Report on Airborne Geophysical Surveys EL 4439 Nicholson River

Abstract

Slightly less than two thousand square kilometres within the EL 4439, Nicholson River which consisted for the most part of poorly drained, clay flat country were covered by a magnetometer/spectrometer survey flown by Austirex International in May 1986. The position of the survey is shown in Figure 1. A total of 8,118 line kilometres were flown at a line spacing of 250 metres, while the nominal terrain clearance was 70m. The aircraft was fitted with an alkali vapour magnetometer measuring total field intensity, and a four channel (Potassium, Uranium, Thorium and Total Count) gamma ray spectrometer. Further details concerning the acquisition and processing of these data are set out in Appendix A. From the large scale survey, ninety magnetic responses were detected, later reduced to twenty-five. Fourteen small areas were outlined which encompassed these responses, and these areas were flown using SPL's helicopter-borne magnetometer system. The line spacing for these surveys was 100 metres and the terrain clearance was 30 metres. See Appendix A for further details. From the helicopter data, eleven anomalies were selected which were considered to have the highest probability of having a kimberlitic source, and these were examined on the ground. The ground work consisted of profiling with a portable proton magnetometer, the taking of magnetic susceptibility samples, and loam samples. This report presents the data from these various surveys.