Loop Array Antenna in a Borehole for Directivity to a Horizontally Polarized Wave

• Published in: IEEE transactions on geoscience and remote sensing Vol. 60; pp. 1 - 10
• Main Authors: Ebihara, Satoshi, Matsumoto, Masayuki
• Format: Journal Article
• Language: English
• Published: New York IEEE 2022; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Algorithms; Antenna arrays; Antenna measurements; Antennas; Azimuth; Boreholes; Dipole antennas; Direction of arrival; Direction of arrival (DOA); directional borehole radar; Directive antennas; Directivity; Electromagnetic fields; Elevation angle; field experiment; Field tests; Horizontal polarization; horizontally polarized wave; Loop antennas; loop array antenna; Method of moments; method of moments (MoM); Mutual coupling; Plane waves; Radar; Radar antennas; Radar measurements
• ISSN: 0196-2892; 1558-0644
• DOI: 10.1109/TGRS.2022.3177444

We propose a directive antenna for borehole radar using a horizontally polarized wave in a vertical borehole. This antenna is an array of several horizontal loop antennas, arranged vertically. When a plane wave is incident on the antenna, the differences in arrival times at the array elements enable us to estimate the direction of arrival (DOA). We present a simple model representing the arrival time difference and propose an algorithm for DOA estimation. Furthermore, we synthesized the array signal by analyzing the electromagnetic field using the method of moments (MoM) and applied the proposed algorithm to the signal. We found that mutual coupling between antenna elements affects the DOA estimation and that a space of about 3 cm between antenna elements prevents mutual coupling. Using the MoM analysis, we simulated a cross-hole measurement to demonstrate the antenna's ability. We carried out field experiments in wet soil to examine the antenna's ability. The proposed loop array antenna in a water-filled borehole received a direct wave from a source in another borehole. After applying the proposed algorithm to the measured data, we found that we could estimate the DOAs to the source with an error of less than 15° in the azimuth angle and 10° in the elevation angle.