Ocean Bottom Seismometer

The OBS24 (24bit, 4channels Ocean Bottom Seismometer Electronics)

24 bit 4-ch digitizer / recorder Removable CF data storage FAT32 filing system Ultra low power consumption 0.5W 1-250sps continuous recording Wide response sensor 0.2-98Hz BW High sensitivity 1000V/m/s OCXO or Atomic Clock timing or Atomic Clock 5*10-10 sec time drift Timed or Command release Submersible up to 12km Operating Temp -20 to +70oC

The OBS 24 offshore seismograph, can record microseismic events occurring under oceanic sea bottom and below water depths up to 10Km. It uses atomic clock for accurate timing. The Instrument’s main structure is a glass sphere of 43 cm of diameter inside which all the electronic units and the submarine type battery packs are placed. 

We have placed the 3-component seismometer outside the sphere held by a side arm that deploys the geophone once the OBS is on the sea bottom. In the tri-axial geophone case, magnetic accelerometers of type SM-6 I/O with a sensitivity of 28.8 V/m/s is used. Three perpendicular, one vertical and two horizontal sensors are placed inside a special housing to support the 600 atm. water pressure. The coupling of the overall geophone has been optimized in order to maximize sensitivity.

The sensor electronics are the same as those used in our LTS-100 wide band sensor, which is using the force-balance principle in order to extend the frequency bandwidth of the geophones, from 4.5Hz to a wide frequency band 0.2Hz to 98Hz. Sensitivity is high, like 1000V/m/sec. Using this technique, the OBS has a ideal sensor for local and regional seismicity monitoring. In additional, a hydrophone is used with a pre-amplifier of gain 4.

The recording unit is consists from three different stages. The input stage implements the force-balance circuitry, the 4-channels digitizer, which does the A/D conversion and the recorder, which stores the data into a removable Compact Flash type card (CF). The recording systems runs a proprietary embedded DOS-compatible file system, in order the recorded data be processed by any DOS or Linux compatible PC. The recording file system is FAT-32 compatible allowing the usage of large volume CF (64Gb). This size is able to store 4-channels data being sampled @ 250sps, for a period at least 4 months. The digitizer has very high dynamic range, greater than 135bB, allowing it to be able to record micro-events. Power consumption is very low.

Timing of the data is a very important issue, and has to be precisely done, even in the absence of GPS signal. In order to achieve high time accuracy, the DPLL unit of our SR-24 and CR-24 seismic instruments, have been redesigned in order to operate for long period, with minimum drift. Two version of the OBS-DPLL unit are available: One that user a extremely precise OCXO crystal oscillator, with accuracy +/-5ppb (5*10e-9 sec) and another version that uses Atomic Clock with 1,5*10e-10 sec precision.

The DPLL – RTC system uses a 12 channels GPS receiver in order to be synchronized while the OBS is still out of the water. Once is has been synchronized, the GPS is automatically switched off and the time synchronization of the digitizer is taken from the DPLL. After the end of the recording period, and when the OBS is out of the water, the GPS is switched on again, in order to allow the system to measure the overall drift of the acquisition period. Since this drift is linear, because the temperature at the sea bottom is very stable, the user is able to apply time correction to the recorder data, if necessary, using the appropriate software. Of course there is absolutely no need for any time correction when the DPLL is using the atomic clock as time reference.

The OBS is attached on an anchor weight that drives and places the instrument at the sea bottom. Between the OBS sphere and the metallic base, there is a mechanism, which is able to release the sphere from the metallic base, and let is free to come up because of the buoyancy. This mechanism, can be operate with two ways. The first one receives the release command from the surface using an ultrasonic frequency, and the second one uses a timer, which releases the instrument after some pre-defined period of time. For maximum safety, both mechanisms are used.

The precise positioning of the OBS on the sea bottom is done by acoustic triangulation which makes use of underwater acoustic signals. By interrogation a slant range distance between the OBS on the sea bottom and the shipboard acoustic transponder unit is measured with an accuracy of 1 m. By shifting the ship position at several sites and measuring each distance between the OBS and each ship sites, we can locate the OBS position within several meters. The positioning of the ship is made by a differential GPS method.

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