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Introduction

Conventional seismic reflection surveys are largely constrained to sources detonated on the surface of the earth. Correspondingly, most of the advanced imaging techniques of proven value have been developed to migrate these types of data sets. One subset of sources generally excluded from the purview of seismic imaging is those originating at depth, including both active (e.g. VSP) and passive (e.g. micro-fracture) generated energy. The predominant reason for their omission is because of a lower applicability during the exploration stage. This is largely due to longer acquisition times, lower average source S/N ratios, the need for three-component data, and the irregularity of source distributions. However, the generality of wave-equation methods allows for the adaptation of exploration-oriented imaging techniques to novel non-exploration geometries. This fact provides the motivation for identifying a number of candidate experiments where buried sources may be used to image the subsurface using adapted imaging techniques.

Shot-profile migration shows an impressive adaptivity to acquisition geometries incorporating subsurface sources. One of this method's strengths is that reconstructions of source wavefields originating in the subsurface easily honor the geometry of individual experiments. These wavefields are used, in turn, to image components of both the forward- and back-scattered wavefields. This is accommodated by a proper selection of the sign of the exponential used to propagate the wavefields. Further, the ability to choose different velocity models by which to propagate each wavefield adds the ability to image with converted energy modes.

We propose the application of seismic imaging methods to a suite of problems centered on the utilization of energy originating from buried sources. Geometries potentially suitable for imaging include both natural and induced fracture point sources, teleseismic plane waves, downhole VSP, similar to Harwijanto et al. (1987), and borehole tomography. All of these applications require a minor amount of pre-processing of the scattering wavefields contained in recorded data, the construction of appropriate source wavefields, and the generation of appropriate compressional and shear velocity models for use in the shot-profile migration algorithm.

To these ends, we present an overview of the processing flow, and point of some of the difficulties likely to be encountered when imaging with buried sources. After a general discussion of the methodology, we then apply the processing flow to the case of teleseismic imaging for a demonstration and to provide a proof of concept.


next up previous print clean
Next: Methodology Up: Shragge and Artman: Imaging Previous: Shragge and Artman: Imaging
Stanford Exploration Project
7/8/2003