Ocean Bottom Unit
A tsunami wave, generated by an earthquake, produces a sudden pressure change. Ocean Bottom Units, situated offshore at the seafloor, register this pressure anomaly and report it to the surface GPS-buoy, from where it is sent to the data and warning center via satellite.
The Ocean Bottom Unit (OBU), along with the surface GPS-buoy, is one of the two main offshore marine systems. The OBU is an instrument, which is placed at the seafloor of the continental margin to measure and record seismic and pressure data. The system was developped by IFM-GEOMAR together with KUM GmbH. The frame, constructed by KUM GmbH, holds the buoyancy bodies that ensure a safe rise for recovery of the instrument. The floatation is rated, as are all other components of the system, for a maximum water depth of 6000 m. Attached to the frame are a radio beacon, a flash light, and a flag for retrieving from aboard a vessel. The system components are mounted to the OBU frame and consist of pressure cylinders holding the data logger and batteries as power supply, an acoustic release unit, an acoustic modem, and the actual sensors to measure the data.
The OBU is equipped with a broadband Güralp seismometer and a Differential Pressure Gauge (from Scripps Institution of Oceanography). Additionally, it measures the absolute pressure with a Paroscientific Intelligent Depth sensor, manufactured by DIGIQUARZ. The data of these sensors are recorded by the use of a data logger, the Marine Tsunameter Seismocorder, which is designed by SEND GbmH for long-time recordings of low frequency bands. The pressure cylinders protect the recorder as well as the batteries from the damage through water and pressure at depth. Communication and data transmission to the surface GPS-buoy occur via the acoustic modem (Develogic), which is connected to the data recorder. The acoustic release unit, a K/MT562 made by KUM GmbH, holds the OBU attached to the anchor during the measurement and releases the OBU from the anchor for recovery and maintenance of the instrument. While deployed to the seafloor the entire system rests horizontally on the anchor frame. This ensures minimum water current sensibility at the ground (during measurement). After releasing its anchor weight the instrument turns 90° into the vertical and ascends to the surface with the floatation on top.
The OBU is a stand-alone instrument that can operate for more than one year before maintenance and renewal of the power supply for the data logger and the acoustic data transmitter.
Until the end of 2009, a total of 10 offshore instrument pairs, OBU and GPS-buoys, will be installed within the GITEWS along the Indonesian margin for tsunami detection.
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