Micropulsation and induction array study near Ballarat, Victoria

Abstract

Airborne magnetometers now have resolutions of typically less than 0.1 nT, and the location of sub-1 nT crustal anomalies is becoming increasingly important, particularly for the delineation of weak intrasedimentary magnetic sources in prospective petroleum provinces. Natural time-variations of the geomagnetic field, with periods ranging from fractions of a second to diurnal harmonics of several hours, are a source of noise for the accurate delineation of such crustal anomalies. Time variations of period greater than a few minutes are usually successfully removed by the tie-line system employed for aeromagnetic surveys (e.g. Reeves, this volume; Campbell et al., this volume). Hence, micropulsations of the geomagnetic field, which have periods of less than a few minutes and typical amplitudes of a few tens of nT at most, are important likely noise sources in high-resolution aeromagnetic surveys. The total-field spatial characteristics of micropulsations across Australia are relatively unknown. With such short periods, micropulsations will induce currents in local subsurface conducting bodies, and also in surface waters. These induced currents will produce their own secondary fields, which sum with the inducing fields to produce a complex spatial variation. Highly susceptible rocks will also affect the characteristics of pulsations. The Australian Geological Survey Organisation (AGSO) is currently investigating the morphology of such total-field micropulsations, with the ultimate aim of their accurate removal from total-field aeromagnetic records. During March 1992, in a collaborative project between AGSO and The Flinders University of South Australia (FUSA), a series of FUSA ring-core fluxgate magnetometers, with a resolution of 0.1 nT, were deployed in an array across Ordovician shales, south of Ballarat. Also deployed were a Narod ring-core magnetometer and a Helium-vapour total-field instrument. The shales were chosen as they have only a small aeromagnetic signature. Long trains of Pc4 micropulsations were observed, with a period of 60 s, and peak-to-peak amplitudes up to 3 nT. The micropulsation activity was found to be spatially uniform across the array, which spanned only a small area of width 40 km.