A scientific point of view.

Is it possible to make direct measurements of the properties of waves in the sea? The answer to this possible question of some is: yes, it has been possible to measure waves, especially height, their main characteristic, since the beginning of the last century. Other basic parameters such as wave period and direction are also calculated by most current sensors available.

But, why should we measure the waves at sea? The waves can be measured both in deep waters and in coastal regions. In fact, they behave differently when at different depths. The ongoing activities in these environments are totally influenced by sea state and may become unviable due to excessive energy from the waves, the main parameter when designing a sea state. In the coasts and beaches environments, for example, we could observe countless forms of interaction between people, machines, and the ocean. Water sports such as surfing, bodyboarding, diving, fishing, recreational bathing, are examples of activities that require sea conditions data, either for reasons of optimization, in the case of the surfer who wants to enjoy waves with better formation and size, whether for safety reasons for bathers on a busy beach. In addition to this direct contact between people and the sea, there is huge concern and attention on the influence of the wave on coastal structures like ports and harbors, which connect products all over the world being one of the main means of transfer and economic interaction between cities and countries.
The navigation business needs low energy in sea conditions to operate safely. For this reason, wave monitoring, the main energy vector that acts in the oceans, is done carefully by most ports in the world. This investigation can be done by analyzing the basic parameters of a wave such as significant height, period, and direction or in more detail including information about the wave directional energy spectrum and current velocities induced by waves under some structures.

How to measure the waves?

There are several ways to measure waves on the ocean surface. Currently, there are technologies capable of taking measurements at any depth of the sea. The instruments are made to act exclusively at a certain depth, under a certain period of time governed by its supply of memory and internal energy, or in direct contact with an external energy source. At the beginning of the last century, science developed forms of measuring waves, based on different physical principles, some of which are simpler and others more complex.

Initially, they were able to measure the height of the waves using pressure sensors installed on the seabed. As it spreads and passes through the sensor, the wave generates a change in the height of the water column at that point, thus changing the local pressure. This pressure variation, evidenced by the passage of the highest part of the wave (crest) and the lowest part (hollow) is interpreted as the height of the wave. A little later came the directional buoys, which from the movements that buoy suffered due to the surface water particles accelerations as waves passing through  (heave, pitch, roll) it was then possible to infer the directionality of the wave field and consequently the directional spectrum of the waves and their derived parameters, significant height, period and direction. These methods can be considered the most classic, but today there is already a wide spectrum of possibilities for measuring waves. Today this can also be done using instruments that infer the speed of currents below the surface, by satellites, radars and even by video. Traditional methods such as buoys, pressure and speed sensors, collect information from waves at a point on the sea surface. But, these typical oceanographic instruments are expensive, complex to build, hard to deploy and require constant and specialized maintenance, turning the wave measurement in the sea a not simple neither cheap task.

Others techniques are able to measure the wave field over an area, representing in a more realistic way the true dynamics of the waves in that region, since they are irregular and can have different heights even within the same measured area of 20x20m, for example. This special measurement is only possible through measurements by stereo-video, radars and synthetic aperture radars onboard satellites.

typical oceanographic instruments are expensive, complex to build, hard to deploy and require constant and specialized maintenance, turning the wave measurement in the sea a not simple neither cheap task. "

Matheus VieiraOcean wave researcher, PhD candidate Centre for Marine Technology and Ocean Engineering - Portugal

In general, installing a sensor at sea to measure waves requires high costs and reliance on low energy conditions at sea for direct measurement instruments. Given this, a recent way of sampling the waves has gained popularity and called attention, precisely for circumventing these problems mentioned. We are talking about filming the waves and measuring them based on calculations based on stereoscopic principles.

Video-cameras are now accessible in most places and at a lower cost than traditional wave measurement methods. Making wave measurement even more popular, recently in the year 2020, it was found that it is possible to spatially sample the waves, and thus extract their properties, from the filming of Smartphones (Vieira, et al 2020). It is expected that in the near future, in view of the advancement of these technologies, wave measurement will be closer to people outside the academic world, and will become popular among the various sea users.

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