We derived an equation giving a relationship between the average depth of a stratum boundary and the power spectrum of theg g_zz component of a gravity gradient tensor. It was found that the relationship was nonlinear to the wave number on a semi-logarithmic scale and that the average depth was estimated to be shallower than the actual one if a conventional linear approximation would be employed. We applied the equation obtained in this study to the g_zz component of the gravity gradient tensor observed in the Kuju geothermal area of central Kyushu, Japan, and obtained results that are consistent with the average depth estimated by spectrum analysis of gravity anomaly. On the other hand, we found that the equation could not estimate the average depth of the stratum boundary at the lowest wave number range. In addition, it was shown that the wave number range giving the same average depth as that given by spectrum analysis of gravity anomaly shifts to the higher wave number range, and its range becomes broader.

Integrated geophysical survey result has been used for the simple soil type classification of the river levee. As for an another application, an interpretation method using the interpretation template based on the rock physics model is developed to estimate two dimensional profile of useful geotechnical parameters. In this method, clay content and porosity are estimated from the S-wave velocity and resistivity by using the interpretation templates. The clay content and porosity are employed to infer the soil type and the looseness of the soil, respectively. In this study, a velocity model based on the granular media and a parallel circuit resistivity model are adopted for unsaturated soils. The rock physics models used in this study are hybrid model developed by both theoretical and empirical approaches for a practical method. Clay content and porosity are not estimated by an inversion; instead these are estimated by using the interpretation templates that are created by the forward modeling beforehand. The latter approach is easy to be applied and has a benefit to evaluate the sensitivity of the parameter estimation. Several input parameters required for the rock physics model are assumed from the laboratory measurements of artificial soil samples. For a case study, clay content and porosity have been estimated from the integrated geophysical survey at a river levee. There are six excavated locations in the survey interval; thus we can compare the estimation and actual soil type and condition at those locations. As a result, I confirmed that silty soil exists at the location which the estimated clay content is high, while gravel exists where the clay content is low. In addition, predicted porosity is high at the location which the levee body consists of loose soil. These results indicate that the soil type and the looseness can be evaluated from the surface by the integrated geophysical survey.

The Taiwan Power Research Institute plans to perform a field experiment on sedimentary rocks at 2500 m depth at a CO₂ geological storage site near a coastal area in Taichung-city. Application of electromagnetic methods is being planned to monitor changes in resistivity caused by injected CO₂. It is necessary to measure the initial resistivity of the layers, before large-scale CO₂ injection tests. For the CSMT (Controlled Source Magneto-Telluric) method, the exploration depth has been previously limited to less than 1000 m at most sites in Japan, because the distance between an electric current source and a survey site cannot be separated enough because of the influence of artificial noise. In this study, we conducted simulations using numerical models in order to evaluate the effectiveness to monitor the CO₂ reservoir at a depth of 2500 m. As a result, the total amount of injected CO₂ reached 100 Mton, apparent resistivity increased 5%. We conducted a field test at the site using the great depth CSMT method. We set the current source at a distance of 15 km away from the site and we measured apparent resistivity and phase data at the frequencies of 8192 - 0.015625 Hz using a new electromagnetic exploration instrument with high spectrum resolution controlled by GPS synchronization (Johmori et al., 2010). We could obtain good data by processing with a digital filter and average E/H vector for adjacent survey stations, regardless of the noises at the site. We could also measure the data at the low frequency of 0.0625 Hz by performing a near-field correction considering the signal source. Furthermore, the resistivity profiles obtained by the 1D and 2D inversion techniques matched well with the resistivity of boring core samples and electrical logging data from 2000 to 3000 m depth at the site. In conclusion, we confirmed the effectiveness of this technique.

The 2011 off the Pacific coast of Tohoku Earthquake occurred on March 11, 2011, and caused enormous damage on Japan. The Tokyo Bay and the Tone River downstream regions were widely damaged by liquefaction. In this research, we carried out a surface wave survey at the Tone River northern coast area in Katori, Chiba, Japan which belongs to the Tone River downstream region, and obtained the S-wave velocity (V_s) structure down to about 30 m below the ground surface. As a result, the subsurface of the survey area is classified into three layers; the sandy surface layer which has relatively high V_s, the low V_s silty layer, and the high V_s sandy basement layer. Using the obtained V_s distribution, we investigated whether 1.5 m below the ground surface of the survey area would liquefy or not when it will be shaken by the same scale earthquake as the 2011 off the Pacific coast of Tohoku Earthquake. As a result, it was presumed that liquefaction would not occur on most of the survey line. Moreover, the estimated domains where liquefaction occurred correlated well with those estimated by geological surveys and also well with marshy area distributions in Meiji Era. These results showed that liquefaction resistance estimation using V_s distribution was useful.

In this report, ground shaking characteristics in the residential area around Mashiki Town, Kumamoto Prefecture, Japan, are evaluated by dense microtremor array measurements. First, we conducted microtremor measurements at 365 sites in the residential land. H/V spectra were then calculated using the measurement records. Finally, fine distributions based on the peak (ridge) and trough frequencies of the H/V spectra were illustrated in the residential land. The relationships between the evaluated H/V spectra and the seismic damage will be useful in the future study on the strong motion estimation in the residential land of during the 2016 Kumamoto earthquake sequence.

