Removal of the host rock response from time domain electromagnetic data by transforming to, and filtering in the frequency domain

Abstract

The time domain electromagnetic method is mostly used for the mapping of confined conductors. When interpreting the data obtained from such a survey it is necessary to determine and subtract the response due to the host rock. or overburden from the survey data. This is done in order to make a correct interpretation of the anomaly responses in the residual signal. In this thesis an overview of the existing host rock removal methods is given where after an alternative frequency domain filtering method is discussed and advocated. At first the filtering method is applied on theoretical combinations of host rock and plate responses in order to determine the applicability limits. Thereafter two case histories are studied with special emphasis on the comparison between the traditional and the filtering host rock removal methods. It was found that the speed of data reduction was greatly increased by use of the filter method. Furthermore, the decay constants calculated from results obtained by the filtering method correlates well with the corresponding decay constants calculated from traditional method results. It was also found that the isolated anomalies tend to be of a higher frequency than the corresponding anomalies obtained by means of the traditiona1 host rock response removal methods. However, the shape with regard to the side lobes, and the position of the anomalies are correct. Removal of the host rock response from time domain electromagnetic data by transforming to, and filtering in the frequency domain