Suppressing Near-Receiver Scattered Waves from Seismic Land Data

When upcoming body waves travel through a heterogeneous near-surface region, the continuity of the wavefront can be diminished by scattering. We discuss a multichannel method to predict and subtract near-receiver scattered waves, such that the continuity and trace-to-trace coherency of wavefronts increases. We apply this method to a part from a field-data set which was acquired in an area with significant near-surface scattering. We show that the method increases trace-to-trace coherency in a reflection event. Moreover, application of our method improves the results obtained from application of a dip filter only, because we remove parts of the scattered wave with apparent velocities that are typically passed by the pass-zone of the dip filter.Shell GameChangerDutch Technology Foundation (STW)Shell International Exploration and Production, Inc

A Short Note on Modeling Wave Propagation in Media with Multiple Sets of Fractures

Wave propagation and scattering in fractured formations have been modeled with finite-difference programs and the use of equivalent anisotropic media description of discrete fractures. This type of fracture description allows a decomposition of the compliance matrix into two parts: one accounts for the background medium and another accounts for the fractures. The compliance for the fractures themselves can be a sum of compliances of various fracture sets with arbitrary orientations. Non-orthorgonality of the fractures, however, complicates the compliance matrix. At the moment, we can model an orthorhombic medium (9 independent elastic constants) with the two orthogonal fracture sets. However, if the fractures are non-orthogonal, this results in more general anisotropy (monoclinic) for which we need to specify 11 independent parameters.. Theoretical formulation shows that the finite difference program can be extended to simulate wave propagation in monoclinic media with little additional computational and storage cost.United States. Dept. of Energy (Award No. DE-FC26-02NT15346)Massachusetts Institute of Technology. Earth Resources Laborator

Finite Difference Modeling of Seismic Responses to Intersecting Fracture Sets

Fractured reservoir characterization is becoming increasingly important for the petroleum industry. Currentmethods for this task are developed based on effectivemedia theory, which assumes the cracks or fractures in a reservoir are much smaller than the seismic wavelength. A discrete fracturemodel has to be used for large-scale fractures. We describe an approach of using a finite difference method for modeling seismic wave propagation in rock formations with intersecting fracture sets. We then use the code to study the behavior of seismic waves, particularly scattering due to such fracture sets with various spacing and compliances. The scattering pattern due to fractures varies azimuthally. We find that converted PS and PSP waves from the bottom of the fractured layers show strong interference by the scattered waves. We observe coherent scattered waves in shot gathers parallel to the fracture orientation and significant backscattering at near offsets and forward scattering at far offsets in the gathers normal to the fracture orientation. When two sets of fractures are present, scattering becomes stronger and more complex scattered waves appear in the gathers. The scattering becomes stronger with increasing the fracture compliances and decreasing spacing (still on the order of seismic wave length). When the fracture sets are not orthogonal to each other, the gathers still show coherent scattering in the fracture orientations. Azimuthal characteristics of the scattered waves may be used to analyze fracture orientations, spacing, and relative compliance of intersecting fracture sets.Shell GameChangerMassachusetts Institute of Technology. Earth Resources Laborator

Computation of 3D Frequency-Domain Waveform Kernals for c(x,y,z) Media

Seismic tomography, as typically practiced on both the exploration, crustal, and global scales, considers only the arrival times of selected sets of phases and relies primarily on WKBJ theory during inversion. Since the mid 1980’s, researchers have explored, largely on a theoretical level, the possibility of inverting the entire seismic record. Due to the ongoing advances in CPU performance, full waveform inversion is finally becoming feasible on select problems with promising results emerging from frequency-domain methods. However, frequency-domain techniques using sparse direct solvers are currently constrained by memory limitations in 3D where they exhibit a O(n4) worst-case bound on memory usage. We sidestep this limitation by using a hybrid approach, calculating frequency domain Green’s functions for the scalar wave equation by driving a high-order, time-domain, finite-difference (FDTD) code to steady state using a periodic source. The frequency-domain response is extracted using the phase sensitive detection (PSD) method recently developed by Nihei and Li (2006). The resulting algorithm has an O(n3) memory footprint and is amenable to parallelization in the space, shot, or frequency domains. We demonstrate this approach by generating waveform inversion kernels for fully c(x,y,z) models. Our test examples include a realistic VSP experiment using the geometry and velocity models obtained from a site in Western Wyoming, and a deep crustal reflection/refraction profile based on the LARSE II geometry and the SCEC community velocity model. We believe that our 3D solutions to the scalar Helmholtz equation, for models with upwards of 100 million degrees of freedom, are the largest examples documented in the open geophysical literature. Such results suggest that iterative 3D waveform inversion is an achievable goal in the near future.Shell GameChangerMassachusetts Institute of Technology. Earth Resources Laborator

