Tides are the result of the gravitational attraction of the sun and the moon on the earth. The attraction of the moon is far greater than the attraction of the sun due to the close proximity of the earth and the moon. The sun is 360 times further from the earth than the moon. Therefore, the moon plays a larger role than the sun in producing tides. Every 27.3 days, the earth and the moon revolve around a common point. This means that the oceans and other water bodies which are affected by the earth-moon system experience a new tidal cycle every 27.3 days.
Because of the physical processes which occur to produce the tidal system, there are two high tides and two low tides each day. Because of the angle of the moon with respect to the earth, the two high tides each day do not have to be of equal height. The same holds true for the two low tides each day. Tides also differ in height on a daily basis. The daily differences in tidal heights are due to the changing distance between the earth and the moon.
There are several different types of tides. Neap tides are especially weak tides that occur during the moon's quarter phase. When this happens, the sun's and moon's respective pulls cancel each other out, so there isn't as much difference between high and low tides. Spring tides occur during the full moon and the new moon. They are especially strong high tides because the earth, the sun and the moon are aligned, so the sun and moon pull together against the earth. In addition, there are local influences on the tides. Some of those factors are:
1:Shape of the land
2:Shape of the ocean floor (bathymetry)
3:Depth of water
4:Restrictions to flow (narrow inlets to bays, etc.)
Tide gauge is a device for measuring sea level and detecting tsunami. Tide gauges that are close to the earthquake would be able to detect the rise in the sea level that a tsunami would produce.
There are fundamentally four types of sea level measuring technology in common use:
Stilling well and float: in which the filtering of the waves is done through the mechanical design of the well.
Pressure systems: in which sub-surface pressure is monitored and converted to height based on knowledge of the water density and local acceleration due to gravity. Such systems have additional specific application to ocean circulation studies in which pressure differences are more relevant than height differences.
Acoustic systems: in which the transit time of a sonic pulse is used to compute distance to the sea surface.
Radar systems: similar to acoustic transmission, but using radar frequencies.
In order to confirm the tsunami waves following an earthquake, to monitor the progress of tsunami and later for cancellation of warning, it is essential to monitor the coastal sea level changes. A network of tide gauges may be the only way of detecting a Tsunami in cases where seismic data is not available or when the Tsunami is triggered by events other than an earthquake.
As part of the Indian Tsunami Early Warning System, a real time network of Tide gauges has been established by Survey of India (SOI) and National Institute of Ocean Technology (NIOT). The network is designed to measure and monitor the progress of tsunamis from the two Tsunamigenic source regions of Indian Ocean in real time. The network comprises of 50 (36 by SOI and 14 by NIOT) state-of-the-art tide gauges transmitting real time data through satellite communication to INCOIS at Hyderabad, SOI at Dehradun and NIOT at Chennai simultaneously for processing and interpretation. In addition to Indian tide gauge network, the real time tide data from the Global Sea Level Observing System (GLOSS), in Indian Ocean is being received at INCOIS via internet.
Each field station is equipped with two types of sensors in the tide gauge system: Pressure sensors and acoustic sensors, to measure tide levels. The pressure sensors can be fixed directly in the sea to monitor sub-surface pressure. The sensor is connected by a cable that carries power and signal lines to an onshore control and logging unit. The sensor is usually contained within a copper or titanium housing with the cable entering through a watertight gland. Material used for the housing is chosen to limit marine growth. The acoustic tide gauges depend on measuring the travel time of acoustic pulses reflected vertically from the sea surface. This measurement is made with the acoustic transducer mounted vertically above the sea surface, located inside a tube that provides some degree of surface stilling and protects the equipment.
Each tide gauge measures the sea level by sampling for every one minute and transmits it for every 5 minutes. The filed stations are equipped with necessary power and communication facilities. The real time data from field stations is transmitted simultaneously to the central receiving stations established at INCOIS, SOI and NIOT.