Andrew Fitzpatrick

Exploration for kimberlites through a complex regolith cover — A case study in the application of AEM in the deeply weathered Archaean Yilgarn Craton, Western Australia.

Regolith electrical structures associated with a small kimberlite dyke located in the Yilgarn Craton, Western Australia are examined. This study was concerned with determining The kimberlite is hosted by a deeply weathered granitic host. The saprolite associated with the granite and the kimberlites are conductive as defined from downhole geophysical logging. The kimberlite shows as a conductor in ground EM (SIROTEM and GEM-2 systems) surrounded by a resistive host. It also shows a small conductive response in 56Khz DIGHEM data but is not apparent in lower frequency data or in SALTMAP.a fixed wing time domain airborne EM system. The geometry of the latter suggests that any response of the kimberlite is likely to be lost in a coupling of transmitter with conductive wedges of granitic saprolite to the north and south of the blow. Stitched sections of 1D LEI’s for SIROTEM, DIGHEM and SALTMAP show similar trends and suggest that the regolith electrical structure is effectively modelled as a layered earth, with some exceptions. An AEM “response map” in this area is essentially a map of the variation of regolith thickness and conductivity.

Practical 3D Inversion of AEM Data for Environmental Applications in Complex Regolith Settings

This paper presents the application of an innovative approach to 3D Inversion of airborne electromagnetic (AEM) data which renders the routine Inversion of entire surveys realistic from the computational perspective. the method uses a moving footprint, and is based on the 3D integral equation method for computing data and sensitivities, and the re-weighted regularized conjugate gradient method for minimizing the objective functional. Specifically, we examine results from several well constrained AEM surveys that target the geometry, distribution and water quality associated with palaeochannel aquifers which are developed in contrasting, variably complex regolith settings. the TEMPEST and SkyTEM surveys targeted anastomozing coarse sands and gravels beds in a sandy clay background. the surveys also targeted the abruptly changing and varying basement (due to changes in lithology and structure) which defines the base of the alluvial aquifer and/or the weathered regolith. Results from the 3D Inversions are compared against drill hole geological and other geophysical data. the integrated Interpretations are able to define the location and groundwater salinity within the palaeochannels. As a result of the surveys and drilling, local farm water and irrigation systems are being developed.

Magnetic modelling and geological modelling come together at the Kintyre uranium deposit

We present a case study whereby unconstrained magnetic modelling accurately defined the altered host lithology of the Kintyre Uranium deposits, verified by detailed geological modelling. The Kintyre Uranium deposits are hosted by a sequence of iron and carbonate rich meta-pelites, which makes it an ideal target for magnetic prospecting. As part of the resource definition, magnetic modelling and geological modelling were performed over the Kintyre deposit independently. In the process of further refinement of the magnetic model through incorporation of geological constraints it was determined the two models were already highly complementary and further modelling was not warranted, particularly at the resolution of the magnetic data. This case study demonstrates that in some geological environments, unconstrained geophysical models can adequately map stratigraphy & structure for drillhole target generation.

REGIONAL, RECONNAISSANCE SCALE AEM SURVEYS TO BETTER DEFINE SURFACE WATER - GROUNDWATER PROCESSES BENEATH LARGE UNREGULATED RIVER SYSTEMS.

The groundwater resources in northern Australia offer the greatest potential for future irrigation development and are a potential source for water supply to other regions of Australia. The Fitzroy River with its associated alluvial aquifer in northern Western Australia has been considered in planning for Perth’s future water supply and for agricultural development but its potential needs to be informed by detailed understanding of groundwater-surface water interactions occurring along its extent, and in particular must consider and account for the consequences that might arise when extracting groundwater from shallow and deep aquifers linked to this river system. While the Fitzroy is one of Australia’s largest unregulated rivers characterised by braiding channels within a wide floodplain, knowledge concerning the extent and significance of its floodplain storage is limited. This paper considers results from the analysis and interpretation of a regional scale longitudinal transect (~274 line kms) of SkyTEM helicopter EM data to help elucidate river-bed processes occurring along its extent. The AEM data were acquired to provide an indication of the variation in groundwater quality and related aquifer characteristics associated with different parts of the river. Conductivity-depth sections derived from their inversion using a 1D spatially constrained inversion (SCI), were examined against available hydrochemical, environmental tracer (including 222 Rn and Cl-), and hydrogeological data sampled longitudinally. They provided further insight into the Fitzroy’s alluvial aquifer system and its links with the underlying Canning Basin sediments. The results demonstrate the value of regional, reconnaissance scale AEM surveys to better define groundwater processes beneath large unregulated river systems.

Continuity And Current Mapping - New Laboratory Techniques For the Study of the Electrical Properties of Sulphide Rocks

Electrical petrophysical measurements of sulphide mineralisation (galvanic resistivity, inductive conductivity and induced polarisation) are commonly conducted on drill core samples to provide information for quantitative interpretation of exploration survey data. These measurements provide a general view of the properties of an orebody but give little information about the influence of ore textures and mineralogical variations on current flow. Two new mesoscopic laboratory techniques have been developed as part of an on-going study of scale dependent electrical properties of sulphide rocks (Roach, Lewis and Jablonski 1997). These methods specifically address the relationship between ore textures and electrical parameters.