Towards AEM bathymetry and conductivity estimation in very shallow hypersaline waters of the Coorong, South Australia*
Julian VrbancichDefence Science and Technology Organisation (DSTO), 13 Garden Street, Eveleigh, NSW 2015, Australia.
Email: julian.vrbancich@dsto.defence.gov.au
Exploration Geophysics 44(2) 63-69 https://doi.org/10.1071/EG12051
Submitted: 16 August 2012 Accepted: 3 January 2013 Published: 29 January 2013
Abstract
The Coorong is a shallow (typically 1.5 m) narrow coastal lagoon extending ~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 essentially closed, being connected to the North Lagoon via a choke point. 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 fair agreement with known bathymetry in areas of high salinity. The conductivities of waters ranging from saline to hypersaline in the North Lagoon and upper half of the South 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 ~8–10 S/m at its southern end and in the South Lagoon. These values underestimate the known strong salinity gradient (~0.6 to ~13 S/m respectively) observed from a sparse distribution of fixed conductivity meters located in the Coorong. The application of AEM in this region is challenging because of the very large range of water conductivities and because the average water depths are comparable to the typical residuals between known depths and depths derived from AEM data in previous studies in Australian coastal waters. These results do however show that AEM has the potential to remotely map shallow water depths, and water conductivity gradients using known bathymetry to monitor hypersalinisation in these significant wetland areas where changes in the ecology have been linked to high salinity.
Keywords: AEM, airborne electromagnetic bathymetry, Coorong, hypersalinity.
References
Brookes, J. D., Lamontagne, S., Aldridge, K. T., Benger, S., Bissett, A., Bucater, L., Cheshire, A. C., Cook, P. L. M., Deegan, B. M., Dittmann, S., Fairweather, P. G., Fernandes, M. B., Ford, P. W., Geddes, M. C., Gillanders, B. M., Grigg, N. J., Haese, R. R., Krull, E., Langley, R. A., Lester, R. E., Loo, M., Munro, A. R., Noell, C. J., Nayar, S., Paton, D. C., Revill, A. T., Rogers, D. J., Rolston, A., Sharma, S. K., Short, D. A., Tanner, J. E., Webster, I. T., Wellman, N. R., and Ye, Q., 2009, An ecosystem assessment framework to guide management of the Coorong. Final Report of the CLLAMMecology Research Cluster, July 2009: CSIRO Water for a Healthy Country National Research Flagship, Canberra. Available at http://www.clw.csiro.au/publications/waterforahealthycountry/cllamm/CLLAMM-Final-Report-Ecosystem-Assessment.pdfJackson, P. D., Taylor-Smith, D., and Stanford, P. N., 1978, Resistivity-porosity-particle shape relationships for marine sands: Geophysics, 43, 1250–1268
| Resistivity-porosity-particle shape relationships for marine sands:Crossref | GoogleScholarGoogle Scholar |
Munday, T., Fitzpatrick, A., and Berens, V., 2008, Lower Lakes, South Australia: results from a pilot study using helicopter EM data to define surface water-groundwater interactions: CSIRO Technical Report No. P2008/2376, October 2008.
Reid, J. E., and Vrbancich, J., 2004, A comparison of inductive-limit footprints of airborne electromagnetic configurations: Geophysics, 69, 1229–1239
| A comparison of inductive-limit footprints of airborne electromagnetic configurations:Crossref | GoogleScholarGoogle Scholar |
Vrbancich, J, 2011, Airborne electromagnetic bathymetry investigations in Port Lincoln, South Australia: comparison with an equivalent floating transient electromagnetic system: Exploration Geophysics, 42, 167–175
| Airborne electromagnetic bathymetry investigations in Port Lincoln, South Australia: comparison with an equivalent floating transient electromagnetic system:Crossref | GoogleScholarGoogle Scholar |
Vrbancich, J., 2012, Airborne electromagnetic bathymetry and estimation of bedrock topography in Broken Bay, Australia: Geophysics, 77, WB3–WB17
| Airborne electromagnetic bathymetry and estimation of bedrock topography in Broken Bay, Australia:Crossref | GoogleScholarGoogle Scholar |
Vrbancich, J., and Fullagar, P. K., 2007, Improved seawater depth determination using corrected helicopter time-domain electromagnetic data: Geophysical Prospecting, 55, 407–420
| Improved seawater depth determination using corrected helicopter time-domain electromagnetic data:Crossref | GoogleScholarGoogle Scholar |
Vrbancich, J., Whiteley, R. J., Caffi, P., and Emerson, D. W., 2011, Marine seismic profiling and shallow marine sand resistivity investigations in Broken Bay, NSW, Australia: Exploration Geophysics, 42, 227–238
| Marine seismic profiling and shallow marine sand resistivity investigations in Broken Bay, NSW, Australia:Crossref | GoogleScholarGoogle Scholar |
Webster, I. T., 2010, The hydrodynamics and salinity regime of a coastal lagoon – The Coorong, Australia – Seasonal to multi-decadal timescales: Estuarine, Coastal and Shelf Science, 90, 264–274
| The hydrodynamics and salinity regime of a coastal lagoon – The Coorong, Australia – Seasonal to multi-decadal timescales:Crossref | GoogleScholarGoogle Scholar |