Consideration of Oblique Incidence in 3-D Imaging of a Planar Interface With a Circular Dipole Array in an Air-Filled Borehole

• Published in: IEEE journal of selected topics in applied earth observations and remote sensing Vol. 13; pp. 3711 - 3722
• Main Authors: Ebihara, Satoshi, Kotani, Shyuhei, Fujiwara, Kengo, Kimura, Yuta, Shimomura, Takaaki, Uchimura, Ryota
• Format: Journal Article
• Language: English
• Published: Piscataway IEEE 2020; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Algorithms; Antenna arrays; Antennas; Boreholes; Computer simulation; Critical elevation angle; Dipole antennas; dipole array antenna; Dipoles; Direction-of-arrival estimation; directional borehole radar; Errors; Imaging techniques; Mathematical models; oblique incidence; plane wave; Production methods; Radar; Radar antennas; Radar arrays; Radar imaging; Reflected waves; Rocks; Zirconium
• ISSN: 1939-1404; 2151-1535
• DOI: 10.1109/JSTARS.2020.3004479

We consider the effects of oblique incidence of a wave on the creation of a 3-D image of a planar interface with an array-type directional borehole radar. In this study, we focus on the situation where the radar probe is close to the planar interface. In such circumstances, the reflected wave from the planar interface may be incident on the receiving array antenna at very steep elevation angles. As a result, borehole effects cause differences in the arrival times of the wave at the array elements, and consequently some errors emerge in imaging the planar interface. Observing the arrival time differences, we present an algorithm to compensate those errors in creating a 3-D image of a planar interface. Computer simulations predict that the errors may occur when the circular dipole array antenna is in an air-filled borehole in rock. Numerical simulations show that our proposed algorithm generates a 3-D image of an interface around an exact position, whereas conventional methods produce some spurious images opposite to the correct position (i.e., out of position by 180°). We then applied the proposed method to analyze reflected waves from a real-world fault in rock. A 3-D image of the fault could be successfully created, which was not possible using the conventional method.