BOREHOLE INSTRUMENTATION

Objectives

A major limitation of our understanding of active tectonic processes largely comes from the fact that we lack in situ long-term observations in the oceans where many areas of major tectonic activity are found. Since the DSDP era, there have been many attempts to utilize boreholes for such observations. For example, recent circulation obviation retrofit kit (CORK) deployments to measure pressure and temperature changes in sealed boreholes are beginning to produce interesting results. ODP continues to recognize the importance of long-term observatory objectives (Ocean Drilling Program, 1996). Tomographic studies using earthquake waves propagating through the Earth's interior have revolutionized our understanding of mantle structure and dynamics. Perhaps the greatest problem facing seismologists who wish to improve such tomographic models is the uneven distribution of seismic stations, especially the lack of stations in large expanses of ocean such as the Pacific. The ION project, an international consortium of seismologists, has identified gaps in the global seismic net and is attempting to install digital seismometers in those locations. One of the highest priorities of ION is to install a station beneath the deep seafloor of the northwest Pacific. A primary objective of Leg 195 was to install a permanent seismic observatory (WP-1) at Site 1201, situated in the western Philippine Basin (Fig. F87), which would become a long-term borehole seismological observatory. The WP-1 seismic observatory is surrounded by Inuyama and Taejon stations to the north, Ishigakijima and Baguio stations to the west, Davao City and Jayapura stations to the south, and Guam, Minami Torishima, and Ogasawara stations to the east.

A global seismographic network was envisaged by the Federation of Digital Seismographic Networks to achieve homogeneous coverage of the Earth's surface with at least one station per 2000 km radial distance in the northwestern Pacific area. Thus, the WP-1 seismic observatory at Site 1201 will provide invaluable data, obtainable in no other fashion, for global seismology. Data from this observatory will help revolutionize studies of global earth structure and upper mantle dynamics by providing higher-resolution imaging of mantle and lithosphere structures in areas that are now poorly imaged (Fig. F88). In addition, this observatory will provide data from the backarc side of the Izu-Ogasawara and Mariana Trenches, giving greater accuracy and resolution of earthquake locations and source mechanisms. The observations of seismic surface waves as well as various phases of body waves from earthquakes at the Philippine Sea plate margins will provide sufficient data to map differences in plate structure among different basins comprising the plate (e.g., the West Philippine, Shikoku, and Parece Vela Basins). There are only a few studies with limited resolution on the lithospheric structure of these areas (Kanamori and Abe, 1968; Seekins and Teng, 1977; Goodman and Bibee, 1991).

There are indications that the subducting Pacific plate does not penetrate below the 670-km discontinuity and that it extends horizontally (Fukao et al., 1992; Fukao, 1992), but the resolution of these studies is poor (>1000 km) beneath the Philippine Sea and the northwestern Pacific, especially in the upper mantle, where significant discontinuities and lateral heterogeneities exist (Fukao, 1992). The WP-1 seismic observatory is a crucial network component in determining whether the Pacific plate is penetrating into the lower mantle in the Mariana Trench but not in the Izu-Ogasawara (Bonin) Trench (van der Hilst et al., 1991; Fukao et al., 1992; van der Hilst and Seno, 1993). In addition, the WP-1 seismic observatory will allow imaging of the subducting slab to determine how the stagnant slab eventually sinks into the lower mantle (Ringwood and Irifune, 1988).

NEREID-195 System Outline

We outline the NEREID-195 system of the WP-1 borehole broadband seismic observatory in this section (Fig. F89). The details of each component are described in separate sections. Like other existing oceanic borehole observatories (JT-1 and JT-2) (Sacks, Suyehiro, Acton, et al., 2000), there is a nearby coaxial transoceanic telephone cable (TPC-2) to use for data recovery and power. However, the WP-1 observatory installation is designed as a stand-alone system with its own batteries and recorder. Thus, once instruments are installed in the hole, they will be serviced for data analyses, distribution, and archiving. We plan to connect data, control, and power lines to the TPC-2 cable owned by University of Tokyo after confirmation of data retrieval.

The two seismometers are designed to be placed near the bottom of the hole, each housed in a separate pressure vessel. Both sensors are feedback-type broadband seismometers (Guralp Systems, Ltd., CMG-1T). Two separate cables are required to connect the sensors uphole. The signals are digitized in the sensor packages and sent in digital form to the seafloor packages. The seafloor package (multiple-access expandable gateway [MEG]-195) (see Table T15 for abbreviations) serves to combine the digital data from the two seismometers to a single serial data stream. It also distributes power to the individual seismometers. The data are stored in digital format in a separate module (the storage acquisition module [SAM]-195) after being sent via an RS-232C link using Guralp Compressed Format (GCF) protocol. The SAM-195 has four 6-GB SCSI 2.5-in hard disks capable of storing more than six channels of 0.5-yr-long continuous data of 24-bit dynamic range at 100-Hz sampling rate. In this case, there are three channels for each of the seismometers. We plan to change the hard disks in the SAM-195 to those with a capacity of 15 GB. Using four 15-GB hard disks, the recording period of the SAM-195 is extended to ~1.5 yr. The seismometers are emplaced in the borehole permanently; they are cemented into the hole to assure good coupling. The MEG-195, on the other hand, may be serviced by a remotely operated vehicle (ROV) or submersible. The MEG-195 can be physically replaced and accepts commands and software upgrades through the SAM-195. The SAM-195 must be replaced by an ROV or submersible before the hard disks become full, which is ~1.5 yr with the future design. The SAM-195 also provides a communication link to the borehole system while the station is being serviced by the ROV or submersible. The SAM-195 can send part of the data to the surface across a serial link to check the health of the system. The SAM-195 measures the time difference between the clocks in the SAM-195 and the MEG-195. Before deployment and after retrieval of the SAM-195, the time difference between the SAM-195 and the Global Positioning System (GPS) clocks is measured on board. Because the MEG-195 controls the timing of the whole borehole system, we can adjust the system timing to universal time coordinated using the data from the SAM-195. All the necessary power is supplied from the lithium battery unit (LBU). The LBU consists of two units. Each unit is composed of 16 cells provided by Yuasa, Japan (model CL-1300L). Each cell has a voltage of 2.7 V at 0°C with a capacity of 1300 Ah. We connected eight cells in series and two series in parallel; therefore, each unit has a dominant voltage of 21.6 V at 0°C with a capacity of 2600 Ah. Each battery unit has two titanium spheres with a diameter of 65 cm because eight cells are housed in one titanium sphere. It is expected that the battery system will provide power to the system for ~2.5 yr.

