The OBS32 offshore seismograph, can record microseismic events occurring under oceanic sea bottom and below water depths up to 10Km. It uses atomic clock or OCXO crystal for accurate data 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.
The 3-component seismometer located outside of the sphere being held by a side arm that deploys the seismometer once the OBS is on the sea bottom. Three perpendicular, one vertical and two horizontal sensors have been placed inside the special seismometer housing 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 so the sensor provides a wide frequency response as 0.1Hz (10sec) to 98Hz. Sensitivity is also high, such as 1500V/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 pre-amplifier.
The recording unit consisted 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 the microSD memory card. The recording system runs a proprietary embedded DOS-compatible file system, in order the recorded data be able to be processed by any DOS or Linux compatible PC. The recording file system is a FAT32 compatible so it allows the usage of large volume memory cards (64Gb). This size is able to store 4-channels data being sampled at 250sps, for at least 4 months period. The digitizer has very high dynamic range, greater than 138dB at 100sps, so it is able to record micro-events. Overall power (sensor electronics + digitizer/recorder electronics) consumption is very low only 0.65W.
Timing of the data has to be accurate, even in the absence of GPS signal at the bottom of the sea. In order to achieve high time accuracy, a super low drift DPLL unit has been designed in order to operate for long period, with minimum drift. Two versions of OBS-DPLL units are available: One that uses an extremely precise OCXO crystal oscillator, with accuracy +/-5ppb (5*10e-9 sec) and a second version that uses an 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 the system has been synchronized, the GPS is 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, in order to measure the overall drift generated during the acquisition period. Since this drift is linear, because that the temperature at the sea bottom is very stable, the user is able to apply time correction at the recorder data, using 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 is able to release the sphere from the metallic base, and make it free to come up to the surface because of the buoyancy. This mechanism, can be operate with two ways. At first way, it receives a release command from the surface coming from an ultrasonic frequency commander, and at second 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 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, the OBS position can be located with within several meters accuracy. The positioning of the ship is made by a differential GPS method.
Orientation of the instrument once at sea bottom, is recorder by an integrated electronic compass. The orientation information is stored at the instrument’s log file. So the user reading the log-file, is able to know the exact sensor orientation.