Detection, recording and transmission of submarine buoy signals

The technology of the future tsunami warning system in the Indian Ocean

The buoys planned for future tsunami detection in the Indian Ocean will be deployed up to 6000 meters deep in the ocean, where they will have to function for a year without further intervention. In the event of an alarm, however, the measurement data will not take a year to reach the surface, but only a few seconds.

After the devastating tsunami of 26. In December 2004 in Asia, the Indonesian government signed a cooperation agreement with the Federal Republic of Germany for the rapid establishment of an early warning system. Currently, an experimental system is being prepared as part of the first stage of the tsunami early warning system. Automatic measuring stations will be installed at two locations on the seabed of the Sunda Trench off Indonesia’s western coast. These will each consist of a buoy and an ocean bottom unit ("OBU").

The measuring stations record all relevant data, such as seismic, position, water column height, absolute prere and differential prere, as well as their own operating information. The data collected on the seafloor are preprocessed there, transmitted by acoustic data transmission similar to the communication system of dolphins to the buoy on the sea surface and bundled with the data collected there. All information is then transmitted by radio to the Garuda news satellite, which is in geostationary orbit over Indonesia at 123° east longitude. The satellite in turn transmits the information to the appropriate warning center for further processing.

Detecting, recording and transmitting submarine buoy signals

In order to detect tsunamis with a warning time of 30 minutes, deviations of only 10 cm from the normal tidal range (ca. 50 cm) to detect. Such small deviations from the normal level can be detected at sea depths of 4000-5000 m, as is the case with tsunamis.B. off Sumatra is typically recorded with quartz prere sensors installed on the seafloor.

The approach of sinking seismometers and prere sensors with ocean bottom sensor systems (OBS) in the deep sea for up to one year, storing the measured data and then recovering the OBS ultrasonically controlled, has been developed by the University of Hamburg and IFM-Geomar Kiel since the mid-1980s. For early tsunami detection, it is then additionally necessary to transmit the prere data obtained from the seafloor by acoustic signal transmission with underwater modems to a relay buoy anchored next to the OBS, which forwards the data to a geostationary satellite that establishes the connection to the evaluation center and the Internet.

What are OBS and OBU?

An OBS is an autonomous system that is launched from the ship and sinks to the seabed thanks to an anchor weight. There it registers with a hydrophone the water sound and with a geophone finest movements of the seabed. At the end of the measurement campaign, the anchor is released by an ultrasonic signal emitted by the ship and the OBS starts its own ascent to the sea surface.

The abbreviation ‘OBS’ stands for ‘Ocean Bottom Seismometer’, because these systems were originally developed only for the investigation of seismic zones in the sea. In the meantime, however, they are also used for the exploration of oil and gas deposits with the help of active seismic and even electromagnetic quantities. Therefore, the term ‘Ocean Bottom Sensor’ is now more common. The OBS systems can be used in water depths of up to 6000 m.

An OBU is an OBS additionally equipped with a high resolution prere sensor for tsunami detection and an acoustic modem for data transmission. In addition, the hydrophone will be replaced by a differential prere sensor that can more accurately register the water sound generated by an earthquake.

The basis of the sensor signal processing on the seabed is the data logger Geolon-MTS from Send, which was developed in 1996 by Klaus Schleisiek and Dr. Ulrich Hoffmann in Hamburg and specializes in the acquisition and processing of signals measured under extreme conditions. In addition to sensor signal processing and storage, the prere data are analyzed by a tsunami detection program for indications (typical patterns of prere changes) that are typical for a tsunami. If a tsunami signature is detected, a warning is transmitted from the OBS via the underwater modems to the buoy and from there via the satellite to the evaluation center.

In the opposite direction, the data center can retrieve the following sensor data from the OBS at any time:

  1. Prere data averaged over five minutes each, generally transmitted to the buoy only every six hours to save power
  2. A three-hour history of prere data collected every 15 seconds. In this way, in the event of a tsunami warning, the evaluation center can understand why the detection program had reported a tsunami
  3. Prere data at 15-second intervals, transmitted to the buoy every two minutes, allowing the further development of the prere after a warning to be tracked
  4. A complete section of seismic data and prere data of a given date of z.B. 15 minutes Long, so that in the event of an earthquake the data from the "Earthquake monitoring center" on the seabed can be included in the analysis. This data is transmitted on request only in sections, as the amount of data is too coarse for continuous transmission to the evaluation center.

The OBS needs annual maintenance to read out the stored data and to replace the batteries. After maintenance, the complete seismological data (X, Y, Z and P waves) and the prere data will be available for scientific evaluation.

Participants in the development of the tsunami warning system

At the consortium Experimental ocean-bottom unit for the first phase of the German-Indonesian tsunami early warning system The following companies are involved in the project:

  1. BSS Behrens Schiffs- und Schweibtechnik GmbH, Hamburg: is building the buoy
  2. Develogic GmbH, Gerlingen: builds an underwater modem link with intelligent power consumption control
  3. Evo Logics GmbH, Berlin: builds an underwater modem link that works according to bionic principles (dolphin communication)
  4. K.U.M. Umwelt- und Meerestechnik Kiel GmbH, Kiel: builds all mechanical parts of the Ocean Bottom Unit (OBU) like the prere cylinders and the support system with the buoyancy bodies
  5. Nautilus Marine Service GmbH, Bremen: will anchor the buoy
  6. Send Signal Elektronik GmbH, Hamburg: builds the recorder to which the sensors (seismometer, differential prere gauge, prere sensor) are connected and on which the tsunami detection runs, controls the communication via the modems to the buoy, and is responsible for the project management of all components from the seabed to the buoy.
  7. Space-Tech GmbH, Immenstaad: is responsible for the software on the buoy and for the communication via the satellite and is responsible for the overall project management during the experimental phase.

Accompanying and leading institutes:

  1. Bundeswehr Research Institute for Waterborne Sound and Geophysics, Kiel: assesses the use of underwater modems
  2. Geo-Forschungszentrum Potsdam: supervises and manages the construction of the buoy and the acquisition of the GPS data. The program management lies with Dr. Jorn Lauterjung.
  3. IFM-GEOMAR, Kiel: manages the construction of the OBU and the installation cruise on the research vessel "Sun".