Ocean waves are transient oscillations of the sea surface, generally resulting from the transfer of energy from wind blowing across the water. Waves can also be generated by massive disturbances (tsunamis) or by the gravitational pull of the moon and sun (tides). Accurately measuring wave characteristics is fundamental for numerous human activities. This information is required for maritime safety, marine weather forecasting, and coastal engineering projects, such as the design of harbors and seawalls. Long-term wave measurements are also essential for climate modeling, helping scientists track changes in storm intensity and wave height trends globally.
Direct Measurement Using Surface Buoys
The most traditional and direct method for measuring wave characteristics involves specialized surface buoys that float and follow the motion of the water. These in-situ instruments are moored in a fixed location, where they record the movement of the ocean surface in real-time. Modern wave buoys typically use an accelerometer-based system or a GPS receiver to precisely track the buoy’s vertical and horizontal displacement.
Directional wave buoys are equipped with additional sensors, such as gyroscopes and magnetometers, to measure the tilt and rotation of the hull. This combination of vertical heave, pitch, and roll motion allows for the calculation of three essential wave parameters: significant wave height, average wave period, and wave direction. Significant wave height is calculated as the average height of the highest one-third of the waves recorded over a specific time interval.
Once collected, the processed data is immediately transmitted to shore stations or satellite networks using satellite communication or radio telemetry. This allows for the assimilation of near-real-time observations into operational wave models, which is necessary for timely warnings and forecasts. Because they directly experience the surface motion, buoys are considered the gold standard for validating other wave measurement techniques.
Submerged Sensors and Pressure Measurement
An alternative to floating buoys is the use of instruments fixed to the seafloor, which measure the pressure changes caused by passing waves. Pressure transducers are common bottom-mounted sensors, frequently deployed in relatively shallow water, typically less than 15 meters deep. As a wave crest passes, the weight of the water column above the sensor increases, registering a higher pressure, and the inverse occurs when a wave trough passes.
This pressure signal is not a direct measurement of the surface wave height because the pressure fluctuations attenuate exponentially with depth. To convert the recorded pressure variations back into the true surface wave profile, a mathematical transfer function derived from linear wave theory must be applied. The accuracy of this conversion decreases in very shallow water where wave motion becomes more non-linear, often requiring empirical correction factors.
Other bottom-mounted devices, like the Acoustic Doppler Current Profiler (ADCP), also contribute to wave measurement. The ADCP uses acoustic beams to measure the orbital velocity of water particles as waves pass overhead. By analyzing the time series of these orbital velocities and applying wave kinematics, the ADCP can derive the wave height and directional spectrum. Some ADCPs also include a pressure sensor for redundancy and use a vertical acoustic beam to track the distance to the surface.
Remote Sensing via Satellite and Coastal Radar
Remote sensing techniques offer a non-contact alternative to in-situ instruments, allowing for the measurement of wave parameters over large geographic areas. Satellite altimetry is a primary space-based method where instruments transmit a radar pulse vertically toward the ocean surface and measure the travel time of the reflected signal. This measurement of the two-way travel time allows scientists to determine the distance from the satellite to the sea surface, which is used to calculate the ocean’s height profile.
The shape of the returned radar echo, known as the waveform, is also analyzed to determine the significant wave height. A rougher sea surface, indicative of higher waves, causes the radar pulse to reflect back over a longer period, broadening the slope of the waveform. Satellites continuously orbit the globe, providing massive spatial coverage and essential data for global climate studies and long-range ocean forecasting.
Closer to the shore, land-based High Frequency (HF) radar systems offer continuous, near-real-time wave monitoring over a wide coastal area. These systems transmit high-frequency radio waves over the ocean surface and analyze the backscattered signal using the principle of Bragg scattering. By measuring the Doppler shift in the returning radio waves, the radar can compute the speed and direction of the dominant surface waves. This technology is valuable for coastal safety and marine operations, providing a comprehensive picture of wave conditions without requiring instruments to be deployed in the water.