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A Magnetotelluric survey of the North Perth Basin: A technical case study

Original motivation of this study was to understand important structures with a proven geothermal signature associated with high temperatures in the shallower basin and high flow rates in the aquifers. Anomalous temperatures are recorded around the Beagle Ridge and significant flow rates observed near the Urella Fault, factors important to unconventional geothermal prospects. The focus of this study was therefore a detailed geophysical investigation of several Geothermal Exploration Permits (GEPs) in the North Perth Basin. Two Magnetotelluric (MT) surveys are conducted over target areas in the North Perth Basin and adjoining tectonic domains to provide information about the electrical conductivity regime of the basin and western margin of the Yilgarn Craton. Existing geophysical data in this part of the basin are sparse and electrical data for the basin in general is limited to shallow Time Domain Electromagnetic (TDEM) data targeting superficial aquifers. High-resolution MT data, acquired between 2011 and 2013, provide information on mid-deep crustal rocks. In addition, new gravity data and joint interpretation of these data sets are undertaken to improve the geological model of the North Perth Basin and test some existing hypotheses. We present a best practice case study and workflow for data acquisition and filtering, robust dimensionality analysis and removal of distortion effects from impedance tensor estimates. 1D and 2D inversions are found to be largely sufficient for the majority of these data while 3D modelling provides an additional tool to verify results. Finally, modelling of gravity data and integrated interpretation ensures robust geological models for the area are consistent with all data available. We conclude with several inferences about the geology in this area. 1) Electromagnetic (EM) and gravity data does not seem to support significant crustal thinning beneath the basin. 2) The Dandaragan Trough appears deeper than generally modelled and 3) extremely high conductivities persist to depth in the basin.