| Summary: | The article reviews the development of the volumetric borehole strain gauge. In response to the current problems of insufficient stability and bandwidth and low calibration accuracy of the volumetric strain gauge, a TY-2B-type small volumetric borehole strain gauge was developed with innovative improvements in the hydraulic sensor, control circuit, and calibration method. The improved hydraulic sensor improves the accuracy and reduces the instrument’s volume; the improved control circuit increases the sampling rate, bandwidth, and the instrument’s stability; the innovative piezoelectric ceramic calibration technology raises the reliability of the monitoring data. The test results show that the improved TY-2B volumetric strain gauge has a low power consumption of less than 3 W, good long-term stability, high sensitivity with a resolution of 10-11 ε, and suitable high-frequency and low-frequency. It has a sampling rate of 100 Hz and can acquire complete seismic strain waveforms with precise and stable solid tide waveforms. It is small and light, with a reduced outer diameter of Φ89 mm for Φ100 mm drilling, a length of 1300 mm, and a weight of 45 kg for easy transport and installation. After 15 years of laboratory and field station testing, it obtained good monitoring data and demonstrated its highly sensitive seismic reflection capability. The observed response of the volumetric strain station in the northern section of Longmen Mountain to the 2010 Yushu earthquake and the 2023 Turkey earthquake shows that the TY-series high-precision volumetric strain gauge is not only a static strain gauge but also a broad-frequency strain seismograph with dynamic-static calibration capability. It has a unique advantage over pendulum seismometers in that it can observe both the long-term slow deformation and accumulation of deformation in the earth’s crust and the transient subtle features of crustal rupture and deformation. The long-term trends of the monitoring curves obtained from the Qingchuan–Hanzhong volumetric strain stations since the Wenchuan earthquake and the Guangzhou station since 2021 are consistent with the regional geological characteristics reflected by the seismic and tectonic geological data, indicating that the TY-2B volumetric strain gauge can meet the needs of geological scientific research and geological hazard observation.
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