Measurements of spontaneous electrical potential (or self-potential, SP) for mineral exploration have been conducted on land. More recently, for exploration of hydrothermal ore deposits (by observing marine electric fields), underwater SP surveys are also conducted. Observed electric field data have smaller amplitudes than those obtained on land. Therefore, the noise must be removed to emphasize self-potential signals in the observed data. However, conventional "stacking" techniques cannot eliminate coherent noise which has a coherent component among the parallel observed data. This paper describes the use of a proposed noise removal technique using independent component analysis (ICA), which can decompose multi-component mixed signals into independent signals.

We tested the application of ICA to electric field data measured using deep-towed self-potential exploration. The data were obtained far from hydrothermal active areas and ore deposits. Results show that we extracted and removed noise that is difficult to extract by conventional stacking. Then we found no large marine spontaneous electric field, and inferred the causes of noise. Subsequently, to the observed electric field data we added a hypothetical marine electric signal from ore deposits to simulate the SP field. After applying ICA to the artificial electric field data we identified and eliminated the noise The hypothetical signal was extracted more clearly than when using conventional stacking.

Based on successful application to the test data, we applied ICA to electric field data obtained by an autonomous underwater vehicle cruising in a hydrothermally active area. Results show that we extracted the marine electric field deriving from sub-seafloor sources. Such extraction was not possible using conventional stacking This report also presents discussion of the causes of noise.

The time-domain induced polarization (IP) method is commonly used for mineral exploration. A new 2.5-D FEM program in which Cole-Cole parameters are incorporated was developed for realistic analysis of time-domain IP data in this study. Using the program, transient changes of a resistivity pseudo section can be calculated, while traditional methods can only use pseudo sections of apparent resistivity and chargeability. The program will be useful for analysis of time-domain IP data because it can simulate the transient changes of a resistivity pseudo section due to the changes of time constant and frequency exponent in addition to chargeability.

A 2.5-D inversion program that estimates resistivity and three Cole-Cole parameters from observed IP data was also developed in order to characterize the differences of ore type. The inversion process contains two steps. First, ordinary 2.5-D resistivity inversion is done at each time step to calculate the resistivity of each rectangular block. Then, resistivity and three Cole-Cole parameters of each block are calculated using transient changes of block resistivity. As a result of the test inversion using the IP model with different IP parameters, individual IP parameters were estimated correctly. Additionaly it was found that two IP anomalies with the same resistivity and chargeability could be characterized by time constant and frequency exponent. This 2.5-D inversion, which can estimate time constant and frequency exponent, in addition to resistivity and chargeability, will be widely used for mineral exploration.

While JOGMEC implemented geothermal business over the past 5 years, there has been a growing recognition as to effective use of geothermal support program by JOGMEC. This is thanks to the backup measures from the government and in response to the strong gain of momentum of the companies in geothermal development. (Egashira, et al., 2014)

On the other hand, in the Long-term Energy Supply and Demand Outlook, July 2015, the Ministry of Economy, Trade and Industry (METI) put geothermal power as "base load power source", which can be produced stably and by low cost, and set a goal to increase the present geothermal power ratio in the total power generation (the energy mix) by 3 times up to 2030.

Aiming for the achievement of the energy mix, JOGMEC will make efforts to enhance collaboration with all the parties concerned and accelerate geothermal development.

In this report, we explain about the current situation of and the issues surrounding geothermal power in Japan, and JOGMEC's support programs, recent activities and its future prospect as well.

The airborne gravity gradient (AGG) survey, the airborne transient electromagnetic (ATEM) survey and the aeromagnetic (AMAG) survey were applied to the project screening geothermal promising areas in Japan. With more detailed results from AGG, we observed gravity gradient lineaments and ring structures in the vicinity of high gravity gradient anomalies that indicate intrusive rocks of possible heat sources. Those lineaments and ring structures can be related to fracture zones where geothermal reservoirs could develop. Also some of the conductive anomalies captured by ATEM and the low magnetic anomalies by AMAG may respectively suggest cap rocks and hydrothermal alteration zones that are indications of geothermal activity. While each of those features detected through the airborne surveys does not directly lead to promising geothermal zones, the zones where all such features overlap one another possibly have high geothermal potential. Combining temperature profiles acquired in boreholes with the airborne geophysical survey results enables evaluation of geothermal potential.

We studied various methods of filter analyses and inversion techniques, which have been used to extract the features suggesting geothermal indications, and revealed their characteristics and also points that require attention and consideration.

There were some issues that we had to address when carrying out the airborne geophysical surveys in Japan. However, prominent advantages of the airborne geophysical techniques outweigh those issues; detailed and uniform data are acquired for large areas, regardless of ground conditions, and are processed and analyzed under the same condition. Wider applications of the airborne geophysical methods for various targets, other than geothermal exploration, are expected.

