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

Abstract

ABSTRACTThis 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.