The ocean has a large capacity for environmental changes. Recent anthropogenic effects overwhelming the marine ecosystems may exceed the capacity and ecological functions to maintain its resilience. To achieve the sustainable development goals on marine environments, the practical environmental management plan which consists of standardized protocols for monitoring and assessment will be necessary. The techniques of environmental impact assessment and monitoring for biota and ecosystems, which have complexity and locality caused by diversity of species and habitats, are indispensable elements to operate the management plan. The advanced techniques, based on the molecular genetics and metagenomics and the fine technology for imaging data, have made progress in the methods of biological/microbiological study and monitoring. The application of advanced techniques will strongly support the operation of management plan and sustainable use of marine resources. It must be indispensable in the consistent environmental management for sustainability to understand the fundamental principles and technologies of advanced methods and ensure the consistency of the existing data and the assessment criteria.

The development of seabed mineral resource has been attracting attention as there is a growing concern about the future supply of metal resources. Seafloor massive sulfide deposits, which bear high grade metals including zinc, lead, and copper are formed by precipitation of metal elements from seafloor hydrothermal fluid circulation systems. Many active seafloor hydrothermal systems have been located at around volcanic seamounts within Japanese EEZ, which strongly suggests high potential for metal resources on the seafloor in this area. The "Next-generation technology for ocean resource exploration" was conducted as an SIP Project of the Cabinet Office from 2014 to 2019 with a goal of technology development/transfer of systematic exploration method for ocean mineral resources. "Integrated Survey System for Concealed SMS Deposits" are established by combining various survey methods and "Survey protocol for seafloor massive sulfide deposits" are organized to define target areas using three-step approach; regional survey, semi-detailed survey and detailed survey. The exploration trials conducted in 2018 by private sectors were successful in identifying unknown subseafloor ore body based on the newly developed survey protocol. As another important operation, drilling campaigns using D/V Chikyu were conducted three times in active hydrothermal fields in the Okinawa Trough. Analyses of the obtained sediment cores and logging data revealed that occurrence of sulfide minerals is likely to be controlled by sedimentary strata, and that hydrothermally altered sediments of specific physical properties are distributed across wide region so that detected by geophysical exploration methods. Synthesis of these obtained results would be facilitated by comparison with knowledge from explorations of Kuroko-type VMSDs in northeast Japan. Progress in geophysical exploration technology is not only useful for effective mineral resource exploration but also applicable to get crucial information for understanding the genesis of seafloor massive sulfide deposits.

Marine mineral resources such as seafloor hydrothermal deposits, cobalt-rich crusts and manganese nodules have been discovered around Japan. The authors have developed new survey tools for exploration of deep-sea mineral resources and examined the efficiency of the survey system. In 2018, an integrated survey including core sampling was conducted in uninvestigated hydrothermally active area. In this paper, we present high-resolution subseafloor image obtained from acoustic survey using a deeply towed hydrophone cable as part of the integrated survey. The spatial resolution (lateral and vertical) of this survey system is on the order of meters. We identified the distribution of fracture structures such as faults and fissures that would be the hydrothermal fluid path from the acoustic survey records. In addition, by comparing with the results of the coring survey, we were able to identify the layer where the hydrothermal sulfide mineral was deposited in the past. In this area, we consider that fault activity continues from the distribution of cracks in the surface sediments, but because the specific terrain suggesting the existence of faults is not found on the seafloor, meaning that the sedimentation rate is fast. These results collectively suggest that the survey area is under the environment where hydrothermal activity is existing, but it is difficult to form a large hydrothermal deposit due to the high deposition rate. From these outcomes from the acoustic survey, it is demonstrated that valuable information indicating the relationship between hydrothermal activity and seafloor sediment including sulfide ores, that is difficult to understand only from the core sampling, can be obtained from our high-resolution acoustic survey.

