Gaining value from historical AEM data

Australia is covered with a significant number of AEM surveys, primarily acquired over the last three decades. Most of these data were collected with now obsolete systems or system variants. Unfortunately, many of these historical datasets contain unknown, uncertain or poorly measured system parameters and undocumented or poorly documented metadata such as acquisition and processing methods. With an increased emphasis being put on getting more information from AEM datasets, their quantitative use requires that they be interpreted using full non-linear inversion methods. Unfortunately, that requires information on system geometry, transmitter waveform, and other processing parameters. Where this information is lacking, uncertainties in derived models for the conductivity structure increase. In recent years, state geological surveys, and government water departments along with Geoscience Australia have been acquiring large spatially extensive AEM surveys with more modern AEM systems. Most recent of which, the AusAEM program, has been acquiring AEM over large regions of Australia. Although these large AEM surveys are typically acquired with relatively large line spacing which limit their application for fine-scale mapping, they do provide a basis for standardising historical datasets, therefore providing more accurate estimates of subsurface conductivity and extending their value and relevance. Here, we present a methodology and results from a recent AEM interpretation project in which historical AEM datasets have been employed to address groundwater and mineral exploration applications. The historical data have been standardised with respect to the more modern regional AEM data thereby extending the application of both data sets for finer scale studies.