Development of Precise, Continuous Monitoring System of Sound Velocity In Situ

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A precise, continuous monitoring of sound velocity is one of the useful tool for estimating stability and shielding effect of the rock surrounding the underground structure. When radio-active species and other environment-pollutants are kept in such an under ground structure, we have to monitor surrounding rocks with ex- tremely careful attentions. The main aim of this study is to establish a system for measuring sound velocity of the rocks in situ with a resolution of ppm. When the measurement path has to be longer than the size of the structure, we have to measure the sound velocity along the path more than a hundred meters. We have to be also careful to select transmitter, because we have to contrnue to measure very long time. The transmitting wave have to be free from damaging the rocks. As the amplitude of the wave, particularly with high frequency decreases exponen- tially with propagation, we have to measure very Low signals. In order to eliminate noises an averaging method was employed. This method requires very high reproducibility of the transmitting wave. A laminated piezo-electric transducers was used for transmitter in this study, because of low power and of high reprodu- cibility. In an underground drift of the Kamaishi Mine the sound velocity is measured continuously with half hour intervals now. The maximum measurement path was 126 m. The best resolution of the sound velocity was estimated as about ppm. A periodical change of the sound velocity with an interval of about 12.4 hours was found due to the Earth tide. The maximum variation of the sound velocity during 12.4 hours was a couple of ppms. Laboratory measurement of the pressure sensitivity of the rock suggests that such a small change of a couple of ppms corresponds to the small stress change of about 3 hectopascals, which agrees well with the order of the Earth tide.