Directional borehole radar system design and antenna calibration for accurate direction estimation

Mankind has been curious to “see” beneath the earth’s surface for a variety of reasons; be it for mining exploration, geological/archaeological research, in search of suitable locations for oil/gas reservoir or nuclear waste disposal. Borehole radar (a special form of Ground Penetrating Radar - GPR) is one of the successful technologies for such purposes. Conventional borehole radar systems employ ordinary dipole as a receiving and transmitting antenna. Therefore, full 3D mapping requires expensive and time consuming tomographic measurements using several boreholes. Hence, there is great research interest in a more convenient and cost-efficient single-hole directional borehole radar. The radar system requires a directional antenna to determine the azimuthal angle at which reflected electromagnetic waves reach the antenna. Among the few types of directional antenna, Adcock direction finding antenna and orthogonal loop antenna are most commonly used. The radiation pattern of an antenna is distorted by the loading effect of the surrounding medium. For example, an Adcock antenna’s radiation pattern is deformed when placed in an off-center position inside a borehole. This eventually leads to a significant error in the estimated angle of arrival of reflected waves. Similar effects are also observed when the borehole immediate surrounding has inhomogeneous composition. This thesis introduces a novel antenna calibration solution that can be used to correct the effects of antenna eccentricity and borehole medium inhomogeneity. The calibration method requires the characterization of the directional antenna through in-situ antenna port measurements, which are later used to remove the undesired effects from the measured radar signals. An analytical analysis of antenna transfer function that was done to identify the most important calibration parameters is also presented. The thesis starts by giving a summary of the underlying radar and electromagnetic theories as well as the principles of directional antenna. Following that the special design requirements and challenges of a borehole radar system are presented. Next, the theoretical concept of antenna calibration, the proposed calibration method and its implementation in hardware are discussed. Finally, simulation and experimental results, which validate the proposed calibration solution, are presented. At maximum antenna eccentricity, the estimated azimuthal angle had mean error of more than 60°. After calibration this error is reduced to less than 2°. A second, more practically oriented, innovative task dealt within this thesis is the implementation of cable-less synchronization between the radar receiver and transmitter. This increases probe reliability by avoiding the fault-prone optical cable connection between the transmitter and receiver. Moreover, it allows the construction of a more robust, well modularized probe which can withstand the extreme conditions in deep mining grounds.