To obtain high-quality data, a suitable instrument, as described in "Borehole Instruments," must be in intimate contact with the host rock. This is accomplished by cementing robust instruments in the bottom of an open hole in competent, indurated rock. At Site 1179, we planned to install and cement the seismic instruments within basaltic basement.
One of the complications of subseabottom installations arises from having to cope with irreducible ship heave during hole entry. Because heave may be a meter or more, cables linking the instruments with the seafloor data handling units have to be protected from stresses arising out of relative motion between the units and the hole wall and between the units and any insertion tools. Although the passive and active heave compensators can be used during hole entry, the instrument string has to hang from the rig floor without compensation while pipe is being added. Pipe is added every 10 to 30 m for ~475 m of hole penetration. In addition, for the cement to set properly, the instrument package has to be completely undisturbed for about a day after the cement is introduced.
The technique we have developed to satisfy the installation requirements listed above is illustrated in Figure F3. The instrument package is supported on 4.5-in diameter casing pipe that hangs on the base of the reentry cone at the seafloor. This has two advantages: (1) the pipe provides a conduit for cement pumping and (2) it also keeps the package stationary once its support (riser/casing hanger) lands on the hanger at the base of the cone. After cementing, the drill pipe from the ship can be uncoupled and withdrawn, leaving the casing pipe in the hole. The cables are protected by strapping them to the casing pipe and are also protected from wall contact by centralizers (Fig. F4). Therefore, there is no motion between the cables and the support tube (casing pipe) and no contact with the borehole walls. Strapping the cables to the support tube minimizes the tension in the cables. Armored cables are not required, and the cable structure is such that it is almost neutrally buoyant in seawater, further minimizing long-term stress on the cables.
It has been found that cement pumped through a pipe into a water-filled hole does not penetrate much below the pipe opening, tending rather to force its way upward. To make a strong plug below the seismometers, a 3.2-m-long extension tube called a stinger is coupled to the bottom of the borehole instrument assembly (BIA) that supports the seismometers. This ensures that the seismometers are sealed off from the bottom of the hole and that a strong cement plug extends well below the lower seismometers.
The cement is pumped through the casing pipe, the BIA, the stinger, and then up around the BIA into the 10.75-in casing. In Hole 1179E, the open hole was 86.9 m, the top end of the stinger was 11 m above the bottom of the hole, and cement was pumped up from the bottom of the stinger, filling ~198.9 m of the hole (Fig. F3).
To avoid water circulation in the borehole column, which may cause seismic noise, the entire hole should be filled with cement. However, a long column of cement makes an overpressure at the position of the seismometers because of the cement density of ~2.0 g/cm3. The cables from seismometers may be damaged by the overpressure because a pressure limitation of the borehole cables is ~6500 m. Therefore, we adopted a length of ~200 m for the cement fill, a compromise between filling the hole and limiting the overpressure.
Figure F5 shows the BIA. Each of the two sensors has its own cable to the seafloor unit. There are a number of reasons why this plan was adopted rather than having a single armored cable carrying all the signals. Because we do not know the exact depth of installation until the hole has been drilled and the formation evaluated, the downhole cable cannot be cut and terminated ahead of time. Cable termination with an underwater mateable connector (UMC) is a delicate operation and took ~16 hr for the two connectors used during Leg 191. With flexible cable, enough slack can be provided that errors in the termination length can be tolerated. With armored cable, this would be impossible and the termination would be extremely difficult to accomplish onboard ship.
An overriding concern has been the long life of the installation. A 10-yr goal is necessary if we are to achieve all the scientific objectives. Our experience with long-term land installations is that cable leakage and electronic component failure are the most likely sources of data termination. For this reason, we use multiple cables and much of the electronic circuitry is contained in a removable seafloor unit, as described in "Seafloor Instruments".
The BIA is designed to prevent the instruments from being damaged during the installation in the borehole and to secure a conduit for cement pumping. The main frame of the BIA is a 0.076-m-diameter × 7.1-m-long steel pipe with two blades, which have an angle of 62° between them (Figs. F6, F7, F8, F9). The steel pipe serves as a conduit for cement. The 5.5-m-long middle part of the pipe is shifted toward outside so that two ocean borehole seismometer (OBH) sensors (S/N T1036 and T1037) and their cables can be emplaced there. Two OBH sensors are situated and fixed on the frame pipe in the area between two blades. The two blades protect the instruments from being hit and abraded by the hole. The surface of the frame pipe, where the pressure vessels of the OBH sensors touch, is covered with fiberglass cloths to insulate the instruments from the frame pipe (Figs. F10, F11). A 3.2-m-long stinger pipe with centralizers is bolted onto the bottom of the BIA. Cement that is pumped into the hose flows through the drill pipes, the BIA frame pipe, and the stinger. At the last moment, cement floods out from the lower end of stinger and rises upward to fill the space between the instruments and the borehole.