Recovery of 3D IP distribution from airborne time-domain EM

Abstract

A materialsâ?? chargeability is commonly considered to be the most diagnostic physical when exploring for disseminated mineralization. It is also useful in many environmental and engineering applications. Traditionally, the presence of chargeable material is detected using the induced polarization technique. While this technique has been successfully applied in the mineral exploration industry for a number of years, its application is not always practical. Exploring on a reconnaissance scale can be limited by the time required to place the transmitter and receiver electrodes. The technique can also fail in certain geologic environments. A highly resistive overburden may make it impossible to inject enough current to excite a measurable IP response. Conventional IP is not the only technique that is sensitive to chargeable material. Any electromagnetic method applied in the presence of chargeable material will be affected. Unfortunately, the effects are often hard to recognize in the data. For the particular case of coincident loop time-domain EM data, negative transients - soundings with a reversal in sign of the received fields - are diagnostic of chargeable materials. This property can also be extended to center loop systems, including many airborne systems. Negative transients are commonly observed in airborne TEM systems, such as Fugroâ??s AeroTEM system or Geotechâ??s VTEM system. Despite the fact that negative transients can be directly related to the presence of chargeable material, relatively little has been done to try and interpret them directly. In this work, we develop a three dimensional inversion routine to recover the distribution of chargeable material from IP affected airborne time domain electromagnetic data. The technique is applied to a variety of synthetic examples in order to showcase when this approach should be expected to be successful. Finally, the technique is demonstrated on a field data set.