A Novel Application of Time Reversed Acoustics: Salt Dome Flank Imaging Using Walkaway VSP surveys

GEOPHYSICS, VOL. 71, NO. 2 (MARCH-APRIL 2006); P. A7–A11, 4 FIGS. 10.1190/1.2187711In this paper we present initial results of applying Time-Reversed Acoustics (TRA) technology to saltdome flank, seismic imaging. We created a set of synthetic traces representing a multilevel, walkaway VSP for a model composed of a simplified Gulf of Mexico vertical-velocity gradient and an embedded salt dome. We first applied the concepts of TRA to the synthetic traces to create a set of redatummed traces without having to perform velocity analysis, moveout corrections, or complicated processing. Each redatummed trace approximates the output of a zero-offset, downhole source and receiver pair. To produce the final salt-dome flank image, we then applied conventional, poststack, depth migration to the zero-offset section. Our results show a very good image of the salt when compared to an image derived using data from a downhole, zero-offset source and receiver pairs. The simplicity of our TRA implementation provides a virtually automated method to estimate a zero-offset, seismic section as if it had been collected from the reference frame of the borehole containing the VSP survey.Massachusetts Institute of Technology. Earth Resources LaboratoryUnited States. Air Force Research Laboratory (Contract F19628-03-C-0126)Shell Gamechange

A Novel Application of Time Reversed Acoustics: Salt Dome Flank Imaging Using Walk Away VSP Surveys

In the past few years, there has been considerable research and interest in a topic known by various names, such as Time Reverse Acoustics (TRA), Time Reverse Mirrors (TRM), and Time Reverse Cavities (TRC), which exploits reciprocity and the time symmetric property of the wave equation. Very little of this work has been directed at the seismic exploration imaging problem. In fact, most of the work has had application in sonar, medical and non-destructive testing applications. Here we present some initial results of applying this technology to the seismic imaging of a salt dome flank. We create a set of synthetic traces representing a multi-level, walk away VSP for a model composed of a simplified Gulf of Mexico vertical velocity gradient and an embedded overhanging salt dome. To process these data, we first apply the concepts of TRA to the synthetic traces. This creates a set of stacked traces without having to perform any velocity analysis or complicated processing. Each of these stacked traces is equivalent to the output of a spatially coincident, or zero offset, down hole source and receiver pair. Thus we have the equivalent of a zero offset seismic section as if it were collected from down hole sources and receivers. After having applied the TRA concepts, we then apply conventional post stack depth migration to this zero offset section to produce the final image of the salt dome flank. Our results show a very good image of the salt. In fact, the image created is nearly identical to an image actually using data from down hole, zero offset source and receiver pairs. The simplicity of the TRA implementation provides a virtually automated method to create a stacked section as if it had been collected from the reference frame of the borehole containing the VSP survey.Massachusetts Institute of Technology. Earth Resources Laborator

Imaging Salt Dome Flank and Dipping Sediments Using Time Reversed Acoustics

In this paper we define the theory and basic principles to move (redatum) the surface shots from a walk away VSP to be as if they had been located in the borehole. We will refer to this theory using several of the terms used in the literature including Time Reverse Acoustics (TRA), Seismic Interferometry (SI) and Virtual Source (VS) technology. Regardless of the name, the theory is built upon reciprocity and the time symmetry of the wave equation. We apply these TRA principles, together with prestack depth migration, to produce images of a modeled salt dome flank. We create a set of synthetic traces representing a multi-level, walk away VSP for a model composed of a simplified Gulf of Mexico vertical velocity gradient and an embedded overhanging salt dome. The sediment reflectors in the model dip up towards the salt dome flank. The energy from the surface shots is bent into turning rays by the linear v(z) gradient which illuminate the steeply dipping sediments and overhanging salt edges. The illuminating energy is reflected and scattered from these surfaces and then captured by the downhole VSP receivers. To simplify the processing of these data, we move (redatum) the surface shots into the borehole using our TRA or seismic interferometry principles. This removes from the seismic traces the entire, potentially complicated, path from the surface shot location to the borehole without having to perform any velocity analyses or moveout corrections. Each of these new (redatummed) traces mimics the output of a down-hole source and down-hole receiver pair. We apply prestack depth migration to these new traces to produce the final image of the beds and the salt dome flank which agree very well with the original model structure.United States. Air Force Research Laboratory (Contract F19628-03-C-0126)Massachusetts Institute of Technology. Earth Resources Laborator