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.
Use of a look-ahead geophysical method to delineate water-bearing features ahead of a tunnel boring machine, The
In the field of underground construction and tunneling, comprehensive geologic investigations are both time-intensive and expensive, and can be logistically challenging. As such, it is common to encounter unexpected ground conditions while tunneling. To reduce the inherent uncertainty in underground construction operations, the use of a geophysical look-ahead system is proposed. This system will aid in the development of a model of ground conditions ahead of the excavation face at the Rondout West-Branch Bypass Tunnel. By performing both reflection and tomographic borehole ground penetrating radar surveys, discrete features such as cracks and voids, as well as bulk features such as fractured and saturated regions can be identified. Identifying potential water bearing features allows for the application of appropriate pre-excavation grouting procedures, reducing the likelihood of tunnel support degradation and annulus grout washout and helping to preserve the project’s critical path timeline. In the Fall of 2017, proof-of-concept radar surveys were conducted from the face of the Rondout West-Branch Bypass starter tunnel. Over the Summer of 2018, numerous geophysical look-ahead surveys were conducted through holes drilled ahead of the tunnel boring machine. A possible correlation between radar data as well as probe drill behavior and tunnel boring machine performance is explored, while additional ground truth observations confirm the efficacy of the geophysical method in question.
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.