Recently, an array microtremor survey which is one of the non-destructive geophysical methods has been widely applied in Japan and overseas, to estimate the S-wave velocity profile of the subsurface structure. To improve the quality of data and reduce the variability of results that depend on the measurers and analysts, there are guidelines independently established by institutions and research groups in Japan, Europe, and US. At the same time, institutions and research groups in Japan and Europe are conducting international blind predictions for the array microtremor survey, to verify the estimation accuracy of the S-wave velocity profile and the variability of results estimated by the array microtremor survey. Although the array microtremor survey has been widely applied in Japan and overseas, there is no internationally common guideline on the array microtremor survey. Therefore, it is important to establish the international common guideline on equipment specifications and survey procedures for the array measurement of microtremor, and the data analysis. First, in this commentary, we outline the development process of international standards on the array microtremor survey in the international and domestic deliberation activities of ISO/TC 182 (Geotechnics). We, second, explain the scope and basic requirements of an international standard for the array microtremor survey. The developed international standard ISO 24057:2022 "Geotechnics - Array measurement of microtremors to estimate shear wave velocity profile" is a new international common guideline. Therefore, by complying with the basic requirements for array measurements of microtremors and the data analysis in 24057:2022, it is expected the quality of data will be improved and the variability of results will be reduced.

　The Biot's modulus M is a kinds of storage moduli, and expresses the relationship between K_d and K_u, the bulk moduli under the two conditions: drained and undrained. The word "storage" means the instantaneous elastic responses preceding the fluid flow caused by pressure gradients in the pore fluid. The K_u is the same as the Gassmann's equation often expressed as a complex formula with many parameters. The Gassman's equation changes to a simple formula if we use the modulus M. However, M often puzzles people studying proelasticity. The parameter M is analogous to the bulk modulus of water in a PET bottle. When non-gas water fills a thin wall bottle, the water pressure is not so much increased because of the swelling of the bottle. On the other hand, when gas water is in a thick wall bottle, the swelling of the bottle is not remarkable, causing more increase in water pressure. The former and the latter correspond to the low and high effective moduli of fluids. The bottle hardness does not affect the fluid pressure under the drained condition, but the bottle hardness affects the fluid pressure under the undrained condition. This article seeks to interpret the meaning and the role of M and intends to clarify and schematically illustrate the basic meanings and concepts of parameters appearing in poroelasticity.

　The Biot's second P-wave is well known as a kind of sound wave that propagates through the fluid in a pore network inside the porous rock. In the Japanese science community, however, researchers lack interest in the second P-wave. This article will show background information about Biot's second wave and the relationships between equations and parameters relevant to waves propagating in porous rocks by solving the elasticity equations of motion for a fluid-solid system. The equation is basically an eigenvalue problem of displacement vectors for fluid and solid. Through an example of the solutions for sandstone, we see the physical characteristics of the second P-wave. The diffusion coefficient in porous rock is derived. The coefficient contains the three bulk moduli; those of drained and undrained conditions, and the Biot modulus. A well-considered experiment demonstrated the second wave is the sound wave.

　Collecting samples, measuring their physical properties, and analyzing their constituent minerals are essential to understand the spatial scale of mineral resources on the deep seafloor and the origins of their formation. In particular, vertically continuous samples obtained by drilling surveys are indispensable to assessing the effectiveness of geophysical explorations. In this study, sub-seafloor drillings using a seafloor-seated drilling system were conducted at a promising location of a seafloor hydrothermal deposit. The main objectives of this study are to expand the sample information of submarine hydrothermal deposits and to evaluate the usefulness of the self-potential survey method for buried hydrothermal deposits. Drillings were conducted at two sites, one in an area with both hydrothermal signatures and self-potential anomalies and one in an area with hydrothermal signatures but no self-potential anomalies. Sulfide mineral precipitations were confirmed in the core collected in the area of self-potential anomaly. In contrast, the core collected in hydrothermal signatures was mainly composed of volcaniclastic material and showed minor sulfide mineralization. Detailed physical property measurements and chemical analyses were performed on the core of the self-potential anomaly area. First, electrical resistivity measurements were performed on the entire core using a multi-sensor core logger. Then precise electrical property measurements were performed using the spectral induced polarization method at one point per section. Elemental concentration analysis by X-ray fluorescence spectrometry and mineral identification by X-ray diffraction analysis was also conducted, and mineral assemblage was determined in conjunction with drilling descriptions. The results showed the intermittent occurrence of conductive sulfide minerals such as galena and pyrite over an area of approximately 20 m. The distribution of concentrations of these conductive sulfide minerals corresponds well with the distribution of points where the induced polarization effect is strong. On the other hand, the distribution of high induced polarization effect points did not coincide with that of low electrical resistivity points. In conclusion, this study shows that hydrothermal signatures do not necessarily indicate the embryo of seafloor hydrothermal deposits and that self-potential surveys can detect buried mineralization zone. The induced polarization effect was again shown to be effective in detecting sulfide minerals. In contrast, the insufficiency of electrical resistivity alone was suggested for estimating the scale of submarine hydrothermal deposits by geophysical surveys.

