Depth to basement calculation in Southern Thomson, Queensland

Abstract

Depth to crystalline basement is a major consideration when conducting minerals exploration. The thickness of sedimentary cover concealing mineralised basement rock is a primary factor in determining whether economic greenfields exploration is viable. A good knowledge of depth to basement is therefore critical for tenement selection and drill hole targeting. New airborne magnetic data released in 2012 allows the calculation of a depth to basement surface in the southern Thomson area adjacent to the Queensland - New South Wales border in south Queensland. Available drill hole data shows basement depths from about 100m to over 3km in the study area but is too sparse to create a reliable surface. A combination of different automatic depth to basement techniques, including Euler deconvolution and Naudy, were used. These techniques were preferred due to a regionally extensive high frequency, low amplitude magnetic signature attributed to shallow sources in one of the cover sequences in the study region. This signature created difficulties for depth to basement calculations which involved the use of higher order derivatives. Geosoft's Located Euler calculation was preferred in areas where remanence was evident as it uses the analytical signal grid. This technique was used in combination with standard Euler. Naudy depths were also compared to the Euler solutions, particularly in areas where the drill holes indicated the basement was relatively shallow (where smoothing associated gridding can cause overestimation of source depths). Seismic data available in the north of the study area was interpreted and used as a secondary (along with the drill hole data) quality control check on the basement surface. The calculated depth to basement surface was used to create voxet model for further analysis using gravity and magnetic inversion. The final depth to basement surface defines an area of shallow basement in the south-west of the study area.