MONITORING OF A HOT SPRING AQUIFER USING REPEATED MICROGRAVITY MEASUREMENTS IN THE NORTHERN PART OF BEPPU, JAPAN

Abstract

Repeated microgravity measurements for monitoring a hot spring aquifer have been conducted in the northern part of Beppu, Northeastern Kyushu, Japan. We used not only a SCINTREX CG-5 relative gravimeter but also a Micro-g LaCoste A10 absolute gravimeter to assess the gravity changes at the reference station (BGRL). Repeated microgravity measurements were conducted five times at three or four month’s intervals from April 2014 to July 2015 at 8 benchmarks including the reference station. Through comparison between the gravity and groundwater level data, we concluded that the obtained gravity changes at BGRL are caused by the seasonal groundwater level changes since the calculated porosity (23.6%) is close to the measured porosity of alluvial fan sediments (20.0%). The distribution map of gravity changes in the second period (from July 2014 to November 2014) shows huge difference in fluctuation amplitude of the observed gravity between the central area (B1, B2, B3) and the northern area (C1, C2, C3, TERUYU). We assume that is due to the thickness of alluvial fan sediments, which is widely distributed in Beppu district, based on a three dimensional gravity analysis. More gravity and groundwater level data should be accumulated, and a hydrological modeling is necessary to extract the gravity changes caused by the binary power plant operation.