Treading where ground geophysics can’t venture – the use of airborne geophysics to characterise remote freshwater lens systems in tropical island settings in Northern Australia.

Abstract:

Despite being the primary source of freshwater supply for indigenous communities, small island groundwater resources in the tropical north of Australia are often poorly characterised. The hydrogeology of these systems also remains generally poorly understood. In part this is linked to their remoteness, the practicalities of undertaking ground investigations in isolated areas, but also to culturally-related access issues. Extending this knowledge is critical to groundwater management in such settings, and is increasingly important where the effects of climate change and the projected future water needs have the potential to compromise the limited fresh groundwater reserve through saltwater intrusion, over pumping and pollution.

Geophysical, and in particular electrical and electromagnetic methods have been used extensively in characterising fresh water lens systems in Island settings, providing information that is less expensive and time consuming to acquire than direct sampling approaches. However, information provided by ground methods (direct or indirect) is often limited by site access in culturally sensitive areas, as is often the case in northern Australia. Airborne geophysics, most notably airborne electromagnetics (AEM) offers an efficient and effective alternative to employ in extending the conceptual hydrogeological framework for remote islands. It also circumvents the access issues. We describe results from an “island-scale” survey over Milingimbi Island in Australia’s Northern Territory, demonstrating the spatial comprehensiveness of the hydrological data acquired and the value of the data for extending relatively sparse spatial information from existing bore fields and more recent ground geophysical surveys. The helicopter EM data used, maps the extent and thickness of the fresh groundwater lens system, and defines the geometry and extent of the salt water interface around the Island. When coupled with surface NMR soundings, and available hydrological information, estimates of the available fresh water resource have been defined suggesting it is greater than previously thought. The inverted airborne geophysical data has been used to determine the chloride content of the lens and constrain the development of a calibrated steady state numerical groundwater model.