Ocean Waves

Surface waves are formed by winds either locally as "wind waves" with shorter periods, or further away as “swell" with longer periods. A rare case of extremely long period of ocean waves are tsunamis from e.g. earthquakes and landslides. Because ocean waves can travel far and can contain high energy it is important to measure ocean waves to understand their impact on shipping fishing, tourism, constructions, shorelines, water mixing, sediment resuspension, gas exchange etc.

Methods to measure Ocean Waves:

What instruments are used to measure ocean waves? There is a range of technologies to measure ocean waves including microwave from space, various types of radars and spot measurements from below or at the surface. Microwave and radar give an overview but cannot provide the same level of detailed information as the spot measurements do. In addition spot measurements with instruments and/or buoys are normally well suited to measure other relevant parameters as well e.g. currents, wind, temperature, salinity, particles, oxygen, algae etc.

Waves vary on time scales of seconds to minutes to hours. To obtain reliable information on wave dynamics requires instruments that are installed and measure continuously over longer time periods. Because waves are variable in their character a minimum calculation period of 15 minutes is recommend to assess the prevailing conditions.

 At Aanderaa we have focused on three different systems for spot measurements of ocean waves including:

Motion based waves with the Aanderaa MOTUS stand-alone directional wave sensor is designed to accurately measure waves from surface located platforms. Most MOTUS sensors are combined with Current and water quality measurements and operated in Real-time from our standard MOTUS Buoy. The MOTUS sensors features innovative electronic and mechanical filters that enables accurate wave measurements from surface platforms of any size and shape. Examples of use on third party platforms range from small autonomous surface vehicles to 10m diameter steel buoys. In general surface located wave measurements give the highest quality wave information and the possibility to simultaneously distinguish between energy and spectrum from wind formed waves and swell from different directions.  

Acoustic based directional wave sensors are mounted below the surface. They use an upward facing ADCPs (Acoustic Doppler Current Profiling) sensors to measure the orbital velocities as close to the surface as possible. With this technology it is also possible to obtain water current information simultaneously. To increase the accuracy of the directional wave measurements Aanderaa has developed a unique adaptive pulse technology that automatically switches between 3 different acoustic modes (two types of broadband and narrowband) depending on the wave height. The Acoustic Wave Measurements are done with SeaGuardII instruments and normally combined with pressure based wave measurements, for redundancy and independent verification, and with other water quality parameter.

Pressure based sensors are mounted below the surface. By sampling in bursts non-directional waves and mm changes in the water level are detected at 2 or 4 Hz. The Aanderaa pressure based wave sensors include highly sensitive pressure elements and user selectable cut-off frequencies that pushes the limits for measuring shorter period/smaller waves from deeper locations. The pressure based wave sensors are often used in real-time connected to SmartGuard loggers or SeaGuardII instruments, combined with other parameter (link to water quality), or cable connected to third party loggers using serial Rs232 and/or Rs422 format.

The three Aanderaa methods to measure waves have been extensively compared and field verified. Please read our white papers.

    Motion Based Wave (MOTUS)

    		 Acoustic Based Wave (SeaGuardII DCP Wave) Pressure Based Wave (Wave & Tide Sensor)
    Depth
    • 5 to several 100 m
    • Maximum 40 meters depth*
    • Maximum 40 meters depth*
    Parameters
    • Height, Period, Steepness, Irregularity of Sea, Energy spectrum, Raw data time series.

    • Directional wave spectrum, Peak wave direction, Directional  spreading, Long Crestedness, Orbital ratio spectrum, Fourrier coefficients spectra, Separate between swell and wind driven waves
    • Height, Period, Steepness, Irregularity of Sea, Energy spectrum, Tide, Temperature, Currents, Raw data time series
    • Directional wave spectrum, Peak wave direction, Directional  spreading, Orbital ratio spectrum, Fourrier coefficients spectra
    • Height, Period, Steepness, Irregularity of Sea, Energy period, Tide, Temperature, Raw data time series
    Benefits
    • Highest accuracy
    • Add currents and water quality
    • Easy to maintain
    • At surface
    • Combine wave, current, tide and temp from one instrument
    • Add water quality
    • At bottom
    • Robust and easy to install sensor
    • Low cost alternative stand-alone or in a system
    • Below surface

    * Deeper installations is possible but will reduce the possibility to measure short period waves. 

