BOREHOLE INSTRUMENTS

The OBH package consists of a three-component seismometer and a 24-bit digitizer (DM24) assembled in a grade five titanium pressure cylinder 1.2 m long × 12.7 cm outside diameter (OD). The cylinder is designed to withstand pressures at 10,000 m water depth, and all of the cylinders were 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 (Fig. F12). 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 F13. 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 × 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.

Block diagrams of the feedback system are shown in Figure F14. In order 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 short period performance of the Leg 191 seismometer was improved over the one deployed during Leg 186 by an improved displacement transducer circuit. Figure F15 illustrates the instrument self-noise curves for vertical and horizontal components of the OBHs in comparison to those from Leg 186. The OBH for Leg 191 is >10 dB quieter at frequencies >2 Hz than the one deployed during Leg 186.

The 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 ~2.4 × 10^-10 m/s/bit for the velocity outputs and 8.0 × 10^-8 m/s²/bit for the mass position outputs. 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-191 seafloor data module through a 450-m-long cable. The communication link to the MEG-191 is a four-wire 38,400-bps (bits per second) RS422 serial link. As well as sending the signal data, the DM24 receives the time reference signal from the MEG-191 and synchronizes the OBH clock to the MEG-191 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-191 clocks becomes >20 ms. The OBH clock in the DM24 records the time in the digitized records.

The voltage range for the OBH is 10-36 V. Because the 450-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 serially 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 450-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-191 to isolate the power ground from that in the MEG-191, and its efficiency is ~80%. The overall power consumption of the OBH is ~3.3 W during normal operation. When the masses are locked, each OBH consumes 0.2 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-191 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 the deployment. During Leg 191, this date was set to 15 September 2000 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 of the range of the mass movement.

The DM24 is a modular intelligent digitizer developed by Guralp Systems, Ltd. The schematic diagram of the DM24 is shown in Figure F16. 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 Motorola 56002 Digital Signal Processor (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 an 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 "Guralp Compressed Format" in "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.