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How Tsunami-Meters and Tide Gauges Work

A tsunami-meter is a pressure sensor deployed at sea that can detect very-low-amplitude waves of only a few centimetres. When a wave sweeps by, pressure increases as the volume of water above the sensor rises. Tsunami-meters are used for recording tsunamis and predicting regional or remote tsunamis’ development and impact. There are two types of tsunami-meter :

either the sensor is linked to an undersea cable, through which data are transmitted. This has the advantages of low maintenance costs and damage risk, but also two constraints: the tsunami-meter cannot be set up very far from the coast (only up to 150 km) and a strong earthquake can break the cable. Cable-linked tsunami-meters are mainly used by Japan.

or the sensor is placed on the sea floor and transmits the recorded data by acoustic link to a buoy at the surface, which relays the information via satellite. These arrays, known as DART (Deep-Ocean Assessment and Reporting of Tsunamis) buoys, were developed in the US starting in 1997 as part of a national tsunami hazard mitigation programme. The sensors are amazingly accurate, as they can detect one-centimetre waves 6000 metres deep. They can also be set up in the middle of the ocean and provide real event forecasting.

They are, however, very costly to install and maintain. According to information obtained by our report writer, the instruments cost € 70,000 to € 200,000, installation € 100,000 and yearly maintenance checks € 50,000 to € 70,000, bearing in mind that the array has to be replaced every 5 to 10 years.

A tide gauge is an instrument that measures sea level at a given point. It is generally located in a harbour and sometimes hooked up to a GPS station. It is mainly used to measure tides with the data rarely being transmitted in real time, but stored and then collected once a day or once a month. As tide gauges are located in inshore waters, they cannot be used to predict tsunamis heading towards the area. They do, however, serve a dual purpose. They are part of an early-warning system that provides neighbouring countries, regions and islands as well as civil safety agencies with valuable information for deploying immediate emergency measures when high waves are detected.
The measurement data on wave size, number and arrival times can also be used for reconstructing the event and for simulation models. In order to contribute to disaster readiness, however, they must be able to transmit their data in real time.

(taken from www.senat.fr)