Interpretation of ground VLF-EM data in terms of vertical conductor models

Abstract

The very-low-frequency electromagnetic (VLF-EM) method has been used extensively for the detection and delineation of weak conductors such as those formed by water and/or clay-filled fracture and shear zones in the Precambrian rocks of the Canadian Shields as part of the Canadian Nuclear Fuel Waste Management Program. However, no satisfactory method of interpreting VLF anomalies over such conductors was available when the field data became available in the early 1980s. A study was undertaken to develop a method for quantitative interpretation of ground VLF-EM data over two-dimensional (2D) sheet-like conductors of low conductance, finite depth and depth extent, embedded in a resistive host rock of finite resistivity, using characteristic curves. Numerical modelling of the VLF response of such conductors revealed that the axes-ratio (inappropriately termed ellipticity in VLF literature) variations are much smaller than the corresponding tilt angle variations, and much less sensitive to the depths of the conductors. Extensive ground VLF-EM surveys over weakly conductive fractures and shear zones at several test areas in Ontario also indicated that the axes-ratio response is not only much smaller than the corresponding tilt angle response, it is often difficult to identify in the field data. An interpretation scheme was devised to determine the conductance, depth and depth extent of such weak vertical conductors located in a host rock of finite resistivity which depends on the peak-to-peak tilt angle response and the horizontal separation between the peaks. A complete interpretation of such conductors requires knowledge of the host rock resistivity and the approximate depth, if the depth extent of the conductor is required. The interpretation method was applied to interpret a VLF anomaly over a complex conductivity structure at East Bull Lake, Ontario which illustrated the limitations of the technique. A ground VLF anomaly from Chalk River, Ontario, was interpreted using the interpretation scheme described, which agrees well with the information from geological mapping and with the numerical forward modelling response.