“Zipangu in the Ocean program,” a Strategic Innovation Promotion Program (SIP) managed by the cabinet office of the government of Japan, was launched for the scientific and technological development of natural resources around the Japan Islands for recent national requirements for marine natural resources. Under this project, “Protocol for the natural hydrothermal resources” was formulated, and we tried to confirm a part of this protocol on a research cruise. In the candidate areas we chose before the cruise, we obtained the bathymetry, back scatter, water column data by the dense surveys used MBES system on a ship and narrow the candidates for AUV surveys. A clear negative self-potential (SP) zone and many mound structures were detected by AUV surveys using an electrometer and a high accuracy MBES system on an AUV in the one area of five candidates for detailed surveys. Many detailed distributions of hydrothermal plumes were also found. However, our survey methods enabled effective data acquisition for hydrothermal deposits exploration. According to the survey results in the known hydrothermal deposit, the negative SP anomaly relates to the hydrothermal deposit on the seafloor and/or under the seafloor. Therefore, this result implies that the SP anomaly detects the sub-surface structures. It is needed to obtain the certain direct evidence with hydro-thermal deposit and the detailed electrical structural analysis to confirm the reliability of our survey results. However, our survey methods obtained effective data acquisition under the hydrothermal deposit survey. New knowledges of this observation are very important guideline for the unknown hydrothermal deposit survey.

For exploration of seafloor massive sulfide deposits, we improved the marine deep-towed electric exploration system which could be handled by vessels of private companies. The survey was conducted at the “Hakurei-site” and “Gondou-site”. At each site, clear spatial changes in the horizontal electric field, which may be due to the Self-Potential (SP), were observed. If the horizontal electric field is spatially integrated, it becomes a potential, but in reality it is not easy to integrate because of the noise of towing and the nonlinear drift of the electrodes. The polarity of the horizontal electric field varies depending on the direction of travel. Therefore, in this study, we show a method to convert a horizontal electric field into a pseudo vertical electric field with the same polarity as the potential by the Hilbert transform, and apply it for mapping the acquired data. On the other hand, in the signal processing of resistivity exploration, a method for removing short period noise relatively easily from a record after removing long period signals is shown. And it was applied to the resistivity analysis. As a result, the sub-seafloor resistivity lower than the seawater value tends to be found in the region where the pseudo vertical electric field was negative. Drilling surveys were conducted at both sites, confirming that the negative anomalies in the pseudo vertical electric field corresponds to the presence of a sulfide layer. This proves that this method is very effective in narrowing down the target areas of a Seafloor Massive Sulfide deposit.

Ocean bottom gravimery can acquire precise gravity data as same as gravimetry on land, and it can acquire clear and large gravity anomalies compared with sea surface gravimetry by using ships due that the ocean bottom gravimetry is conducted near exploration target. We have developed an ocean bottom gravimeter (OBG-3) to use for explorations of offshore deep basement structures and submarine resources such as seafloor massive sulfide (SMS).

As a part of “Next-Generation Technology for Ocean Resources Exploration (Zipangu-in-the Ocean Project)” as a part of the “Strategic Innovation Promotion Program (SIP)” of the Cabinet Office of Japan, ocean bottom gravimetry utilized the OBG-3 was carried out at the “Hakurei Site” in the middle Okinawa Trough. As a result, the observed gravity anomalies at the “Hakurei Site” can be explained by the gravity anomalies calculated from two-dimensional density structure models which referred SMS sections of the boring core samples by the D/V “Chikyu” of JAMSTEC. The measurement time for each station was only 30 minutes including moving time to the next station 100 m away. This high efficiency of the measurements was achieved by introducing the ROV towing-type exploration system newly developed for SIP.

