VSP survey is to delineate the 2D salt flank using the P-wave reflections. Previous study has identified an anhydrate layer as the cap rock of the salt dome. The large impedance contrasts of this anhydrite layer generate strong downgoing P (sediment)-S (anhydrite)-P (salt) waves recorded by downhole geophones. Incidentally, the P-S-P-waves have traveltimes similar to those of the P-wave salt flank reflections, thus contaminating the imaging of the salt flank. Identifying shear-mode contamination requires an accurate velocity model of anhydrite. However, the extremely poor coverage of the single-offset VSP greatly challenges tomographic techniques to determine the lateral velocity variation. We tackle this problem using multiscale DLT, which characterizes the velocity field by a set of deformable layers. We constrain the layer velocities using the check-shot data and invert for the geometric variation. The inverted model indicates that the anhydrite layer has a “thick-thin-thick” lateral variation with offset, and the S-wave in the anhydrite layer helps in imaging the P-S-P-waves along the well track. The estimated anhydrite layer geometry is validated by the kinematic accuracies of P-waves in the data domain and P-S-P-waves in the image domain. Some in-salt dipping structures are determined by multiscale DLT as well. This field data example indicates that multiscale DLT is feasible for estimating velocities using VSP data of the single-offset situation. An accurate velocity model is the key for modeling and adaptive subtraction of the shear-mode contamination related to the salt geometry.

External signal preservation in halokinetic stratigraphy: A discrete element modeling approach

Abstract Subsurface salt movement in the absence of external tectonic forces can affect contemporaneous sediment deposition, mask allocyclic signals, and deform older strata. We used a discrete element model (DEM) to better understand salt-related modification of a sedimentary sequence with an increasing sedimentation rate. This permitted quantification of thinning rates and analysis of the lateral extent of synkinematic layers. Results show realistic evolution of salt-related faults, defining two salt-withdrawal basins, beyond which strata are undeformed. Thinning of stratigraphy is four times greater between the salt flank and crest than between the undeformed zone and flank, confirming an intense zone of halokinetic modulation adjacent to the diapir. Early, slowly aggrading layers are isolated within the salt-withdrawal basin and strongly influenced by salt growth, whereas later, quickly aggrading layers are more laterally extensive, matching inferences made from subsurface and outcrop data. Halokinetic modulation reduces up the stratigraphic section, mirroring observations around the Pierce diapirs, in the North Sea, offshore UK. Our DEM provides quantitative insights into the dynamic interplay between halokinetic and allocyclic controls on salt-stratigraphic relationships.

Inversion for Salt Flank Geometry Using Transmitted P- and S-Wave Travel Times

Accurate delineation of the salt flank is important for oil and gas exploration in areas with salt intrusion. We describe the application of deformable-layer tomography (DLT) to invert for the geometry of salt flank using travel times of transmitted P- and S-waves from surface sources to downhole receivers. The DLT allows us to take advantage of a common situation that the salt velocity is known and generally invariant, but we need to determine the variable geometry of salt flank. We demonstrate our new method using a physical model experiment mimicking the setup of a walkaway vertical-seismic-profiling (VSP) survey. We first use picked P-wave arrivals to invert for the salt flank geometry; the sinusoid shape of the salt flank is estimated fairly by the DLT due to the uneven P-wave raypath coverage. As an improvement, we further incorporate the S-wave arrivals in the DLT. The picking of S-wave arrivals is assisted with modeled S-wave arrivals based on the P-wave DLT model. The DLT solution using both P- and S-wave arrivals delineates the salt flank geometry more accurately than that using P-wave arrivals alone. The salt flank delineation using DLT can complement with the conventional salt proximity and migration methods. It could also serve as constraints or the initial model for the full-waveform inversion of salt geometry.

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.

