YeMiGO: Data Processing and Analysis of Underground Superconducting Gravity Data in South Korea

Abstract

We present the status of the Yemi micro-gravity observatory (YeMiGO), including the installation, operation, and initial analysis of the gravity data. In October 2022, we installed GWR Instruments Inc,’s iGrav (serial #001) superconducting gravimeter (SG) at Yemi underground laboratory (YemiLab) in South Korea. YemiLab is located approximately 1,008 and 118 meters below the Earth's surface and mean sea level, respectively. The noise characteristics were assessed using one month of raw data collected in September 2023 and compared to those of other seismometer stations. The results show the noise level at the SG station, especially in the seismic band, is significantly low and proves the stability of the Lab.  The research findings also indicate that blasting during mining operations at a distance between ~700 and ~900 meters (please confirm this) from the SG impacted the dewar and barometer pressures as well as the tilt balance data. However, no discernible effects were observed in the raw SG data, leading to the hypothesis that the SG tilt system was able to compensate for the resulting vibrations. After 6 months of continuous data recording from 16th November 2022 to 18th May 2023, a calibration factor of -92.17 μGal∙V-1 was estimated using tidal analysis. In November 2023, a new calibration factor of -94.15 μGal∙V-1 was estimated using parallel measurements with FG5-231 provided by the Ministry of Interior, R.O.C. (Taiwan). Having accounted for various environmental effects, including Earth tide, atmospheric pressure, groundwater level, and polar motion, during the initial six months of data, the residual gravity was obtained. Spectral analysis revealed several unidentified residual gravity power spectrum density frequencies, necessitating further investigation. Co-seismic gravity changes resulting from four earthquakes in May 2023 with different magnitudes and within various distances from the SG station were examined. The M6.2 earthquake that occurred 765 km away was linked to the most notable co-seismic gravity alteration, which recorded a value of 0.561 μGal. The mentioned changes decreased gradually and faded away entirely within half an hour after the SG's first arrival.