Detection of underground target using ultra-wideband borehole radar and SAR image formation

The U.S. Army Research Laboratory (ARL) has recently evaluated a commercially available borehole radar to detect targets hidden underground. The goal of the experiment is to demonstrate the feasibility of the borehole radar coupled with ARL's signal and image processing techniques to penetrate various geophysical media for target detection. In this paper, we briefly describe the commercial ultra-wideband borehole radar used in the experiment. A conventional technique to provide the attenuation and velocity maps of the underground area between two holes is called tomography. It requires separate probes for the transmitter and the receiver for the measurement, and generally is more time consuming and laborious. Another technique known as reflection is also widely used. In this mode, the transmitter and receiver travel together as one single unit in one hole to measure the reflection data from surrounding clutter and underground targets. Although this mode is much simpler to operate than tomography, the resulting image has inferior resolution in the cross-range (depth) direction. In our experiment we employ this reflection mode, where a small cylindrical metal target is placed in one hole while the radar (both transmitter and receiver) travels in another hole to measure the target return. To improve the poor cross-range resolution associated with the reflection raw data image, we apply the backprojection image formation algorithm that is commonly used in synthetic aperture radar to form high resolution 2D images. We present the resulting images of background (without target) and with target, and show that the underground target can be easily detected using change detection technique. This paper also compares the measured data with the electromagnetic model prediction of the same target.