Twenty-three Years of Evolving “State-of-the-Art” CORK Borehole Geophysical Monitoring: A Review of Technologies and Case Studies

Abstract:

The first successful attempt to instrument an Ocean Drilling Program borehole for formation pressure and temperature monitoring and fluid sampling was accomplished in 1991 in Hole 857D, and the system there has been in nearly continuous operation since that time. This hole and others that followed have provided many new insights into ocean crustal and subduction zone hydrogeology and geodynamics, while at the same time being the “proving ground” for a number of technological advances in ocean borehole monitoring, including 1) the CORK scheme itself for sealing holes for hydrologic recovery to natural-state conditions after drilling; 2) the use of absolute pressure sensors for monitoring both relative formation pressures and changes in seafloor depth; 3) multi-level completions for pressure monitoring that leave cased borehole interiors open for other instrumentation; 4) the development of ultra-high-precision, low-power digital recording systems for examining the effects on the formation of seismic and microseismic loading; and 5) the proof-of-concept of an optical communications system that eliminates dependence on submersibles or ROVs for data download operations (see Tivey et al., this session). Relatively low-sample-rate data spanning the first part of the more than two decades of operations have shown how large anomalous pressures generated thermally and by deformation can be; how seafloor tidal loading influences formation pressure and can drive an “a.c.” component of flow; and how seismogenic and slow strain can be observed by way of formation-fluid pressure transients. More recent instrumentation has allowed much higher fidelity observations (1 Hz sampling at a resolution of 10^-8 of full-scale), and thus is permitting complementary studies of hydrologic, oceanographic, seismic, and microseismic phenomena. Plans for the future include connections to shore via observatory cable systems, such as those of NEPTUNE Canada and DONET, for unlimited power supply and real-time data acquisition (the first having been connected in 2009), and incorporation of quartz sensors, under development at Paroscientific Inc. and Quartz Seismic Sensors Inc., that are coupled to acceleration for monitoring tilt and ground motion.