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Abstract

Physical modelling experiments have been conducted to study how the degree of anisotropic influence on spatial resolution varies with the orientation of the symmetry axis in transversely isotropic materials. Blocks of Phenolite laminated plastic comprising sheets of paper set in phenolic resin were used to simulate media such as shales having vertical and horizontal axis of symmetries respectively. Cylindrical holes of various sizes were drilled into the bottoms of both models. The hole sizes ranged from 0.83 to 1.17 and from 0.86 to 1.71 times the expected Fresnel-zone diameters in the horizontally layered and vertically fractured Phenolites respectively. These holes while serving as reflectors simulate a range of geological features, e.g. pinnacle reefs, intrusions, seals, and pipes etc.

A three-dimensional physical modelling facility was used in collecting the experimental data. The spatial extents of the reflector boundaries are estimated by marking the half-amplitude points of the horizontal reflector events with respect to the amplitude at the centre of the reflectors. The degree of horizontal resolution is determined by comparing the seismically estimated reflector dimension with the true spatial dimension of the reflector. The resolving potentials of pressure and shear body waves in both media were compared using the collected data. Results obtained indicate that anisotropy can impose varying effects on the spatial resolving power of seismic waves. The degree of these effects depends on the curvature of the wavefront which changes with the orientation of the symmetry axis. Essentially, the lateral extent of discontinuities will be better imaged or resolved in a vertically fractured medium than in a similar medium that is horizontally layered.

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