Fluid activity detection in geothermal areas using a single seismic station by monitoring horizontal-to-vertical spectral ratios

Kyosuke Okamoto,Hiroshi Asanuma,Hiro Nimiya

doi:10.1038/s41598-021-86775-1

Kyosuke Okamoto, Hiroshi Asanuma + Show 1 more

Open Access

https://doi.org/10.1038/s41598-021-86775-1

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Abstract

Subsurface structure survey based on horizontal-to-vertical (H/V) spectral ratios is widely conducted. The major merit of this survey is its convenience to obtain a stable result using a single station. Spatial variations of H/V spectral ratios are well-known phenomena, and it has been used to estimate the spatial fluctuation in subsurface structures. It is reasonable to anticipate temporal variations in H/V spectral ratios, especially in areas like geothermal fields, carbon capture and storage fields, etc., where rich fluid flows are expected, although there are few reports about the temporal changes. In Okuaizu Geothermal Field (OGF), Japan, dense seismic monitoring was deployed in 2015, and continuous monitoring has been consistent. We observed the H/V spectral ratios in OGF and found their repeated temporary drops. These drops seemed to be derived from local fluid activities according to a numerical calculation. Based on this finding, we examined a coherency between the H/V spectral ratios and fluid activities in OGF and found a significance. In conclusion, monitoring H/V spectral ratios can enable us to grasp fluid activities that sometimes could lead to a relatively large seismic event.

Highlights

Subsurface structure survey based on horizontal-to-vertical (H/V) spectral ratios is widely conducted
Based on the numerical calculation of ambient noise propagation in the model shown in Fig. 3, H/V spectral ratios were observed at the receiver in a borehole that simulated YAE6
We found a temporary drop in H/V spectral ratios in a geothermal area in Japan

Summary

Introduction

Subsurface structure survey based on horizontal-to-vertical (H/V) spectral ratios is widely conducted. It is reasonable to anticipate temporal variations in H/V spectral ratios, especially in areas like geothermal fields, carbon capture and storage fields, etc., where rich fluid flows are expected, there are few reports about the temporal changes. We observed the H/V spectral ratios in OGF and found their repeated temporary drops These drops seemed to be derived from local fluid activities according to a numerical calculation. The H/V spectral ratio was proposed as a seismic response approximate of subsurface S-wave structures known as the Nakamura method[1]. In this method, the H/V spectral ratio observed on the ground surface is considered as the resonance frequency and amplification of ground motions. The behavior of elastic wave velocities of porous media is complex because it is a function of effective stress (confining pressure minus pore pressure), porosity, saturation, pore fluid (e.g., water, vapor, gas, and air), temperature and other factors that change matrix parameters like chemical reactions

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