Jun Nishijima

Landwater variation in four major river basins of the Indochina peninsula as revealed by GRACE

We estimated mass variations in four major river basins—the Mekong, Irrawaddy, Salween and Chao Phraya river basins—of the Indochina Peninsula using the newly released GRACE (Gravity Recovery and Climate Experiment) monthly gravity field solutions of UTCSR RL02 (University of Texas at Austin, Center for Space Research Release 02), JPL RL02 (Jet Propulsion Laboratory Release 02) and GFZ RL03 (GeoForschungsZentrum Potsdam Release 03). The estimated variations were compared with that calculated from a numerical model. The results show that there is a good agreement between the GRACE estimations and the model calculation for the Mekong and Irrawaddy basins, while the aggreement for the Salween and Chao Phraya basins is poor, mainly due to the spatial scale of the areas concerned. The comparison over the combined area of the four river basins shows fairly good agreement, although there are small quantitative discrepancies. The amplitudes of the annual signals of the GRACE solutions are 0.9- to 1.4-fold larger than that of the hydrological model, and the phases are delayed about 1 month compared with the model signal. The phase differences are probably due to improper treatments of the groundwater storage process in the hydrological model, suggesting that the GRACE data possibly provide constraints to the model parameters.

Integrated gradient interpretation techniques for 2D and 3D gravity data interpretation

The Obama geothermal field is located on the western part of Kyushu Island, Japan. This area has importance due to its high geothermal content which attracts sporadic researchers for study. In 2003 and 2004, Obama was covered by gravity surveys to monitor and evaluate the geothermal field. In this paper, the surveyed gravity data will be used in order to delineate and model the subsurface structure of the study area. Gradient methods such as analytic signal and vertical derivatives were applied to the gravity data. The available borehole data and the results of the gradient interpretation techniques were used to model the Obama geothermal field. In general, the obtained results show that the gradient interpretation techniques are useful to obtain geologic information from gravity data.

Monitoring groundwater variation by satellite and implications for in-situ gravity measurements

In order to establish a new technique for monitoring groundwater variations in urban areas, the applicability of precise in-situ gravity measurements and extremely high precision satellite gravity data via GRACE (Gravity Recovery and Climate Experiment) was tested. Using the GRACE data, regional scale water mass variations in four major river basins of the Indochina Peninsula were estimated. The estimated variations were compared with Soil-Vegetation-Atmosphere Transfer Scheme (SVATS) models with a river flow model of 1) globally uniform river velocity, 2) river velocity tuned by each river basin, 3) globally uniform river velocity considering groundwater storage, and 4) river velocity tuned by each river basin considering groundwater storage. Model 3) attained the best fit to the GRACE data, and the model 4) yielded almost the same values. This implies that the groundwater plays an important role in estimating the variation of total terrestrial storage. It also indicates that tuning river velocity, which is based on the in-situ measurements, needs further investigations in combination with the GRACE data. The relationships among GRACE data, SVATS models, and in-situ measurements were also discussed briefly.

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.

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.

Application of A10 Absolute Gravimeter for Monitoring Land Subsidence in Jakarta, Indonesia

To investigate the cause of land subsidence occurring in Jakarta, Indonesia, we have conducted absolute gravity measurements near or at the existing GPS points in 2008, 2009, 2010, 2011 and 2013, by using a portable absolute gravimeter Micro-G LaCoste A10 serial number 017 (MGL A10-017). The A10 measurements, particularly those conducted before 2010, were adversely affected by the influence of the high-temperature and high-humidity climate, which caused problems with the ion vacuum pump, laser controls and unstable behavior of the data processing personal computer. Moreover, some of the gravity points were lost mainly due to construction work. For these reasons, the number of reliable gravity data was restricted. Nevertheless, the observed gravity changes along the northern coastal area of Jakarta showed an increase in gravity, which was consistent with the GPS results. The analysis of the height changes versus the gravity changes suggested that the subsidence was caused by soil compaction and/or groundwater level changes, which are likely connected with excessive groundwater extraction.

A Coastal Aquifer Study Using Magnetotelluric and Gravity Methods in Abo Zenema, Egypt

Magnetotelluric (MT) soundings and gravity methods were employed to study the deep freshwater aquifer in the area north of Abo Zenema city on the eastern side of the Gulf of Suez, Egypt. Seven MT sites and 48 gravity stations were surveyed along northeast–southwest profiles as close as possible to a line perpendicular to the coast of the Gulf of Suez. The MT survey was conducted using high and low frequencies to investigate shallow and deep areas, respectively. One-dimensional inversion was conducted using a heuristic inversion scheme of the Bostick algorithm. The MT data were also inverted with a 2-D smooth model inversion routine using the nonlinear conjugate gradient method to infer variation in vertical and lateral resistivity inside the Earth. A 100-Ohm-m homogeneous half-space initial model was used to invert the TE mode data only. Then, the inverted model obtained from the TE mode data was used as an initial model for inversion of the TM mode data. The inverted model thus obtained from the TM mode data inversion was used as an initial model for the inversion of the joint TE and TM responses. Two-dimensional (2-D) forward modeling of the gravity data was conducted using the 2-D polygon method of Talwani’s algorithm for an arbitrarily shaped body and was based on the subsurface information from the MT survey and the available information about the geological structure of the study area. This method enabled us to obtain the basement structure of the coastal aquifer in the study area. The results from the analysis and the interpretation of MT and gravity data were used to detect and delineate the groundwater coastal aquifer in the study area.

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.

New Absolute Gravity Measurements in New Zealand

To enhance and extend the absolute gravity (AG) measurements in New Zealand, we conducted new measurements using a FG5 (#210 of Kyoto University) in January and March 2016. The measurements in the North Island were made at two existing points (the Warkworth Radio Astronomical Observatory and Wellington A) and at one newly established point at the Wairakei Research Centre, Taupo. The gravity measurements in the South Island were made at five existing AG points; Godley Head, Mt John, the University of Otago, Helipad and Bealey Hotel. At each point more than 4,000 drops were made and AG values were determined with measurement uncertainties better than 3 μGal (mostly better than 2 μGal) at 130 cm instrument height. The values are compared with those of the 2015 campaigns. Although the differences of about 10 μGal were observed at Wellington A and Godley Head, those at the other points were within 5 μGal. At points in the Southern Alps we combined AG with relative gravity measurements and achieved good agreement with the 2015 results. Definite values for long-term gravity trends at the points in Southern Alps and Christchurch could not be obtained from the survey, but the results are consistent with those of the previous studies. Further measurements are planned to accurately determine these gravity changes.