Robert J. Whiteley

Velocity inversion and the shallow seismic refraction method

Abstract Velocity inversion in the subsurface is one of the most serious limitations of the shallow seismic refraction method. Inversion can occur whenever a geological layer has a lower velocity than that of the overlying layer and is more common than generally believed. Unrecognised inversion layers can create considerable errors in depth interpretation. The magnitude of these errors is examined and theoretical equations for a single velocity inversion in a multilayered earth are presented. In certain situations inversion layers can be identified and incorporated in a modified interpretational procedure using these equations. The methods for recognising velocity inversions are reviewed. A field example from a high-way investigation in Australia is also discussed. It is concluded that a combination of drilling and seismic refraction using both shallow shots and shots within the low velocity layer can, to some extent, reduce errors associated with velocity inversions. If conventional seismic refraction alone is used to solve the shallow velocity inversion problem then more sophisticated field and processing procedures are required to assist reliable identification of later events on refraction records.

Comments on: Palmer, D., 2010. Is visual interactive ray trace an efficacious strategy for refraction inversion? Exploration Geophysics, 41, 260-267 *

Palmer’s paper addresses visual interactive ray tracing (VIRT), specifically referring to Whiteley (2004) and Whiteley and Leung (undated). In these papers, field examples of seismic refraction data from Welcome Reef and Mt Bulga were re-interpreted using VIRTandwavepatheikonaltraveltime(WET)Tomography.These different approaches to near surface refraction interpretation assume very different earth models, from the discrete or ‘blocky’ modelsforVIRTtothecontinuousvelocitymodelsforWET.This enhanced flexibility permits more realistic seismic modelling in near-surface materials, spanning soils, weathered rock and fresh rocks. My comments and discussions address many of the technical deficiencies, inconsistencies, unsubstantiated statements and claims in Palmer’s paper. They are directed at improving the rigour with which near-surface refraction interpretations are undertaken and the quality of these interpretations. In the title, Palmer poses an incompletely defined question. It is not clear actually what ‘visual interactive ray trace’ is,

Seismic refraction characteristics of the Elura ore body and regolith

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.