The geophysical study of Drybones kimberlite using 3D Time Domain EM Inversion and 3D ZTEM inversion algorithms

Abstract

Two airborne EM surveys were conducted over the Drybones kimberlite, NWT, Canada. A VTEM helicopter time domain EM survey was flown by Geotech in 2005, followed by a ZTEM helicopter tipper EM survey flown in 2009. The data sets collected over the kimberlite were inverted and interpreted first by Geotech, Ltd using a 1D TDEM conductivity-depth transform for VTEM data and a 2D inversion for ZTEM data. The data sets were transferred to UBC-GIF for 3D inversions and comparative analysis. Both the VTEM and ZTEM data have now been inverted in 3D and the results compared with the drill data over the kimberlite and with older results of previous geophysical interpretation. The 2D and 3D ZTEM inversions show the same general structure but the 3D inversion has greater electrical conductivity contrast and may have fewer artefacts in zones of low sensitivity. On a large scale the conductivity structure, recovered from the 3D inversion of time domain data, is comparable to that from the CDI transform, but the 3D result shows a more complicated distribution of the conductive properties. The 3D magnetic inversion had not been done previously and it shows susceptible material to depths of approximately 400 m, which is consistent with the known geometry of the kimberlite.