Exploration Geophysics - Volume 13, Issue 3, 1982

Crustal thickness and average seismic velocity can be determined by fitting the spectral ratio of the vertical and horizontal radial components of teleseismic long-period seismic P-waves to the theoretical spectral ratio of feasible crustal models. The vertical and horizontal radial transfer functions are calculated by the Haskell-Thomson matrix formulation pointwise in the frequency domain, by estimating the transmission coefficient of a given layered system for each of a sequence of input frequencies at the base. The character of the spectral ratios is affected mainly by the total crustal thickness and average velocity. Knowledge of these parameters is particularly valuable in combination with complementary data from deep crustal seismic reflections at near vertical incidence. Comparison tests using data from the ASRO/HGLP installation at Charters Towers (near the Queensland continental margin, Australia) reveal that the mean crustal thickness at Charters Towers is about 40 km and the mean Moho P-wave velocity contrast is 0.8. These values are consistent with independently derived models of ‘normal’ continental crust.

Magnetic data available for use in correlation with MAGSAT data in the Australian region include observatory recordings, first-order and third-order vector regional magnetic surveys, long airborne scalar profiles, and approximately 75% coverage of the continent with detailed aeromagnetic mapping of total intensity. Reconnaissance gravity coverage is virtually complete, and deep seismic sounding refraction and reflection surveys have been carried out at widely scattered locations. There are scattered data from geothermal investigations. Magnetic and gravity coverage is also available over much of the surrounding marine area, and in selected areas in Antarctica. It is planned to use this material in a comprehensive study of the broader litho-spheric features of the area.

Fluctuations of the earth’s magnetic field with time can be a source of error of up to tens of nanotesla in magnetic surveying or prospecting. The best defence against possible difficulties such fluctuations may cause lies in understanding their origin, and their geographical uniformity or non-uniformity. The geographical uniformity of magnetic fluctuations has now been measured over several different areas of the Australian continent as part of research projects to examine natural electromagnetic induction taking place in the earth. The present paper shows examples of the results and gives some general description of magnetic fluctuations characteristics at coastlines, inland, and offshore. A major zone of non-uniform magnetic fluctuations has been detected in two places and may run north-south right across the continent.

Seismic data acquired by the Bureau of Mineral Resources (B.M.R.) for crustal studies are recorded on magnetic tape as frequency modulated (FM) signals accompanied by an amplitude modulated (AM) serial time code. A playback system is described which permits accurate recovery of seismic signal amplitudes from FM tape recordings: an advantage over AM systems which, because of the effects of many unknowns such as tape quality, require continuous calibration for absolute amplitude recovery. By using the recorded time-code carrier frequency to sense tape speed, the playback system corrects for speed changes in the original recording and compensates for variations in its own speed characteristics. The novel speed control system, combining both frequency and phase control, ensures accurate tracking, rapid response, and a wide acquisition range. An adaptable detection threshold ensures that the recovered and decoded time signal is sufficiently reliable to be used for computer controlled event searching. The data processing task is further aided by overall computer control of the playback system, and programmable digitizing modes.