Probe rotation effects on direction of arrival estimation in array‐type directional borehole radar

Abstract

This paper discusses the relationship between feeding line delay compensation and direction of arrival estimation in an array-type directional borehole radar. In this radar, since the space available for an array antenna is limited by the borehole diameter, direction of arrival estimation is based on small travel time differences among array elements, and an accurate compensation of feeding line delays is important. Computer simulation confirmed that the radar probe rotation in a borehole leads to errors in the direction of arrival estimation of greater than 15° if the delays associated with the feeding array antenna elements are not compensated to within 0.1 ns. This may be caused by a failure to measure the time delays of electrical circuits around the feeding points of the antenna elements. In this case, we suggest that the lengths of system components other than the coaxial cables should be kept to less than 3 cm. Based on these investigations, we developed an array-type directional borehole radar for a geotechnical project to locate foundation piles. In a field experiment, we confirmed that direction of arrival estimation errors were below about 15°, although the radar probe rotated through more than 180° during the measurement, thanks to correct compensation of the cables. With the correct compensation, we demonstrated three-dimensional location of a buried cylindrical conducting object, which was located at 2 m from the radar in wet soil. We were able to estimate the reflection point position with an accuracy of 34 cm, which is the averaged error of the three-dimensional location, while allowing the radar probe to rotate. tool used in such projects (Daniels 2004). Soundings made with GPR have provided valuable information on objects buried in the subsoil up to several metres below the ground surface. Due to the sounding depth limitation of GPR, borehole radar has started to be utilised at construction sites. Borehole radar is a type of GPR (Slob, Sato and Olhoeft 2010) in which frequencies between 20 MHz and 100 MHz are used for long-range detection. Most conventional borehole radar systems for single borehole application use two vertical dipole antennae, which are omnidirectional. This type of radar has been applied to the detection of buried foundation piles (Yamashita and Toshioka 2002), and it is useful in two-dimensional space measurement for the determination of the depth and the radial distance (range) of targets parallel to the measurement borehole. However, to detect and range objects such as foundation piles in three-dimensional (3D) space using a single borehole, a directional borehole radar system is needed. As a directive antenna for the directional borehole radar, a cavity-backed antenna has been used; this system mechanically rotates the antenna inside the borehole

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

“…From now on in this paper, we assume that only the receiver is a CAB, while the transmitter is an omnidirectional dipole antenna. The previous literatures [14] [12] [22] [16] on the CAB also adopted this assumption. It is to be noticed that similar conclusions may be obtained even if the transmitter is a CAB.…”

“…We estimate the dip angle  of the planar interface from the slope of the approximate lines. In this estimation, the equation (17) in Appendix C is useful.…”

“…1. Note that, if the vertical axis is the receiver position Dr instead of zr, we need to use the negative value of (17).…”

“…A linear prediction-based DOA estimator may also be employed for 3-D imaging with the CAB [16]. The directional borehole radar with CAB is now being used for commercial purposes [17].…”