Use of the EM-34 instrument in groundwater exploration in the Shepparton region

Abstract

Since the late 1970's, the Rural Water Commission of Victoria (RWC) has been offering a groundwater exploratory drilling service to landholders in the irrigation areas of the Shepparton Region. The aim of this service has been to encourage exploration for shallow aquifers in the Shepparton Formation and in turn to encourage groundwater pumping for water-table control. The selection of drilling sites in early exploration programs was based on geomorphological expressions of prior stream activity. Later, the dipole-dipole resistivity survey method was used. While giving good resolution of targets in the sedimentary profile, this method was found to be too restrictive in covering large areas of a property. In 1986, the RWC acquired a Geonics EM-34 unit to become the main geophysical tool in groundwater exploration. Since this time, the unit has been used on over 60 individual properties, as well as being used for broader scale exploration surveys on the Riverine Plains. The EM-34 unit has proved to be a useful tool in defining areas where shallow sand aquifers occur in the Upper Shepparton Formation. The RWC uses the horizontal dipole technique with a constant coil separation of 20 m. This technique has allowed good line coverage of up to 7500 m in a day, while allowing fair resolution of the target within the top 15-20 m of the Shepparton Formation. Experience has shown that a number of non-rigorous categories can be applied to the various ranges of conductivity readings. Such categories aid in the interpretation and selection of drilling targets.