Crustal imaging of prospective geothermal basins using magnetotellurics: a case study of the Renmark Trough in South Australia

Abstract

The presence of anomalously high geothermal gradients in South Australia has led to increased geothermal exploration and an associated demand for cost effective methods to explore the subsurface of a region before drilling. This case study is aimed at delineating the basement structure of prospective geothermal basins, in particular the Renmark Trough tenement held by Petratherm in South Australia, using magnetotelluric (MT) and geomagnetic depth sounding (GDS) data. The effectiveness of the MT data has been compared to both gravity and seismic data in the area and 1D resistivity models have been used to estimate the temperature gradient range in the area. Broadband and long-period MT data were obtained in 2009 5 km northwest of Renmark, South Australia, along two, two-dimensional profiles perpendicular to the Hamley Fault and across the Renmark Trough. The MT data fit all two-dimensional smooth inversions produced to a root-mean square (rms) misfit of 2.2 or less. The broadband resistivity model shows a higher resistivity basement (40 -100 Ωm) below low resistivity (0.3 -12 Ωm) sediments in a half graben structure of 4 km depth, apparently shallowing to 3-3.5 km and forming a graben structure 10 km north along the lower resolution long-period profile. Comparison between MT, gravity and seismic data revealed an additional low permeability structure in the MT model between the highly conductive sediments and seismically constrained basement. Magnetotelluric resistivity data identified basement depth at 4km in a seismically complex area where seismic data are unable to image due to the presence of high angled faults. Due to a lack of constraints in the Renmark area, rough temperature gradient calculations have resulted in large ranges for the gradient in the deepest section of the trough, with 75-110 ºC/km for a mainly sand lithology (Cooltong 1 drillhole) or 50-75 ºC/km for a mainly silt lithology (North Renmark 1 drillhole). With additional information from relogging existent well bores and drilling new wells, the temperature calculations could be more tightly constrained.