Seismic refraction characteristics of the Elura ore body and regolith

Robert J Whiteley

doi:10.1071/EG07026

RESEARCH ARTICLE

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Seismic refraction characteristics of the Elura ore body and regolith

Robert J Whiteley

+ Author Affiliations

- Author Affiliations

Coffey Geotechnics Pty. Ltd., 8/12 Mars Road, Lane Cove West, NSW 2066, Australia.
Email: bob_whiteley@coffey.com

Exploration Geophysics 38(4) 242-253 https://doi.org/10.1071/EG07026
Submitted: 20 April 2007 Accepted: 4 September 2007 Published: 6 December 2007

Abstract

Major exploration difficulties occur when the distinctive characteristics of ore targets are obscured or modified by physical property variations within the regolith. Improved exploration success in the highly variable Australian regolith requires greater understanding of this medium which can only be achieved with new or improved exploration technologies.

The Cobar District of New South Wales is one of the world’s most active mineral exploration regions with a variable regolith. This region has yielded substantial mineral wealth and the Elura ore body is one of the largest massive sulphide deposits to have entered the Cobar regolith. Previous pre-mining, shallow seismic refraction data over this ore body have been re-interpreted using visual interactive ray tracing and wavepath eikonal tomography. This improved interpretation approach has been integrated with the most recent geological knowledge, weathering history and the seismic properties of the shallow ore and host rocks to refine the seismic characteristics of the Elura ore body and regolith. The interpretations have confirmed the earlier qualitative interpretation that the Elura gossan and the altered ore zone form a local, low-velocity plug extending to a depth of ~100 m within the shallower, higher velocity weathered and fresh siltstone host rocks. The margins of this plug are well defined in the refraction interpretation as they form strong seismic wave diffraction sites at the base of the surrounding regolith. The base of this plug, representing the altered massive sulphide ore, also tends to have a lower seismic velocity than the fresh host rocks. Velocity information on the deeper gossan, supergene zone could not be obtained directly from the first-arrival seismic data as this region is laterally hidden. It is clear from this interpretation that the base of the regolith over the Elura ore body and margins are highly irregular and not well represented as a single continuous refractor as required by less sophisticated refraction interpretation approaches.

This case study shows that detailed seismic refraction, supported by improved interpretation techniques and petrophysical testing, provide detailed regolith information and have increased exploration potential for massive sulphide targets that enter or are close to the regolith.

Key words: Elura ore body, Cobar, seismic refraction, regolith, VIRT, WET.

Acknowledgments

The assistance of Dr Don Emerson is gratefully acknowledged. Dr Barry McMahon kindly provided copies of the analogue seismic records from the early seismic refraction survey with unpublished reports. I thank Dr Richard Bates, an anonymous reviewer and the editor for their efforts with the manuscript.

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