NEREID-191 Borehole Seismic Observatory
NEREID-191 is a broadband seismic observatory (Fig. 6) designed to last for many years deep beneath the sea. Because there are no coaxial transoceanic telephone cables near Site 1179 to utilize for data recovery and power, the NEREID-191 installation was designed as a stand-alone system with its own batteries and data recorder. Two seismometers are installed in Hole 1179E, each housed in a separate pressure vessel. Both sensors are feedback-type broadband seismometers (Guralp Systems Ltd., CMG1T). The seismometers in the borehole are cemented into the hole to make good coupling to the host rock and to avoid noise caused by water circulation near the seismometers. Two separate cables are connected to the sensors and carry signals and power between these instruments and other system modules at the seafloor. The signals are digitized in the sensor packages and sent in digital form to a data-recording unit in the seafloor electronics package.
The seafloor package, called the MEG-191, combines the digital data from the two seismometers into a single serial data stream and distributes power to the individual seismometers. The data are stored in digital format in a separate module, called the SAM-191, via an RS232C link. The SAM-191 has four 18 Gb SCSI hard disks, which allow more than 1.5 yr of continuous data recording with 24-bit dynamic range at a 100-Hz sampling rate from the two seismometers. The MEG-191 can be physically replaced by an ROV or submersible and accepts commands and software upgrades through the SAM-191. The SAM-191 is designed for servicing by an ROV or submersible. Before the disks become full, it can be swapped for an "empty" unit. Additionally, the ROV or submersible can download part of the data stream via a serial link, so operators can check the health of the borehole system. The SAM-191 also measures the time difference between the clocks in the SAM-191 and the MEG-191. Before deployment and after retrieval of the SAM-191, the time difference between the clock within the SAM-191 and the global positioning system (GPS) clock is measured so time-based corrections can be applied.
All station power is supplied from the seawater battery (SWB) system. The SWB can supply up to ~24 W with a >400 kWh capacity. Its energy comes from electrolytic dissolution of the magnesium anode. The condition of the SWB system is continually monitored by the power control system (PCS), and data from the PCS are recorded in the data logger (DL). In addition, the PCS controls the power switch and shuts off the observatory, for the protection of the system, if the SWB provides either too much or too little power.
Hard Rock Reentry System
The HRRS is being developed to provide ODP with the ability to establish a reentry casing on sloped and fractured hard rock outcrops on the seafloor. The system uses a Model 260 downhole fluid hammer developed by SDS Digger Tools of Canning Vale, Western Australia, along with a bit to advance the hole while casing is installed simultaneously. At present, 13.375-in casing is being used in the prototype development program. The rough sea states encountered during Leg 179 tests demonstrated the need for more robust bits that could withstand the torque, lateral pivoting (i.e., rocking) movements, and weight-on-bit fluctuations experienced during this first offshore trial. All three of these parameters contributed to the premature failure of the bits tested during that leg (Pettigrew, Casey, Miller, et al., 1999).
The next generation of bits developed for the Leg 191 HRRS testing program was tested on shore. Corrections and improvements to the bits were made based on the observations of these land tests. Despite the limited onshore testing, the next generation of bits appear much superior to those used during Leg 179.
Two new bit types were developed for testing during Leg 191; these include underreamer and ring-type bits. Two different versions of the underreamer bits were tested as well as two versions of ring-type bits. Underreamer bits have retractable arms to open a larger hole than the pilot bit onto which they are mated. Ring bits are composed of two major parts that include a casing shoe and pilot bit. The casing shoe has a ring of tungsten carbide buttons that works in tandem with the pilot bit. However, unlike the underreamer bits, which are totally recovered at the completion of the installation process, the casing shoe is left in the hole on the bottom of the casing after the pilot bit and hammer are withdrawn.
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