Interpretation of VLF-EM anomalies of 3D structures by using linear filtering techniques

For almost four decades now, the VLF-EM method has been used to locate different conducting structures. The interpretation is usually carried out on a profile anomaly where a possible estimation of the depth and the dip can be obtained. The maps are only used, after applying the Fraser filter, to best position the anomaly and no further quantitative interpretation is given. In this paper a linear filtering technique is developed based on the X- and Y-derivatives of the VLF-EM anomaly. This technique helps in determining the exact dimensions of the causative structures as well as their depths regardless of their conductivities. This work also shows that the known Fraser filter that is usually applied on a profile data can be adapted, for the first time, in studying quantitatively 3D structures if it is applied to raw data in the X- and Y-directions. It can now be used to estimate the dimensions of the targets.