Frequency-domain least-squares generalized internal multiple imaging with the energy norm

Recorded seismic data contain various types of scattered energy, including those corresponding to multiples. Traditional imaging techniques are focused on single-scattering events and, thus, may fail to image crucial structures, such as salt flanks and faults that sometimes are illuminated only by the multiple scattered energy. The recently introduced generalized internal multiple imaging (GIMI) method offers an opportunity to image multiples by projecting the recorded data back into the subsurface, followed by an interferometric crosscorrelation of the subsurface wavefield with the recorded data. During this process, the interferometric step converts the first-order scattering to a tomographic component and the double-scattering forms the primary reflectivity. Dealing with a large volume of data consisting of full wavefields over the image space renders the interferometric step computationally expensive in the time domain. To make the implementation of GIMI tractable, we formulate its frequency-domain version. Moreover, we use the energy norm imaging condition to separate the reflectivity portion from the tomographic component. We demonstrate these features with numerical experiments.

Use of prismatic waves in full-waveform inversion with the exact Hessian

The truncated Newton method uses information contained in the exact Hessian in full-waveform inversion (FWI). The exact Hessian physically contains information regarding doubly scattered waves, especially prismatic events. These waves are mainly caused by the scattering at steeply dipping structures, such as salt flanks and vertical or nearly vertical faults. We have systematically investigated the properties and applications of the exact Hessian. We begin by giving the formulas for computing each term in the exact Hessian and numerically analyzing their characteristics. We show that the second term in the exact Hessian may be comparable in magnitude to the first term. In particular, when there are apparent doubly scattered waves in the observed data, the influence of the second term may be dominant in the exact Hessian and the second term cannot be neglected. Next, we adopt a migration/demigration approach to compute the Gauss-Newton-descent direction and the Newton-descent direction using the approximate Hessian and the exact Hessian, respectively. In addition, we determine from the forward and the inverse perspectives that the second term in the exact Hessian not only contributes to the use of doubly scattered waves, but it also compensates for the use of single-scattering waves in FWI. Finally, we use three numerical examples to prove that by considering the second term in the exact Hessian, the role of prismatic waves in the observed data can be effectively revealed and steeply dipping structures can be reconstructed with higher accuracy.

Imaging below a complex overburden with borehole-seismic data

We have determined how the measured polarization and traveltime for P- and S-waves can be used directly with vertical seismic profile data for estimating the salt exit points in a salt-proximity survey. As with interferometry, the processes described use only local velocities. For the data analyzed in this paper, our procedures have confirmed the location, inferred from surface-seismic data, of the flank of a steeply dipping salt body near the well. This has provided us more confidence in the estimated reservoir extent moving toward the salt face, which in turn has added critical information for the economic evaluation of a possible new well into the reservoir. We also have found that ray-based vector migration, based on the assumptions of locally plane wavefronts and locally plane formation interfaces, can be used to create 3D reflection images of steeply dipping sediments near the well, again using only local velocities. Our local reflection images have helped confirm the dips of the sediments between the well and the salt flank. Because all parameters used in these processes are local and can be extracted from the data themselves, the processes can be considered to be self-sufficient.

Improved seismic interpretation of a salt diapir by utilization of diffractions, exemplified by 2D reflection seismics, Danish sector of the North Sea

Development of salt diapirs affects the hydrocarbon trapping systems in the Danish sector of the North Sea, where the reservoirs mainly consist of chalk. Seismic imaging and interpretation of the salt structures are challenging, primarily due to the complex geometry of the salt bodies and typically strong velocity contrast with the neighboring sediment layers. The quality of seismic imaging in the North Sea is highly dependent on the quality of the estimated velocity model. We have studied diffracted arrivals originating from the salt flanks and adjacent sedimentary structures using a diffraction imaging technique. The diffracted waves carry valuable information regarding seismic velocity and the location of geologic discontinuities, such as faults, fractures, and salt delimitations. We apply a plane-wave destruction method to separate diffractions from our stacked data. We optimize imaging based on diffraction analysis by using a velocity continuation migration technique, which leads to an estimation of the optimum focusing velocity model. We determine that the diffraction-based approach significantly improves the seismic imaging adjacent to the salt diapirs and the neighboring layers when compared with a standard approach in which we mostly ignore the diffractions. The new poststack time-migrated results provide detailed information that optimizes our interpretation of the salt diapir itself (e.g., the width of the salt neck) as well as the sediment layers related to the rim synclines. Processing schemes such as prestack depth migration and full-waveform inversion may potentially provide high-resolution images of the salt structures. We only account for diffractions in nonmigrated stacked data to better constrain seismic velocity and improve imaging around the salt diapir. The obtained results are critical for reservoir characterization.

