The multiple-access expandable gateway (MEG) is composed of a combiner/repeater module (CRM), A/D converter modules (DM24), a strainmeter interface module, and a PDM seismometer (Fig. F13). The major roles of the MEG are to acquire signals from each sensor, and, for analog sensors, to convert their analog signals to digital data and send out the converted digital data to the SAM via a single serial link, together with an accurate time stamp.
All electrical components are stored in an 8.5-in outside-diameter 316 stainless steel canister (Fig. F14). A titanium UMC from Ocean Design, Inc., is installed on its top bulkhead. The UMC is an interface to the SAM and the SWB and is joined by a 20-ft-long oil-filled cable to the power supply access terminal (PAT). On the bottom bulkhead, four stainless-steel Ocean Design, Inc., UMCs are connected to the long cables to the downhole sensors. The MEG frame is a part of the riser pipe assembly that stands in the center of the reentry cone. The MEG can be removed from the frame by pulling up on a rope attached to eyebolts on its top. Thus the MEG is a replaceable component on the seafloor. Two stainless-steel pins are attached each to the top and bottom of the canister to guide the MEG into the frame and to mate the connectors smoothly. The UMCs, whose conductor pins have their own ejecting force, are mated by the >100-kg weight of the MEG in water.
The DM24 is a modular intelligent digitizer (Fig. F15) developed by Guralp Systems, Ltd. Each DM24 has three single-ended analog input channels to 24-bit A/D converters as well as additional three-component 16-bit A/D channels. Every DM24 consists of five stacked circular printed circuit boards (PCBs) in the MEG, or rectangular PCBs in the OBHS. The 24-bit digitizer utilizes the Crystal Semiconductor (CS) 5321/2 chipset and Motorola 56002 digital signal processor (DSP). The CS5321/2 digitizes the 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 individually have various decimation factors that allow multiple data output rates to be selected simultaneously. The sampling by 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 static random access memory, and sends it through the serial link to the outside module in GCF. If a packet is lost during data transmission, it can be recovered from the receiver side by a procedure called "block recovery protocol."
GCF, used in transferring data throughout the system, enables sharing of a single transmission line by many different time series data channels. It also incorporates status messages in American Standard Code for Information Interchange (ASCII) characters. Each data transmission in GCF is a packet containing either a data block or a status block. The GCF packet consists of an identification character, transmission serial identification (ID), data/status block contents, and 2-byte checksum characters. The transmission serial ID increases by 1 every packet. Serial numbering enables a receiver to detect loss of a GCF packet, which will result in a request to resend the lost packet. Each data block comprises data in multiples of a full second starting on an exact second. The data block consists of a block header and a compressed data block. The basic attributes of the data (e.g., 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) are stored in a block header. The set of system ID and stream ID identifies the source of the data. The assignment of the system ID/stream ID to each source is listed in Table T1. The data are compressed by recording 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 block header as that of data blocks but is identified by a sps field of 0 and a compression-byte value of 4. After the block header, status information in ASCII characters follows and is terminated by a NUL character. 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 that of the CRM and of the DM24s.
The H8 processor also controls the DM24 real-time clock (RTC), which has battery backup power. The processor receives an external time reference signal through the serial link and synchronizes the internal RTC to the reference. The external time reference can be either a GPS or "Stream Sync" time base signal. But in this case all the DM24s in the MEG and the OBHS are configured to use the Stream Sync signal, a set of clock synchronization characters sent by an upstream module through the serial data link of the DM24. The signal consists of 2-byte characters sent every second that encode date and time information over a 1-min sequence. The advantage of the Stream Sync is that it can utilize a low band-width link such as 9600 bps, which is the link speed between all the DM24s and CRM.
Each DM24 has a clock that stamps time on the data sent to CRM. These less-accurate clocks are separate from the precision clock in CRM, which is the master clock for the whole system. Thus, the clock in DM24 must always be synchronized to the master clock in CRM.
