Maximising the lateral resolution of near-surface seismic refraction methods *

Derecke Palmer

doi:10.1071/EG08119

RESEARCH ARTICLE

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Maximising the lateral resolution of near-surface seismic refraction methods ^*

Derecke Palmer

+ Author Affiliations

- Author Affiliations

School of Biological, Earth and Environmental Sciences, The University of New South Wales, Sydney, NSW 2052, Australia. Email: d.palmer@unsw.edu.au

Exploration Geophysics 40(1) 85-98 https://doi.org/10.1071/EG08119
Submitted: 1 September 2008 Published: 27 February 2009

Abstract

The tau-p inversion algorithm is widely employed to generate starting models with most computer programs, which implement refraction tomography. This algorithm emphasises the vertical resolution of many layers, and as a result, it frequently fails to detect even large lateral variations in seismic velocities, such as the decreases which are indicative of shear zones. This study demonstrates the failure of the tau-p inversion algorithm to detect or define a major shear zone which is 50 m or 10 stations wide. Furthermore, the majority of refraction tomography programs parameterise the seismic velocities within each layer with vertical velocity gradients.

By contrast, the Generalized Reciprocal Method (GRM) inversion algorithms emphasise the lateral resolution of individual layers. This study demonstrates the successful detection and definition of the 50 m wide shear zone with the GRM inversion algorithms. The existence of the shear zone is confirmed by a 2D analysis of the head wave amplitudes and by numerous closely spaced orthogonal seismic profiles carried out as part of a later 3D refraction investigation. Furthermore, an analysis of the shot record amplitudes indicates that a reversal in the seismic velocities, rather than vertical velocity gradients, occurs in the weathered layers.

The major conclusion reached in this study is that while all seismic refraction operations should aim to provide as accurate depth estimates as is practical, those which emphasise the lateral resolution of individual layers generate more useful results for geotechnical and environmental applications. The advantages of the improved lateral resolution are obtained with 2D traverses in which the structural features can be recognised from the magnitudes of the variations in the seismic velocities. Furthermore, the spatial patterns obtained with 3D investigations facilitate the recognition of structural features such as faults which do not display any intrinsic variation or ‘signature’ in seismic velocities.

Key words: GRM, near-surface, RCS, resolution, seismic refraction, 2D, 3D.

Acknowledgments

I am indebted to Steve Hearn, Roman Pevsner, and an anonymous reviewer for many insightful comments. As usual, this paper would not have been published without the indefatigable efforts of Lindsay Thomas.

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^* ^*Presented at the 19th ASEG Geophysical Conference & Exhibition, November 2007.