Environmental Requirements

Site 1201 is geographically situated in a large gap of the global seismic network (Fig. F87) where no seismic observatory exists within 1000 km. The seismic image of the Earth's structure beneath this area, especially in the upper mantle, will remain ambiguous without a seismic station in this area. There are several requirements that must be fulfilled for the permanent installation of a seismic observatory so that the expansion of the global network to the ocean is truly effective. The seismic noise of an observatory should be as small as possible. The number of observed seismic phases depends on the magnitude of the seismic noise in the same frequency band as the seismic phases from earthquakes. Therefore, the reduction of seismic noise at the site directly enhances the value of the observatory. There are many sources of seismic noise. Environmental seismic noise caused by microseisms, infragravity waves, and water currents at the sea bottom are commonly recorded by ocean seafloor observatories. Each environmental seismic noise has a significant characteristic frequency band. At frequencies <0.1 Hz, seafloor seismic observations are significantly degraded by the noise caused by water currents at the seafloor. The magnitude of this type of noise can be higher than that of almost any long-period teleseismic phase. Escaping the noise due to water currents by shallow burial of the seismometer in sediment or borehole installation was suggested and was tried in several pilot experiments (e.g., Stephen et al., 1999). Because a lower noise level is expected in a borehole rather than at the seafloor or in shallow sediment, especially long term, permanent seismic observatories should be installed in boreholes at the sea bottom. Installation in a deep borehole seems to eliminate the effect of flow noise, but noises characteristic of borehole installation, such as turbulence in the water column of the borehole, might reduce that advantage. For high-sensitivity measurements, pressure fluctuations resulting from ocean long waves or temperature changes can be noise sources. Any water motion near the sensor is also a potential noise source. Therefore, the seismometers must be grouted inside the borehole to avoid noise from water motion and to be optimally coupled to the surrounding rocks. From the experience of Leg 186, during which borehole geophysical observatories were installed on the inner slope of the Japan Trench, it was determined that the infragravity wave was the dominant noise source, with frequencies between 0.004 and 0.02 Hz (Araki, 2000). Using theoretical estimation of strains, it was found that the acceleration of the ground as a result of tilt by the infragravity wave is largest in a sediment layer but becomes negligible below the top of the basement. This is because the sediment has a large P-wave velocity (V_P)/S-wave velocity (V_S) ratio, whereas the V_P/V_S ratio of the basement is smaller. The depth of the horizontal traction maximum is mostly determined by the wavelength of the applied pressure signal at the seafloor, although shear strength is also an important parameter. The wavelength of infragravity waves is more than a few kilometers in the frequencies of interest. Consequently, the horizontal stress takes a maximum value at the bottom of the sediment column unless the sediment thickness is extreme. Horizontal traction is maximum at the bottom of the sediment layers, whereas the vertical traction is maximum at the seafloor, a consequence of the traction-free boundary condition at the seafloor. A seismometer in deep sediment will suffer from large horizontal infragravity wave noise, whereas a seismometer in shallow sediment will suffer from large vertical infragravity noise. Therefore, it is necessary to install a seismometer in igneous basement rather than in sediment to reduce the noise caused by infragravity waves; the seismic noise from infragravity waves in the horizontal component is smaller by >40 dB in basement than in sediments. During Leg 191, broadband seismometers were installed ~70 m below the sediment/basement interface in the northwestern Pacific Basin (Kanazawa, Sager, Escutia, et al., 2001). Although we have only 1-hr records from the seismometer at present, the noise level in the horizontal component estimated to be caused by the infragravity wave decreased by at least 20 dB.

Requirements

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 1201, we installed and cemented the seismic instruments into the 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 from 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 ~580 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.

Methods

The technique we have developed to satisfy the installation requirements listed above is illustrated in Figure F90. 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 (see Fig. F91). 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 1201E, the open hole length was 53.0 m, the top end of the stinger was 14.5 m above the bottom of the hole, and cement was pumped up from the bottom of the stinger, filling ~184.3 m of the hole (Fig. F90).

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 the 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 F92 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 ~14 hr for the two connectors used during Leg 195. With flexible cable, enough slack can be provided that errors in the termination operation 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 (see "Seafloor Instruments").

Borehole Instrument Assembly

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 x 7.1-m-long steel pipe with two blades, which have an angle of 62° between them (Figs. F92, F93, F94). The steel pipe serves as a conduit for cement. The 5.5-m-long middle part of the pipe is shifted toward the outside so that two ocean borehole seismometer (OBH) sensors (S/N T1023 and T1038) 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 cloth to insulate the instruments from the frame pipe. A 3.2-m-long stinger pipe with centralizers is bolted onto the bottom of the BIA. Cement pumped into the hose flows through the drill pipes, the BIA frame pipe, and the stinger. At the bottom, cement floods out from the lower end of stinger and rises upward to fill the space between the instruments and the borehole.

