Ocean bottom seismometer
Ocean-Bottom-Seismometers ( OBS ) are used to research the sea floor using undersea wave measurements and their evaluations, which have a variety of properties. They are seismic devices , including geophones , (land) seismometers , hydrophones and streamers . Both land seismometers and streamer ships have contact with GPS and therefore know both their own position and the absolute time with high precision. The time standard in particular plays a major role in seismics, since here distances or layer thicknesses are measured using the transit times of sound waves . Deviations in the range of milliseconds already produce a noticeable error in the analysis.
The OBS, on the other hand, is a completely autonomous system, i. H. without (cable) connection to the ship. A connection to the GPS is not possible. Therefore, an OBS needs a highly precise clock that must not lose its normal time either over the entire period of use or over a wide temperature range. An OBS is able to be under water for a longer period of time, which depending on the design can be several months.
While land seismometers and streamers usually have a fixed connection to the data center and transmit their data in real time, the data must be stored at the OBS. The third major difference is the autonomous energy supply. Streamers get their energy from the ship, land seismometers either from the data center or from solar panels or small wind turbines.
The requirements for an OBS are high and partly contradict each other: highly precise and temperature- stable quartz oscillator , fast, precise A / D converters for the smallest voltages and computers with real-time operating systems on the one hand and robust, energy-saving technology on the other. Today's systems can be used up to a water depth of 6000 m at a pressure of 600 bar. The energy consumption is less than one watt, the service life up to a year.
The core components of the OBS are a three-axis seismometer and a hydrophone as sensors (in Fig. 2, covered) as well as the data recorder for recording and storing the seismic data (in the pressure-resistant titanium tube, 5) including the batteries (6). These components are attached to a support frame that is additionally equipped with sufficient buoyancy (4), an anchor weight (7) and an acoustic trigger (3) for dropping the anchor. The OBS is usually equipped with a radio transmitter and a flashlight (1) to simplify recovery. An important component of the OBS is also the shell, which has to be processed shockproof.
As an autonomous system, it is let into the water from the exploration ship and sinks to the sea floor thanks to its anchor weight. There it registers the water-borne sound (pressure waves) with a hydrophone and the finest movements of the sea floor with the seismometer (pressure and shear waves or also P and S waves, see seismic wave ). After the investigation period, the anchor is released with an acoustic signal emitted by the ship and the OBS rises to the surface of the water.