SEAFLOOR INSTRUMENTS

The multiple-access expandable gateway (MEG-191) is composed of a combiner/repeater module (CRM) and power conditioning/distribution module (PDM) (Fig. F17). The major role of the MEG-191 is to acquire signals from each sensor and send out the converted digital data to the SAM-191 data recorder across a single serial link.

Within the MEG-191, all of the electrical components are stored in an 8.5-in OD titanium pressure vessel (Fig. F18). 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-191 and the SWB and is joined by a 20-ft-long ROV cable to the power access terminal (PAT), in which the SWB and SAM-191 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-191 connect each OBH at the bottom of the borehole to the MEG-191. 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. F18) that connects electrically with the downhole seismometers. 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 handle attached to the MEG frame. At the bottom of the frame, a set of levers with a latch mechanism is linked to the handle to push the vessel out. After UMC disconnection, the vessel can be pulled out of the frame with a rope. Thus, the MEG-191 may be replaced even after being deployed on the seafloor. The retrieval of the MEG-191 may be necessary if it malfunctions.

To guide the vessel correctly into the MEG frame, four 0.010-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, 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 an ROV is necessary. The MEG pressure vessel is electrically isolated from the frame to prevent galvanic corrosion.

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-191 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 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 sps, 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 T2. Simple data compression is done 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 sps 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-191 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). 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, 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 of the full scale from the zero position.

The CRM collects digital data from all the DM24s in the OBHs. The serial link between the CRM and the DM24 in the MEG-191 is a transistor-transistor-logic (TTL) level interface to minimize power consumption in its line driver/receivers. An RS422 serial interface is used to connect the CRM to the OBH to ensure a link of sufficient quality over a cable length of 450 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 an accidental electrical leakage to seawater. The data collected by these serial interfaces are handled by an Hitachi H8 microprocessor. The data are buffered in an 8-MB silicon file in order and are transmitted to the SAM-191 recorder through a high-speed 57,600-bps RS232C 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 better than 1 × 10^-8 (Fig. F19).

When the ROV plug from the battery frame is connected to the MEG-191, 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. 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-191 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-191 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-191 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-191 also checks the hourly supplied voltage average from the SWB. If the average voltage over two successive hours falls below the threshold (23.05 V), the CRM in the MEG-191 shuts off one of the OBHs to conserve power consumption. The CRM confirms that the SWB has little power using two successive measurements of the voltage from the SWB. Because the storage acquisition module (SAM-191) needs a large current for writing data to the disk, there is a possibility that the voltage of the SWB can drop temporarily. Writing data to the disk typically takes ~35 min. When the voltage of the SWB is greater than 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-191 switches on the OBH that had been shut down. When the CRM in the MEG-191 finds that the hourly average voltage is smaller than the low threshold (21.47 V), the MEG-191 and the OBHs will be completely shut off for 24 hr. After 24 hr, the CRM will try to boot again.

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 T3) and reports the status of power distribution, clock synchronization, and intersystem communication. The status message from the CRM in the MEG-191 contains ~400 bytes and the CRM in the SAM-191 contains an additional ~100 bytes of information about the data buffer status.

When the CRM in the MEG-191 receives a set of characters to request a command session from the upstream SAM-191, 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-191, and CRM) are summarized in Table T4. The command session is finished by a close command from upstream or by a timeout 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.

The watch-dog 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.

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-191 utilizes two channels for the OBHs; the CRM in the SAM-191 uses three 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 SWB 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 25.0 V or for 24 hr. This temporary shutdown will ensure that the SWB battery will recover its normal operation. 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.