The authors used resistivity tomography, a kind of geotomography, to investigate ground water flow in rock. Ground water flow through rock often is complex because it is affected by faults and fractures. The authors used resistivity tomography with a low resistivity salt tracer to detect and estimate faults and fractures, which show up as thin sheets. Prior to field experimentation, the authors conducted numerical experiments to examine the characteristics of a reconstruction algorithm and evaluate its applicability to rock investigations. These experiments indicated the successful reconstruction of the model structure and the possibility to detect the resistivity change caused by injection of a salt tracer. Next, the authors applied these techniques to a rock investigation at a dam site. It had been known that the site contains three faults that would affect the flow of ground water. The electrodes used in measurement were placed in a vertical borehole, a horizontal adit and on the ground surface in such a way as to surround the objective area. Resistivity tomography was conducted three times, once before injection of the salt tracer, one day and six days after the injection. During this period, the conductivity of ground water was also measured in the adit. The results show a complex resistivity distribution, and provide images of a fracture zone. The resistivity changes caused by the salt tracer clearly reveal the motion of the tracer. The tomographic images of the area agreed with previously available geological infomation. These numerical and field experiments showed that resistivity tomography is effective not only for the investigation of structures in rock, but also for estimating the ground water flow when used with a salt tracer.
The detailed map of the groundwater discharge on a tunnel wall must be a useful information to evaluate the groundwater flow in rock mass. A technique of the evaporation measurement that was newly developed by the present authors seems to be an effective tool to accurately measure the small value of the discharge of water coming from a low permeable rock mass. The vapor flux that is mainly transported by the molecular diffusion in the laminar flow layer formed just near the wall is measured by using two couples of humidity and temperature sensors. The applicability of the technique and some characteristic features of the vapor flow in the vicinity of the wall, i. e., the thickness of the laminar layer and the vapor flow from a high permeable fracture are theoretically as well as experimentally studied. It is found that the technique is highly practical.
For environmental reasons, it became virtually difficult to investigate the underground structure in urbanized areas using explosive sources. Therefore, an easy and covenient survey method is now required. An array observation of microtremors may be one possible procedure to answer this demand, because it can be made anywhere, even in urban areas and much easier than the other exploration methods. From the above consideration, the observation of short period microtremors were carried out using a cross array of seismometers near an urban area in Honjo city, Akita prefecture, on August, in 1987. Their phase velocities in the frequency range 1 Hz to 4 Hz were determined by the frequency-wavenumber spectral analysis (F-K method) of the data of an array. The observed phase velocities were compared with the theoretical dispersion curves computed for a six layered model obtained by PS loggings. As a result, it is shown that the F-K method can be successfully applied to short period microtremors. This result suggests that the utilization of short period microtremors is one of practical methods for estimating a shallow velocity structure in urbanized areas.
A Knowledge of the mechanical properties of weak rocks such as shale, sandy shale and coal is essential for ground control in undeground coal mining. Most tests which are usually used to determine the strength of rock require the specimen to be in the precisely prescribed shape. However, with weak rocks it is often difficult, or practically impossible in some cases, to prepare the specimen as required by these tests. From this point of view, rod penetration test has been carried out, because in this method an unprepared specimen can be used. The results of this study are summarized as follows: 1. The size of specimen should be at least 10 times the rod diameter. 2. Size effect of rod on penetration strength Srp is caused by difference in fracture mechanism of rock around the top the penetrated rod. 3. Penetration modulus Erp is not dependent on the rod size. 4. Uniaxial strength Sc and tangent Young's modulus at 50% of compressive strength E50 can be estimated with penetration strength Srp and penetration modulus Erp as follows; Sc=0.11·Srp, E50=2.3·Erp. 5. It is possible to estimate the strength reduction with water quantitatively by this test.