Ocean waves play a defining role in coastal erosion and global shipping routes. Measuring these waves involves quantifying the transfer of energy across the ocean surface using sophisticated instrumentation. Scientists define the sea state using three primary characteristics: height, period, and direction. Wave height is the vertical distance between the crest and the trough, often summarized as the Significant Wave Height (SWH), which is the average height of the highest one-third of waves. The wave period is the time it takes for two successive wave crests to pass a fixed point, and wave direction indicates the path the energy is traveling. A variety of techniques, ranging from submerged sensors to orbiting satellites, are employed to capture these parameters.
Fixed Sensors and Submerged Pressure Gauges
Wave measurement often involves deploying stationary sensors fixed to the seafloor or secured to rigid structures like piers. Submerged pressure transducers, or pressure gauges, detect fluctuations in hydrostatic pressure caused by the changing weight of the water column as waves pass.
The sensor converts the measured pressure into a corresponding surface elevation to calculate wave height. However, the pressure signal is naturally attenuated as it travels downward through the water. Deeper placement reduces the sensor’s effectiveness in detecting pressure changes from shorter, higher-frequency surface waves.
Mathematical models are applied to the pressure data to accurately reconstruct the surface wave height and compensate for signal loss. Pressure transducers are typically deployed in shallower water or mounted on fixed platforms. Acoustic Doppler Current Profilers (ADCPs) measure waves by sending acoustic pulses into the water column, measuring the orbital velocity of water particles beneath the surface. This process allows for the derivation of wave height and directional information.
Floating Measurement Buoys
Floating measurement buoys gather real-time, in-situ wave data by tracking their movement across the water surface as waves pass. These instruments provide a direct measurement of the sea state. Buoys are categorized as either moored (anchored to the seafloor) or drifting, with moored buoys typically ranging from 1.5 to 12 meters in diameter.
The buoy’s measurement capability relies on internal sensors, primarily accelerometers and gyroscopes. Accelerometers capture the vertical motion, known as heave, by measuring changes in speed and direction as the buoy rises and falls. An onboard computer processes this raw acceleration data using double integration, converting the measurement into a precise record of vertical displacement that corresponds directly to the wave height.
Directional buoys incorporate additional sensors to track rotational movements, including pitch, roll, and yaw. These rotational measurements, combined with the heave data, allow the buoy to determine the direction from which the waves are propagating. Modern buoys often integrate Global Positioning System (GPS) technology, using precise positional changes over time to calculate both wave height and direction.
Non-Contact Remote Sensing Methods
Non-contact remote sensing methods measure waves without requiring equipment to be placed in the water. These techniques are valuable for obtaining broad spatial coverage and for use where deploying fixed sensors or buoys is difficult. This field is dominated by two main categories: shore-based radar and satellite altimetry.
Coastal radar systems, such as those using marine X-band radar, are deployed on shorelines or fixed offshore platforms to monitor waves locally. These systems transmit microwave signals toward the water surface and analyze the returned signal, or backscatter, to determine wave characteristics over a radius of several kilometers. Specialized instruments use Frequency Modulated Continuous Wave (FMCW) technology to precisely measure the distance to the water surface by timing the signal return.
Satellite altimetry offers a global perspective on wave conditions, providing data over vast ocean areas inaccessible to buoys or coastal radar. Orbiting satellites are equipped with radar altimeters that transmit energy pulses toward the Earth and measure the time it takes for the echo to return. The total time of the two-way trip is used to calculate the distance from the satellite to the ocean surface.
To determine the Significant Wave Height (SWH), the altimeter analyzes the shape and intensity of the returned radar echo, known as the waveform. The presence of waves causes the signal reflected from wave crests to return before the signal reflected from wave troughs, spreading out the return time. Measuring this spread allows the altimeter to precisely calculate the SWH.