During the cruise CK16-05 aboard the D/V Chikyu, we conducted natural gamma-ray logging with a through-the-bit logging system by using a memory-type geothermal exploration tool that has so-far been used only in terrestrial geothermal wells. This logging system was firstly tried during the cruise CK16-01 to reduce the cost and increase the efficiency of drilling surveys, but issues from vibration and collision impact within the drill hole were found. Based on these results, we re-examined and modified the logging tools and operation methods. As a result, we successfully obtained highly accurate and continuous data of natural gamma-ray intensity, temperature, and pressure within the drill holes at five sites in the Izena Hole, mid-Okinawa Trough, and accomplished significantly lower-cost logging at one order of magnitude lower than that of the Logging While Drilling (LWD) system. At three sites within the Izena Hole hydrothermal field, the gamma-ray intensity exhibited significant variations along with lithological changes composed of pumice, hemi-pelagic sediment, massive sulfide, silicified rock, and a K-rich altered clay layer. On the other hand, at the other two sites located ca. 330 and 670 m N-NNW from the hydrothermal sulfide mound, subseafloor strata were dominated by pumice with minor amounts of hemi-pelagic sediment and the gamma-ray intensity correlated well with the density and porosity of the core samples determined by the multi-sensor core logger (MSCL) and discrete sample measurements. Our results demonstrate that gamma-ray logging with the through-the-bit logging system is highly effective in identifying and characterizing the subseafloor lithology as well as the thickness of the massive sulfide layer, and can compensate for the often inevitable poor core recovery rate during coring operations at seafloor hydrothermal deposits. The versatility of this system could be further improved with additional sensor measurements (e.g., resistivity) in future drilling operations at seafloor hydrothermal fields.

Environmental impact assessment (EIA) is an indispensable process for mining activity. The discussion on legal framework of EIA for deep-sea mining is continued in domestic and international committees. The international standards of technical protocol for commercial-based deep-sea EIA are under development stage. It is required how to conduct the deep-sea survey and monitoring with economic efficiency of process, and how to keep the data quality. The project of the SIP next-generation technology for ocean resource exploration (hereinafter referred to as “the SIP project”) aimed at the development of a practical method incorporating advanced research technologies and knowledges, and the establishment of a strategically evaluating procedure with economic efficiency. The rapid inspection method based on an image analysis for benthic meiofauna has been established as application of imaging flowcytometer. The integrated analysis of meiofauna community is studied by using combined information of morphological data from flowcytometer and genetic data from metagenomics. As for the long-term observation on seafloor, we established practical method of long-term observation using portable/standalone observation platform “Edokko Mark 1”, which is a commercial product of the domestic manufacturer, designed as a long-term monitoring device on the deep seabed. We conducted a sea-floor mapping using AUV “Hobarin”, which can cruise keeping on height of several meters, to collect the detailed topography of seabed and the information on habitat condition. The method of turbulence observation, which may be essential data for accurate prediction of turbid water diffusion, was established. These new methods and procedures were published as several technical protocols for environmental impact assessment conducting surveys and monitoring on the stage of resource exploration. The manual of new commercial-based EIA protocol for deep-sea mining were compiled by the concept of “strategic environmental assessment” recommended by the Ministry of the Environment. These manuals anticipate user’s needs for strategic approach to make a sustainable development with environmental considerations regarding seabed mining. In the future, the knowledges and experiences of the SIP project will enhance the business progression of private companies, and lead the international marine industry.

The first geophysical exploration by the Japanese scientist is a gravity survey conducted by Motonori Matsuyama at the Faculty of Science in Kyoto Imperial University in 1915 on the Jalute Atoll in the Marshall Islands using the Eötvös’s gravity-variometer. The purpose of this survey is to estimate the depth of bedrock below the coral reef. In order to minimize the effect of temperature change, the measurements could be begun after sunset under the best conditions before morning. As the results, the probable depth of the bed rock was 243 m for rock density of 2.6 g/cm³, and 466m for rock density of 3.0 g/cm³. The first geophysical exploration conducted in Japan is the magnetic survey by Gaichi Yamada at the Faculty of Engineering in Kyoto Imperial University in 1919 at the Takano mine in Hyogo Prefecture, using the Thalēn-Tiberg magnetometer. As the survey results, the clear magnetic anomalies are obtained near the outcrop. The first electrical prospecting in Japan is the self-potential method conducted by Fumio Oda at the Faculty of Engineering in Kyushu Imperial University in 1924. However, the author could not find the paper that described the detail of the survey. The first electrical prospecting published as a paper is the self-potential method carried out in 1925 by Yoshikata Fujita at the Faculty of Engineering in Kyoto Imperial University using the Schlumberger’s apparatus at the Yanahara mine in Okayama prefecture. As the result, the equipotential line having a clear negative potential center near the left bank of Yoshii River is obtained. The first seismic survey is a refraction survey conducted by Seizo Haeno at the Faculty of Science in Tokyo Imperial University in 1931 to determine the thickness of the riverbed gravel layer at the planned site for the construction of a hydroelectric power plant dam in Yamagata prefecture. A radio seismometer developed by Haeno was used. As the results, it is obtained that the thickness of the gravel layer is 5.5 m to 6.3 m.

