Improved near surface mapping in groundwater studies:
Application of fast sampling time domain EM surveying methods

There are a limited number of methods currently in use to gather information on the various hydrogeological/environmental problems that are part of "everyday" life. Traditionally, groundwater problems have been evaluated and then monitored using a carefully designed network of wells where water depth and quality are measured on a regular basis. In recent years, some of the various mining and petroleum oriented geophysical techniques have been modified from their deeper applications to sample at shallower depths. Often the goal of these surveys is to help geologists and engineers determine whether their assumptions on well location and water flow are correct. Techniques that have been used include shallow seismics, DC resistivity, ground penetrating radar, frequency domain electromagnetics (FDEM), and time domain electromagnetics (TEM).

Recent advances in sampling speed, circuitry speed, and data recording have allowed the development of TEM techniques where data can be taken faster (and therefore start closer to the surface), and with better resolution of the top 15-50 m. These techniques include the Zonge Engineering NanoTEM system and the fast sampling modifications to the SIROTEM-3 system.

This paper briefly summarises the TEM results from three separate study areas encompassing a range of hydrogeological and environmental problems, each of some immediate importance in Australia at this time. The first study, at the Stockyard Plain Disposal Basin (SPDB) near Waikerie, South Australia, examines the changing hydrological environment around a groundwater disposal basin in the Murray-Darling system. The second study, in the Willaura Catchment in Victoria, examines water mobility in an evolving dryland salinity system. The third study, at an abandoned mine site in New South Wales, attempts to delineate the extent of acid-mine drainage in the area around the mine.