Fractal scaling of Airborne EM data: an indicator of lithology and fluid connection?

Abstract

Electrically conducting fluids within the subsurface account for a large portion of the Earth’s electrical response and therefore mapping of groundwater and regions of high salinity can be undertaken using Airborne Electromagnetic (AEM) methods. While the majority of nearsurface conduction is generally due to fluids in porous or fractured media, there is always an ambiguity in determining the cause of the electrical response. For example, clay layers and mineralization may also significantly enhance conduction. Analysis of how the Earth’s resistivity varies with spatial-wavelength through determining the associated fractal dimension is proposed as an additional tool that can assist in differentiating anomalies of similar magnitude but dissimilar sources.

AEM, using the TEMPEST and VTEM systems, has recently been collected by Geoscience Australia through the Onshore Energy Security Program over the Pine Creek region in Northern Territory. Approximately 21,000 line-km of TEMPEST and 9000 line-km of VTEM were collected. A moving window is passed across each line of data creating small overlapping subsets on which the fractal dimension is calculated. These values are then gridded to create a map of the fractal dimension of the Pine Creek AEM survey data. Such maps are compared with existing geological and hydrological information allowing for inferences to be made on the differing sources of anomalism.