The calculation program which can do the numerical simulation of seismic waves first created in Japan is a Days-2 Code which is made by the author. The Honshu-Shikoku Bridge Authority has planned the world's first method of submarine blasting with no free surface. Therefore, a numerical simulation was conducted using the Days-2 Code in 1974 to predict all phenomena generated by this special blasting. Honshu-Shikoku Bridge Authority conducted a test blasting of this method in 1975. The actual measurement results of the ground vibration generated by the test blasting were compared with the prediction results by the numerical simulation, and it was confirmed that the both were coincide quite well. In addition, the maximum ground vibration measured was about 7 times larger than the value calculated using the blast vibration estimation formula conventionally used. From this, the effectiveness of the numerical simulation by the Days-2 Code was confirmed.

The initiation of magnetic exploration in Japan has been verified by reviews of literature. In our society, a magnetic exploration for iron deposits at the Takano Mine, Hyogo Prefecture conducted by Gaichi Yamada, Kyoto Imperial University in 1919 was treated as the first magnetic and geophysical exploration in Japan (Editorial Board, 1948). Whereas, Sato (1985) introduced a magnetic exploration at the Kamaishi Iron Mine, northeastern Japan, conducted by Shuzo Sekino, Geological Survey of Japan (GSJ) in 1891 (Otsuka, 1893) and suggested that it can be treated as the first one in Japan. As magnetic exploration was limited inside Sweden at that time, this exploration consists of inclination and declination measurements only at nine sites and the explanation and interpretation of the result seem to be insufficient. It is very hard to judge if the suggestion is correct since the geologic map on which measurements sites were drawn is not available even if intensive searches through the network of libraries. Field notebooks which would include the measurement procedure and information of environment of the measurement sites of Sekino’s survey should have been kept in GSJ at that time. However, the office of GSJ was burnt down by the Great Kanto Earthquake in 1923 and various research materials including field notebooks were lost (Hisamatsu, 1956). Sekino, himself was very busy with his own business of surveying and might have lost his intension to continue magnetic surveys because of a geological controversy over the Fossa Magna and its magnetic characteristics proposed by his former master, Dr. Naumann. In this way, Sekino’s magnetic exploration at the Kamaishi Mine had been forgotten in history and was not treated as the first magnetic and geophysical exploration in Japan. In 1926, 35 years after Sekino’s first trial, Yoshizo Fujita, Kyoto Imperial University conducted an intensive magnetic exploration at the Kamaishi Iron Mine and was successful in finding a new iron deposit (Fujita, 1928). Coincidentally, the new deposit was found very close to the Sahinai Path where an anomalous declination was measured by Sekino.

The first seismic survey for oil exploration in Japan was conducted by a Japanese oil company at Futsudo area located in north-western part of Akita prefecture in 1940. The results reported by Y.Ishida(1943) were briefly reviewed, and the seismic profile of the recent 3D survey acquired in 1985 was reviewed for comparison. Since then, the exploration seismology in Japan has been developing for more than 80 years, and the policy of Japanese government for oil and gas exploration has an important effect upon the development of the survey equipment and software. After the Pacific War (1941-1945), in order to promote the exploration of the domestic oil, the GHQ had been supporting the Japanese oil company for a decade to install the modern seismic instruments. The national project (the first 5-year plan discussed and determined in the governmental committee) for the purpose of exploration and production of domestic oil fields started in 1950, and the project had continued for 45 years, up to 8-th 5-year plan (1995 to 2000). The development of the field data acquisition and processing/analysis system had been motivated by the change of the exploration target (shallow complex structure to deeper structure, continental shelf to deep sea on offshore area, shallow water and transition area, structural trap to stratigraphic trap, etc.). Especially, the digital recording system, developed in 1960's and combined with the digital telemetry system in 1970's, made it possible to acquire the large volume of data more than 1000 channel, which enabled to acquire large spread data and 3D data.