DM24s receive the time reference signal and trim their clock oscillators to synchronize with the reference. The measurement of the clock offset and the trimming is made once a minute. The mechanism typically keeps the clock offset between CRM master clock and these slave clocks within 1 ms. DM24s also report the clock offset in units of ms in the status block.
Another DM24 feature is that it is interactively configured or commanded through the same serial link that is used for the data transmission. With a simple command line, control of the sensor attached to the DM24 is possible, such as mass unlock/lock or mass centering of the OBHS. Also, 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.
Analog signals from the strainmeter are digitized by the DM24 with 24-bit resolution. Nine channels from three DM24 modules are employed for the three-component strainmeter, whereas three channels of a single DM24 are used to digitize the single-component strainmeter. The PMD seismometer and the tiltmeter share a common DM24. The PMD seismometer channels are attached to the 24-bit digitizers, and the two-component tiltmeter signals are connected to 16-bit digitizers. The DM24 dedicated to the PMD seismometer and the tiltmeter differs from the others in that it has three extra circuit boards and a system process to control motors to level the tiltmeter sensors.
The CRM takes digital data from all the DM24s. Serial links between the CRM and the DM24s in the MEG are transistor-transistor logic level interfaces to minimize power consumption of the MEG's line driver/receivers. An RS422 serial interface is used between the CRM and OBHS to ensure a link of sufficient quality over a 1-km-long cable. The data collected by these serial interfaces are handled by a Hitachi H8 microprocessor, buffered sequentially in an 8-MB silicon file and transmitted to the SAM through a high-speed (57600 bps) RS232C serial interface.
The H8 processor controls its precision master RTC in the same manner as for the DM24 RTC. The master RTC is a temperature-compensated precision clock, and the trimming of the oscillator by the processor results in an accuracy exceeding 10-8. Absolute time of the master RTC can be set by sending a stream sync signal from a surface ship via an ROV-BOB connection (see "BOB").
On power-up of the system (e.g., when the SWB is connected to the MEG) the CRM begins executing the system program. The CRM also allows the system program to be reloaded from the high-speed serial link. After 20 min of standby time to assure a complete cable connection by the ROV, the CRM begins to handle data transmission and powers up all the DM24s and the sensors sequentially. The power-up sequence of the sensors is as follows:
The CRM monitors all units. If after 1 hr the CRM has not received valid data from a unit, it will repeat the start-up sequence for that unit. This process repeats as many as 10 times if the unit continues to fail to respond. In the future, this will be modified to allow manual override.
Another process running on the CRM monitors the output from the tiltmeter and automatically nulls the output by sending the DM24 a command to drive the leveling motors if the output is >40% of full scale for more than half an hour. Each time the CRM powers up the motor drive circuitry, it makes two attempts to null the tiltmeter output. After nulling, it will not attempt to null for another 8 hr.
When the CRM receives a character to request a "command session" from the upstream SAM, it stops sending data and switches to command-session mode. In command-session mode the CRM provides features to control many other modules (i.e., master RTC, the PDM, and the strainmeter interface) in a simple command set. The available commands (those of the DM24s, SAM, and BOB, as well as of the CRM) are summarized in Table T2. The command session is finished by a "go" command from the upstream or by a time-out. When the upstream requests the CRM for a connection to a DM24, the CRM stops sending data to the upstream and relays the request of connection to the DM24. The CRM maintains the command-session link until the upstream finishes the session.
The watchdog fail-safe timer that is a part of the H8 microprocessor reset circuit is employed to reset the CRM. The timer should be triggered at least every 1.5 s. The multitasker of the CRM system normally does this every second. If there is some failure of the CRM program, no triggering of the timer results in a reboot of the system.
The power distribution module (PDM) is a single round circuit board that switches and distributes power to all the sensors, the DM24s, and the SAM. Supply voltage for the PDM can vary from 16.5 to 75 V. For all the DM24s, the PDM supplies power simply by switching the power on; for the SAM, it first regulates the supply to 18 V. A voltage drop in series diodes (4.4 V) plus that in the cable resistance (~24 ) ensures that the supply for the OBHS is below 36 V. The other DM24s operate from 36 to 75 V.