　The behavior of higher-order terms of the complex coherence function (CCF), which vary with incident waves of microtremors, have been investigated for phase velocity measurements with a two-sensor array using both theoretical and actual results. In this study, for the incident characteristics, the source coefficient theoretically defined in Shiraishi et al. (2005) by the incidence zimuth and intensity ratio of the incident waves was used. And phase velocities have been measured at three different locations, one in an urban area surrounded by major roads and two in rural areas near (about 300 m away) or far (about 800 m away) from major roads, and the phase velocity measured through two-sensor arrays were compared with the results of SPAC technique as a reference.

　As the result, the following three properties have been clarified: 1) The frequency characteristics of the source coefficient which multiplied on the second-order term of the CCFr (real part of CCF) that gives the largest error varies with SITE and array, and the time variation is extremely large (coefficients of variation range from a few tens to over 100%). 2) The relative error of the two-sensor array (in the range of －20 to 30%) is closely related to the frequency characteristics of the source coefficients, and its sign and absolute value determine whether the error is over estimation or under estimation. 3) The process of generating source coefficients was traced by estimating contribution ratio of incident waves from F-K power spectra obtained from urban observation site. The sign and value of the coefficients were closely related to the azimuthal distribution of the contribution ratio, and even anisotropic distributions sometimes resulted in minute values giving correct phase velocities. The source coefficients derived from the F-K analysis are consistent with those from the SPAC technique.

　From these results, it is clear that the source coefficients are closely related to the incident characteristics to the array and the behavior of errors in the estimates, and we conclude that they are useful indicators for realizing high-precision phase velocity measurements with a two-sensor array.

　Differential subsidence is likely to occur near bedrock in rockfill dams, which may cause shear deformation caused by huge earthquakes. Further erosion of the embankment through this shear deformation zone by reservoir water filtration is a major concern. In this process, fine grains are gradually eroded, and a high permeability zone is formed. We suggested locating of the high permeability zone in the embankment by repeated DC electrical surveys while the reservoir water level was lowered in the periodic inspection of the rockfill dam. Using the difference tomography technique, we obtained a 2D resistivity change cross-section from measured data at a dam site. Positively, we expectedly detected a zone of remarkable change in resistivity near the right bank abutment. Next, assuming that the high permeability zone was formed by the washout of fine particles, we estimated the hydraulic conductivity, electrical resistivity, and cross-sectional area of the high-permeability zone using the Kozeny–Carman equation and the Glover’s equation for granular medium consisting of sand and clay. Based on these estimates, we performed a numerical simulation using a resistivity model in which the resistivity of groundwater within pores in the high-permeability zone changes due to the lowering of dam water level. The resulting remarkable resistivity change zone predicted by the simulation was almost consistent with the measured resistivity change zone. High-permeability zone is likely to exist at a location where fine grains are eroded due to shear deformation, particularly in the upper part of the steep change in bedrock slope near the abutment. In addition, it is possible to evaluate the usefulness of the DC electrical survey in advance by conducting numerical simulations based on the permeability model proposed in this paper. We plan to verify the validity of the DC electrical survey results after repair work is completed at the survey dam site.