INSTALLATION TECHNIQUES

A major objective of the installation of the observatories is to monitor the changes in the deformation state of the overlying strata in an active subduction zone. To achieve this, a suitable instrument, described in "Borehole Instruments", has to be in intimate contact with the rock. This is accomplished by cementing a resilient instrument in the bottom of an open hole in competent, indurated rock. Although all instruments benefit to some extent by being emplaced in competent rock, this is critical for successful strain monitoring. Depths at least 1 km below seafloor are generally required in areas with thick sediment cover.

One complication of subseafloor installations results from having to cope with irreducible ship heave during hole entry. Since heave can be 1 m or more, cables linking the instruments with the seafloor data handling units have to be protected from stresses created by the relative motion between the cables and the hole wall and between the cables and any insertion tools. Even though the heave compensator is used during hole entry, the instrument string has to hang from the rig floor with no compensation while pipe is being added, and this is repeated every 10-30 m for more than 1 km of hole penetration.

In addition, in order for the cement to set up properly, the instrument package has to be completely undisturbed for ~1 day after the cement is introduced.

The technique we developed to satisfy the installation requirements is illustrated in Figure F2. The instrument package is supported on 4-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 After cementing, the drill pipe from the ship can be uncoupled and withdrawn leaving the casing pipe in the hole.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 being strapped to the casing pipe and also are protected from wall contact by centralizers (Fig. F77 in the "Site 1150" chapter); 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 almost neutrally buoyant in seawater, further minimizing the long-term stress on the cables.

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. On land it is possible to have a clean hole down to the bottom and to place a grout slug down on the bottom. This is optimum. In seafloor holes it is impossible to clear all the cuttings from the bottom of the hole. In Hole 1150D, for example, the bottom 7 m was found to be filled with detritus when checked with a wiper trip. In addition, the bottom of the cement column could be diluted and have rather poor strength. To make a strong plug below the instrument, an ~3-m-long extension tube called a "stinger" is coupled to the bottom of the strainmeter. This ensures that the strainmeter is sealed off from the bottom of the hole and that a strong cement plug extends well below the instrument.

The cement is pumped through the casing pipe, the coupling tube, the strainmeter, and the stinger, and then up around the instrument string and into the 10-in casing. In Hole 1150D the open hole was 105 m, and the end of the stinger was 17 m above the bottom of the hole. Cement was pumped through the bottom of the stinger, filling ~150 m of the hole.

The amount of cement fill has to be a compromise. Since the cement density is ~2000 kg/m³, a column of cement will cause an overpressure at the position of the instruments. This is desirable because it will tend to close microfractures and also be forced into cracks in the formation; however, too much overpressure causes hydrofracturing. Maximum pressure estimation requires knowledge of the tensile strength of the rock as well as of the pore pressure. Since neither is known well enough to make the calculation reliable, we adopted a conservative value of 150 m.

Figure F3 is a schematic of instrument installation. Each of the four sensors has its own cable to the seafloor unit. There are several reasons why this plan has been adopted rather than having a single armored cable carrying all the signals. Since we do not know the exact installation depth 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 critical operation and takes ~12 hr for four connectors. With the flexible cable used, enough slack can be provided so that errors in the termination operation can be corrected. With armored cable this would be impossible and the termination would be extremely difficult to accomplish on the ship. Since some of the signals are noisy digital and others are low-level analog, multiple cables give minimum cross talk in the >1-km-long shared path.

An overriding concern has been the longevity of the installation. A 10-yr goal is necessary to achieve all of the scientific objectives. Our experience with long-lived on-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 in a seafloor removable unit, described in "Seafloor Instruments".