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Sub-seafloor Borehole Observatory and its Application in Ocean Drilling
Xianda Zhan, Shun Li, Binbin Guo, Qingjie Zhou, Duanxin Chen, Yonggang Jia and Chaoqi Zhu

Sub-seafloor downhole observation represents a milestone in the development of scientific ocean drilling. Since the first system was installed in 1991, this technology has evolved through a series of successive generations—CORK (Circulation Obviation Retrofit Kit), ACORK (Advanced CORK), Wireline CORK, CORK-II, L-CORK (Lateral CORK), and LTBMS (Long-Term Borehole Monitoring System)—progressively expanding from simple formation-fluid monitoring to interdisciplinary, multi-parameter observation, fluid sampling, and in-situ testing. These systems have provided long-term, continuous, real-time data central to major discoveries in marine geology, hydrology, geochemistry, and biology. This paper reviews the origin and technical evolution of sub-seafloor downhole observation, synthesizes the key scientific discoveries enabled by more than three decades of CORK deployments, and—drawing on representative case studies such as the Nankai Trough off Japan, where CORK technology has captured frontier phenomena including shallow slow-slip events (SSEs)—discusses the technological gaps that remain in current global observatory coverage. We further examine how emerging deep-sea drilling capabilities, including newly commissioned vessels such as China’s Mengxiang, may help close these gaps and expand the global network of sub-seafloor borehole observatories into previously unsampled tectonic and hydrological settings.

Keywords: subseafloor borehole observatory, ocean drilling, CORK, seafloor observation network, Long-Term Borehole Monitoring System (LTBMS)