    MOTUS Directional Wave Buoys

    Collect reliable environmental data with MOTUS Wave Buoy

    The MOTUS (movement in Latin) Wave Buoy is a perfect solution for collecting environmental data in coastal areas. The platform used for collecting data is the robust MOTUS buoy which has proven to be an excellent platform for providing high accuracy and reliable meteorological and oceanographic data. The MOTUS Wave Buoy can effectively combine various Hydrological and Meteorological parameters. Basic parameters as wind, waves and currents can easily be expanded to include sensor packages with different water quality sensors. Data is controlled and synchronized by the data management systems that Xylem Analytics offers which also controls and powers the real time communication. This provides a low power efficient system offering long-term deployment capabilities.

    The MOTUS Wave Buoy combines the ultra-low power and field proven solutions we have from developing oceanographic and water quality sensors, as well as 60 years of experience operating in AtoN to provide you with an optimal platform based on the SB 138P buoy, a rugged lightweight polyethylene buoy deployed in more than 40 countries all over the world. By selecting Xylem as your partner in your projects, you are on track for the lowest overall cost of ownership and a versatile platform that can be tailored to your needs. The MOTUS Wave Buoy is developed for both scientific and commercial use.

    SeaGuardII DCP Wave


    The SeaGuardII DCP Wave is a 600kHz Doppler Current Profiler able to measure directional wave parameters and currents from a bottom mounted installation. The Acoustic Wave software 5759 used by the DCPS implements unique features to improve the wave measurement accuracy by optimizing the signal to noise ratio.

    • Maximum deployment depth is 40m in normal scatter conditions.
    • Available as a self-recording instrument, it is easily integrable into a real-time system offering reliable two-way communication.
    • Redundant wave measurement for QA/QC can be implemented by adding the wave and tide sensor 5218.

    Simple solution to measure waves, currents and water quality in one instrument

    • Independent measurement of waves and currents
    • Field validated data-comparison with MOTUS buoy, Datawell Waverider and pressure based non directional wave sensor
    • Measures wave from centimetres to 20 meters
    • 2Hz or 4Hz wave sampling
    • Processing of up to three columns in parallel, provides flexible setup with both surface referred and bottom referred cells.

    Adaptive pulse technology automatically optimizes wave measurement accuracy
    The transmission pulse is automatically adapted to the current sea conditions to provide best measurement achievable; a low noise broadband mode is used for smaller waves, an extended range broadband mode used for medium range waves and a narrowband mode is applied for higher waves.


    Wave and Tide Sensor

    The Wave and Tide Sensor is a compact fully integrated sensor for measuring wave and tide conditions.

    The 5218 sensor is designed to be mounted on the Aanderaa SeaGuard top-end plate or connected to SmartGuard using AiCaP CANbus or in other measurement systems using the RS-232 interface.

    The 5218R sensor is designed for use with long cables by means of RS-422 full duplex interface.

    5218/5218R is based on a silicon piezoresistive pressure sensor.

    The pressure measurements are sampled and temperature compensated by an advanced Digital Signal Processor. The sensor application areas are in fixed installations, either deployed in a seabed installation in shallow waters, or mounted onto a fixed structure in the upper water column.

    The tide measurement is an average of the hydrostatic pressure measured over a time period of 10 seconds to 8 minutes (integration time configurable by the user). The update interval is between 1 second and 255 minutes.

    The wave measurements are based on the pressure time series measured over a time period of 64 second to 17 minutes (configurable by the user). The update interval is between 1 second and 255 minutes.

    The wave and tide sensor output parameters are:

    Since all calibration and temperature compensation data are stored inside the sensor, the parameters are by default presented directly in engineering units without any external calculation. The sensor also provides raw data of the pressure and the temperature measurements.

    5218 is a compact yet intelligent sensor designed to be used with Aanderaa SmartGuard or SeaGuard as well as in other measuring systems.

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    Worldwide Representatives for Aanderaa

    MOTUS Buoy outside Ålesund, Norway


    The Norwegian Coastal Administration has tested out the use of the Aanderaa MOTUS smart buoy at Fauskane, northwest of Ålesund in Norway. The buoy has been fitted with additional equipment such as wave sensor, wind gauge, power meter and GPS antenna. Thus, it supplies data right from the field that can benefit all seafarers.

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