The geophysical surveys described in this report were performed during 1935-1937 by the Aerial, Geological and Geophysical Survey of Northern Australia, but the results were not fully published at that time. As mining development has shown that the Tennant Creek Field, originally considered as a goldfield only, now has possibilities as a base metal field, it was considered desirable that the leading features of the surveys and the results thereof be presented in a unified report. The report is confined mainly to geophysical aspects and refers only to the detection of magnetic bodies. Three types of anomaly were observed, and are classified as minor, major and regional. The minor anomalies appear to be of little significance, and the surveyed areas were not large enough to completely delineate the regional anomalies, which are almost certainly associated with the regional geology. The major-type anomalies are well suited to interpretation by theoretical methods. A method of interpretation based on polarization by induction has been applied to these anomalies, and testing performed to date indicates that this method shows the position of the body causing an anomaly with sufficient -accuracy for the siting of an exploratory drill hole. In general, a reasonable estimate can be made also of the extent in depth of the body. More complete evaluation of the magnetic results would 'be assisted by accurate delineation and interpretation of the regional anomalies. An airborne magnetometer survey, with this object in view, has recently been completed by the Bureau and should give valuable information on the structure and limits of the field. Illustrations are presented showing contour maps of most of the major anomalies, and, for the most important areas, observed magnetic profiles are shown alongside profiles based on theoretically determined dimensions of the bodies causing the anomalies. Four appendices give details of the mathematics involved in the method of interpretation which has been adopted.