Ocean Borehole Seismometer

The OBH package consists of a three-component seismometer and a 24-bit digitizer (DM24) assembled in a 1.2-m-long grade five titanium pressure cylinder (12.7 cm OD). The cylinder is designed to withstand pressures at 10,000 m water depth, and the T1023 cylinder was tested at a pressure of 72 MPa. The two seismometers are model CMG-1T units made by Guralp Systems, Ltd. Each consists of three orthogonal sensors stacked vertically in the canister with a vertical sensor above two horizontal sensors (Figs. F95, F96). Two OBHs were mounted on a BIA frame lengthwise with 3-m spacing.

Mechanically, the vertical OBH sensors are of the Ewing type and the horizontal sensors are of an inverted pendulum type. Mechanical details of the vertical sensor are shown in Figure F97. The inertial mass is a boom supporting a transducer coil. The boom consists of a solid machined beam. The vertical sensor mass is supported by a prestressed triangular spring to compensate for its weight and has a natural period of ~0.5 s. The horizontal sensor mass is centered by an unstressed flat triangular spring and has a natural period of ~1 s. The effective mass of each sensor is ~250 g. The springs are connected to the frames with a temperature compensating thread that minimizes the effect of temperature variation. A compact design is achieved chiefly by the short stiff springs and short boom.

The adjustments required for operation consist of leveling the boom of the vertical sensor and tilting the bases of the horizontal sensors to center the mass movements in their equilibrium positions. Adjustments are made by small (1 cm diameter x 3 cm long) direct current (DC) motors operating gear mechanisms to tilt the bases of the horizontal sensors and to apply a small extra force to the vertical sensor's boom.

Before and during borehole installation, the instrument may be subjected to severe motion that can damage the mass support hinges. Consequently, the masses have to be locked securely in their frames and the hinges released. This operation is performed by a small motor-driven clamp, which is controlled by a command to the DM24 digitizer.

The sensors employ feedback to expand their bandwidth and dynamic range. The response of the sensor is determined by the characteristics of the feedback loop. The mass position is sensed by a capacitative position sensor. The voltage from the sensor, which is proportional to the displacement of the mass from its equilibrium position, is amplified and fed to a coil on the mass. The current in the coil forces the mass to its equilibrium position. With a high loop gain, motion of the mass is essentially prevented and the feedback voltage is then a measure of the force, and thus the acceleration, applied to the mass.

Figure F98 shows block diagrams of the feedback system. To obtain stable performance over the whole frequency range, the feedback-loop phase shift has to be carefully controlled by compensation components in the forward and feedback paths in the system. There are two feedback paths; one consists of a single capacitor in parallel with a resistor, and the other consists of a noninverting integrator in series with a resistor. The arrangement gives a double pole at specific frequencies. The system velocity responses are defined by a transfer function identical to that of a conventional long-period sensor with a velocity transducer whose natural resonance period is set at 360 s with the damping factor at 0.707. The velocity output (flat to 100 Hz) is fed through a low-pass filter (<50 Hz) before the digitizer. The mass position output can be used for periods >360 s.

The single-end output signals, such as velocity and mass position, are digitized by the DM24 digitizer. A detailed description of the DM24 digitizing module is given later in this section. The velocity outputs are digitized at 100 samples per second (sps), and the mass position outputs are digitized at 4 sps. The digitizer was programmed to produce decimated optional velocity outputs at 20 sps, although the sampling frequencies of velocity channels can be changed by commands. The sensitivities are ~5.0 x 10^-10 m/s/bit for the velocity outputs and 8.0 x 10^-8 m/s²/bit for the mass position outputs (Table T16). The DM24 also digitizes the signal from a temperature sensor in the OBH cylinder at a resolution of 12.87 mK/bit. All the digital data are sent to the MEG-195 seafloor data module through a 560-m-long cable. The communication link to the MEG-195 is a four-wire, 38,400-bit per second (bps) RS-422 serial link. As well as sending the signal data, the DM24 receives the time reference signal from the MEG-195 and synchronizes the OBH clock to the MEG-195 clock. The precision of the OBH synchronization is typically within 200 µs. The OBH clock is resynchronized to the reference if the time offset between the OBH and the MEG-195 clocks becomes >20 ms. The OBH clock in the DM24 records the time in the digitized records (Fig. F99).

The voltage range for the OBH is 10-36 V. Because the 560-m-long cable resistance is ~11 , the supply voltage can be varied in response to the power consumption of the OBH instrument. A large power consumption is required when the sensor mass unlock/lock is performed. This large current in the power line may cause damage to the DM24 processor. Therefore, an electrolytic capacitor with 4700 µF of capacitance and 63 V of resisting voltage is employed to eliminate undesirable effects caused by voltage fluctuation. A 100- resistor is connected in series with the capacitor to limit the charging current of the capacitor. The limitation of current in the power line is important to protect the power supply. A diode is also connected to the resistor in parallel for discharge of the capacitor. The capacitor must discharge quickly when power is turned off. The 560-m cable that contains both the power supply and the data link is connected to the OBH by an eight-way underwater connector (SEACON MSSK-8-BCR) attached to the top bulkhead. The power consumption of the OBH instrument is ~2.5 W during regular operation of the sensor. Power loss in the long cable is expected to be ~0.15 W. The OBH power is supplied through a DC/DC converter in the MEG-195 to isolate the power ground from that in the MEG-195, and its efficiency is ~80%. The overall power consumption of the OBH is ~2.9 W during normal operation. When the masses are locked, each OBH consumes 0.3 W more power.

The microprocessor in the DM24 controls various functions of the sensors such as unlocking/locking the masses and bases of the horizontal sensors and centering the masses. These controls are initiated by a command sent from the MEG-195 or automatically by the program in the OBH system. The OBH system is programmed to start unlocking the sensors and centering the masses after a programmed date, which must be set after deployment. During Leg 195, this date was set to 6 May 2001 for all sensors. Another task related to control of the sensor is auto centering. The masses are recentered whenever they deviate from the center position by a more than half the range of the mass movement.

