HR: 1340h
AN: G33A-0888    [Abstracts]
TI: Hydrological Modeling of Groundwater Disturbance to Gravity Signal for High-accuracy Monitoring of Volcanic Activity
AU: * Kazama, T
EM: takujin@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Okubo, S
EM: okubo@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AB: Gravity observation is one of the effective methods to detect magma movements in volcanic eruptions [e.g., Furuya et al., J. Geoph. Res., 2003]. Groundwater-derived disturbances have to be corrected from gravity variations for highly accurate monitoring of volcanic activities. They have been corrected with empirical methods, such as tank models and regression curves [e.g., Imanishi et al., J. Geodyn., 2006]. These methods, however, are not based on hydrological background, and are very likely to eliminate volcanic signals excessively. The correction method of groundwater disturbance has to be developed with hydrological and quantitative approach. We thus estimate the gravity disturbance arising from groundwater as follows. (1) Groundwater distributions are simulated on a hydrological model, utilizing groundwater flow equations. (2) Groundwater-derived gravity value is estimated for each instant of time, by integrating groundwater distributions spatially. (3) The groundwater-derived gravity, as the correction value, is subtracted from observed gravity data. In this study, we simulated groundwater flow and groundwater-derived gravity value on the east part of the Asama volcano, central Japan. A simple hydrological model was supposed, consisting of homogeneous soil, lying on a flat impermeable basement. Hydraulic conductivity, which defines groundwater velocity, was set as 2.0×10^-6[m/s], which is consistent with typical volcanic soils. We also observed time variations of watertable height, soil moisture and gravity simultaneously during the summer of 2006 at Asama volcano, and compared the observations with the theoretical values. Both simulated groundwater distributions and gravity changes agree fairly well with observed values. On variations of water level and moisture content, rapid increase at the time of rainfalls and exponential decrease after rainfalls were illustrated. Theoretical gravity changes explained 90% of the observed gravity increase (+20μgals) for the heavy rainfall (200mm) of mid-July 2006. These facts showed that even a simple hydrological model can reproduce characteristic variations of groundwater and gravity at the same time. We believe that hydrological simulation with more sophisticated model (such as 3D inhomogeneous soil lying on a curved basement) will enable us to estimate groundwater disturbance more accurately. Improved groundwater correction will reveal detailed magma movements in volcanic eruptions.
DE: 1217 Time variable gravity (7223, 7230)
DE: 1829 Groundwater hydrology
DE: 5114 Permeability and porosity
DE: 8419 Volcano monitoring (7280)
SC: Geodesy [G]
MN: 2007 Fall Meeting