Small-diameter directional borehole radar system with 3D sensing capability

This paper describes a new directional borehole radar system and its field testing. The system uses a thin radar probe (57 mm diameter) and a circular dipole array directive antenna. The radar is of the step frequency type with a network analyzer. Through careful antenna design, we were able to achieve the compact radar probe and precise measurement at frequencies between 5 and 500 MHz. All the associated surface electronics for the radar system can be fit into a small carrying case. The radar probe includes a triaxial accelerometer, a triaxial compass, an angular velocity sensor and a thermometer. Data from these sensors can be used to compensate for the rotation and inclination of the radar probe, and this enables us to locate reflection points in 3-D space correctly. All the data acquired by the radar probe were sent to the processing electronics via an optical link, and the data was updated in real time. Our field testing confirmed that system accuracy for determining arrival directions was better than 10 degrees between 30 and 180 MHz in wet soil. We demonstrated 3-D location of a buried cylindrical conducting object, which was set 2 m from the radar in wet soil. After system calibration and signal processing, we were able to estimate the reflection point position with an accuracy of 41 cm.