Acehtsunami

In the deep sea, a tsunami has a small amplitude (a smaller amount than 1 metre) but very long wavelength (hundreds of kilometres). This means that the incline, or steepness of the wave is very small, so it is practically untraceable to the human eye. However, there are ocean observing instrument that are intelligent to detect tsunamis.

Tide Gauge

Tide gauges measure the elevation of the sea-surface and are primarily used for measuring wave levels. Most of the tide gauges operated by the government department of Meteorology’s National Tidal Centre are SEAFRAME stations (Sea Level Fine Resolution Acoustic Measuring Equipment). These consist of an aural sensor connected to a vertical tube open at the lower end which is in the dampen. The aural sensor emits a sound beat which travels from the top of the tube downward to the water surface, and is then reflect back up the tube. The distance to the water level can then be considered using the travel time of the pulse. This system filters out small-scale effects like wind-waves and has the capacity to measure sea-level change within 1mm accurateness.

The tide determine at Cocos Island observed the tsunami on December 26th 2004 as it passed by the island, as shown in these explanation made during December.

Satellites

Satellite altimeters calculate the height of the ocean surface in a straight line by the use of electro-magnetic pulses. These are sent down to the ocean surface from the satellite and the height of the ocean surface can be resolute by knowing the speed of the rhythm, the site of the satellite and measuring the time that the pulse takes to go back to the satellite. One trouble with this kind of satellite data is that it can be very meager - some satellites only pass over a exacting location about once a month, so you would be fluky to spot a tsunami since they travel so quickly. However, during the Indian Ocean tsunami of December 26th 2004, the Jason satellite altimeter happen to be in the right spot at the right time.

The below picture shows the height of the sea surface (in brown) measured by the Jason satellite two hours behind the initial earthquake hit the region southeast of Sumatra (shown in violet) on December 26, 2004. The data were taken by a radar altimeter on board the satellite along a track traverse the Indian Ocean when the tsunami waves had just overflowing the entire Bay of Bengal. The data shown are the differences in sea surface height from previous observations made along the same track 20-30 days before the quake, showing the signals of the tsunami.

The DART System

In 1995 the nationwide Oceanic and Atmospheric Administration began developing the Deep-ocean appraisal and Reporting of Tsunamis system. An array of stations at present deployed in the Pacific Ocean. These stations give full information about tsunamis while they are still far off shore. Each station consists of a sea-bed bottom force recorder which detect the passage of a tsunami.. The data is then transmitted to a exterior buoy via sonar. The surface buoy then radios the information to the Pacific Tsunami Warning Center via satellite. The bottom force recorder lasts for two years while the surface buoy is replaced every year. The system has significantly improved the forecasting and warning of tsunamis in the Pacific.