An Assessment of ``In-Stream'+D47 Survey Techniques along the Murray River, Australia

Abstract

A number of tools have been developed to help understand the processes of salinisation at work along the Murray River in Southern Australia. Four techniques that have been used to help investigators either directly measure the salt load entering the river, or to image the distribution of conductivities under the river are examined here. They include Run-of-River surveys (ROR), in-stream towed NanoTEM, in-stream towed Resistivity, and Helicopter EM (specifically using the RESOLVE FDHEM system). Each technique has strengths and weaknesses related to its mode of operation and the approach adopted in field data collection. Run-of-River samples the water salinity directly and then attempts to estimate river salt load and source location. It provides a direct measure of the salt entering the river but a) only provides salt load information and b) generally only provides information on a kilometre scale. The other three techniques are all geophysical techniques that do not directly inform the investigator about salt loads in the river, but provide information about conductivity distributions in the sediments under the river, which then may be related to salt loads. Each of the geophysical techniques sample the instream environment at three to 20 metre intervals, and provide information from near the river surface to depths of between 10 and 40 metres below the surface. Data may be displayed as depth sections, or as contoured depth slices prepared to examine different levels beneath the river bottom.