VLF-EM surveys at Chalk River, Ontario

Airborne DIGHEMII electromagnetic (EM) and airborne and ground Very Low Frequency Electromagnetic (VLF-EM) surveys were carried out at Chalk River prior to September 1979. All surveys were run in two directions perpendicular to the azimuths of the two VLF transmitters used (NAA, Cutler, Maine and NSS, Annapolis, Maryland). Both airborne and ground data show responses corresponding to the fracture sets mapped geologically. the order of increasing sensitivity of the techniques to small rock fractures is airborne DIGHEMII, airborne VLF-EM and ground VLF-EM. Comparison of azimuthal distribution of EM anomaly sets with geological mapping demonstrates that a VLF-EM survey in one direction only will not give a complete picture of fracture distribution. With two transmitters whose azimuths are roughly orthogonal, and with a survey in the corresponding two directions responses will be observed from all fractures that are electrical conductors. In the Chalk River area, overburden is generally resistive, and does not strongly influence results. In areas of thick or conductive cover, VLF-EM techniques should be used with caution