Egu General Assembly Conference Abstracts, May 2, 2010
We report on results from post-eruptive time-lapse gravity investigations performed between 1999 ... more We report on results from post-eruptive time-lapse gravity investigations performed between 1999 and 2004 at Unzen volcanic complex, Shimabara Peninsula, Kyushu Island, southwest Japan. One of the most active volcanoes in Japan, Unzen saw its most recent eruption between 1990 and 1995. We demonstrate that annual residual gravity variations reflect subsurface mass changes most likely coupled to changes in shallow
Memoirs of the Faculty of Engineering Kyushu University, Jun 1, 2006
ABSTRACT Shimabara Peninsula is located in southwestern Japan. There are three geothermal areas a... more ABSTRACT Shimabara Peninsula is located in southwestern Japan. There are three geothermal areas as well as a volcano on this Peninsula. In this study, we attempt to delineate the subsurface structures of the area using integrated interpretation techniques on gravity data. This study describes data processing and interpretation methods for gravity data. This processing was required in order to estimate the depths to the gravity sources, and to estimate the locations of the contacts of density contrast. The power spectral analysis was used to delineate the regional and the local components of the Bouguer anomaly. A bandpass filter was applied in order to separate the local from the regional Bouguer anomaly. Three methods were used for estimating source depths and contact locations: the horizontal gradient method, the analytic signal method, and the Euler deconvolution method. The depth estimation resulting from the respective methods were compared, and the contact locations combined into an interpretative map showing the direction for some contacts.
The Fukuoka area is located in the southwestern part of Japan. The Yokote-Ijiri area, located in ... more 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.
Unzen volcano is located in Shimabara Peninsula (Nagasaki prefecture), Kyushu Island, southwest o... more Unzen volcano is located in Shimabara Peninsula (Nagasaki prefecture), Kyushu Island, southwest of Japan, and one of the most active volcanoes in Japan. The most recent eruption occurred in 1990 to 1995. The possibility of existing or degenerating of a new hydrothermal system beneath Unzen volcano associated with the last eruption, was investigated by using repeat gravity monitoring since August 1999. The variations in observed gravity depend significantly on changes in shallow groundwater level changes. A good correlation of gravity with precipitation is observed with a phase lag about 3 months in some regions in the study area. The water level changes are studied by Unzen Scientific Drilling Project porosity data. The results show that the downflow of permeating rain water is dominant.
Our purpose is to predict earthquake disaster in Fukuoka city. A predictive estimation of ground ... more Our purpose is to predict earthquake disaster in Fukuoka city. A predictive estimation of ground motion requires an accurate subsurface structure. But the detailed subsurface structure in the western Fukuoka city was still unknowndue to lack of gravity data and uneven distribution of deep boring data. So we conducted densed gravity measurements to obtain a detailed Bouguer anomaly map and estimated subsurface structure based on the measured gravity data. We used Scintrex CG-3+ and CG-5 relative gravimeters for the gravity measurement and we measured the elevation of the measurement point by using network RTK-GPS.As a result of drawing a map, we found a steep slope of Bouguer anomaly which runs from west to east across the western Fukuoka city. And we also found out the slope is a boundary which divides the area into the granite layer in the south and the Paleogene layer in the north. By the prism modeling of the subsurface structure in this area, granite was found in about 30m depth ground level in the south and in about 250~300m depth in the north. Therefore the depth of the granite is approximately 200~250m deeper on the north side of the boundary than that in south.
We usually create a geothermal conceptual model based on various exploration data. After that, we... more We usually create a geothermal conceptual model based on various exploration data. After that, we construct a numerical model for geothermal resource evaluation. In this study, we selected a hot spring area in Fukuoka city where a numerical model was constructed in the previous study as a study area. And we tried to conduct preprocessing and post-processing of a numerical model and to calculate geothermal resources in the hot springs area by using GIS. As a result, GIS provided us a more objective model which reflects exploration data better than the past model. Though it is difficult to judge which model is correct, we can say that using GIS is very useful in terms of conducting numerical simulation objectively and efficiently. In evaluation of geothermal resources, we calculated resources by volumetric method. We usually use the average values of temperature, physical properties, etc. for the calculation in volumetric method. However, GIS can compute the resources by more strict calculation method, namely GIS can calculate the resources with distribution of temperature and physical properties. Assuming that the lower limit temperature for using geothermal resources is 40 , the estimated abundance of heat is 1.14×10 13 kJ.
