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Aanderaa Data Instruments AS – TD302
Appendix 4: Calculations of wave parameters (5218/5218R)
Raw Wave data are simply the pressure measured at the location of the instrument.
The pressure changes according to changes in distance between the surface and the instrument. As the distance
increases, the pressure increases. As the distance decreases, the pressure decreases.
The wave parameters are calculated from wave records containing 2
N
samples where N = 8, 9, 10, 11. The pressure
is sampled at either 2 or 4 Hz so that the duration of the wave records ranges from 64 seconds to 17 min (1024
seconds)
Processing of raw data
In general, the measured water pressure consists of a hydrostatic and dynamic components:
(
)
dyn
atm
P
P
gh
P
+
+
=
ρ
The dynamic part is due to surface wave motion. To reveal the dynamic pressure, the measured time series are pre-
processed in a two-step manner:
First, the contribution from the atmospheric pressure,
P
atm
, to the absolute pressure is subtracted from the samples.
In AiCaP CANbus mode, the sensor uses the fixed atmospheric pressure set by the user in
Sensor Configuration
, refer
chapter 5. In RS232/RS422 the atmospheric
Air Pressure
is a property that must be set by the user, refer e.g. chapter
6.
Second, the hydrostatic pressure is calculated by subtracting the atmospheric pressure from the time series samples.
The hydrostatic pressure is used to calculate the deployment depth.
Calculation of Wave Spectrum:
The dynamic pressure time series are used to calculate the wave spectrum. This involves Fourier transformation of
the time series using a Fast Fourier Transform (FFT) algorithm and scaling of the power spectrum to compensate for
the damping of the dynamic pressure.
Power Spectrum Scaling:
The wave motion at the sea surface causes a dynamic pressure that can be measured using a pressure sensor
deployed somewhere between the seabed and the sea surface. The magnitude of the observed dynamic pressure
depends on the surface wave period and the sensor deployment depth.
The deeper the sensor is deployed the more the dynamic pressure is damped. The shorter the surface wave period
the faster the damping of the dynamic pressure. Hence, the power spectrum must be scaled to correct for the