DM24 Digitizer

The DM24 is a modular intelligent digitizer developed by Guralp Systems, Ltd. The schematic diagram of the DM24 is shown in Figure F100. Each DM24 has three single-ended analog input channels to 24-bit analog-to-digital (A/D) converters, as well as additional eight-component 16-bit A/D channels. Each DM24 consists of rectangular printed circuit boards in the OBH. The 24-bit digitizer utilizes the Crystal Semiconductor CS5321/2 chipset and the Motorola 56002 DSP. The CS5321/2 digitizes signal at 2000 sps, and the data are processed by the 56002 DSP to give lower sample-rate data. The high sample-rate data are filtered and decimated in four cascaded stages. The first stage decimates the data by 10 to give 200 sps. The following three stages can have various individual decimation factors that allow multiple data output rates to be selected simultaneously. Sampling by the CS5321/2 is triggered by a Hitachi H8 16-bit microprocessor. The H8 processor receives data from the DSP, buffers it in 512 KB of S-RAM memory, and sends it through the serial link outside the module in GCF (see "Seafloor Instruments"). Transmission of the data by the processor is intelligent, so that even a lost packet during transmission can be recovered by handshaking upstream in a block recovery protocol.

MEG-195

The MEG-195 is composed of a combiner/repeater module (CRM) and power conditioning/distribution module (PDM) (Fig. F101). The major role of the MEG-195 is to acquire signals from each sensor and send out the converted digital data to the SAM-195 data recorder across a single serial link.

Mechanical Design of the MEG Frame

In the MEG-195, all of the electrical components are stored in an 8.5-in OD titanium pressure vessel (Fig. F102). The vessel is sealed at the top and bottom by bulkheads. On the top bulkhead, a UMC from Ocean Design, Inc. is installed. The UMC, which has four conductor pins, is an interface to the SAM-195 and the LBU and is joined by a 20-ft-long ROV cable to the power access terminal (PAT), in which the LBU and SAM-195 are stored. On the bottom bulkhead, four titanium UMCs are installed and connected to the long cables that connect the seismometers to the seafloor electronics. The UMCs at the bottom of the MEG-195 (Fig. F103) connect each OBH at the bottom of the borehole to the MEG-195.

The UMC on the top bulkhead has a latch mechanism, whereas the bottom UMCs are stab-mating connectors that require continuous stabbing force to maintain connection. The required stabbing force is 36.4 kg total for the four connectors, which is provided by the weight of the MEG pressure vessel (70.9 kg in air; 37.3 kg in water without contents). The vessel is inserted to a MEG frame (Fig. F102). The MEG frame is part of the riser/hanger assembly that stands up in the center of the reentry cone. The MEG frame holds the vessel and aligns the bottom bulkhead connectors to the UMC receptacles on the stab plate, which is at the bottom of the frame. The MEG pressure vessel can be removed from the frame. The bottom UMC connections can be disengaged safely by operating a lever attached to the MEG frame. At the bottom of the frame, a set of flippers with a latch mechanism is linked to the lever to push the vessel out. After UMC disconnection, the vessel can be safely pulled out of the frame with a rope. Thus, the MEG-195 may be replaced even after being deployed on the seafloor. The retrieval of the MEG-195 may be necessary if it malfunctions.

To guide the vessel correctly into the MEG frame, four 12.7-mm-diameter titanium pins are attached to the sides of the vessel. Two pins are at the top and two are at the bottom. The pins slide into the slots of the MEG frame and define the orientation of the vessel, which allows the UMCs at the bottom of the vessel to mate smoothly upon insertion. There are also two plastic wedges on the top side of the vessel, which together with wedges on the MEG frame side, are designed to increase the space between the vessel and the frame side members. This allows the vessel to be easily reinserted into the frame if replacement by ROV is necessary. The MEG pressure vessel is electrically isolated from the frame to prevent electric corrosion. The plastic wedges at the top of the vessel meet with the titanium wedges at the top of the frame, and the bottom of the titanium vessel meets with the lower plastic wedges at the frame because a plastic-plastic contact may become stuck by increasing the friction during insertion or extraction of the vessel.

Guralp Compressed Format

GCF is a format that allows many different time-series data channels to share a single transmission line. The format is used to transfer data throughout the NEREID-195 system. It can also transfer status messages in ASCII characters. Each GCF-format data transmission is an information packet containing either a data block or a status block. The GCF packet consists of an identification character (G), transmission serial identifier (ID), data/status block contents, and 2-byte checksum characters. The transmission serial ID increments by one for every packet. The serial number enables the receiver to recognize a lost GCF packet and will result in a request for the lost packet to be resent. The data block is used for time-series data transfer, and the status block is used for the sensor status information. Each data block stores data in multiples of a full second, starting on an exact second. The data block consists of a header and compressed data. In the 16-byte header, the most basic attributes of the data are stored, such as system ID, stream ID, date and time of the observation data, number of samples per second, number of data in the block, and type of compression in the data block that follows. The set containing system ID and stream ID identifies the source of the data. The assignments of the system ID/stream ID by each source are listed in Table T17. Simple data compression is made by the first and last complete values in each block and the difference values between adjacent samples. The bit lengths of the difference values are all the same in a data block and are 8, 16, or 32 bits, depending on the maximum first difference in the signal in that data block. The status block has the same header as that of data blocks, but its samples-per-second field is set to zero and the compression byte has a value of four. After the header, status information in ASCII characters follows. The status block transfers many different types of information, such as boot messages, progress reports of seismometer mass control, and measurements of clock offset between the CRM and the DM24s.

