Yasuhiro Fujimitsu

Mapping hydrothermal altered mineral deposits using Landsat 7 ETM+ image in and around Kuju volcano, Kyushu, Japan

To evaluate the conventional methods for mapping hydrothermal altered deposits by using Landsat 7 ETM+ image in and around Kuju volcano is the prime target of our study. The Kuju volcano is a mountainous composite which consists of hornblende-andesite lava domes and associated lava flows. We used the colour composite, band ratio, principal component analysis, least square fitting and reference spectra analysis methods. The colour composite and band ratio methods showed very clearly the hydrothermal altered deposits of clay minerals, iron oxides and ferric oxides around the fumaroles. The principal component analysis using the Crosta technique also enabled us to represent undoubtedly the altered hydroxyl and iron-oxide mineral deposits of this region concentrating around the fumaroles. Least square fitting method illustrated the goethite, hematite and clay alteration region. Finally the target detection method for reference spectral analysis by using ENVI 4.3 detected the representative hydrothermal altered minerals around Kuju volcano fumaroles area. Therefore, all the methods showed high efficiency for mapping hydrothermal altered deposits especially iron-oxide minerals such as hematite, goethite and jarosite, which are alteration products of hydrothermal sulfides around the fumaroles.

Relation between structure and low-temperature geothermal systems in Fukuoka city, southwestern Japan

The Fukuoka area is located in the southwestern part of Japan. The Yokote-Ijiri area, located in the southern part of Fukuoka city, has several low-temperature geothermal systems, including eleven hot springs. From 1996 to 2008, the Fukuoka area was investigated by gravity survey, using Scintrex CG-3 and CG-3M gravimeters, in an attempt to delineate its subsurface structure. The surveys were intended to improve the understanding of the relation between the geothermal systems and the subsurface structure as well as to locate the active faults in the surveyed area, which are responsible for generating large earthquakes. The gravity data were analyzed using integrated gradient interpretation techniques, such as the Horizontal Gradient (HG), Tilt Derivative (TDR), and Euler deconvolution methods. With these techniques, many faults were detected, including the famous Kego fault, which is an active fault in Fukuoka city. A 2-D gravity model was constructed to show the relationship between the faults and the geothermal systems. The results of the present study will hopefully lead to an understanding of the relationships between the interpreted faults and the location of the low-temperature geothermal systems and possibly aid in future geothermal exploration of the area.

Heterogeneous surface displacement pattern at the Hatchobaru geothermal field inferred from SAR interferometry time-series

Abstract We estimated surface displacements using persistent scatterer SAR interferometry (PS-InSAR) around the Hatchobaru geothermal field, Japan, from 18 ALOS/PALSAR images acquired from July 2007 to December 2010. Generally, geothermal fields, covered with natural targets such as rocky terrain and vegetation, have been one of the difficult targets for PS-InSAR analysis. However, we applied space adaptive filtering to increase the number of pixels for measuring surface displacement. The results of our analysis demonstrate ground subsidence with decaying velocity over the observation period around the geothermal field. The spatial pattern of ground subsidence includes sharp boundaries of subsidence that can be interpreted as fault traces. We demonstrated the usefulness of PS-InSAR analysis with the space adaptive filtering to estimate surface displacements with high spatial resolution and high spatial density around a geothermal field.

A model study of residual oil distribution jointly using crosswell and borehole-surface electric potential methods

Although high resolution can be provided by electrical logging, the measured electrical log range is narrow and is limited to near the well. Borehole-surface electric potential measurements are able to detect a wide enough range but its resolution is limited, particularly for reservoirs with complex oil and water distribution or complicated structure. In this study, we attempt to accurately locate the 3-D reservoir water and oil distribution by combining borehole-surface and crosswell electric potentials. First, the distributions of oil and water in both vertical and horizontal directions are detected by the borehole-surface and crosswell electric potential methods, respectively, and then the measured crosswell potential result is used to calibrate the measured borehole-surface electric potential data to improve vertical resolution so that the residual oil distribution is determined in a lower half-space with three dimensions. The evaluation of residual oil distribution is obtained by investigation of differences between the simulation results of the reservoir with and without water flooding. The finite difference numerical simulation results prove that the spatial residual oil distribution can be effectively determined by combining the crosswell and borehole-surface electric potentials.

