Geomagnetic Depth Sounding Studies in Central Australia

• Main Author: Woods, Dennis Victor
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1979
• Subjects: Aeronomy; Biology; Electromagnetics; Electromagnetism; Fourier transforms; Groundwater; Ionosphere; Magnetic fields; Mathematics; Pacific Rim Studies; Permeability; Physics; Radiation; Regional Studies; Sun; Vortices; Water Resources Management
• ISBN: 9798759902522

Two magnetometer array studies were carried out in the central regions of the Australian continent in 1976 and 1977 using the ANU set of Gough-Reitzel magnetic variometers. Because of the nature of the fluctuation fields recorded in each array, the analysis of collected data divides naturally into two separate topics: 1) the study of the deep one-dimensional electrical conductivity structure beneath Australia using smoothly grading horizontal and vertical components of the fluctuation field; and 2) the study of two- or three-dimensional conductivity structure in central Australia from laterally anomalous fluctuation fields.In the first topic, data recorded at substorm and quiet daily variation periods are analysed by the method of spatial derivatives and then collectively interpreted by forward modelling techniques to give the layered conductivity structure in the upper mantle. Daily variation fields proved most suitable for analysis in this manner, as these data display the least effect of anomalous fluctuation fields and the strongest regional (source field) gradation of the horizontal components. The analysis was initially carried out on the 1976 array data, but in addition , data from the 1971 array in south-eastern Australia were used to do a comparative analysis from another region of the continent.From these initial studies using the spatial derivatives method, it appears that the conductivity beneath south-eastern Australia is greater at a shallower depth than the conductivity beneath central Australia, corresponding to the more recent tectonism of the -Tasman orogenic belt in the southeast of the continent. The limited amount of useful data indicates that the conductivity beneath south-eastern Australia increases to greater than 0.3 Sim at a depth of approximately 200 km. Beneath central Australia a sharp increase in conductivity is indicated at a depth of 350 km from less than 0 .1 Sim to more than 1.0 Sim. A shallower rapid increase in conductivity beneath central Australia, from surface values of less than 0.01 Sim to at least 0.05 Sim, is also indicated by the central Australia data. The details of this shallower increase are poorly resolved but it must occur in the upper 100 km beneath central Australia.Anomalous fluctuation fields in central Australia are investigated using data recorded over the entire geomagnetic depth sounding band-width, from pulsations through substorms to quiet days. The frequency characteristics of these fields indicate that the anomalous fluctuation fields result from EM induction at a relatively shallow depth (i.e . in the crust), and the spatial characteristics of the data indicate that the anomalous telluric currents, which produce the anomalous magnetic fields, are flowing near the surface.To explain both the large areal extent of the anomalous fields over the 1976 and 1977 array areas at substorm periods, as well as the isolated anomalies in south-western Queensland, a combined interpretation is made which includes: 1) large- scale, two-dimensional EM induction in the Great Artesian Basin covering much of the region; and 2) localized channelling of telluric currents either within the basin sediments or into zones of high conductivity within the basement rocks.An alternative interpretation of anomalous two-dimensional induction associated with conductivity contrasts in the basement rocks is discounted partly because it is not required and partly because of the clear current channelling nature of anomaly in south-western Queensland.