The purpose of this study is to develop the method to evaluate permeability distribution in rock masses using elastic wave tomography. This method is based on Biot theory and BISQ theory which describe the dependency of elastic wave velocity on frequency. To verify the applicability of this method to rock masses, we performed laboratory and in-situ experiments. In laboratory experiments, we measured seismic velocity in several frequencies. In in-situ experiments, we performed seismic exploration using three frequencies. In this paper, we present the results of these experiments. From the laboratory experiments results, it is turned out there is the possibility that this method is applicable to various kinds of rock. And from the results of in-situ experiments, we consider that this method also has the possibility of application to the real field.
Recently, underground space came to be used for the various purposes such as underground power plant, oil stockpiling base, underground dam, geologic disposal of high-level radioactive waste, or disposal of low-level radioactive waste. On the occasion of estimation of the safety and the applicability over the purpose, It is very important to figure out permeability distribution in rock masses to know how groundwater will flow the inside of rock masses. Therefore today we Conduct various researches (for example permeability test in borehole or geophysical exploration) to know how groundwater flow through the rock masses. Each method has advantages such as borehole test can estimate permeability directly and domain of geophysical exploration is large. However each method also has disadvantages, for example domain of borehole test is rather small, and geophysical exploration cannot measure permeability directly. Accordingly we have to develop a new method for geophysical exploration that can evaluate extensive permeability distribution In rock masses, We developed the method using elastic wave as the evaluation technique of extensive permeability distribution in rock masses. Firstly the applicability of the basic theory of our method has been verified by laboratory experiments. Then we made in- Situ experiment and discussed the application of this method to a real field which consist of a granite.
The technique we develop is based on Biot theory and BISQ theory (Biot 1941, 1956, Dvorkin & Nur 1993, 1995). These theories describe the dependency of seismic velocity on frequency and relate this dependency and permeability. Biot theory describes that when elastic wave propagates in rocks saturated with water, viscous resistance acts between medium and water varies with changes in seismic frequency. When seismic frequency is low, viscous resistance acts between solid and water because water is forced to participate in a solid's motion, therefore seismic velocity becomes low. When seismic frequency is high, influence of viscous resistance to solid's motion differs, therefore seismic velocity becomes high. BISQ theory describes elastic wave dispersion by the mechanism described in Biot theory and the mechanism that when elastic wave propagates in rocks saturated with water, pore fluid is pushed from thin cracks into the surrounding large pores with pressure in these pores changing in time.