Each DM24 has a clock that adds time information to the data sent to the CRM. These clocks are independent from the precision clock in the CRM, which is the reference. The DM24 clocks have less accuracy; therefore, the clocks in the DM24s must always be synchronized to the accurate clock in the CRM. Each DM24 receives the time reference signal and adjusts its clock oscillator to synchronize with the reference.

The adjustment to the external clock can be performed by either GPS or stream-sync time-base signal; however, all the DM24 clocks in the MEG-195 and OBHs are set up to use the stream sync. The stream-sync signal is a set of clock synchronization characters sent by an upstream module through the serial data link to the DM24. The signal consists of 2-byte characters sent every second; date and time information is transmitted over 1 min. The first character (0x10) of each 2-byte character represents the timing reference, which is accurately synchronized to each second of the clock in the transmitter. The second character represents a part of a date or time. Although the first character is synchronized to the second, the processor actually needs the second character to compare with its own clock. Therefore, the receiver's internal clock will be delayed for the little time it takes to send the 10-bit data in the serial data link (~0.2 ms in a 38,400-bps line between the OBHs and the MEG frame). The difference between the clocks is measured every minute. The adjustment of the clock oscillator is also performed every minute. The time difference between the clocks is typically kept within 200 µs. When the difference between the clocks becomes >20 ms, the DM24 clock is resynchronized to the standard in the CRM.

The DM24s report the time difference in a unit of 25/1536 µs in the GCF status block. The DM24 can be interactively configured or commanded through the same serial link that is used for data transmission. With a simple command line, control of the sensor attached to the DM24 in the DM24 is possible (e.g., mass unlock/lock and mass centering). The system programs are customized to fit the sensor attached to the DM24.

As well as the control on demand of a command from outside, some of the DM24s can have a task that automates the control of the sensors. For example, the DM24 for the CMG-1T seismometer has a process to monitor the mass positions and centers the masses when they deviate by more than half the full scale from the zero position.

Combiner/Repeater Module

The CRM collects digital data from all the DM24s in the OBHs. The serial link between the CRM and the DM24 in the MEG-195 is a transistor-transistor-logic level interface to minimize power consumption in its line driver/receivers. A 38,400-bps RS-422 serial interface is used to connect the CRM to the OBH to ensure a link of sufficient quality over a cable length of 550 m. The serial links from the CRM to the DM24s in the OBHs are optically isolated. The downhole power lines are also isolated with DC/DC converters. Complete electrical isolation of each component is necessary to avoid corrosion in case of accidental electrical leakage to seawater. The data collected by these serial interfaces are handled by a Hitachi H8 microprocessor. The data are buffered in an 8-MB silicon file in order and are transmitted to the SAM-195 recorder through a high-speed 57,600-bps RS-232C serial interface.

The H8 processor controls a precision reference real-time clock (RTC) in the same manner as the DM24. The reference RTC is a temperature-compensated precision clock, and the trimming of the oscillator by the processor makes its accuracy on an order of 1 x 10^-8 (Fig. F104).

When the ROV plug from the battery frame is connected to the MEG-195, the CRM boots the whole system upon system power up. The CRM runs a boot loader in its EPROM first, and the boot loader reads the actual program from EEPROM in its CMOS memory to run the system. The CRM will also allow the system program to be loaded from the high-speed serial data link to update the one in EEPROM. After the system program is started, the CRM begins to handle data transmission and powers up all the seismometers sequentially. The CRM in the MEG-195 executes the command at the time-determined running state, whose number increases every minute from boot time. The CRM controls all the power for itself and the OBH. All of the supplied current for the OBHs connected to the MEG-195 is monitored by the CRM every minute. If the average current for the OBH over 1 min exceeds the preset limit (160 mA), the CRM in the MEG-195 shuts down the power of the OBH with overcurrent and will not automatically power on the OBH for the protection of the power supply circuit. The MEG-195 also checks the hourly supplied voltage average from the LBU. If the average voltage over two successive hours falls below the "good" threshold (20.0 V), the CRM in the MEG-195 shuts off one of the running OBHs to conserve power consumption. The CRM confirms that the LBU has little power using two successive measurements of the voltage from the LBU. Because the SAM-195 needs a large current for writing data to the disk, there is a possibility that the voltage of the LBU can drop temporarily. Writing data to the disk typically takes ~43 min. When the voltage of the LBU is greater than the "good" threshold over 2 hr (e.g., the CRM obtains two successive measurements of an average voltage above the threshold) after shutting down one OBH, the CRM in the MEG-195 switches on the OBH that had been shut down. When the CRM in the MEG-195 finds that the hourly average voltage is smaller than the "low" threshold (18.8 V), the MEG-195 and the OBHs will be completely shut off for 24 hr. After 24 hr, the CRM will try to boot again and see the supply gets back to a "good" condition.

The CRM produces a GCF status message every minute to report the condition of each system. The status message is composed using ASCII strings (Table T18) and reports the status of power distribution, clock synchronization, and intermodule communication. The status message from the CRM in the MEG-195 contains ~400 bytes, and the CRM in the SAM-195 contains an additional ~100 bytes of information about the data buffer status.

When the CRM in the MEG-195 receives a set of characters to request a command session from the upstream SAM-195, it stops sending data and switches to command session mode. In command session mode, the CRM provides features to control many other modules such as the master RTC or the PDM in a simple command set. The available commands (those of the DM24s, SAM-195, and CRM) are summarized in Table T19. The command session is finished by a "close" command from upstream or by a time-out of 1 min. The command session can be established between a DM24 and upstream modules over the CRM through the high-speed link, in what is called a pass-through. When an upstream unit requests the CRM for a connection to a DM24, the CRM stops sending data to the unit and relays the connection request to the DM24. The CRM continues to maintain the command session link until the upstream unit finishes the session. Stream-sync characters from the upstream module are sent even during the command session to maintain stable clock synchronization over the modules.

