A tsunami wave in deep water creates a small but measurable change in pressure that will be maintained for as long as twenty minutes. By monitoring any such changes, subsea detectors can be used to trigger an alarm that sends a warning message to a buoy-mounted receiver on the surface. The buoy, in turn, relays the message via a satellite data link to a control centre that can issue a warning to vulnerable communities.

The Sonardyne system is based on a Compatt 5 subsea transponder that uses the latest Wideband acoustic signal technology to provide robust through water communications in difficult acoustic conditions. Compatt 5s are used extensively in the oil and gas industry where their reliability is regularly trusted for use on the most complex construction survey projects. 

The Compatt 5 may be deployed in water up to seven thousand metres (more than four miles) deep and it is fitted with a sensor that continuously monitors water pressure, saving data every fifteen minutes. Because a reliable early warning of a tsunami can only be obtained close to the sea floor, the Compatt provides the essential means of sending these readings up to the surface. Every hour the Compatt converts the pressure readings into signals which it transmits acoustically to the buoy on the surface.

The satellite communications transceiver on the buoy then automatically forwards the pressure readings to the tsunami monitoring agency ashore. The system can also receive data from the central control so that revised monitoring parameters can be downloaded to the Compatt if required.

The Compatt is programmed to anticipate continual changes in the pressure of the water as these can be caused by influences such as tides, weather conditions and temperature. As these changes can be predicted, a deviation of as little as 3cm from the expected pattern will switch the device into alert mode. This will cause the Compatt to immediately transmit any data that has been saved during the past hour to the surface. It will then take pressure readings every fifteen seconds which it will immediately send up to the buoy for transmission by satellite to the monitoring organisation. As a result, the first warnings of a tsunami occurring several thousand of miles away can be issued within minutes.