Cross‐well tomographic imaging of geological structures in Gulf Coast sediments

Velocity estimation technique using seismic data is often based on time/distance equations which are independent of direction, and even though we now know that many rocks are quite anisotropic, useful results have been obtained over the years from these isotropic estimates. Nevertheless, if velocities are significantly direction-dependent, then the isotropic assumption may lead to serious structural interpretation errors. Additionally, information on angle-dependence may lead to a better understanding of the lithology of the rocks under measurement. VSP and cross-well data may each lack the necessary aperture to estimate more than two velocity parameters for each wave type, and if the data straddle a symmetry axis, then these may be usefully chosen to be the direct velocities (from time-and-distance measurements along the axis) and NMO velocities (from differential time-offset measurements). These sets of two parameters define ellipses, and provide intermediate models for the variation of velocity with angle which can later be assembled and translated into estimates of the elastic moduli of the rocks under scrutiny.If the aperture of the measurements is large enough e.g. we have access to both VSP and cross-well data, we divide the procedure into two independent steps, first fitting best ellipses around one symmetry axis and then fitting another set around the orthogonal axis. These sets of four elliptical parameters are then combined into a new, double elliptical approximation. This approximation keeps the useful properties of elliptical anisotropy, in particular the simple relation between group and phase velocities which simplifies the route from the traveltimes measurements to the elastic constants of the medium.The inversion proposed in this paper is a simple extension of well-known isotropic schemes and it is conceptually identical for all wave types. Examples are shown to illustrate the application of the technique to cross-well synthetic and field P-wave data. The examples demonstrate three important points: (a) When velocity anisotropy is estimated by iterative Paper presented in two parts at the 53rd EAEG meeting, Florence, techniques such as conjugate gradients, early termination of the iterations may produce artificial anisotropy. (b) Different components of the velocity are subject to different type of artifacts because of differences in ray coverage. (c) Even though most rocks do not have elliptical dispersion relations, our elliptical schemes represent a useful intermediate step in the full characterization of the elastic properties.

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“…The model shown in Fig. 20 is very similar to the one obtained by Harris et al (1990), where ray bending has been taken into account.…”

Section: Inversionsupporting

confidence: 60%

Anisotropic Traveltime Tomography

Michelena

Muir

Harris

1993

Geophysical Prospecting

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“…Other recent studies that investigated EOR activities (Inderwiesen and Lo, 1990;Justice et al, 1990) and structural detection (Paulsson et al, 1990;Lo et al, 1990;Harris et al, 1990) have all shown the wide interest that the crosswell experiment has gained over the last few years.…”

mentioning

confidence: 99%

Full‐waveform processing and interpretation of kilohertz cross‐well seismic data

1993

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High‐frequency, cross‐well seismic data, from the Midale oil field of southeastern Saskatchewan, are analyzed for direct and reflected energy. The goal of the analysis is to produce interpretable sections to assist in enhanced oil recovery activities ([Formula: see text] injection) in this field. Direct arrivals are used for velocity information while reflected arrivals are processed into a reflection image. Raw field data show a complex assortment of wave types that includes direct compressional and shear waves and reflected shear waves. A traveltime inversion technique (layer stripping via ray tracing) is used to obtain P‐ and S‐wave interval velocities from the respective direct arrivals. The velocities from the cross‐well inversion and the sonic log are in reasonable agreement. The subsurface coverage of the cross‐well geometry is investigated; it covers zones extending from the source well to the receiver well and includes regions above and below the source/receiver depths. Upgoing and downgoing primary reflections are processed, in a manner similar to the vertical seismic profiling/common‐depth‐point (VSP/CDP) map, to construct the cross‐well images. A final section is produced by summing the individual reflection images from each receiver‐gather map. This section provides an image with evidence of strata thicknesses down to about 1 m. Synthetic seismograms are used to interpret the final sections. Correlations can be drawn between some of the events on the synthetic seismograms and the cross‐well image.

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Seismic methods for imaging physical properties of the Earth

Mavko

Lucet

Mukerji

1995 International Conference on Acoustics, Speech, and Signal Processing

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Cross‐well tomographic imaging of geological structures in Gulf Coast sediments

Cited by 4 publications

References 1 publication

Anisotropic Traveltime Tomography

Anisotropic Traveltime Tomography

Full‐waveform processing and interpretation of kilohertz cross‐well seismic data

Seismic methods for imaging physical properties of the Earth

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