A High-Resolution, Short-Range, Directional Borehole Radar

Abstract

The U.S. Geological Survey (USGS) has recently developed a prototype directional borehole radar system designed to complement a commercial system that the USGS has used for several years for fracture mapping in crystalline rock. The new prototype is designed for higher resolution, shorter range operation. We have numerically modeled several types of antennas using finite difference time domain (FDTD) code. Based on the FDTD numerical modeling, we designed, fabricated, and physically tested a pair of cavity-backed monopole antennas. The antennas were then incorporated into a pulsed time-domain radar system. Tests of the system in a laboratory sand pack and an outdoor pit have demonstrated antenna directionality, good detection of small metal and dielectric targets, and radial resolution of a few centimeters. Such a radar system, with some modifications, might find use in utility and other obstacle avoidance in horizontal drilling operations. An important part of the development of the directional radar is data processing and visualization. We can process our data in a number of ways, including average waveform removal, filtering, and range-gain compensation. After processing, radar data can be displayed in three dimensions using a graphical user interface (GUI). Received waveforms are mapped into a cylindrical volume, where each waveform fills a cylindrical wedge. The angle subtended by each wedge depends on the total number of waveforms acquired azimuthally in the borehole at each depth. The length of the image cylinder is a function of the number of stations occupied by the radar in the borehole. The GUI allows the operator to visualize the data using a variety of tools, including: rotate, zoom, annulus view, slicing, and control of the color table into which data values are mapped.