Chapter 16 - Ocean Waves
16.6 Measurement of Waves
Because waves influence so many processes and operations at sea, many techniques
have been invented for measuring waves. Here are a few of the more
commonly used. Stewart (1980) gives a more complete description of wave measurement
techniques, including methods for measuring the directional distribution
Sea State Estimated by Observers at Sea
This was perhaps the most common observation
included in early tabulations of wave-heights. These are the
significant wave-heights summarized in the U.S. Navy's Marine
Atlas and other such reports printed before the age of satellites.
Satellite altimeters are now the most widely source of wave measurements. Altimeters
were flown on Seasat in 1978, Geosat from 1985 to 1988, ERS-1 & 2 from 1991,
Topex/Poseidon from 1992, Jason from 2001, and Envisat. Altimeter data are used
to produce monthly mean maps of wave-heights and the variability of wave energy
density in time and space. The data are also assimilated into wave forecasting
models to increase the accuracy of wave forecasts.
The altimeter technique works as follows. Radio pulse from a satellite altimeter
reflect first from the wave crests, later from the wave troughs. The reflection
stretches the altimeter pulse in time, and the stretching is measured and used
to calculate wave-height (Figure 16.12). Accuracy is ±10%.
|16.11 Shape of radio pulse received by the Seasat altimeter,
showing the influence of ocean waves. The shape of the pulse is used to
calculate significant wave-height. From Stewart (1985).
Synthetic Aperture Radars on Satellites
These radars map the radar reflectivity
of the sea surface with spatial resolution of 6-25 m. Maps of reflectivity
often show wave-like features related to the real waves on the sea
surface. I say "wave-like" because there is not an exact one-to-one
relationship between wave-height and image density. Some waves are clearly
mapped, others less so. The maps, however, can be used to obtain additional
information about waves, especially the spatial distribution of wave directions
in shallow water (Vesecky and
Accelerometer Mounted on Meteorological or Other Buoy
This is a less common measurement, although it is often used for measuring
waves during short experiments at sea. For example, accelerometers on weather
ships measured wave-height used by Pierson & Moskowitz and the waves
shown in Figure 16.2. The most accurate measurements are made using an accelerometer
stabilized by a gyro so the axis of the accelerometer is always vertical.
Double integration of vertical acceleration gives displacement. The double
integration, however, amplifies low-frequency noise, leading to the low frequency
signals seen in Figures 16.4 and 16.5. In addition, the buoy's heave is not
sensitive to wavelengths less than the buoy's diameter, and buoys measure only
waves having wavelengths greater than the diameter of the buoy. Overall, careful
measurements are accurate to ±10% or better.
Gauges may be mounted on platforms or on the seafloor in shallow water. Many
different types of sensors are used to measure the height of the wave or
subsurface pressure which is related to wave-height. Sound, infrared beams,
and radio waves can be used to determine the distance from the sensor to
the sea surface provided the sensor can be mounted on a stable platform that
does not interfere with the waves. Pressure gauges described in
§6.8 can be used to measure the depth from the sea surface to the gauge.
Arrays of bottom-mounted pressure gauges are useful for determining wave directions.
Thus arrays are widely used just offshore of the surf zone to determine offshore
Pressure gauge must be located within a quarter of a wavelength of the surface
because wave-induced pressure fluctuations decrease exponentially with depth.
Thus, both gauges and pressure sensors are restricted to shallow water or to
large platforms on the continental shelf. Again, accuracy is ±10% or