Exploration Geophysics - Volume 28, Issue 4, 1997

A shear-wave (S-wave) splitting analysis has been carried out on multioffset, shallow VSP data from the Gordonstone coal prospect in the Bowen Basin, Australia. In comparison to previous petroleum-scale investigations, the analysis is complicated by the short time window available for shear-wave analysis, and interference from preceding compressional-wave (P-wave) energy.

A simple preprocessing scheme incorporates highcut filtering, as well as a two-stage coordinate orientation procedure, designed to enhance the shear wavefield for the subsequent splitting analysis. Several single-source algorithms for shear-wave splitting analysis are considered, with the primary aim being determination of the polarisation azimuth of the fast split shear wave. The results from two boreholes, whose shots were at large offsets, show good consistency between the different algorithms, and from shot to shot. Results from the third borehole studied are considered less reliable, especially for those shots having smallest offsets.

The preferred fast shear-wave polarisation azimuth is 060°. This compares well with independent estimates of the direction of maximum horizontal compressive stress in the Bowen Basin. In general, the slow shear wavefield is poorly determined. However, hodogram analysis on some traces permits estimation of the time delay between fast and slow waves. These observations are consistent with a variation of approximately 2% between the fast and slow shear-wave velocities, indicating moderate development of azimuthal anisotropy.

Tomographic estimation of near-surface velocity anomalies has direct application in prestack depth migration. Seismic velocity information extracted from traveltime measurements often suggests unrealistic geological settings. The linear system constructed by ray tracing is typically very large, sparse and poorly conditioned. In most cases it is too large to invert using a generalised inversion algorithm. Instead, a least squares method to solve the system is employed.

Limited angular ray coverage traveltime measures using three synthetic reflection data sets have been shown to allow reliable velocity image reconstruction of anomalous zones that vary up to 50 per cent from the background.

Tomographic imaging using four summation expansion methods, namely, the algebraic reconstruction techniques (ART), and the simultaneous iterative reconstruction technique (SIRT), a conjugate gradient (CG) method and a singular value decomposition (SVD) method, were used to image the synthetic data sets, and the result of each of these tomographic algorithms was evaluated based on their rms misfit and residual, solution stability and sensitively to data noise and algorithmic operation time.

Modifications to the algorithms that stabilise the inversion are shown to improve the velocity images obtained and enhance solution convergence rate. These modifications include the application of damping parameters, weighting schemes, smoothing filters including average and median, implemented both pre- and post-solution as well as convolutional quelling. The effect of noisy traveltime data on tomographic images, and useful methods for dealing with this, are also studied.

Principal directions for an azimuthally anisotropic medium are the directions along which the quasi P and quasi S waves propagate as pure P and S modes. For azimuthal anisotropy, induced by a single set of oriented vertical fractures, two of these principal directions are the directions parallel to and perpendicular to the fractures.

To investigate if conventional P-wave data could be used in fracture detection, we computed synthetic data for an isotropic Taylor shale over a fractured Austin chalk model. We found that the modelled P-wave reflection amplitude variation with offset has different slopes along different directions. We also found that the reflection amplitude at fixed offsets is periodic in 2ϕ, where ϕ is the orientation angle of the shooting direction with respect to one of the principal directions. For fracture induced anisotropy, this principal direction corresponds to the direction parallel to or perpendicular to the fractures. We use this periodic azimuthal dependence to obtain fracture orientation and a qualitative measure of the fracture density from the azimuthal P-wave data.

We applied our technique to real P-wave data, collected over a wide source-to-receiver azimuth. Computations using our method gave an orientation of the principal direction consistent with the general fracture orientation in the area as inferred from other geological and geophysical evidence.