Ocean Bottom Seismometer

The OBS32 (32 bit, 4 Channel Ocean Bottom Seismometer Recorder)

  • 32 bit ADC 4 channel digitizer/recorder
  • Removable microSD data storage
  • Data retrieval from USB port
  • FAT32 filing system
  • Ultra low power consumption 0.55 W
  • 50 – 1000 SPS continuous recording
  • Wide band response sensor 10 sec – 98 Hz
  • High sensitivity 1500 V/m/s
  • OCXO or Atomic Clock timing
  • 5*10 – 10 sec time drift
  • GPS time synchronization at startup
  • Timed or Command release actuator
  • Submersible up to 12 km
  • Operating Temp -20 to +70°C

The OBS32 offshore seismograph, can record microseismic events occurring at the ocean floor and at water depths up to 10 km. It uses the atomic clock or an OCXO crystal for accurate data timing. The instrument is made of a glass sphere, 43 cm in diameter, with all the electronic units and the submarine battery packs inside.

The 3 component seismometer is located outside of the sphere and is held by a side arm that deploys it once the OBS is on the sea bottom. Three perpendicular, one vertical and two horizontal sensors have been placed inside the special seismometer to support the 600 ATM water pressure. The coupling of the overall seismometer has been optimized in order to maximize sensitivity.

The sensor electronics are the same as those used in our S-100 wide band sensor, which is designed according to the force balance principle, thus providing a wide frequency response as 0.1 Hz (10sec) to 98 Hz. Sensitivity is also high, such as 1500 V/m/sec. Using this technique, the OBS becomes an ideal sensor for local and regional seismicity monitoring. In additional, a hydrophone is used with a variable gain preamplifier.

The recording unit consists of three different stages. The input stage implements the force balance circuitry, the 4 channel digitizer for the A/D conversion, and the recorder for storing the data on the microSD memory card. The recording system runs a proprietary, embedded DOS compatible file system, which can process recorded data on any DOS or Linux compatible PC. The recording file system is a FAT32 compatible system allowing the usage of large volume memory cards (64 Gb). This size is able to store 4 channel data, sampled at 250 SPS, for at least 4 months. The digitizer has a very high dynamic range, greater than 138 dB at 100 SPS, so it is able to record microevents. Overall power (sensor electronic + digitizer/recorder electronic) consumption is very low, only 0.65 W.

Timing of the data has to be accurate, even in the absence of a GPS signal at the bottom of the ocean. In order to achieve high time accuracy, a super low drift DPLL unit has been designed to operate for a long period, with minimum drift. Two versions of OBS-DPLL units are available: one that uses an extremely precise OCXO crystal oscillator, with an accuracy of +/-5 ppb (5*10 e – 9 sec), and a second version that uses an atomic clock with an accuracy of 1, 5*10e – 10 sec.

The DPLL RTC system uses a 12 channel GPS receiver in order to stay synchronized while the OBS is out of the water. Once the system has been synchronized, the GPS automatically switches off and the internal DPLL synchronizes the digitizer. After the end of the recording period and when the OBS is out of the water, the GPS switches on again to measure the overall drift generated during the acquisition period. Since this drift is linear due to the temperature at the sea bottom being very stable, the user is able to apply time correction to the recorder data using the appropriate software. Of course, there is absolutely no need for any time correction when the DPLL is using the atomic clock source for 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 releases the sphere from the metallic base, freeing it to come up to the surface due to buoyancy. This mechanism can be operated in two ways. The first way is by a release command from the surface, coming from an ultrasonic frequency commander. The second is using a timer, which releases the instrument after some pre-defined period of time. For maximum safety, both mechanisms are used in parallel.

The precise positioning of the OBS at the sea bottom is done by acoustic triangulation using underwater acoustic signals. The 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’s position at several sites and measuring the distance between the OBS and each of the ship sites, the OBS position can be located within several meters of accuracy. The positioning of the ship uses a differential GPS method.

Instrument orientation, once at sea bottom, is recorded by an integrated electronic compass. The orientation information is stored in the instrument’s log file. The user reading the log file knows the exact sensor orientation.

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