Plane wave electromagnetic measurements for imaging fracture zones

Abstract: Electromagnetic fields from distant radio transmitters interact with the resistivity structures in the earth. They can be regarded as plane-wave fields since their penetration depth is much smaller than their wavelength. In this thesis, plane wave methods have been applied in a number of novel ways to map the resistivity of the bedrock and the sedimentary overburden. The main objective of the field measurements carried out was to map and characterise fault and fracture zones in several areas in northern Sweden. Two different techniques have been applied, the radiomagnetotelluric (RMT) method and the radiofrequency electromagnetic (RF-EM) method. These methods utilise frequencies in both the very low frequency band (VLF, 3-30 kHz) and the low frequency band (LF, 30-300 kHz). They are extensions of the traditional VLF method, which has been widely used for detection of fracture zones in the bedrock and for groundwater exploration. The broader frequency range used in the RMT and RF-EM methods makes it possible to resolve variations of resistivity with depth in addition to lateral variations. A recently developed 2D-inversion scheme for plane-wave excitations of the ground was applied for the interpretation of the data. The computer program was originally developed for the interpretation of low frequency magnetotelluric data. It was found that the 2D-code is a fast and convenient way for quantitative interpretations of both the RMT and the RF-EM data. The results show that detailed information concerning resistivity distribution in the ground down to several hundred meters in the bedrock can be obtained. Fracture zones in the bedrock can be identified and their resistivity, width and dip can be determined.