Improved Mapping Of Conductive Clays And Groundwater Salinity Using Attitude-Corrected Helicopter-Borne Em

The interpretation of airborne electromagnetic (AEM) data has always assumed that the systems maintain a fixed geometry and attitude relative to the earth during survey. However, recent testing has demonstrated that the systems commonly experience significant vertical and horizontal offsets and rotations during flight. Failure to take account of variations in the observed conductivity induced by these movements have limited the potential for AEM systems to accurately resolve shallow conductive features that are often the target in environmental applications.<br>We present some initial results on the effects of variations in EM bird attitude on the observed response from the RESOLVE FDHEM system, in a recent study conducted in South Australia. Bird attitude was measured using three-antenna GPS array mounted on a boom at the front of the bird during survey. The intent of the survey was to define the thickness and extent of near surface, conductive clays as part of a saline water disposal management strategy being developed by the South Australian State Government. These clays have a significant influence on the rates of groundwater recharge in the study area, and the accurate definition of clay thickness was required as an input into a groundwater recharge model, which in turn was used to determine salt flux to the Murray River system from planned and extant disposal basins in the area. It has been shown that correction for normal changes in bird attitude in relatively smooth terrain<br>does create a small but measurable change to the HEM data amplitudes, but virtually no measurable change to the apparent resistivity. However, when precision results are desired for inversion of the data, where the amplitude of the response is the most sensitive parameter to the depth to target layers, the geometric correction may be necessary.