Full waveform inversion of simultaneous long-offset data

SummaryThe standard workflow for velocity model building (VMB) in complex regimes is an interpretive process that requires time-consuming manual intervention, and remains an error-prone process that can produce suboptimal results. We discuss an application of Full Waveform Inversion (FWI) to automate the refinement of legacy velocity models generated by conventional workflows. We demonstrate our solution on a full-azimuth (FAZ) survey acquired in the Gulf of Mexico using dual-sensor streamers and blended sources in the form of simultaneous long-offsets (SLO). The dual-sensor acquisition provides low-frequency data, while the SLO configuration enables the recording of long offsets in excess of 16 km; thereby providing a unique dataset to meet the traditional data dependencies for robust FWI stability. The long offsets and low frequencies were key to using both refractions and reflections to update the deeper parts of the velocity model. Our solution directly inverts the simultaneous data as acquired in the field, and is particularly relevant to the industry growth in blended seismic data acquisition. We also employed an FWI velocity gradient that eliminates the migration isochrones; thereby removing the reflectivity imprint from the model updates. The FWI application to the field survey successfully refined the geometry of the salt bodies, including the base salt and the intra-salt enclosures. It also improved the RTM image; particularly the salt flanks and the subsalt reflectors.

Salt/sediment proximity to delineate salt boundaries using seismic while drilling in the Gulf of Mexico

Between July 2018 and January 2019, Shell acquired two vertical seismic profiling (VSP) surveys in two deepwater wells in the Gulf of Mexico by using a seismic while drilling (SWD) tool. Each survey included a rig source zero-offset VSP and a boat source offset VSP. The main objective of the surveys is to delineate the salt-sediment boundary at the salt base and flank. We design and execute the complex VSP surveys with emphasis on optimization, efficiency, and integration. We develop a comprehensive analysis and processing method to integrate P-wave sediment and salt proximities with converted PS salt proximity. We use SWD-VSP surveys to demonstrate how we define the salt boundary with the integrated results. Our results show that we can delineate the salt boundary with better accuracy and with a high degree of confidence. These successful VSP surveys provide significant business and technical value.

Reverse time migration of transmitted wavefields for salt boundary imaging

The imaging of steep salt boundaries has received much attention with the advent of improved wider azimuth acquisition designs and advanced imaging techniques such as reverse time migration (RTM), for example. However, despite these advancements in capability, there are cases in which the salt boundary is either poorly illuminated or completely absent in the migrated image. To provide a solution to this problem, we have developed two RTM methods for imaging salt boundaries, which use transmitted wavefields. In the first technique, downgoing waves, typically recorded in walkaway vertical seismic profile surveys, are used to image the salt flank via the generation of aplanatic isochrones. This image can be generated in the absence of an explicit interpretation of the salt flank using dual migration velocity models, as demonstrated on a 3D walkaway field data set from the Gulf of Mexico. In the second technique, we extend the basic theory to include imaging of upgoing source wavefields, which are transmitted at the base salt from below, as acquired by a surface acquisition geometry. This technique has similarities to the prism-imaging method, yet it uses transmitted instead of reflected waves at the salt boundary. Downgoing and upgoing methods are shown to satisfactorily generate an image of the salt flank; however, transmission imaging can create artifacts if reflection arrivals are included in the migration or the acquisition geometry is limited in extent. Increased wavelet stretch is also observed due to the higher transmission coefficient. An important benefit of these methods is that transmission imaging produces an opposite depth shift to errors in the velocity model compared with imaging of reflections. When combined with conventional seismic reflection surveys, this behavior can be used to provide a constraint on the accuracy of the salt and/or subsalt velocities.