Estimation and monitoring heat discharge rates using Landsat ETM+ thermal infrared data: a case study in Unzen geothermal field, Kyushu, Japan

The Unzen geothermal field, our study area is active fumaroles, situated in Shimabara Peninsula of Kyushu Island in Japan. Our prime objectives were (1) to estimate radiative heat flux (RHF), (2) to calculate approximately heat discharge rate (HDR) using the relationship of radiative heat flux with the total heat loss derived from two geothermal field studies and (3) finally, to monitor RHF as well as HDR in our study area using seven sets of Landsat 7 ETM+ images from 2000 to 2009. We used the NDVI (Normalized differential vegetation index) method for spectral emissivity estimation, the mono-window algorithm for land surface temperature (LST) and the Stefan-Boltzmann equation analyzing those satellite TIR images for RHF. We obtained a desired strong correlation of LST above ambient with RHF using random samples. We estimated that the maximum RHF was about 251 W/m2 in 2005 and minimum was about 27 W/m2 in 2001. The highest total RHF was about 39.1 MW in 2005 and lowest was about 12 MW in 2001 in our study region. We discovered that the estimated RHF was about 15.7 % of HDR from our studies. We applied this percentage to estimate heat discharge rate in Unzen geothermal area. The monitoring results showed a single fold trend of HDR from 2000 to 2009 with highest about 252 MW in 2005 and lowest about 78 MW in 2001. In conclusion, TIR remote sensing is thought as the best option for monitoring heat losses from fumaroles with high efficiency and low cost.

Repeat Absolute and Relative Gravity Measurements for Geothermal Reservoir Monitoring in the Ogiri Geothermal Field, Southern Kyushu, Japan

Repeat hybrid microgravity measurements were conducted around the Ogiri Geothermal Field on the western slope of Kirishima volcano, southern Kyushu, Japan. This study was undertaken to detect the short-term gravity change caused by the temporary shutdown of production and reinjection wells for regular maintenance in 2011 and 2013. Repeat microgravity measurements were taken using an A-10 absolute gravimeter (Micro-g LaCoste) and CG-5 gravimeter (Scintrex) before and after regular maintenance. Both instruments had an accuracy of 10 μgal. The gravity stations were established at 27 stations (two stations for absolute measurements and 25 stations for relative measurements). After removal of noise effects (e.g., tidal movement, precipitation, shallow groundwater level changes), the residual gravity changes were subdivided into five types of response. We detected a gravity decrease (up to 20 μgal) in the reinjection area and a gravity increase (up to 30 μgal) in the production area 1 month after the temporary shutdown. Most of the gravity stations recovered after the maintenance. The temporal density changes in the geothermal reservoir were estimated based on these gravity changes.

Monitoring heat losses using Landsat ETM+ thermal infrared data - a case study at Kuju fumarolic area in Japan

To monitor heat losses using Landsat 7 thermal infrared data from 2002 to 2010 within the active fumarolic region of Kuju volcano in Japan, we used the Stefan-Boltzmann equation for radiative heat flux (RHF) estimation. Heat discharge rate (HDR) was calculated by using the relationship coefficient of RHF and HDR, obtained from two previous studies. The highest total RHF was found to be about 57.7 MW in 2002 and the lowest was about 21.1 MW in 2010. We found the highest HDR, of about 384.5 MW, in 2002 and the lowest, of about 140.8 MW, in 2010. The RHF anomalous areas were showing a declining trend during our study period. The relationship between the land surface temperature (LST) above ambient and RHF was, as expected, in a strong correlation for each result during our study period. Overall, our study was able to delineate the declining trend of heat losses that supports a previous study of similar declining trend of HDR using steam maximum diameter method from the active fumarolic region of Kuju volcano.

