Joining the dots: How airborne geophysics helps constrain hydrogeological models

Abstract

Hydrogeological models rely on accurate conceptualisations of groundwater flow in the sub-surface. For this we require an accurate interpretation of the sub-surface, regolith, architecture and definition of preferential lines, and obstacles to, movement of water. Traditionally, this information is acquired through judicious use of groundwater bores, combined with expert knowledge and assumptions based on the understanding of the regions hydrogeology. Flow nets are created from water level data and flow parameters determined from point determinations through pump tests. Increasingly, geophysics is being used to help define the sub-surface architecture, identify preferential flow lines and constrain the extents of groundwater models. In particular, airborne geophysics (AG) can provide a contiguous image of subsurface features, defined by the technology being used. Thus, airborne magnetics can define pre-existing, buried river channels from the relict iron oxides on some river gravels; airborne electromagnetics (AEM) can define the preferential flow-lines from the higher conductivity of water saturated sediments. Field mapping and careful calibration of signals is imperative, though this is often an iterative process requiring additional information from new bore holes or cross-comparisons with other technologies. Examples of where AG technologies have greatly aided the development of groundwater models will be shown from regions in South Australia. Both simple (FLOWTUBE), and complex (MODFLOW), models have been enhanced by using AG data.