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.

Inline Delayed-shot Migration in Tilted Elliptical-cylindrical Coordinates

Riemannian wavefield extrapolation, a wave-equation imaging method for efficient one-way propagation of 3D wavefields, is extended to 3D seismic surveys. I formulate an inline delayed-shot migration procedure in 3D tilted elliptical-cylindrical (TEC) coordinate systems. When inline coordinate tilt angles are well matched to the inline source ray parameters, the TEC coordinate extension affords accurate propagation of both steep-dip and turning-wave components important for successfully imaging complex geologic structure. I show that wavefield extrapolation in TEC coordinates is no more complicated than propagation in elliptically anisotropic media, which is easily handled by current finitedifference practice. I apply this approach to a realistic 3D wide-azimuth synthetic derived from a field Gulf of Mexico data set. The resulting images demonstrate the imaging advantages made possible through 3D RWE implementations, including the improved imaging of steeply dipping salt flanks, at reduced computational cost.

Broadband Least-Squares Wave-Equation Migration

We introduce an effective iterative Least-Squares Wave-Equation Migration (LS-WEM) solution for broadband imaging. Least-Squares Migration (LSM) solutions are designed to produce images of the subsurface corrected for wavefield distortions caused by acquisition and propagation effects. They implicitly solve for the earth reflectivity by means of data residual reduction in an iterative fashion, which usually demands intensive computation. The LS-WEM is implemented using a visco-acoustic anisotropic one-way wave-equation wavefield propagator that is able to fully utilize both the broader seismic bandwidth and the high-resolution velocity information from Full Waveform Inversion (FWI). Our implementation combines the one-way extrapolator with fast linear inversion solvers into an efficient migration inversion system. Application to the 2D Sigsbee2b synthetic model improves the sub-salt illumination by balancing the image amplitudes and reducing the effects of the shadow zones, enhances temporal resolution by broadening the frequency spectrum, balances the wavenumber content and improves images of faults and dipping salt flanks. In addition, LS-WEM converges rapidly to the true solution, reducing the data residuals by 90% in only four iterations. Application to real 3D datasets from the Gulf of Mexico and the North Sea demonstrates high-resolution imaging with reduced acquisition footprint effects, improved spatial frequency content, and better structural imaging at all depths.

Prism waves for imaging steep geologic features and sediment terminations against salt flanks: Examples from the Gulf of Mexico

Subsalt imaging in the Gulf of Mexico often suffers from illumination problems where various steep geologic features are not captured with conventional imaging techniques due to the lack of a direct wave path from the source to the steep reflector and back to the receiver. The exact geometry of these invisible salt flanks or salt welds is of great interest to interpreters during salt model building and prospect mapping. This paper discusses a suite of technologies that uses prism waves for imaging geologic interfaces with reverse time migration (RTM). Prism wave RTM imaging represents an example of imaging with multiples, Due to its specific geometric setup, it is often complementary to traditional RTMs. Specifically, the prism wave reflections can illuminate steep features that are invisible otherwise. Furthermore, by utilizing prism wave RTM in “reverse,” one can specify the steep event as the input prism wave generator (often a well-constrained salt boundary) and obtain an alternative (and perhaps cleaner) image of the sediment geometries next to this input event. In the industry, a lot of resources are spent on trying to improve illumination for the geologic features of interest by acquiring additional seismic surveys. With prism wave imaging, one can instead leverage the seismic signal that has already been recorded. We believe that even with moderate prism wave illumination uplift, the interpretive value gained can be quite large. To give the reader a glimpse of what is possible, we present several real data examples highlighting the utility of prism waves in interpretation.