The Monterey Bay ocean bottom observatory
Since its initial installation in 2002, the MOBB station has provided valuable data which help to refine our understanding of the noise characteristics of ocean-bottom seismic installations.
In addition, its offshore location allows scientists to gain a greater understanding of crustal formations between the San Andreas fault and the continental edge.
Although MOBB is currently an autonomous station with replaceable storage in situ, a continuous telemetric link to the Berkeley Digital Seismic Network is planned as part of the MARS networking project.
This will allow MOBB to become a full part of Northern California’s earthquake monitoring network.
The autonomous station consists of a Guralp CMG-1T ocean bottom sensor buried at a depth of 1000 m, 40 km off the coast of California.
A nearby floating platform holds a data logger, battery, current meter and differential pressure gauge.
Every three months, the system is serviced by divers from a research vessel. Recorded digital data is downloaded to the ship over a cable link, and if necessary the battery and data logger are replaced.
The low-power sensor provides the installation with a total power consumption of only 2.2 W, allowing it to run for an entire three-month stretch from a single lithium battery.
The CMG-1T sensor system outputs velocity signals over the full broadband frequency range 0.0027 Hz to 50 Hz. The sensor system is a self-contained unit which can level itself automatically against a tilt of up to 30 °. A patented locking mechanism ensures that the sensor suspension on either vertical or horizontal sensors cannot be damaged under any circumstances.
The self-noise of the CMG-1T cuts the Peterson NLNM at 200 sec (0.005 Hz) at the long period end, and at 18 Hz at the short period part of the seismic spectrum. The CMG-1T is performing well in comparison tests with nearby land-based instruments, with observed long-period noise, especially in the Z component, attributable to ocean currents and sediment reverberation. An understanding of these sources of noise is vital if ocean bottom systems are to take their place alongside land-based installations in the study of the Earth’s behaviour.
The sensor base is installed inside a levelling platform operated with 2 high-torque DC motors. Its design takes advantage of the natural stability provided by a ball-and-socket construction joint: the three sensors are contained in a machined cavity inside an inverted dome, whose exterior is machined to a spherical form. The dome rests in a ring shaped bearing so that it can move freely in any direction. A metal post, mounted centrally under the dome, carries a machined ball which articulates with a cylindrical cavity in a driving block below it. The driving block, moving on a plane surface below the dome, effectively translates the position of the dome (in spherical co-ordinates) into positions on a plane (in polar co-ordinates.)
This mechanism is more stable and more compact than a gimbal arrangement, and also has fewer moving parts. Indeed, a gimbal design would not be stable enough for broadband OBS seismometer applications.
A DC motor rotates the sensor base using a worm drive, whilst the tilt is provided by a second DC motor which positions the bearing block along a pair of parallel slides. Encoders fitted to the motor shafts transmit digital position signals to a microprocessor to enable precise movement without slippage. When the bowl needs to be centred, radial and azimuth adjustments are done until the outputs of an internal 2-axis inclinometer are brought as close to zero as possible. The magnitude of the tilt is calculated from the X and Y readings of the inclinometer, and the azimuth position is determined from the sign information.
The inclinometer output varies over a voltage range of ±4 Volts, with an output impedance below 10 Ω. Although the outputs are not linearly related to the tilt angle, calibration information is provided to the user to establish the tilt of the bowl with reasonable accuracy. In any case, a linear relationship is not required for levelling purposes.
The levelling platform is operated from an internal power supply which is housed within the sensor package. The power for the levelling platform is generated from a small isolated battery system which can be trickle-charged by the main power supply. Normally the instrument switches its inclinometer electronics off after initial installation, because subsequent settling can be accommodated by the sensor components’ own ± 3 ° levelling facility.
The acquisition system is based on the Guralp CMG-DM24, a 24-bit digitizer system which can control all the OBS functions as well as providing state-of-health information to the user. As well as the 24-bit seismic channels, the DM24 provides a number of multiplexed 16-bit channels for the non-seismic instrumentation (e.g. mass position, inclinometer, levelling bowl displacement transducer, internal thermometer and barometer.)
Power consumption and management
The total power consumption of the system, including the precision real-time clock, is 1.8 W. The sensor, digitizers, internal data storage and the inclinometer can be operated over a voltage range of 10 – 36 V. MOBB’s own recording system adds a further 0.4 W to this total.
An intelligent power management system provides all the power requirements of the sensor system while the system is operational, dealing with the greater power needs of the high-torque levelling motors whilst minimizing power usage whilst the system is in operation.
The MOBB ocean-bottom system uses Guralp Systems’ precision real-time clock module. This unit provides a precision timebase for seismic systems, particularly those which cannot access GPS.
Details on the progress of the MOBB experiment can be found at the Monterey Bay Institute MOBB site and in a forthcoming paper (B. Romanowicz, D. Stakes, D. Dolenc, D. Neuhauser, P. McGill, R. Uhrhammer and T. Ramirez, “The Monterey Bay Broadband Ocean Bottom Seismic Observatory”, Annales Geophysicae, submitted.)