ABSTRACT Time-lapse gravity monitoring is a way to assist in building up knowledge of the changes... more ABSTRACT Time-lapse gravity monitoring is a way to assist in building up knowledge of the changes of subsurface condition. Gravity monitoring techniques have been applied to the investigation of dynamic processes in various types of geothermal and volcanic fields. Gravity changes enable the characterization of subsurface processes: i.e., the mass of the intrusion or hydrothermal flow. A key assumption behind gravity monitoring is that changes in earth’s gravity reflect mass-transport processes at depth (Battaglia et al., 2008). Combined absolute and relative gravimeter is an advance method that is newly used for monitoring. Use of an absolute gravimeter as a reference for other relative measurements, a technique called hybrid gravimetry (Okubo et al., 2002), was applied successfully to geothermal studies (Sugihara et al., 2008; Nishijima et al., 2010). We use relative gravimeters (LaCoste-Romberg or Scintrex) and were combined by absolute gravimeter (A-10 Micro-g) to monitor the subsurface condition. A10 absolute gravimeter is a new generation of portable absolute instrument and has accuracy 10 µGal. This method has many advantages for monitoring method. Some of the advantages are the ability to reduce uncertainties caused by regional gravity variations and reduce the drift correction factor. The repeated absolute gravity measurement was conducted in the Kamojang geothermal field between 2011. On the other hand, the repeated relative gravity measurement also was conducted in the same field. Some gravity benchmarks were measured using both absolute and relative gravimeter and is used as the reference benchmarks. The combined absolute and relative gravity measurement can reduce drift correction of relative measurement. In the long time period, the hybrid gravimetry method will improve the result of gravity change data for monitoring in Kamojang geothermal field. This present study introduces a time lapse monitoring method that will enhance reservoir monitoring using repeated precisely-gravity measurements.
Hatchobaru geothermal field is located 5km northwest of Kuju volcano, central Kyushu, and it show... more Hatchobaru geothermal field is located 5km northwest of Kuju volcano, central Kyushu, and it shows a fractured type of geothermal reservoir. We have been conducted GPS and micro-gravity monitoring to detect the ground deformations and mass fluid movement caused by the production and reinjection. Increases in gravity were observed in the reinjection zone and part of the production zone just after the commencement of No. 2 unit. After that, in the production area, a rapid decrease of gravity (up to 200 µgal) was observed. Since 1998 the gravity change became steady. A contour map of recent gravity changes show that the areas of gravity changes became very small. And these areas are limited in the part of the production and reinjection zone. We suppose that the recharge from surrounding area became gradually stable and reached a new equilibrium state. Changes of baseline up to 64 mm, were detected from August 2000 to October 2003. We tried to estimate the location of pressure source ap...
Egu General Assembly Conference Abstracts, May 2, 2010
We report on results from post-eruptive time-lapse gravity investigations performed between 1999 ... more We report on results from post-eruptive time-lapse gravity investigations performed between 1999 and 2004 at Unzen volcanic complex, Shimabara Peninsula, Kyushu Island, southwest Japan. One of the most active volcanoes in Japan, Unzen saw its most recent eruption between 1990 and 1995. We demonstrate that annual residual gravity variations reflect subsurface mass changes most likely coupled to changes in shallow
Memoirs of the Faculty of Engineering Kyushu University, Jun 1, 2006
ABSTRACT Shimabara Peninsula is located in southwestern Japan. There are three geothermal areas a... more ABSTRACT Shimabara Peninsula is located in southwestern Japan. There are three geothermal areas as well as a volcano on this Peninsula. In this study, we attempt to delineate the subsurface structures of the area using integrated interpretation techniques on gravity data. This study describes data processing and interpretation methods for gravity data. This processing was required in order to estimate the depths to the gravity sources, and to estimate the locations of the contacts of density contrast. The power spectral analysis was used to delineate the regional and the local components of the Bouguer anomaly. A bandpass filter was applied in order to separate the local from the regional Bouguer anomaly. Three methods were used for estimating source depths and contact locations: the horizontal gradient method, the analytic signal method, and the Euler deconvolution method. The depth estimation resulting from the respective methods were compared, and the contact locations combined into an interpretative map showing the direction for some contacts.
The Fukuoka area is located in the southwestern part of Japan. The Yokote-Ijiri area, located in ... more 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.
Unzen volcano is located in Shimabara Peninsula (Nagasaki prefecture), Kyushu Island, southwest o... more Unzen volcano is located in Shimabara Peninsula (Nagasaki prefecture), Kyushu Island, southwest of Japan, and one of the most active volcanoes in Japan. The most recent eruption occurred in 1990 to 1995. The possibility of existing or degenerating of a new hydrothermal system beneath Unzen volcano associated with the last eruption, was investigated by using repeat gravity monitoring since August 1999. The variations in observed gravity depend significantly on changes in shallow groundwater level changes. A good correlation of gravity with precipitation is observed with a phase lag about 3 months in some regions in the study area. The water level changes are studied by Unzen Scientific Drilling Project porosity data. The results show that the downflow of permeating rain water is dominant.
