Development of a Giant Magnetostrictive Seismic Source for Constant Monitoring of Underground Physical Conditions

We have developed a giant magnetostrictive seismic source (GMSS), which is a small, high-frequency seismic source for constantly monitoring temporal changes in subsurface physical properties with high precision. The GMSS generates a single force by vibrating the weight in the vertical direction synchronized with GPS time, and excites P and SV waves of arbitrary waveforms with very high reproducibility in the subsurface. The GMSS is intended for constant monitoring of approximately 1 km area of underground, and is essentially maintenance-free even during long-term continuous operation spanning several years. In this study, the GMSS was installed in the Mizunami Crustal Movement Observation Vault (Mizunami-City, Gifu Prefecture) and constantly transmitted elastic wave signals in the 100-200 Hz band using 5 kg vibrating weight. In the experimental site, Toki Granite is widely distributed beneath the Mizunami Group, which is composed of Miocene sedimentary rocks. The GMSS was located in the Mizunami Group, and observations were conducted using the seismometers of borehole observation sites, TGR350 (GL-350 m, distance 353 m from the GMSS) and TRS (GL-505 m, distance 690 m from the GMSS), installed in the Toki Granite. The data stacking period was set to 1 day in order to obtain transfer functions and Green’s functions with sufficiently high signal-to-noise ratio at both stations. During the Kumamoto Earthquake (April 16, 2016, Mj7.3), travel time delays of 60 µs (TRS, travel time 0.185 s) were observed for the P wave and 166 µs (TGR350, travel time 0.148 s) for the SV wave. Since the travel time delay of the P wave propagating almost directly beneath the GMSS was only 23 µs (TGR350), it was estimated that a high-angle crack had opened and reduced the seismic velocity. The travel time delays continued even after the earthquake, reaching approximately 100 µs for the P wave and approximately 250 µs for the SV wave about 1.5 months later, and then both gradually became faster. During the earthquake, the pore water pressure in the Toki Granite (depth 200 m in the Mizunami Underground Research Laboratory) and the groundwater level (TGR350) rose suddenly and continued to rise gradually for about 1.5 months, after which they began to decrease. It is presumed that the earthquake caused an increase in groundwater flow, which increased pore water pressure and porosity, resulting in a decrease in seismic wave velocity. The subsequent decrease in pore water pressure led to a recovery in seismic wave velocity. This paper reports the results of an experiment conducted over approximately seven and a half months, but the continuous transmission test has been ongoing for five and a half years, demonstrating that underground monitoring is possible over long periods with almost maintenance-free.