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.
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.
Redistribution of rare-earth elements during diagenesis of carbonate rocks from the Mid-Proterozoic Newland Formation, Montana, U.S.A.
Abstract REE data for carbonate rocks from the Mid-Proterozoic Newland Formation, Belt Supergroup, show variable degrees of relative REE enrichment when normalized to a composite shale of the Newland Formation. Most conspicuous are relative enrichments in LREE's and positive Eu anomalies. Possible coprecipitation of REE's in carbonates from seawater is much too small to account for the REE enrichment, thus only diagenetic REE enrichment has to be considered. By far the strongest LREE enrichment is observed in carbonates with interbedded shales. During diagenesis REE apparently moved from the shale partings to the adjacent limestone beds. Massive limestone units have considerably smaller REE enrichments (except in proximity to shales), suggesting that REE addition from formation waters is less efficient for supplying REE's than is local redistribution between shale and limestone beds. Early precipitation of diagenetic silica in pore spaces reduces REE enrichment in limestone beds by reducing their permeability. Dolostones are cemented by pervasive early-diagenetic silica and lack significant REE enrichment except for Eu, probably because silica cement prohibited access of REE-bearing pore waters. Positive Eu anomalies indicate reduction of Eu to the divalent state during diagenesis and relatively large mobility of divalent Eu.