Our purpose is to predict earthquake disaster in Fukuoka city. A predictive estimation of ground ... more Our purpose is to predict earthquake disaster in Fukuoka city. A predictive estimation of ground motion requires an accurate subsurface structure. But the detailed subsurface structure in the western Fukuoka city was still unknowndue to lack of gravity data and uneven distribution of deep boring data. So we conducted densed gravity measurements to obtain a detailed Bouguer anomaly map and estimated subsurface structure based on the measured gravity data. We used Scintrex CG-3+ and CG-5 relative gravimeters for the gravity measurement and we measured the elevation of the measurement point by using network RTK-GPS.As a result of drawing a map, we found a steep slope of Bouguer anomaly which runs from west to east across the western Fukuoka city. And we also found out the slope is a boundary which divides the area into the granite layer in the south and the Paleogene layer in the north. By the prism modeling of the subsurface structure in this area, granite was found in about 30m depth ground level in the south and in about 250~300m depth in the north. Therefore the depth of the granite is approximately 200~250m deeper on the north side of the boundary than that in south.
We usually create a geothermal conceptual model based on various exploration data. After that, we... more We usually create a geothermal conceptual model based on various exploration data. After that, we construct a numerical model for geothermal resource evaluation. In this study, we selected a hot spring area in Fukuoka city where a numerical model was constructed in the previous study as a study area. And we tried to conduct preprocessing and post-processing of a numerical model and to calculate geothermal resources in the hot springs area by using GIS. As a result, GIS provided us a more objective model which reflects exploration data better than the past model. Though it is difficult to judge which model is correct, we can say that using GIS is very useful in terms of conducting numerical simulation objectively and efficiently. In evaluation of geothermal resources, we calculated resources by volumetric method. We usually use the average values of temperature, physical properties, etc. for the calculation in volumetric method. However, GIS can compute the resources by more strict calculation method, namely GIS can calculate the resources with distribution of temperature and physical properties. Assuming that the lower limit temperature for using geothermal resources is 40 , the estimated abundance of heat is 1.14×10 13 kJ.
ABSTRACT Time-lapse gravity monitoring is a way to assist in building up knowledge of the changes... more ABSTRACT Time-lapse gravity monitoring is a way to assist in building up knowledge of the changes of subsurface condition. Gravity monitoring techniques have been applied to the investigation of dynamic processes in various types of geothermal and volcanic fields. Gravity changes enable the characterization of subsurface processes: i.e., the mass of the intrusion or hydrothermal flow. A key assumption behind gravity monitoring is that changes in earth’s gravity reflect mass-transport processes at depth (Battaglia et al., 2008). Combined absolute and relative gravimeter is an advance method that is newly used for monitoring. Use of an absolute gravimeter as a reference for other relative measurements, a technique called hybrid gravimetry (Okubo et al., 2002), was applied successfully to geothermal studies (Sugihara et al., 2008; Nishijima et al., 2010). We use relative gravimeters (LaCoste-Romberg or Scintrex) and were combined by absolute gravimeter (A-10 Micro-g) to monitor the subsurface condition. A10 absolute gravimeter is a new generation of portable absolute instrument and has accuracy 10 µGal. This method has many advantages for monitoring method. Some of the advantages are the ability to reduce uncertainties caused by regional gravity variations and reduce the drift correction factor. The repeated absolute gravity measurement was conducted in the Kamojang geothermal field between 2011. On the other hand, the repeated relative gravity measurement also was conducted in the same field. Some gravity benchmarks were measured using both absolute and relative gravimeter and is used as the reference benchmarks. The combined absolute and relative gravity measurement can reduce drift correction of relative measurement. In the long time period, the hybrid gravimetry method will improve the result of gravity change data for monitoring in Kamojang geothermal field. This present study introduces a time lapse monitoring method that will enhance reservoir monitoring using repeated precisely-gravity measurements.
Hatchobaru geothermal field is located 5km northwest of Kuju volcano, central Kyushu, and it show... more Hatchobaru geothermal field is located 5km northwest of Kuju volcano, central Kyushu, and it shows a fractured type of geothermal reservoir. We have been conducted GPS and micro-gravity monitoring to detect the ground deformations and mass fluid movement caused by the production and reinjection. Increases in gravity were observed in the reinjection zone and part of the production zone just after the commencement of No. 2 unit. After that, in the production area, a rapid decrease of gravity (up to 200 µgal) was observed. Since 1998 the gravity change became steady. A contour map of recent gravity changes show that the areas of gravity changes became very small. And these areas are limited in the part of the production and reinjection zone. We suppose that the recharge from surrounding area became gradually stable and reached a new equilibrium state. Changes of baseline up to 64 mm, were detected from August 2000 to October 2003. We tried to estimate the location of pressure source ap...
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