Tsunami buoys are not independent warning systems, there are singular measurement systems. For GITEWS, the buoy systems were developed for the verification of a tsunami in the open ocean. For the first time, modern GPS receivers and various, especially meteorological sensors were installed on the buoys themselves, so that a buoy itself is a measuring instrument. The buoys can be complemented by ocean bottom units; two development lines are available (OBU PACT). Buoy systems can be employed usefully and reliably in the Indian Ocean - as in the other world's oceans - if operation and maintenance can be assured technically, organizationally and financially.
The most important information for a rapid warning, namely the magnitude and fast localisation of an earthquake, cannot be delivered by buoy systems. At the beginning of the project buoy systems were included in the planning, because they belonged to the "classic" instrument pool of the existing tsunami warning systems. The buoys developed within GITEWS belong to the most advanced worldwide. In the course of the project it quickly turned out that the buoy information cannot be on hand in time for the quick warning necessary in Indonesia (5 minutes after the earthquake), however equivalent information can be obtained in the required short time span through the integration of land-based GPS as an innovative element. To at least deliver information within the first ten minutes of an earthquake, the buoys have to be very close to the earthquake zone and thus anchored close to the coast of Indonesia. The possibility of "vandalism" by fishermen who use the buoys to moor their vessels was, however, underestimated, as was the dense ship traffic near the coast.
Offshore instruments are particularly expensive to maintain. The calculation for the operational cost of the Indonesian system as a whole reveals that the maintenance and operation of the buoy systems alone (20 in total, of which 10 from Germany) costs as much as a comparable service for all of the approximately 280 instruments installed on land. Further examinations of cost and benefit took into account that maintenance is laborious, the buoy data usually comes too late for the first and most important warning and that a limited availability of the systems is to be expected due to vandalism. This led to the overall decision to no longer consider the buoy systems for early warning in Indonesia. Due to the development and expansion of the novel land-based measuring system the offshore buoys is no longer necessary for tsunami early warning in Indonesia.
A tsunami wave, triggered by, for example, a great earthquake, generates a sudden change in pressure in the water column. Ocean bottom units that are installed on the seafloor in the open ocean register this pressure change and report it to the GPS buoy on the surface, which in turn pass on the data via satellite to the data and warning centre.
PACT (Pressure based, Acoustically Coupled Tsunami Detector) is a continued development of the established U.S. Dart system. With this device, the water pressure is measured at the ocean floor, so that a tsunami (pressure rise) is recognized immediately. The pressure data is transmitted acoustically through the water body via a modem connection to the buoy. PACT therefore integrates all the modern marine communication and measurement techniques (pressure level, data logger, data analysis, acoustic modem, acoustic releaser, rescue aid) in one device.
OBU (Ocean Bottom Unit)
OBU is a second line of development to the ocean bottom units besides PACT and was developed and built by IFM-GEOMAR and KUM GmbH. In this base unit, three-component broadband seismometers are added to the existing pressure sensors and hydrophones. The data recording is continuous and will initially be stored on a data recorder. In case of sudden pressure change of more than 3 cm water level change, the OBU triggers a tsunami alert and transmits the data via the buoy to the warning center. To save energy, in nominal mode the data are sent only every 48 hours automatically to the warning centre.