AEM bathymetry and conductivity estimation in very shallow hypersaline waters of the Coorong, South Australia

Abstract

The Coorong is a shallow (typically 1 - 2 m) narrow coastal lagoon extending approximately 110 km parallel to the coastline, and forms an extensive wetland area of international significance. It is divided into two lagoons, the North and South lagoons. The northern lagoon section opens into the mouth of the Murray River and the southern lagoon section is closed. During periods of extended drought where there is no flooding to flush the lagoon system, hypersalinisation gradually increases, especially in the southern lagoon section where salinity may be in excess of four times that of seawater. A helicopter time-domain EM (TEM) system was flown along the Coorong, as extensive flood waters from Queensland (2010) were reaching the North Lagoon lowering the salinity. The derived bathymetry from TEM data was shown to be in good agreement with known bathymetry in areas of high salinity. The conductivity of the saline water in the North Lagoon and underlying sediment was estimated from inversion of TEM data using the known water depth as a fixed parameter. The derived conductivity varied from ~1.6 S/m in the north of the North Lagoon to ~8 - 9 S/m at its southern end, underestimating the gradient (~0.6 to ~13 S/m respectively) observed from a sparse distribution of fixed conductivity meters located in the Coorong. These results show that AEM has the potential to remotely map shallow water depths, and water conductivity gradients using known bathymetry to monitor hypersalinisation in these wetlands where changes in the ecology have been linked to high salinity.