Fractures and rock properties estimated by 3D directional borehole radar
Borehole radar is widely used in civil engineering work such as fault and fracture exploration, salt dome investigation, tunnel detection, UXO detection, exploring locations of foundation piles and sheet metal piling walls, and so on. Borehole radar surveys in resistive rock field have been applied successfully due to the lower attenuation of the rock media. Through those studies, many researchers have become interested in the detectability of thin open fractures and their locations. Though there are some numerical computing studies [1] on its detectability related to the open widths of fractures, there are very few experimental field studies. Thus studies using directional borehole radar system are essentially required at borehole sites where geometrical features and open widths of the fractures are well investigated. We are conducting experimental evaluation surveys for our 3D directional borehole radar system, “ReflexTracker®”. In soft ground in an alluvial plain, a single-hole measurement was carried out to evaluate the detection ability of a conductive cylinder buried 2m from the survey hole and found that the system could detect 3D reflectors representing the target and provided information on the direction of arrival wave and distance to the target [2]. We conducted another experimental measurement to evaluate fracture detectability of the system in rock field. The experimental borehole (BOA-200), a vertical borehole with diameter of 86-115 mm reaching 200 m depth in andesite rock, is located in a rock quarry in Ishikoshi Town, Miyagi Prefecture, Japan. Fractures in the borehole and their geometrical features such as strikes, dips and open widths were investigated in detail by borehole scanner [3], and the electrical characteristics were also obtained by a logging survey for all depths. The radargrams obtained by the non-directional antenna indicate that the arrival times and amplitudes were clearly related to rock mass classification and fracture densities, i.e., the higher the density, the larger the amplitude and the earlier the arrival time. Moreover, interpreted relative permittivities were negatively correlated with resistivities. On the other hand, the interpreted 3D reflectors derived from the directional antenna resulted in significantly good fitting with some of the known fractures' extended planes. Through the experimental study in the rock field, the ReflexTracker shows high potential for applications in fracture extractions and rock property investigations.
A Compact Nonsymmetric Wideband Dipole Antenna for Borehole Radar Application
Borehole radar (BHR) is one of the most important types of ground-penetrating radar and has very strict requirements for antenna. Considering the detection resolution and the limited borehole size, the BHR antenna should be both wideband and compact. In this article, a compact nonsymmetric wideband dipole (NSWD) antenna is proposed. One arm of the NSWD antenna consists of several patches with different lengths, and the other one is a hollow metal tube. The combination of patches which are resistively loaded at the ends is used for increasing antenna bandwidth and frequency reconstruction. In addition, a three-section binomial matching transformer is designed for impedances match between antenna ( $120~\Omega$ ) and coaxial cable ( $50~\Omega$ ). Matching transformer and other radar system devices are housed in the hollow metal tube, which makes the BHR system much compact. The central frequency of the antenna is set to 86 MHz for long-range detection. The proposed antenna is fabricated and measured. The measurement results show that the proposed antenna has the advantages of wide bandwidth (~41.8% from 68 to 104 MHz) and high radiation efficiency (~65%). Finally, the antenna (including electronics) is assembled into a polyvinyl chloride pipe to form a BHR prototype, and we used it to conduct a field test in a water-filled borehole. The experimental results show that the NSWD antenna can work well with the BHR system inside borehole.
Archaeological Survey of Dangoyama Kofun by 3DGPR
We presented a three-dimensional ground penetrating radar (3DGPR) system for the archeological investigation in a harsh site, where we have to deal with many trees and/or great topographic variations. This system consists of a commercial GPR system with a laser positioning system. With this system, we can move the GPR antenna freely, on which a positioning detector can record accurate position for each GPR data trace. We applied our 3DGPR system at Dangoyama Kofun, a burial mound, in Fukushima prefecture, Japan. A lot of trees and bushes covered the survey area and cannot be cut. We acquired six datasets in two days in this area. We successfully imaged the boundary between the original surface of the burial mound and the covering soil clearly at about 0.3 m depth around the mound. Furthermore, we found some strong reflections appeared at 0.8 m depth in the top area of the mound, which can be from the artificial parts of the burial mound. The results would greatly aid the following archaeological excavation. In addition, we compared the non-interpolated depth slice with interpolated depth slice at same position and demonstrated that the interpolation process on threedimensional GPR data may introduce some artifacts which might mislead our interpretation.