The watchdog fail-safe timer, which is a part of the H8 microprocessor reset circuit, is employed to reset the CRM. The timer needs to be triggered at least every 1.5 s. The CRM system multitasker normally does this every second. If there is some failure of the CRM program, a failure to trigger the timer results in a system reboot.

Power Distribution Module

The PDM is a unit that switches and distributes power to all the sensors and the CRM. The PDM measures the supply voltage and currents and sends that information to the CRM. The PDM can have a maximum of seven channels of power switches independently controlled by the CRM. The CRM in the MEG-195 utilizes two channels for the OBHs; the CRM in the SAM-195 uses two channels for hard disk units.

Each power channel is switched by separate MOS FET (LH1517) relays. Channels 1 and 2 are different from the others in that they employ two LH1517s to double the switchable load. Current through each power channel is monitored by the CRM every second. To protect other modules, the CRM shuts down a channel if it draws an unexpectedly large average current over 1 min. Shutdown criteria can be different for each component and are configured in the CRM system CMOS memory.

The PDM controls power to the CRM, which controls power to the sensors. This is done by the power management circuit along with a battery backup RTC, which can run on a very small power supply. The PDM monitors the LBU voltage, and if the voltage drops to <17.5 V, the PDM switches off the CRM (and thus all the sensors) to prevent the system from running into an unstable condition. If this situation should occur, the system is kept down until the voltage is restored to 20.0 V or for 24 hr. Within this shutdown period of 24 hr, the system can be restarted by cycling power for 2 min until the PDM automatically shuts the system back off. If there is the need to operate the system intentionally under this situation, we can change the status of the system by sending a "manual" command to the normal sequence of operation. The PDM can also be configured to power the system on a preset date with a wake-up command.

Downhole Link

Two 550-m-long cables, which are tied to the 4.5-in casing pipes, supply power for each downhole OBH as well as transfer data for the MEG-195. The cables have eight conductors each. Each of the long cables is branched to terminate with two, four-conductor female UMCs 1 m below the connectors. Two UMCs for each OBH were necessary because there were no UMCs with more than four conductors that can withstand 6000-m-depth pressures. The OD of all cables is 19.5 mm. The structure of the cable from the center is two layers of conductors covered with an inner jacket made from high-density plastic elastomer (HDPE) and a tension member made of aramyd fibers covered with an HDPE outer jacket. The fiber tension member provides good tensional protection of conductors up to 1800 kgf (N) of maximum tensional load. These cables retain enough mechanical flexibility to allow a bending radius as small as 12 in. The cables are designed to have a low specific gravity (1.05 g/cm³) in seawater. Thus, these cables experience minimal tension in the borehole. They are also strapped to the 4.5-in casing at 1.5-m intervals with centralizers. The cross-sectional area of the conductors is 1.25 mm². The cable resistance for a 700-m length is 11.3 .

The cable for the OBH consists of three DC power supply wires (two negative and one positive), four data transmission line wires, and one signal ground. The electrical characteristics of the data transmission line conform to RS-422 serial communication standards. The digitized seismic signal and the seismometer status are transmitted through the uplink in GCF format. The acknowledge characters for the uplink data and clock synchronization signal are sent by the CRM to the MEG-195 through the downlink. Commands to control the seismometer, which may be issued manually during ROV operations, can also be sent through the downlink. The GCF acknowledge characters, the clock synchronization characters, and the command characters have different formats so that the seismometer can distinguish them.

The assignment of the cable wires, as well as the pin assignment of connector pins on both the MEG-195 UMC and the sensors, is summarized in Table T20. The OBH T1023 and T1038 are terminated by the UMC sets 1 and 2 (as shown by Fig. F103), respectively.

SAM-195

The SAM-195 is the recorder that is mounted on a frame located on top of the PAT battery frame. The SAM-195 is connected through an ROV-operated cable to the MEG-195. When the SAM-195 storage becomes full after ~1.5 yr of recording, an ROV can replace it with an empty SAM-195. Ejection of the SAM-195 is facilitated by a lever mechanism on the frame.

The power to the SAM-195 is supplied in parallel with power to the MEG-195 directly from the LBU, with a typical voltage of 22.4 V at 0°C. The SAM-195 receives data from the MEG-195 through a high-speed (57,600 bps) RS-232C serial link. The four-pin stab-mating male UMC at the bottom of the SAM-195 cylinder connects the LBU power supply and data link for the MEG-195. The SAM-195 buffers the received data in a silicon file that consists of 64 MB of flash memory. The flash file is nonvolatile memory so that data will not be lost even during power loss. When 56 MB of data is buffered, flushing of the buffered memory into a hard disk drive (HDD) is initiated (Fig. F105). The amount of data incoming to the SAM-195 is expected to be ~15 kbps in a standard recording configuration, as summarized in Table T21.

The SAM-195 has four SCSI 2.5-in HDDs, which gives a total of 22.8 GB of storage. These HDDs are powered only on the memory flush. The speed of data transfer is ~1.30 MB/min.

Thus, it takes ~43 min to flush out the 56-MB buffer memory. The directory of the data written on the disk can be browsed by a "dir" command on the CRM in the SAM-195, although the data itself cannot be replayed through the serial link. On the "dir" command, the SAM-195 will reply with a list of system ID, stream ID, date of first data, date of last data, and total amount of data for a stream. The disk drives can be connected to a PC-compatible computer (PC) with a SCSI interface. A PC program called "scsiread" can replay the data written on the SCSI disk GCF format.

In parallel to saving the received data into the buffer memory, the SAM-195 also hands the data to another serial link for communication with an ROV via the four-pin female UMC. The UMC provided by Ocean Design Inc. is on top of the SAM canister. Because we assume an ROV may have a relatively slow link, such as 9600 bps, only slow data channels (those of 20 sps or slower) and status messages are passed to the ROV serial port. When the SAM-195 receives a pass-through request from the upstream ROV, it will organize the session. If the request is addressed to downstream modules, the SAM-195 passes the request to the MEG-195 to reach the addressed module.

