The use of ground EM systems to accurately assess salt store and help define land management options, for salinity management

Abstract

The ability to rapidly and accurately determine the vertical and spatial distribution of soil salinity at a farm or paddock scale is extremely important for land managers and researchers involved in managing land salination. Knowledge of current (shallow) soil salinity levels is required to make informed short-term management decisions as to what pasture, crop or tree species will be the most productive, or should be grown to minimise surface soil salination. Prognosis of future (deeper) salinity development is required for longer-term management. Hence, characteristics of regolith salt store and groundwater depth are essential information. The vertical distribution of salt throughout the regolith is also very important at the regional scale and for calibrating larger-scale airborne EM mapping systems. Accurate site assessment using soil sampling and laboratory analysis is time consuming, expensive and can be highly variable because of large spatial variability compared with sample size and practical sampling density. Clearly, an accurate and efficient system of salinity mapping using portable ground-based geophysical instruments is required. The Geonics EM38 and EM31 instruments have been routinely used to map soil salinity. Recently, automated ground-based data acquisition and real-time mapping systems have been developed. In the past, most use has been made of the instruments' relative readings. However, continued calibration in variable terrain will allow for more practical and absolute use to be made of readings. The Geonics EM34 and EM39 instruments have also been used to determine salinity profiles to depth; however, usually in experimental situations. They have the potential to replace deep drilling (EM34) and drill-sample assay (EM39), and have been used to evaluate airborne EM systems. We have reviewed the use of the four Geonics instruments for salinity investigations and present over thirty calibrations with soil salinity from various terrains in southwestern Australia and compare them with calibrations from eastern Australia. We found that the instruments are reliable and the information they generate is reproducible. In all situations studied, salinity was the dominant contributor to EM response. However, soil type, moisture and temperature can have secondary contributions, which may be more important when EM is used to assess and predict plant response to salinity. We show that for practical purposes ground-based EM can be confidently used in salinity mapping.