Kirishima area is one of the major active geothermal areas in Japan. In 1996, Ogiri geothermal power plant began operation and continuing operation without serious problems by Kyushu Electric Power Co., Ltd. and Nittetsu Mining Co., Ltd.

Japan Oil, Gas and Metals National Corporation (JOGMEC) conducts high-precision surveys using airborne gravity gradient and time domain electromagnetic survey by a helicopter. Airborne Gravity Gradient survey was conducted in 2013 and Airborne TEM was done in 2014 at Ogiri area. Also airborne magnetic survey data is available acquired by MMAJ at Origi area.

Analyses of the gravity gradiometer data and the airborne magnetic survey data at Ogiri area were carried out and the results were also compared with other geophysical data and geological data. Analyses applied for gravity and gravity gradient data were as follows. Surface rock density was estimated from gravity gradient data and this density was applied for the terrain correction. Three kind of average depths of stratum boundaries were estimated and frequencies for band-pass filters were determined by frequency analysis. On the survey line LKS13A-16, two-dimensional inversion was applied to gravity and gravity gradient data in two different range of wavelength: shorter wavelength indicating shallower structure and longer one indicating deeper structures. Two-dimensional inversion of three-layered model was applied for the shallow structure. The second layer with lower density reaches up to surface and thickness of the second layer is large around hot springs and steaming ground of Ginyu. The gravity basement is shallower at Ogiri area by two-dimensional two-layered analysis of deep structure. Analysis using a three-layered model was also applied for the airborne magnetic data. Second layer with low susceptibility reaches up to the surface at geothermal manifestations. A density cross section and a susceptibility cross section were compared with resistivity cross sections acquired by the airborne transient electromagnetic data (HeliTEM) and ground surface MT method. The cross sections show similar characteristics to each other. Resistivity of drilling core samples, closely connected to the degree of alteration, has a positive correlation with density and susceptibility. Low-density and low-susceptibility zone, obtained by interpreting data of airborne gravity gradiometry and airborne magnetic survey can be an indicator of alteration zones associated with geothermal activity.

Japan Oil, Gas and Metals National Corporation (JOGMEC) is engaged in a technology development project for geothermal reservoir exploration. This project aims to develop an effective method to get an accurate image of reservoir structure, and to improve the geothermal model. Most of the geothermal reservoirs in Japan are composed of steeply dipping faults or fracture zones. Gravity and electromagnetic surveys are commonly conducted to identify reservoir structure and find caprocks and geothermal fluids. However, there is a considerable difference between these survey resolutions and distributions of faults or fracture zones detected by drilling or wellbore imaging. To fill this gap, we focused our attention on a seismic method to reveal the detailed reservoir structure with a higher spatial resolution, applying seismic method to geothermal reservoir exploration, conducting a 3D seismic reflection and refraction survey to verify the method, and obtaining experimental solutions.

The survey was designed with goal of obtaining sufficient resolution to identify fractures and faults. We acquired data with the combination of 3,262 shot points and 4,989 receiver points and processed the data with appropriate static correction and noise reduction, which is one of the most important procedures. 3D seismic data volume was interpreted through integrating the seismic data with the well data. Estimated horizon of the upper Nansatsu formation shows consistency with the conventional geological conceptual model. We also applied the geometrical attribute analyses, such as coherency analysis and ant-tracking, to obtain planar structures that indicate the location of horizontal discontinuities of reflection events. Some indications are distributed in the injection zone and the peripheries of the intruded dacite, margins of which are detected as production zones. The 3D refraction tomography and the full wavefield inversion (FWI) results depicted a complicated velocity structure of the volcanic region up to a depth of 1,000 m. Multi-geophysical models from seismic, magnetotelluric, gravity, and magnetic surveys, well information, and other apriori information were evaluated and integrated with geostatistical technique, joint/simultaneous inversion, and multi-attribute analysis. A rock physics investigation of igneous rocks allows us lithofacies classification for geothermal reservoir characterization. The results from a decimation test encouraged the seismic survey in mountainous areas with the pseudo 3D layout.

Geometrical attribute analysis was applied to the 3D seismic data acquired in the Yamagawa geothermal field, Japan. In particular, coherence attributes have been often used to illuminate discontinuous portions of reflection surfaces in the processed seismic image. The coherence attribute showed consistency with the other geometrical attributes of dip/dip azimuth and curvatures. Lineaments extracted from the coherence attribute were consistent with the natural fractures detected on the borehole image logs in the reservoir section of these production wells.

Reliable and reasonable 3D models of physical properties, which can be recovered from 3D inversions of geophysical data, are required to interpret complex geological structures appropriately in 3D. However, the geophysical inverse problem is ill-posed, i.e., non-unique and unstable, especially for the case of 3D problem. Appropriate 3D a priori model can help solving this problem. In this study, we have performed 3D inversions of MT, Gravity and Magnetic data acquired in Yamagawa geothermal area, with 3D a priori models help recovering more reliable and reasonable 3D models of physical properties. Also, these results can help interpreting the complex geological structures in the surveys area.