INTERPRETATION OF INPUT AEM MEASUREMENTS IN AREAS OF CONDUCTIVE OVERBURDEN

Abstract

During the past six years, the INPUT system has established itself as the world’s most widely used airborne electromagnetic system. From experience with the currently available aids for interpretation it became obvious that more research was needed to understand the effects of conductor size, dip, and depth and also the effect of conductance (σt) of dikes and overburden. An extensive program of time‐domain model measurements with a new Mk VI receiver was initiated to provide the needed answers. In the modeling approach, dike‐like conductors and overburden were simulated by thin aluminum sheets, and the effect of host rock was neglected. The results obtained agree well with previous experiments which were conducted mostly in the frequency domain. Because this project surpassed others in its extent and in the variety of models, new conclusions about the nature of certain anomaly types could be made. In the case of dipping dikes under a horizontal layer of infinite extent, the anomaly shapes were found, to change considerably as a function of the conductances of the bodies involved. For certain combinations of dip, depth, and conductance the peaks on channel one changed into troughs. As the anomaly patterns obtained over horizontal strips and some dipping dikes are similar, a recognition problem may arise in this case, The resulting ambiguity in interpretation is best resolved by measuring with an additional horizontal‐coil receiver, and profiles were obtained with this configuration over a number of models. Detailed quantitative interpretation of data recorded in areas of Western Australia, which are covered by conductive overburden, was made possible by their comparison with the Atlas of INPUT Model Profiles.