Power consumption of the SAM-195 is ~1.0 W in the interval between disk transfers. When the SAM-195 disk is running, the power consumption increases up to an average of 11.0 W. The SAM-195 manages power in a similar way to the MEG-195. During disk operations, the LBU voltage is monitored against a preset threshold (20 V), and if it falls below this for 10 min the disk transfer is aborted. This is done in an orderly way; the disk flushing task stops and the disk supply switches off. If this fails to finish in 5 min, the disk supply switches off. The SAM-195 also monitors the average voltage against a limit of 18.8 V and will automatically power down after 2 hr and remain shutdown for 24 hr, as in the MEG-195.

During the disk transfer period, the large power consumed in the SAM vessel results in a rapid increase of the temperature in the vessel by 20°C, as shown in Figure F106. This rapid temperature increase affects the precision clock, as seen in Figure F104, although the effect is minimized by the effective temperature compensation mechanism built into the clock module. The large current drain into the SAM also drops the supply voltage by 0.6 V at the SAM and by 0.35 V at the MEG (Fig. F107).

System Power Consumption

The power for the whole system is supplied by limited LBU power. We initially record the data in the SAM, which is recovered by an ROV. Switching the LBU to the other battery unit when it becomes empty is also performed by an ROV. To plan the schedule of the ROV visits properly, it is important to have a precise knowledge of the power consumption of the whole system, which varies with many parameters, such as the number of running OBHs and sample intervals of the seismic channels.

Table T22 summarizes the result of the power consumption measurement for the NEREID-195 system in Hole 1201E. When all the OBHs are running, we expect the power consumption of the whole system over a long period to be 7.73 W. If we confirm the functionality of both OBHs, we can shut one of them off to run the system at ~4.7 W, saving as much as 3.0 W for one OBH.

Lithium Battery Unit

The power for the NEREID-195 system is supplied by the LBU. The LBU consists of two identical units, which are composed of 16 lithium cells. The lithium cells are housed in a titanium sphere vessel with a diameter of 65 cm. Each unit has two titanium sphere vessels.

The cell is a manganese dioxide/lithium primary battery. Manganese dioxide and lithium are used as anode and cathode, respectively (Fig. F108). The battery uses lithium as a cathode because it has a larger energy density and a smaller self discharge than other types of primary batteries. The manganese dioxide/lithium battery has an advantage of being safer than other types of lithium batteries because the anode is solid. Therefore, it is easy to construct a large-capacity battery using the manganese dioxide-lithium chemical process. We use the Yuasa model CL-1300L as a cell. The CL-1300L has a capacity of 1300 Ah and has a dominant voltage of 3.0 V at 25°C and 2.7 V at 0°C. The battery cells can operate between -20° and 60°C. The weight and dimension of the CL-1300L is 13.7 kg and 224 mm x 241 mm x 127 mm, respectively. The CL-1300L battery can supply a current of up to 2.6 A continuously and has a thermostat for protection against overcurrent and high temperatures. The thermostat is bimetallic and breaks a circuit at temperatures of >80°C, which corresponds to a current of >16 A passing through the circuit. The thermostat closes the circuit again at a temperature of <65°C after shutting down. The weight and volume are 10 times less than those of a lead battery, and the self-discharge rate is <1% per year. The batteries are sealed hermetically in a stainless steel canister. Therefore, the CL-1300L can be housed in a closed vessel. The discharge characteristics of the CL-1300L are shown in Figures F109 and F110. The voltage of the cell is more than the dominant voltage just after start of the discharge but rapidly decreases to the dominant voltage. After reaching the dominant voltage, the voltage of the cell gradually decreases with the amount of discharge. At the end of the discharge, the voltage of the cell rapidly drops to 2.0 V. The power control units in both the MEG-195 and SAM-195 are determined by these discharge characteristics.

Titanium Sphere Battery System

The CL-1300L cells are housed in a spherical titanium pressure vessel with a diameter of 65 cm, which was developed at the Experimental Design Bureau of Oceanological Engineering, Russian Academy of Sciences. The spherical vessel is made from titanium alloy and consists of two hemispheres that are the same size. The weight in air and buoyancy in water of the sphere are 89 and 73 kg, respectively. The sphere houses eight CL-1300L cells on a rack (Figs. F111, F112). To increase the total capacity of the power supply, two spheres are used for an LBU. Eight CL-1300L cells in one sphere are connected in series, and two series are connected in parallel; therefore, each unit has a dominant voltage of 21.6 V at 0°C with a capacity of 2600 Ah. For the parallel connection of two series of CL-1300Ls, a diode is connected in series for each series for safety reasons (Fig. F113). A cable with a dry-mate connector connects each sphere for the parallel connection, and one sphere has a UMC on the top for supplying the power to the entire system. Two spheres are mounted on a titanium frame (Figs. F114, F115). The top and bottom of the LBU are flat panels made of fiber-reinforced plastic drainboard for compatibility with the top panel of the PAT. The top drainboard of the LBU has an access hole for the UMC. The LBU is put into a hole on the top of the PAT-195 and can be pulled out by an ROV after complete discharge. The frame of the LBU is designed for easy replacement by an ROV. Because the PAT-195 has two LBUs, the total capacity of the LBU is ~112 kWh, which corresponds to ~2.5 yr of operation with one OBH, MEG-195, and SAM-195.

