Wave data recording and processing techniques
The method of recording and processing wave data varies according both to the nature of the recording location and to the purpose for which the data are being collected.
In some instances a knowledge of the average height of the highest waves and the average wave period is adequate. At other times a more detailed analysis of the range of wave frequencies and the energy content of each frequency band is required. The wave direction may also be recorded.
1. Recording wave data
There is a variety of methods of recording wave data instrumentally. Four of the most common are described below
A wavestaff is a vertical sensor, usually fixed to a rigid structure such as a pier or oilrig, which measures changes in electrical properties of the sensor as waves cover a greater or lesser proportion of its length.
1.2 Pressure recorder
Pressure recorders are usually mounted on or near the sea bed. Since the pressure is proportional to the height of the water column above the instrument, which is in turn affected by the passage of waves, the variations in pressure may be recorded and subsequently converted to sea surface elevation changes (i.e. waves).
1.3 Accelerometer buoy or 'Waverider buoy'
A 'Waverider buoy' is a surface following buoy anchored to the sea bed by means of an elastic mooring. An accelerometer mounted within the buoy registers the rate at which the buoy is rising or falling as it follows the pattern of waves. By integrating against time, the acceleration signal can be converted to vertical displacement. The displacement values are relayed to a recording station on the shore.
The Ocean Data Acquisition System (ODAS) buoy is a variation on this. Here the buoy is large enough to carry additional oceanographic and meteorological sensors, together with an on-board data recording system.
Some waverider or ODAS buoys also contain an electronic compass and two additional accelerometers (sensing pitch and roll of the buoy respectively) in order to measure the directional components of the wave field.
1.4 Shipborne wave recorder
As ships are too large to accurately follow the rise and fall of each wave, the shipborne wave recorder mounted within the hull, combines the principles of the pressure recorder and the accelerometer buoy.
A pressure sensor, with an opening through the ship's hull one to two metres below sea level and an accelerometer are located in the hull. The accelerometer measures the vertical movement of the ship as it tries to follow each wave and the pressure sensor measures the change in water height along the hull due to the ship's failure to follow accurately. The combination of the two gives the true wave height. Two pairs of sensors, one on each side, are required so that the effects due to rolling of the ship may be cancelled out.
2. Data logging
Data registered by any of the above instruments may be recorded by pen on paper charts, on analogue or digital magnetic tape. Records are usually taken for a standard period at fixed intervals over a total recording period of at least one year (to ensure that seasonal variations in wave climate are accounted for).
3. Processing wave data
Depending upon the instrument and recording method used, wave data may be processed in a number of ways to produce useful statistics which describe the wave climate. These 'processed' wave data fall into the following three categories. Each category contains all of the information available in the preceding one together with additional detail of the wave field.
3.1 Short term statistics
This type of wave data describes the wave field in terms of average values for the duration of each record. The most important values are a measure of the average height of the highest one third of waves during the record (usually called the 'significant wave height' or 'Hs') and the mean wave period (taken as the average time between consecutive crossings of the mean sea level line in an upwards direction; the 'mean zero upcrossing wave period' or 'Tz').
Depending upon the recording method used, these parameters may be calculated in a number of ways, each of which derives either a direct measure of, or a close statistical approximation to, Hs and Tz, in addition to various other short-term statistical values.
3.2 One-dimensional wave spectra
This method of analysis can only be applied to data which have been recorded digitally as instantaneous values of sea surface elevation at fixed intervals (usually every half second) throughout the recording period. A Fast Fourier Transform (FFT) is applied to these data to derive the heave energy spectrum (i.e. the wave field is broken down into discrete frequency bands, and the amount of energy within each band is calculated).
Additionally, a statistical approximation to Hs and Tz may be calculated from the spectral moments.
3.3 Directional wave spectra
In this method the heave, pitch and roll channels of a suitable buoy (each sampled as for one-dimensional spectra) are spectrally analysed to derive the co- and quadrature-spectra of the three axes. The compass channel is used to relate these to true North rather than to the buoy axes, thus giving information on the directional components within each frequency band as well as the heave energy.