David Marchant

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.

We present a new survey methodology to map the distribution of chargeable material in the subsurface using inductive electromagnetic sources and observations of the magnetic fields in the frequency domain. An accompanying inversion algorithm is developed, and the technique is tested on synthetic data.

Induced polarization (IP) surveys are commonly performed to map the distribution of electrical chargeability that is a diagnostic physical property in mineral exploration and in many environmental problems. Although these surveys have been successful in the past, the galvanic sources required for traditional IP and magnetic IP (MIP) surveys prevent them from being applied in some geological settings. We develop a new methodology for processing frequency domain EM data to identify the presence of IP effects in observations of the magnetic fields arising from an inductive source. The method makes use of the asymptotic behaviour of the secondary magnetic fields at low frequency. A new quantity, referred to as the ISIP datum, is defined so that it equals zero at low frequencies for any frequency-independent (non-chargeable) conductivity distribution. Thus, any non-zero response in the ISIP data indicates the presence of chargeable material. Numerical simulations demonstrate that the method can be applied even in complicated geological situations. A 3-D inversion algorithm is developed to recover the chargeability from the ISIP data and the inversion is demonstrated on synthetic examples.