MODELING OF BROAD-BAND AIRBORNE ELECTROMAGNETIC RESPONSES FROM SALINEENVIRONMENTS

The removal of vegetation for the development of nonirrigated agricult ure and the associated increase in groundwater recharge and discharge has caused significant areas of salinization of surface soil and water resources in Australia. At least three types of salt profiles are kno wn to indicate the relative magnitude of recharge. These profiles may be differentiated by their resistivity structure. Since a broadband ai rborne electromagnetic (AEM) method offers the possibility of readily obtaining resistivity soundings, modeling was carried out to investiga te the ability of a broadband AEM system to distinguish different salt profile types. Salt profile types may be represented by a four-layer resistivity model. The use of a broadband AEM system to distinguish th e relative magnitude of the resistivity of a layer of high salt accumu lation and the underlying layer forms the basis for efficiently identi fying areas of high or low recharge. Where the resistivity of the unde rlying layer is greater than that of the salt accumulation, high recha rge is indicated, and a lower resistivity of this layer implies low re charge. The response of each of the salt profile models was calculated in the frequency domain and then inverted back to a layered model. Wi th noise added to the calculated responses, the inversion results show that the depth, thickness, and resistivity of a layer of high salt ac cumulation can be resolved by AEM measurements. Furthermore, the resis tivity of this layer can be distinguished from the resistivity of the underlying layer. A high-recharge profile may therefore be differentia ted from a low-recharge profile with AEM measurements. Since the quadr ature component of the AEM response is relatively unaffected by noise caused by the primary field, the effect of using solely the quadrature component of the response was examined briefly as a second part of th e AEM modeling investigation. It is found that simultaneous inversion of the quadrature part of the spatial components measured along the li ne of flight and in a vertical direction gives results similar to thos e when both the in-phase and quadrature parts of these components are used in the inversion.