At testing of the equipment before the deployment, the LBU supplied the power to the whole system. During the testing, voltage and current from the LBU were measured by the seafloor equipment, as described in "Seafloor Instruments." The testing was carried out on the deck of the ship on 5 April 2001. Because the temperature was >25°C, the voltage from the LBU without a load was >25.0 V. After connecting the equipment, the voltage of the LBU immediately started to decrease. During a large load (~1.5 A of current) caused by the power-up of the disks in the SAM-195, the LBU showed a temporary decrease of output voltage.

Battery Frame

The LBU is mounted on the cylindrical PAT frame, as shown in Figure F116. Two units of the LBU are stored in holes in the upper part of the PAT. The PAT frame is made of ordinary steel and is coated with epoxy paint to protect it from corrosion. The titanium frames of the LBUs are isolated from the PAT with polyvinyl chloride insulators. The top of the PAT is a flat panel made of fiber-reinforced plastic drainboard. The top of the LBU is the same level as that of the PAT at its proper position. The wide, flat panel serves as the ROV platform. The panel is reinforced by a metal frame and is circular when viewed from above. The diameter of the top panel is 3.2 m. The basal "leg" of the PAT is also circular and the diameter is 3.66 m, which corresponds to the diameter of the reentry cone. The basal leg is 0.24 m in height. The leg keeps the PAT stable on the reentry cone. In the center of the PAT frame, coaxial rings are placed to guide the PAT smoothly over the riser assembly during the installation. The hole in the center of the top panel provides space for the MEG frame. The total height of the PAT is 2.64 m to avoid contamination of seafloor mud during an ROV operation. The vertical position of the MEG-195 on the riser is set to allow ROV operations. The top of the MEG-195 is 608 mm above the top panel of the PAT. The PAT also holds the SAM-195 recorder beneath its top panel. The top part of the SAM-195 sticks out of the panel. The UMC at the bottom of the SAM bulkhead is mated by gravity into a UMC receptacle, mounted on the stab plate placed at the bottom of the SAM hole, when the SAM-195 is dropped into the hole. During the dropping operation, the hole keeps the SAM-195 canister in the upright position. The SAM-195 key is lowered to the guiding wedges prepared inside the hole. At this time, the connectors on the bottom bulkhead of the SAM-195 canister are correctly positioned to the mating orientation of the UMC. The SAM-195 can be set and ejected by an ROV-operated lever mechanism in the recorder frame (Fig. F117). The lever can be locked at two positions, one at the mated position of the SAM-195 and the other at the released position. By the use of the locking positions, an ROV can easily replace the SAM-195 canister. The cable from the UMC at the bottom of the SAM hole receptacle has a T-junction; one branch is for connection to the LBU, and the other goes to the MEG-195. Both branches are terminated with UMCs, which are mounted on the PAT top panel. Two other cables (the ROV cables) are installed on the panel; one is used to connect the UMC receptacle on the top panel to the MEG-191 canister, and the other cable is for connection between the UMC plug to the LBU. The ROV cables are fastened to the top panel initially by fastening mechanisms. The ROV cable end for the MEG-195 canister is fixed at a parking connector for the ROV UMC receptacle. An ROV will take off the fasteners and connect the ROV UMC receptacle to the MEG-195 canister. The other end of the T-junction branch cable is connected to the LBU during the deployment. After complete discharge of one LBU, an ROV must change the connection to the other LBU.

Operation of WP-1 Observatory

The downhole OBH sensors (see "Borehole Instruments") are not functioning until the NEREID-195 system is activated by an ROV. Kaiko (Fig. F118), a Japan Marine Science and Technology Center (JAMSTEC) ROV designed to operate in water depth up to 11,000 m, is scheduled to visit Site 1201 to activate the WP-1 observatory.

The operations by the ROV are as follows:

1. Dive to Site 1201 with the SAM-195.
2. Remove the dummy SAM from its seating frame located on the top of the PAT.
3. Insert the SAM-195 onto the seating frame.
4. Make a connection between the MEG-195 and the PAT.
5. Check the status of the NEREID-195 system using the SAM/ROV interface.
6. Return to the surface.

(See "Seafloor Instruments" for MEG and SAM information; see "Power Supply" for PAT information.)

An ROV dive depends on sea conditions. Poor sea conditions may prevent the ROV from diving to activate the WP-1 observatory during its planned March 2002 visit to Site 1201. The SAM-195 can record automatically using its timer. If the SAM-195 were installed on the PAT at the time of installation and the WP-1 observatory is not activated at the scheduled time, the SAM would start to work and consume battery power while the OBH sensors and the MEG were inactive. For this reason, the SAM-195 will be installed at the time of the ROV visit.

After the SAM-195 is installed on the PAT, the NEREID-195 system activates as soon as the electrical connection between the MEG-195 and the PAT is made using the UMCs. Power is supplied to the OBH sensors, the MEG-195, and the SAM-195 from the LBU. Once this connection is made, data from the OBH sensors start to flow into the SAM-195 on the PAT, which can store 60 GB of data, sufficient to record 490 days of observations.

The ROV can check the status of the system via a SAM/ROV interface that allows RS-232C communication between the SAM-195 and the ROV. If there are no problems, the ROV needs to swap the SAM-195 at least once every year to ensure no loss of data continuity. The battery life is estimated to be ~1.5 yr. After the first observation period, the ROV will change the connection from LBU 2 to LBU 1 and recover LBU 2, which was used for the first observation period. After complete consumption of LBU 1, we will deploy a new LBU to continue observations.

The seafloor downhole observatory is still in the pilot study stage. There is no routine setup that one can rely on. In our design, the OBH sensor string is unrecoverable for the reasons given in "Installation Techniques." This necessitates that the string be composed of highly reliable instruments (see "Borehole Instruments"). On the other hand, the seafloor components are virtually all replaceable and serviceable by an ROV. The SAM-195 is replaced at each ROV visit. The MEG-195 can be pulled out of its seating frame and reinserted by an ROV, although the operation is more complex than other tasks.