DOI: 10.4225/03/5897c95185804
title: Electromagnetic mapping for crustal structure undercover in north central victoria

description: Across central Victoria the north south strike of the prospective Palaeozoic outcrop has been well documented, but to the north these rocks are obscured from traditional survey methods by the Cainozoic sedimentary cover sequence of the Murray Basin and traditional survey methods are ineffective. The new electromagnetic (EM) data presented in this thesis augment previous seismic reflection data and provide additional constraints on the geologic interpretation by further resolving the tectonic evolution of the Lachlan Fold Belt, consequently aiding in both current and future mineral exploration programs.

Comprising a total of 119 soundings, magnetotelluric (MT) data were acquired over two major field campaigns within the Western Lachlan Fold Belt – in April/May 2008 and August/September 2009. Transient Electromagnetic (TEM) data were then subsequently collected with the principal aim of providing high frequency static shift corrections at locations coincident with 105 of the MT sites. Further tangential TEM studies (originating from in field observations) were also completed, with consideration of various effects shown to induce distortion, or be otherwise detrimental to EM data quality (including cultural noise, super paramagnetisim and near surface electrical anisotropy), leading to methods by which these phenomena could be reduced or eliminated.

The phase tensor approach has also been applied to the MT data, further reducing the complications caused by galvanic distortion and thus providing enhanced insights into the dimensionality of areas that play host to complex and/or electrically distorted terrains. Subsequently, a new method is proposed for utilising the TEM roaming receiver configuration in order to ascertain the direction and magnitude of near surface electrical anisotropy, through measurement of the diffusion rate of transient signal. The theoretical developments of the techniques are discussed, along with the corresponding methodologies for data collection, processing and modelling. 

The final 2 D MT inversion models (generated using the WinGLink® software package for MT data) demonstrate common structural trends to be present between the transects, thus allowing the exposed geology to be confidently extrapolated northwards, through central Victoria and into regions where the underlying structure is hidden by the extensive Murray Basin cover. The MT models substantiate previous seismic data indicating an intrazone thrust fault system within the Bendigo Zone, with listric geometries connected in the mid crust; the Whitelaw Fault in particular, appears highly significant for future mineral prospecting.

The Heathcote Fault Zone is unambiguously defined both by its geology and geophysical response; the boundary between the Bendigo and Melbourne Zones, it provides an excellent reference for all subsequent interpretations. The electrical conductivity structures are distinct between these major subdivisions, indicating a different tectonic evolution for the Bendigo and Melbourne Zones. Results from the phase tensor analysis provide supporting evidence for an anisotropic lower crust which persists into the mantle.