New Wave Direction and
Spectrum Measurement Technique
Andrew L. Kun1
(akun@falmouth.com)
Alan J. Fougere2
(afougere@falmouth.com)
This paper describes a new measurement technique for wave spectrum and direction determination. The proposed technique combines acoustic current meter measurements with measurements from high precision pressure sensors. This technique allows the user to measure surface wave amplitude and direction, along with tide and tidal flow, from a single subsurface instrument. If the single instrument is combined with three additional "synchronized" pressure sensors, a complete six-wire-equivalent wave measurement can be attained.
Introduction
Both the scientific and the commercial community have a great interest in measuring wave direction and spectra. The measurement of ocean and higher frequency coastal wave spectra has been previously done by various instruments (Earle, 1984). Often the measurements were done solely with pressure sensors. Ideally the wave spectral information could also be computed from current meter data, by using the vertical water displacement information. However acoustic current meters (Brown, 1992) cannot provide high precision measurements of vertical water displacement. This is a problem, especially when the amplitude of the waves of interest is small (several cm). A better way to acquire wave amplitude spectrum information is by calculating the spectrum from pressure sensor data and direction from the orbital flow velocity measurements.
The technique proposed in this paper combines acoustic current meter measurements with measurements from one or more subsurface pressure sensors in order to acquire high precision measurement of wave direction and spectra.
Instrument Hardware
Figure 1 shows the outline of the instrument housing with the acoustic current meter fingers sticking out. The current meter uses eight acoustic transceivers to create four acoustic paths. The flow velocity is measured by observing the phase shift of the sound along three of the four acoustic paths. One path is always disregarded - this is the path that is contaminated by the wake from the center support strut.
The instrument also includes a 3-axis fluxgate compass which measures the Earth's magnetic field and a 2-axis electrolytic tilt sensor which measures the instrument's angle to the vertical. Using the compass and the tilt sensor we can determine the heading of the instrument, and consequently estimate the flow direction in Earth-coordinates.
The instrument measures pressure using a Silicon machined 0.01% F.S. precision pressure sensor. The pressure sensor provides wave height information, and it can also provide tide information.
The instrument is controller by a 16-bit microprocessor. Data is stored on a PCMICA flash memory card. Since the instrument is battery operated the flash memory approach is very important since flash memory is non-volatile therefore recorded data will not accidentally be lost due to loss of battery power.
Figure 1 Outline of the instrument housing
Measurement Technique
In its simplest form the proposed measurement technique combines data from an acoustic current meter and one subsurface pressure sensor. The instrument is fixed in a frame and the frame is mounted on the ocean floor, as shown in Figure 2. The current meter measurements provide the horizontal wave and current direction information. The pressure sensor data in combination with the current meter data is used to determine the wave spectrum, based on readily available spectral analysis techniques (Tucker, 1992). The high precision pressure sensor also allows for accurate tide information. Vector averaging of the acoustic current sensor data also yields mean flow speed and direction. Note that since the instrument does have a tilt sensor it does not have to be mounted in a vertical position, since tilt can be measured and corrected for.
Figure 2 Instrument positioning
By combining a current meter and pressure sensor setup (as shown in Figure 2) and three subsurface pressure sensors positioned in a non-collinear fashion in a frame, six degree-of-freedom measurements of wave direction and wave spectrum can be implemented, again with tide information available. This idea is outlined in Figure 3, where the instrument's elements are represented by circles, and the frame is represented by the solid line connecting the circles. Note that the positioning of the pressure sensors with respect to the current meter/pressure sensor assembly influences the directional sensitivity of the system.
The instrument is intended to be used in three modes:
- bursting mode,
- vector averaging mode, and
- complete on-board spectral analysis mode.
In bursting mode the instrument saves every data point that it acquires into memory. The data can later be processed. In vector averaging mode the instrument performs vector averaging on the sampled data and only stores the vector averages in memory. These averages are later downloaded to a computer and processed. The instrument can also perform complete on-board spectral analysis. This mode can be useful in telemetry type applications, where spectral and directional data can be sent over low-bandwidth communication lines used in oceanography.
Figure 3 Configuring the instrument with multiple pressure sensors
Conclusion
The new measurement technique described in this paper provides a low-cost means to perform high accuracy wave directional spectrum measurements. The technique relies on the proven technology of the acoustic current meter. The instrument is capable of high sampling rates of 5-6 Hz. The instrument also provides information on tides. The instrument operates on low power. The power consumption is less than 250 mW. This enables the user to deploy the instrument for months at a time.
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