Choosing an AEM system to look for Kimberlites — A modelling study

Abstract

Will time domain or frequency domain AEM systems be more effective when looking for kimberlites? In this study, a 3D integral equation program was used to contrast the responses of a kimberlite pipe with a weathered clay cap in a resistive host environment for several types of AEM systems. Three towed bird time-domain systems were simulated, an idealised Geotem 25 Hz, and Geotem 75 Hz system (step and impulse responses) plus a Tempest 25 Hz pure step system. For the type of model studied, on-time B and sub-millisecond off-time dB/dt gave the best responses. The horizontal component response was much more informative than the vertical component. The response of a generic helicopter EM system with five frequencies, ranging from 500 Hz to 50000 Hz was also simulated. Both coplanar and coaxial configurations were used with transmitter-receiver offsets of 6 and 8 m. A horizontal dipole coplanar wingtip system was also modelled using the same frequencies. When measuring the horizontal component, the three systems gave exactly the same percent anomaly response. PPM (parts per million) responses were identical in shape but differed in magnitude by a factor of the separation distance cubed. The horizontal component had more shape information than did the vertical component. For the model studied, maximum discrimination above background occurred between 5000 to 10000 Hz for the quadrature component. The in-phase anomalous response increased monotonically with frequency. The kimberlite targets used in this study were defined to be poor conductors and therefore time-domain definition required accurate measurements either during on-times or at sub-ms off-times. Thus helicopter-borne frequency-domain EM systems offered the best practical choice for this type of target. A model component analysis showed that although the weathered clay cap was the main part of the EM response, the unweathered kimberlite could be detected in the absence of the cap. The presence of a kimberlite pipe underneath the cap showed a broadened response compared with the response of the cap alone.