Each power channel is switched by separate solid-state relays controlled by the CRM so every component can be switched independently. The current through each power channel is monitored by the CRM every second. To protect other modules, the CRM shuts down a channel if that channel draws an unexpectedly large current for more than 30 s. The criteria to shut down, which is configured in the CRM system complementary metal oxide silicon memory, can be different for each component. The current maximum and minimum limits are set to 200 mA and 28 mA, respectively, for the OBHS and DM24s; the SAM upper limit is set to 500 mA.
The strainmeter interface, other than the DM24 for the strainmeter, monitors the signal and controls the valves of the strainmeter. It reports strainmeter status information to the CRM when some change occurs on these valves or upon request from the CRM. The information includes the status of all valves, signal voltages of all the channels, and the supply voltage for the interface (see "Electronic Control and Signal Conditioning").
Four 1-km-long cables, which are tied to 4-in-diameter casing pipe, supply power for each downhole sensor as well as transfer data to the MEG. Cables for the CMG-1T seismometer and the PMD seismometer are of eight conductors each; tiltmeter and strainmeter cables are of 12 conductors each. The outside diameter of all cables is 19 mm. The cable consists of two layers of conductors covered with an inner jacket of high-density plastic elastomer (HDPE), a tension member of aramid fibers, and an HDPE outer jacket. The fiber tension member provides tensional protection for conductors up to 1800 kilograms force (kgf) of maximum tensional load. But these cables still have enough mechanical flexibility to allow a bending radius as small as 12 in. The cables have densities of 1.12 g/cm3 (12-conductor) and 1.05 g/cm3 (8-conductor) so that they are almost neutrally buoyant in seawater. Thus these cables experience minimal tension in the borehole. They are also strapped to the 4-in casing at ~7-m intervals. The cross-sectional area of conductors is, for both 12- and 8-conductor cables, 1.25 mm2 for those in the inner layer and 0.9 mm2 for those in the outer layer. Cable resistances for 1 km are 16 for 1.25 mm2 conductors and 20 for 0.9 mm2 conductors. Thicker conductors are chosen for the lines carrying larger currents to lower the voltage drop in the cables.
The assignment of the cable wires and the pin assignment of connector pins on both the MEG UMC and the sensors are summarized in Table T3.
The SAM is the recorder mounted in the top of the battery frame. The SAM is connected through an oil-filled cable to the MEG. When the storage capacity of the SAM becomes full after ~1 yr of recording, an ROV can replace it with an "empty" SAM. Ejection of the SAM is aided by a lever mechanism on the frame (see Fig. F16).
The SAM receives data from the MEG through a high-speed (57,600 bps) RS232C serial link, which buffers the data in a silicon file that consists of 64 MB of flash memory of which 56 MB are allocated for buffering. The flash file is nonvolatile memory so that data in memory will not be lost even during loss of power. The amount of data incoming to the SAM is expected to be 12 kbps in a standard configuration of recording (i.e., three channels of the CMG-1T seismometer at 100 sps, three channels of the PMD seismometer at 20 sps, nine channels of the strainmeter at 50 sps, and two channels of the tiltmeter at 4 sps). In this configuration, the buffer memory will be filled in about half a day.
As the buffer becomes full, the SAM flushes the contents of the memory to a hard disk drive (HDD). The SAM has four HDDs of 18 GB each, for a total of 72 GB of storage. The HDDs are powered only on the memory flush, and only one disk drive is activated to spin up, whereas the other HDDs are kept in standby mode. It takes ~1 min to transfer 1 MB of data onto a disk.
