Title:

The Second Seismic Study at the Geothermal Field in Southern Kyushu, Japan Using an Optical Fiber System and Surface Geophones

Authors:

Junzo KASAHARA, Yoko HASADA, Haruyasu KUZUME, Hitoshi MIKADA, Yoshihiro FUJISE,

Key Words:

DAS, DTS, imaging, seismic method, feasibility study,VSP

Conference:

Stanford Geothermal Workshop

Year:

2020

Session:

Geophysics

Language:

English

Paper Number:

Kasahara2

File Size:

1136 KB

View File:

Abstract:

Following our first seismic study at the Medipolis geothermal field in southwestern Japan in 2018, we conducted a second seismic study at the same geothermal field in 2019. We installed an optical-fiber system for distributed temperature sensor (DTS) and distributed acoustic sensors (DASs) measurements. We deployed the optical-fiber system at a 1,545-m depth in the IK-4 borehole. The temperature was measured to be 272.8 °C at a 920-m depth and 152.8 °C at a 1,530-m depth. We operated a MiniVib seismic source at five locations and performed a frequency sweep of 10–75 Hz 480 times each day, for seven days. We cross-correlated the seismic records and the source signature and stacked the correlated data to enhance the S/N. Stacking for 480 or 960 times considerably improved the arrival waveforms. Based on an analysis of DAS data, we constructed the 2D seismic profile. We estimated three major hydrothermal layers, at depths of 800–1,000 m, 1,300–1,600 m, and 3,600 m. The zone around 3,600 m suggests a high Vp/Vs value and the possible presence of a fluid layer.


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