Research on Heavy Oil Thermal Recovery by CO2 Steam Flooding with Help of Combination of Borehole-Surface Electric Potential and Cross-Borehole Electric Potential:

With an increasing tendency towards more demand for energy resources, the supply of energy as a focus of global strategy is attracting more and more attention from the world. However, on the one hand, conventional hydrocarbon resources are decreasing gradually, and therefore it is definitely an urgent task to search for renewable and replaceable resources at the present time. On the other hand, it has been proved that the total reserves of heavy oil are already up to 1105×108 tons around the world, which means that exploring heavy oil can be a beneficial supplement for alleviating the shortage of oil and gas. Moreover, it is noteworthy that because the heavy oil can be exploited by heated CO2, collecting and consuming CO2 during the production process will help to relieve global warming. In this study, we take the feasibility of heavy oil recovery by CO2 steam into consideration only from the viewpoint of geophysics. In the process of research, with the help of borehole-surface electric potential and cross-borehole electric potential, the entire procedures from heating heavy oil reservoir and optimizing the location of well to deciding the layer of perforation are exhibited completely. In the course of calculation, potential distributions corresponding to a point source of current are acquired by solving the Poisson equation using a direct and explicit finite difference technique for a lower half-space with 3-D distribution of conductivity. As for computation of a large sparse matrix, the technique of nonzero bandwidth storage and the Incomplete Cholesky Conjugate Gradient method are adopted. The consequences prove that with the assistance of cross-borehole electric potential combining with borehole-surface electric potential, the project of heavy oil recovery by CO2 steam is feasible and effective.

Monitoring Hot Spring Aquifer Using Repeat Hybrid Micro-gravity Measurements in Beppu Geothermal Field, Japan

Repeat hybrid micro-gravity measurements were conducted to detect the gravity change caused by hot spring water production around Beppu in eastern Kyushu, Japan. An A10 #017 absolute gravimeter (Micro-g LaCoste) and a CG-5 #549 gravimeter (Scintrex) were used for this study in intervals of three to four months at eight gravity stations. According to the results obtained with the absolute gravimetry, a gravity change of up to 33 μgal was detected at the Beppu Geothermal Research Laboratory (BGRL) reference station. The observed absolute gravity was compared with the groundwater level, and there was a good correlation between the gravity changes and the groundwater level changes. Based on the precipitation, groundwater level, and soil character, the effect of the water content changes in the unsaturated zone was estimated precisely by using a Gwater-1D. This calculation can explain that the gravity seasonal changes were caused by the groundwater level changes. After removal of noise effects (e.g., tidal movement, precipitation, and shallow groundwater level changes), the residual gravity changes, which were measured by the relative gravimeter, were subdivided into two types of responses. Gravity changes up to 90 μgal were observed from April 2014 to July 2015. After that, gravity became stable, except for small seasonal changes.

Interpretation of gravity and gamma-ray spectrometry data in low temperature hydrothermal systems, southeastern part of Fukuoka, Japan

The Hakata hot springs area is located in Fukuoka City, which is in the southwestern part of Japan. Gamma-ray and gravity surveys were conducted to understand the relationship between the low-temperature hydrothermal systems and geophysical data of the area. The depth of the reservoir basement, which was derived from gravity data, gradually deepens toward the east; it includes some steep depth gradients in the Hakata hot springs area. High intensities of gamma-rays were detected around these gradients. In addition, higher hot spring temperatures and flow rates can be observed in this area. These results indicate that some part of the level of the basement where the hot springs are concentrated is a part of the Kego Fault and is similar to the fracture zone created by past activities of the fault. Moreover, these steep depth gradients act as a path for hot spring water from the deeper side of the granitic body to the surface.