The reflection seismology, developed as a major tool for exploration of oil and gas resources that produce economic value, has been applied to other fields such as earthquake disaster prevention/mitigation, CO₂ underground storage, etc. The effort to find and promote the new application fields will be required for further expansion of the technology.

Operation restriction during earthquakes with human hands and earthquake disaster prevention system/Earthquake Early Warning (EEW) system automated have been already installed in many railways, Japan and overseas, and therefore, when an earthquake occurs, those restriction and systems have ensured the safety and the stability of the railway. On the other hand, when operating those restriction and systems for railways, each railway operator has adopted an operation method specialized for the own planning of railway operation, and there are no unified guidelines and/or specifications in Japan. Also, there are no internationally shared guidelines. Therefore, it was strongly requested to create quality-guaranteed guidelines for countries and regions where there is a concern about the occurrence of earthquakes. To reduce the risk for customers caused by an earthquake and to reduce the downtime for railway operations, an international standard for a planning of railway operation to ensure the safety and the stability has been developed.

At first, in this commentary, we outline the development process of international standards in the railway field. We, secondary, explain the development of international standard for the planning of railway operation in the event of earthquakes. The developed international standard ISO 22888:2020 "Railway applications-Concepts and basic requirements for the planning of railway operation in the event of earthquakes" is a new international common guideline. Therefore, by complying with the specified concept and basic requirements in ISO 22888, it is expected a planning of railway operation in the event of earthquakes that ensures quality will be executed.

Airborne electromagnetic (AEM) exploration system is one of innovative technologies to visualize underground structure by flight. Conventional AEM systems require a helicopter in order to load large and heavy equipment. Therefore, those systems are not applicable to an investigation of small areas, because of excessive running cost including flight permissions, and risk assessment and management. In recent years, drones (multicopters) have been applied in various fields and the performance of drones has been improved. Present study has developed an AEM using a drone instead of a helicopter. The key point for development was the weight saving of equipment loaded on the drone. Three types of AEM systems have been developed in order to apply them depending on the survey depth and/or the condition of the survey site. The D-GREATEM system is appropriate for linear area survey and requires a grounded wire source. The D-TEM [GLS] system is appropriate for wide area survey and also requires a grounded wire source. The D-TEM [ALS] system, using two drones, does not require a grounded wire source. Test flights showed that resistivity cross-sections obtained from the drone-AEM systems is consistent with those from conventional electrical prospecting. The D-GREATEM and D-TEM [GLS] systems were found to have a relatively deep depth of investigation (more than 100 m), compared to D-TEM [ALS] system. Nevertheless, the D-TEM [ALS] system is effectively applicable to an area where the grounded wire source is not allowed by the conditions of the survey area. These three methods can be used to survey narrower areas and shallower geological structures with higher spatial resolution, compared to a conventional helicopter AEM system. Eventually, those techniques would allow us efficient and cost-effective surveys.

Kuju volcano comprises a series of volcanic mountains located in Oita prefecture. Mt. Iwo lies in Kuju volcano, where a phreatic eruption occurred in 1995, and still continues to eject smoke and ash. The 2016 Kumamoto earthquake occurred on April 16, 2016 and its aftershocks distributed around Kuju volcano. The 2016 Kumamoto earthquake could influence on the activity of Kuju volcano. In this study, therefore, in order to monitor the recent surface displacement of Kuju volcano, we applied SAR interferometry and SAR interferometry time-series analysis (SBAS analysis). For the analysis, we used 17 scenes of ALOS-2 data acquired between August 28, 2014 and November 14, 2016.

As the analysis results during the period of the data used in this study, significant surface displacements were not found around Kuju volcano except Mt. Iwo. Before the 2016 Kumamoto earthquake (August 18, 2014 - February 25, 2016), ground subsidence was observed at Mt. Iwo. Then, a large-scale uplift of about 4 cm were observed immediately after the earthquake (April 18, 2016 - June 13, 2016). After that (from June 13, 2016), ground subsidence was observed again at the same subsidence rate as before the earthquake. These results suggest that the influence of the 2016 Kumamoto earthquake would be temporary, and there was no clear change of trend of surface displacement in the long-term perspective.

