Black Range and Vernon Hill: A comparison of two airborne EM surveys in south east Australia

In south east Australia during the first half of 1996, Rio Tinto Exploration Pty. Limited commissioned QUESTEM airborne time domain electromagnetic (AEM) surveys at Black Range in western Victoria and Vernon Hill in northern South Australia. The aim of these surveys was direct detection of conductivity anomalies: base metals in Cambrian greenstones at Black Range and diamonds in the Precambrian Yoolperlunna Inlier at Vernon Hill. Neither was successful in their primary objective, and the data from each survey are used here to investigate the value of AEM in geological mapping.

At Black Range, lacustrine deposits along the drainage features which cut through many of the major geological units. This conductive surficial material dominates the AEM survey data. Away from the streams, it is still possible to discriminate the major rock types of the area but not successfully map the boundaries between them.

In contrast, conductive surficial material at Vernon Hill was less extensive and the QUESTEM data appears to be providing a considerable amount of geological information. Although very little geological mapping has been done in the area, the close relationship between magnetic and conductive domains within the survey area and the evidence for structural boundaries in the topography which coincide with boundaries of the same conductive domains is very compelling.

The data show that, in areas where there is limited obscuration by surficial conductors, it is possible to use airborne electromagnetics to map geology. In such circumstances the AEM information is complementary to that provided by magnetics so that information from one enhances the interpretation of the other and vice versa. In this way, the use of both data sets provides more than twice as much information.