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.
Analysis of fractures detectability by borehole radar
Fracture is an important geologic phenomenon which is crucial for petroleum and geothermal exploration and exists in various scale and geometry in nature. Borehole radar is an important tool which can image single fractures several to tens meters away from borehole in thousands of meters depth. However, the detectability of various fractures is not clear. We analyzed the radar response to a thin fracture using plane wave theory, and found the primary reflection and multiple reflections cancel each other. The fact increase the difficulty of fracture detection. We use sub-cell FDTD technique to synthesize borehole radar response to fractures from 0.0005m to 0.02m wide, and filled with water or air. It is found that water-filled vertical fracture is easier to be detected than air-filled fracture, and the fracture width affect the reflected signal very much. The wider the fracture, the strong the reflected signals. Also, large dynamic range is required for weaker fracture signals detection. This kind of simulation is helpful for the fracture detection and evaluation.
Design of a Wideband Antenna for a Narrow Borehole Radar Using FDTD Modeling
For the design of a narrow wideband antenna (with diameter less than 3 cm) to serve in a borehole radar in the band [0.5,1.5] GHz for moisture content measurements, we have considered five different geometries. These geometries have been modeled using the finite-difference time-domain approach. The frequency- and time-domain responses have been analyzed in detail in the transmission configuration in the presence of a dielectric soil. We present radargrams for the multiple offset gather configuration. This study will help to define the best antenna geometry for the present application.