Furthermore, in order to estimate the mechanism and the source depth of the surface displacement of Mt. Iwo, we employed analytical deformation models of the crust. As a result, we found that the depth of the pressure source both before and after the 2016 Kumamoto earthquake corresponds to the depth of the steam pool (at the depth of 0.4 km). On the other hand, we estimated that the surface displacement immediately after the 2016 Kumamoto earthquake have a pressure source located deeper than the fluctuation before and after the earthquake.

To clarify the effect of surface geology on seismic motion in Ofunato City, Iwate Prefecture, a survey on detailed seismic intensity distribution was done using questionnaires for the off-Miyagi earthquake ( Mj 7.1 ) occurred at May 26, 2003. The questionnaire revised by Ohta et al.(1998) was used for 5 calculating seismic intensities. 3,387 questionnaires were distributed for parents of students of 14 elementary schools and 8 junior high schools in Ofunato City. The seismic intensities estimated from questionnaires were averaged for 250m square meshes to clarify the spatial variation of seismic intensities. To avoid differences among individuals for questionnaire survey, the effective meshes where the number of the questionnaire was more than three were used for analysis. The number of the 10 effective mesh was 212. The seismic intensities were ranging from 6.4 to 4.1, and the average was 5.1 in Ofunato City. It was revealed that the seismic intensity at the central area of Ofunato City was large, and the one around the area was small.

In order to clarify the difference of seismic intensities for each area in Ofunato City, we carried out conventional microtremor array observations at 14 sites. The array consists of four vertical 15 seismometers with an interval of 6 m. Phase velocities of Rayleigh waves were estimated from array records of vertical microtremors by spatial auto-correlation (SPAC) method. Average S-wave velocities (AVS) between a surface and 30 m depth were estimated from phase velocities of Rayleigh waves with a wavelength of 40 m by using empirical relations proposed by Nagao and Konno (2000). We compared to the estimated AVS distribution with the seismic intensity distribution. The correlation coefficient 20 between the seismic intensity averaged with a 250m mesh and AVS30 was -0.68. Because the geological structure changes abruptly in a small area in Ofunato City, we calculated the seismic intensities averaged from the observation point to a radius of 100 m to 500 m, and examined the correlation between the seismic intensities and the average S wave velocities (AVS10 to AVS30). It was revealed that the AVS was small at the area where the seismic intensity was large. As a result, it 25 was found that the correlation coefficient between the seismic intensity averaged with a radius of 250m and AVS30 was -0.82. It was also clarified that the correlation coefficient of AVS20 was -0.81 as well as the coefficient of AVS30.

The electromagnetic fields excited by horizontal and vertical electric dipoles in stratified media are derived using vector potentials which have been studied in past decades. However, unfortunately, it is difficult to find papers and/or textbooks which describe the derivation of the electromagnetic fields in detail. The main objectives of this paper are to uniquely derive the electromagnetic fields due to electric dipoles in stratified media in detail, and to develop and to demonstrate the computation code which has flexibility. The transmitter dipole and receiver can be located arbitrary in stratified media with anisotropic and/or complex resistivity in the developed technology. The results of synthetic studies show that the developed code can compute accurate and reliable electromagnetic fields. The developed technology can be applied not only for the 1D electromagnetic forward and inverse modeling, but also for the background field and Green's tensor computations which are required for the 3D electromagnetic forward and inverse modeling based on the integral equation method. Further, the technique may be used for the primary field computation required for the 3D electromagnetic forward and inverse modeling based on the differential equation methods with secondary field formulation.

Imaging of the permeation state in the chemical grouting could allow us to optimize the pressure and volume of the injected grouts, to improve the quality of ground improvement. Though the electrical resistivity tomography (ERT) is a promising monitoring method, the assumption in the conventional inversion technique that the resistivity distribution does not change largely during measurement prevents us from monitoring it constantly. So a new analysis method which can take the influence of the resistivity change during measurement into account was proposed in the previous report (Shimizu and Oda, 2019). The proposed method has following three features: i) The measurement data during chemical grouting is divided into multiple time steps, and all time steps are solved with integrated observation equation. ii) Parameters are set to be the difference in the electrical resistivity between two time steps. iii) The difference in measurement time within one time step is considered in the calculation.

