On August 26, 2019, the deep-hole integrated geophysical observation system of the long-term observatory of the Chinese Continental Scientific Drilling project (CCSD) was successfully placed into the deep-hole and received signals in real time (Fig.1).The deep-hole geophysical observation system was installed to expected design depth: two sets of seismographs and three sets of geotherms were placed at the main hole of 1498 m, 2367 m, 3006 m and 3499 m respectively.The temperature of the deepest installation position is up to 98 ℃, the pressure is up to 35 MPa, and the minimum inner diameter is only 175 mm.It is the deepest deep-hole geophysical observation system in China (Fig.2).

Fig.1.Engineers install deep-hole in-situ seismometers.

Fig.2.Records of Huaian M1.9 earthquake (epicentral distance is 112 km).a-Vertical component of broadband seismometer on ground;b-vertical component of 1500 m depth seismometer; c-vertical component of 2500 m depth seismometer; d-vertical component of 3000 m depth seismometer; e-vertical component of 3500 m depth seismometer.

1.Establishment of long-term observatory of CCSD

Chinese Continental Scientific Drilling project (CCSD)was started drilling in 2001 and completed drilling in 2005 with drilling depth to 5158 m.In 2007, the former Ministry of Land and Resources of China approved the establishment of a Long-term Observatory based on the Institute of Geological,Chinese Academy of Geological Sciences (CAGS), to conduct integrated geophysical observation in main hole.It is located at the intersection of the Su-Lu ultrahigh-pressure metamorphic belt and the Tan-Lu fault zone, where the Tancheng earthquake (M8.5) of 1668 and the Heze earthquake (M7.0) of 1937 occurred.

2.Decade progress of long-term observatory

CCSD long-term observation station is surrounded by intact bedrock and plagioclastic (monobaric) granitic gneiss which containing garnet and biotite.Firm and stable bedrock is suitable for geophysical observation.Over the past ten years, research team of long-term observation station has obtained continuous and stable data with a 450 m borehole integrated geophysical observation instrument and seven broadband three-component seismometers which around the CCSD main hole.

3.Significance of observation network

In order to accurate monitor the seismic activity and better understanding lower crust deformation, the research team designed the integrated geophysical observation network after years of repeated argumentations and tests.The observation network including trinity measuring method: deep-hole integrated geophysical observation system, borehole comprehensive geophysical observation instrument and broadband seismometers which installed on the ground.

Compared with sub-surface observation, in-situ observation at deep-hole can effectively eliminate the ambient noise, such as human activity and noise from tide, and improved observation sensitivity by 1-2 orders.The geophysical observation network can simultaneously observe and record multiple geophysical and geochemical data, including strain, geomagnetism, geothermal, seismic, tilt, absolute gravity and radon leakage.Cross-referenced multiple longterm continuous data can be used to study the crustal activity in the eastern China and reveals the adjacent area lithosphere deformation.It may help scientists to better understand the geodynamics process in ocean-continent conversion.

4.Contact

Long-term observatory of Chinese Continental Scientific Drilling Project

Address: CCSD Site, Donghai County, Jiangsu Province,222300, China

Institute of Geology, CAGS

Address: No.26 Baiwanzhuang Street 100037 Beijing,China

Tel: +86-10-68999684

Email: zengxiangzhi@cags.ac.cn