The HDDs are powered by NiMH batteries in SAM, while the other components run directly on power from the MEG. The voltage of the NiMH battery pack is 14.0 at full charge. Use of the NiMH batteries prevents the transient large surge current of the HDDs from affecting the rest of the system. These batteries are recharged after the HDDs have consumed approximately half the capacity of the batteries on each buffer flush. After the batteries are fully charged, the charging mode is switched to trickle charge. Power consumption of the SAM is ~1.6 W in noncharging mode. When the batteries are being charged, the power consumption is higher. The limit of the charging current, typically 150 mA, can be specified by a user.
The directory of the data written on the disk can be browsed by "dir" command on the infrared (IR) port, though the data cannot be replayed through the serial link. On the dir command, the SAM will reply with a list of system ID, stream ID, the date of the first data, the date of the last data, and the total amount of data for a stream. The disk drives can be connected to personal computers (PCs) with a SCSI interface. A PC program called "Scream!" can replay the data written on the disk in GCF.
In parallel to saving the received data into the buffer memory, the SAM also sends the data, together with SAM status information, to the IR port on its top, which is the link for the BOB module. The IR serial link also allows the SAM to receive a command-session request from the upstream BOB module; if the request is addressed to the CRM or downstream modules the SAM relays them to the CRM.
The BOB module is a temporary recording device for short-term use (Fig. F17). An important purpose of the BOB module is to make an initial diagnosis of the system status by ROV either by watching light-emitting diodes (LEDs) on top of the BOB module or by communicating to the system through an ROV cable connected to the BOB module.
A Subconn eight-pin underwater connector is attached to the side of the BOB module. The connector functions both as a serial link to an ROV and as a power switch for the BOB module. The serial link allows communication from a surface ship to the whole system via a cable to the ROV. The speed of the ROV serial link can be configured over a wide range (75 bps-112 kbps) so that it can match the capability of an ROV. The BOB module is run by two internal 18-ampere-hr (Ah) lithium batteries, and the batteries are switched by connecting either the ROV cable or a switch plug to the connector.
An ROV may place the BOB module on top of the SAM so that the IR ports on the SAM and BOB module are aligned for communication. The BOB module receives data sent by the SAM through IR port and stores it in its flash memory. The capacity of the flash memory is 256 MB, which corresponds to storage of about a 2-day record for a standard sensor setting.
The LED matrix display on top of the BOB module shows the status of the MEG, SAM, and sensors. Each pixel of the LED matrix display corresponds to a particular set of system ID and stream ID. The LED has two colors: green indicates a reception of the data block and red indicates a reception of the status block. The assignment of the position of the LED and the stream ID (cf. Table T1) is shown in Table T4. The LEDs are visible through a 50-mm-diameter sapphire glass window on top of the BOB module.
The power consumption of the BOB module is ~1 W, and the data written in the flash file are readable through the ROV serial link.
The whole system is operated with limited SWB power. Although a short-term power increase can be supplied by an accumulator in the battery system, the system will fail eventually if the long-term average power consumption is more than the capacity of the battery system. Thus, the power consumption of the system was carefully evaluated before the deployments for the systems in both Sites 1150 and 1151.
Figure F18 shows the result of the power consumption measurement for the NEREID system in Hole 1151B. The power consumption is 18.1 W when the SAM charges the NiMH batteries for the HDD and 15.8 W otherwise. These values are a little greater than the expected values at the seafloor because they were measured while the sensors were experiencing large ship-driven motions.
In the NEREID system for Hole 1150D, the three-component strainmeter was installed. The system for Hole 1150D consumes ~2 W more power than does the system for Hole 1151B in which a single-component strainmeter was deployed. The difference in the power consumption comes mainly from (1) the two additional DM24s for the Hole 1150D system that digitizes six extra channels and (2) the additional sensors in the strainmeter. The increase of the data obtained also shortens the disk-writing interval, which results in an increase in the duty cycle of the NiMH battery charging.
The SWB power supply is rated at 18 W, but the actual power depends on environment factors (see "Seawater Battery"). For either system power consumption is almost at the limit of the capacity of the SWB system. Plans are in hand to modify the electronics to decrease power consumption.