Arrival Times of Plane Waves Obliquely Incident on a Dipole Array Antenna in a Borehole

摘要

In this article, we investigated the arrival times of plane waves incident on a circular dipole array in a borehole (CAB). Our theoretical and experimental findings suggest that the elevation angle of the received wave strongly affects plane wave arrival times in a borehole installation. If a plane wave is incident at the critical elevation angle (CEA), there is little arrival time difference among the signals at the CAB. On the other hand, when the plane wave is incident at an elevation angle steeper than the CEA, the wave arrival order at the CAB becomes opposite to the normal incidence case. In order to clarify the reason why these phenomena occur, we decompose a transmission matrix, which represents the effect of the borehole, into three components. We found that the azimuthal component of the electric field contributes to the observed phenomena significantly, and two of the three decomposed components cancel each other out at the CEA. For verification of the phenomena in a cross-hole borehole radar measurement, we performed a numerical simulation and carried out experimental verification at a field test site. Both data sets confirmed that the arrival time differences among the signals at the CAB were minimized at the elevation angle predicted to be the CEA. In addition, we found the reverse of the incident wave's arrival order at the CAB.