Though the advantage of the proposed analysis method was confirmed by numerical experiments in the previous report, it was not confirmed by actual data. Although Gaussian noise was added to the data artificially in the numerical experiments, the noise characteristics would be different from that of actual data. Therefore the model experiment which simulates chemical grouting was performed in this study. As a result, it was found that the proposed analysis method performs accurately by the comparison of electric resistivity distribution based on measurement with that based on electric resistivity tomography.

In this study the proposed analysis method, the conventional analysis method and Kalman Filter were compared. Datum at each time step are analyzed independently without considering the context in the traditional analysis method. Kalman filter is the analysis method by which the state at each time step is calculated sequentially by repeating the prediction and the update based on measurement from the first time step in turn.

As a result, it was found that the proposed analysis method is the most accurate regardless of time step among three methods.

Petrophysical data, including magnetic properties of rocks are very effective for interpreting results of magnetic surveys. However, details of rock magnetic properties have been examined rarely in parallel with magnetic survey data. Whereas, apparent magnetic lows are observed in the Musadake geothermal area, eastern Hokkaido by a recent high-resolution aeromagnetic survey of JOGMEC to estimate geothermal potential of the area. In this study, we have made laboratory measurements of density, magnetic susceptibility and natural remanent magnetization (NRM) of rock samples from outcrops in the area to evaluate its application to an interpretation of the magnetic anomalies.

Block samples had been collected from three outcrops: the right bank of the Ikeshomanai River (Site A), the Shura River (Site B), and the right bank of Kutekunbetsu River (Site C) and had been shaped into cylindrical specimens. All the specimens from the Site B and some specimens from the Site C indicate high NRM intensities (≥ 1.0 A/m) with high Qn ratios (≥ 1.0) and negative NRM inclinations. Whereas, all the specimens from the Site A and some specimens from the Site C show low NRM intensities (≤ 1.0 A/m) with low Qn ratios (≤ 1.0) and positive NRM inclinations.

AC demagnetization experiments were also conducted for selected specimens from the three outcrops and all the selected specimens were found to show negative NRM inclination, suggesting its reverse magnetization as the initial magnetization. This result is consistent with the obvious low magnetic anomalies observed over the rock sampling sites and our results imply that the low magnetic anomalies are not attributed to geothermal signs, but rather to the distribution of reversely magnetized volcanic rocks. According to photomicrograph and X-ray fluorescence analyses conducted additionally, the rock sample from the Site A is seen to lack opaque minerals by alteration, causing its low magnetic properties.

On a basis of good agreements between the petrophysical property and magnetic anomaly, it is concluded that examinations of petrophysical properties of rocks are useful when interpreting magnetic survey results in the study area.

The effects of localized source currents on flat Earth’s magnetotelluric (MT) responses have been reported especially in terms of the variations in period and subsurface structure. However, the bias within MT responses of spherical Earth arising from variations in the position of the source current is not reported. This study assumes the loop current for a source and varies the focus latitude, altitude and radius. The bias within MT responses of spherical Earth at 20 and 200 s are calculated at various positions of the loop source current. A slight change in the source’s position causes a shift in the MT responses. The bias becomes large especially at the altitude of 100-150 km from Earth’s surface, where the E layer greatly contributing to the ionospheric electrical process exists. Such bias can be detected in real data analysis because the focus latitude and altitude of the source current vary temporally. Therefore, we should evaluate the ionospheric environment in the discussion of temporal changes in subsurface resistivity structure on the basis of MT responses.

