Ocean Bottom Seismometer
The OBS32 (32-bit, Four-Channel Ocean Bottom Seismometer Recorder)
Overview
The OBS32 offshore seismograph can record microseismic events occurring at the ocean floor and 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.
+ Additional Details
The 3 component seismometer is located outside 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 to maximize sensitivity.
The sensor electronics are the same as those used in our S-100 wideband sensor, which is designed according to the force balance principle, thus providing a wide frequency response of 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 addition, 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 four-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 can store four-channel data, sampled at 250 SPS, for at least four months. The digitizer has a very high dynamic range, greater than 138 dB at 100 SPS, so it can record micro events. Overall power (sensor electronic + digitizer/recorder electronic) consumption is very low, only 0.65 W.
The timing of the data has to be accurate, even in the absence of a GPS signal at the bottom of the ocean. 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 (510 e – 9 sec) and a second version that uses an atomic clock with an accuracy of 1, 510e – 10 sec.
The DPLL RTC system uses a 12-channel GPS receiver 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 can apply time correction to the recorder data using the appropriate software. Of course, there is no need for any time correction when the DPLL uses the atomic clock source for time reference.
The OBS is attached to an anchor weight that drives and places the instrument at the sea bottom. Between the OBS sphere and the metallic base, a mechanism 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: by a release command from the surface, coming from an ultrasonic frequency commander or by using a timer, which releases the instrument after a pre-defined time period. 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 ship site, the OBS position can be located within several meters of accuracy. The positioning of the ship uses a differential GPS method.
Once at the bottom of the sea, instrument orientation 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.
+ Features
- 32-bit ADC four-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
- Wideband 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