We conducted 2D microtremor array measurements to delineate a deep subsurface S-wave velocity model in Tsukuba City. Total length of the survey line was 12 kilo-meter, and the survey line was divided to five sub-linear arrays with overlap. Forty single-component nodal seismograph (McSEIS-AT) with geophone (natural frequency is 2 Hz, SUNFULL; PS-2B) were used for the measurement of each sub-array, and the measurement time for each sub-array was 45 minutes. Total number of observation points was 200. A huddle test was performed before the array measurement, and high coherence was observed in a frequency range of about 0.2 to 10 Hz between any pairs of the geophones. Phase velocity dispersion curves were obtained at every 400 m by CMP-SPAC method. The number of the obtained dispersion curves was 24, and the frequency range of the valid dispersion curves was about 0.3 to 5 Hz. The phase velocity dispersion curves by linear array were consistent with the dispersion curve obtained by cross-shaped array conducted at several cross-sections on the survey line. In addition to the linear array measurement, single-station microtremor measurements using three-component nodal seismograph (McSEIS-AT3C) was conducted at 20 points on the survey line. We observed that the peak frequency of H/V spectral ratio changed from about 0.25 to 1 Hz. The H/V spectral ratio and the phase velocity were used in a multi-station inversion with Genetic algorithm. The results of the multi-station inversion were consistent with PS logging. The inverted depth of the top of the basement varies from about 600 m to 200 m. The result of the multi-station inversion showed that the upper depths of intermediate sediment layer and of basements (Vs3.0 km/s) were gradually changed along the survey line and seems to be more appropriate than the result of individual inversion.

An algorithm is proposed to calculate leaking (leakage or leaky) modes in layered elastic solids. Leaking modes in elastic solid layers propagate faster than the normal modes (Rayleigh wave) and are dispersive. Amplitudes of leaking modes attenuate during propagation, dissipating the wave energy into the lower-most layer which is semi-infinite downward; while in contrast, normal modes propagate without energy leakage, showing no amplitude attenuation when the internal friction is ignored. By introducing complex wavenumbers of which imaginary parts represent attenuation, phase-velocity dispersion curves of leaking modes can be calculated by conventional matrix methods. The characteristic amplitude responses of leaking modes in a layered structure are calculated from integral transformations of surface displacements due to an impulse force. Values of integrals are partially obtained from Cauchy’s residues at the leaking mode poles on complex plains. The theorem of implicit function yields amplitude frequency responses in leaking modes. We obtained dispersion curves and expected amplitudes of leaking modes for a paved road model with underlying subgrade and unconsolidated soil layers in the frequency range 2–100 Hz. The model represents a typical near-surface structure in urban areas. If we use a surface seismic source for generating surface wave, observed seismic waves at the pavement surface may contain leaking modes that mainly propagate through near-surface layers of the pavement and subgrades. Leaking modes will provide a new aspect for interpreting observed dispersion curves, even if the high velocity portions in dispersion curves exceed the S-wave velocity of the lower most layer. Studies of leaking modes will be important for estimating near-surface ground structures from observed dispersion curves, especially for the case of paved roads or frozen soil areas, where the conventional normal-mode-based approaches have been giving ambiguous or questionable results.

A microtremor survey has been carried out around Kushimoto, Wakayama, Japan, and its results were published by Hata et al. (2015) on the current journal. Survey results on specific misconduct in research activities have been opened recently (Osaka University, 2019) and the manuscript on the microtremor survey around Kushimoto was also surveyed. However, the first author' behaviors were doubtful for the manuscript but not specified in the report.

Thus, we have decided to verify the results of the microtremor survey around Kushimoto, and carried out microtremor measurements at 357 sites in the area. We applied the same procedure for the analyses as described in the manuscript by Hata et al. (2015). The results obtained through our study agrees with those through the previous study for spatial distribution of the predominant frequencies of horizontal-to-vertical spectral ratio (H/V). However, systematic differences of the predominant frequencies were found around limited area and the reason why the differences are found may be some problems of the previous studies, because of our robust observations. Although we carried also out microtremor observations at 7 sites, where are very close to the K-NET Kushimoto, a strong motion site, there are no similar shapes of H/V shown in the previous survey in our seven H/Vs. Ellipticity of the fundamental mode of Rayleigh wave is calculated analytically using the velocity structures at the site of K-NET Kushimoto, which is publicly available on the web pages of K-NET. A predominant frequency of the ellipticity obtained by our analysis agrees with one shown in the previous study, though the side shape and frequencies of dips by us are not agreeable to ones of the previous study. The previous study showed very similar site amplification properties obtained through the microtremors and earthquake ground motions. We pointed out that similar characteristics are rarely obtained through the microtremors and earthquake ground motions. The authors can provide our raw data of microtremors in win format used in this report, if researchers and personnel request them.