Collecting data1 is a crucial part of scientific inquiry. To study waves and the ocean sea level, scientists usually gather data through the use of instruments. They collect information about the shape of the seabed, the composition of the water, wave characteristics and currents.
Measuring instruments are becoming more complex and refined as technology advances, so we can measure with greater precision2. The advent of the GPS system started a new era of instruments for the study of waves, combining positional sensing with other measuring instruments.
Measuring sea level
To measure sea level accurately, scientists use bubbler pressure3 gauges. These instruments measure the back pressure from blowing bubbles of air down a hole – the higher the pressure needed to blow the bubbles, the deeper the water. You can think of a bubbler gauge as being like a milkshake – it’s easy to blow through the straw and make bubbles when the milkshake’s nearly gone, but much harder when the glass is full! Bubbler gauges are important for detecting tsunamis4 because they measure sea level at short intervals (1 minute).
Measuring waves
Buoys can be used to measure the height, period and direction of waves. The buoy can even measure its own acceleration5 – this can tell scientists whether it is falling from the top of a high wave into a trough.
Tsunami6 buoys are connected to underwater pressure gauges, which can provide important water-level information about possible tsunamis as they speed past. There is a network of tsunami buoys in tsunami-prone areas of the Pacific Ocean. The buoys play a crucial role in alerting the public about potential tsunami waves. Within 9 minutes of the March 2011 Honshu earthquake, the Pacific Tsunami Warning Center in Hawaii issued a tsunami alert for the Pacific region.
Looking at the seabed
Most of the instruments that ‘look’ at the seabed are acoustic (they use sound instead of light). This is because sound penetrates further through water than light. Using multibeam and side-scan sonar7 systems, researchers can take ‘photographs’ of the seabed. They can also get information about what the seabed surface is made of (sand, mud, rock and so on) and the material below the seabed itself.
The choice of sound frequency8 matters because low-frequency sounds penetrate further but higher frequencies give greater resolution9. To measure at seabed depths, researchers might use frequencies down to 50kHz, but if there’s an area of particular interest, they might use 250–500 kHz to look at detail on the seabed. Very high frequencies (like ultrasound10 waves) don’t penetrate well at all.
Interference11 patterns caused by the reflection12 or backscatter of the sound waves by various things on the seabed can set up characteristic images that are recognised by the scientists. For example, a shellfish like the morning star13 Tawera spissa) is the size of a 20 cent piece and is too small to be picked out on a 500kHz side-scan system – but when there is a bed of them, interference causes a distinctive pattern so that they can be identified. Interference patterns are determined by the size of the organisms and their density14 on the seabed.
Looking at what’s in the water
Sound waves can also tell us about the content of the ocean. If sound waves hit a moving object, their frequency changes – this is called the Doppler shift. The Doppler shift tells us about what is in the water column15 between the sound detector and the seabed in terms of suspended sediment16, plankton17 density and so on. It enables measurements of the size and velocity18 of these particles.
Light systems using infrared19 light are also used to measure concentrations of suspended sediments20, especially silt21 and mud. Infrared light is useful because there are no natural sources of that radiation22 in the ocean – a measure of the backscattering (or reflection) of the transmitted infrared rays gives a measure of the amount of suspended sediment present in that region of the ocean.
Nature of science
Scientific knowledge is based on observations of the natural world. Often, these observations are in the form of data from measuring instruments. The more precise the instrument, the more precise the data that can be gathered.
Related content
These New Zealand researchers are working to undertand more about waves and tsunamis:
- Dr Rob Bell – NIWA researcher, specialising in ocean waves, including storm surges and tsunamis, with a particular interest in sea-level changes
- Dr Richard Gorman – NIWA researcher, focused on waves, numerical modelling and wave forecasting
- Dr Willem de Lange – Earth sciences university lecturer involved in numerical modelling, coastal processes and climatic hazards and tsunami research.
Activity idea
Use a Mexican wave to demonstrate how waves transfer energy and to help your students visualise the wave behaviours of reflection, constructive interference and shoaling.
Useful links
In this Facebook video from August 2020 watch experts, including GNS Science research seismologist Bill Fry, talking about the science behind how DART buoys detect tsunamis Water wave and what happens next.
Visit UNESCO's International Tsunami Warning Center website.
The National Data Buoy Center (USA) website provides background on the network of DART® tsunami buoys in the Pacific Ocean. The site also contains an interactive map of buoys in the network, with real-time water-level data.
- data: The unprocessed information we analyse to gain knowledge.
- precision: The closeness that repeated measurements show under unchanged experimental conditions.
- pressure: The force per unit area that acts on the surface of an object.
- tsunami: A series of massive waves generated in the ocean usually by earthquakes, volcanic eruptions or submarine and coastal landslides, but they can also be caused by the impact of meteorites from outer space.
- acceleration: The rate at which an object speeds up, slows down or changes direction.
- tsunami: A series of massive waves generated in the ocean usually by earthquakes, volcanic eruptions or submarine and coastal landslides, but they can also be caused by the impact of meteorites from outer space.
- sonar: A method of detecting, locating, and determining the speed of objects through the use of reflected sound waves. A sound signal is produced, and the time it takes for the signal to reach an object and for its echo to return is used to calculate the object's distance.
- frequency: 1. How often something occurs within a specified time. 2. The number of waves per second that pass a given point or the number of waves produced per second by a source.
- resolution: In microscopy, the ability to distinguish two separate points or objects as independent. The resolution of a microscope provides a measure of the level of detail it can be used to reveal. In an image, the degree of sharpness. Resolution is measured by the number of dots per linear inch in a hard-copy printout, and the number of pixels on a display screen.
- ultrasound: Any sound whose frequency is above the range of normal human hearing (greater than approximately 20 kHz). Used in medical fields as an imaging technique and also used by animals in nature for navigation.
- interference: The simultaneous presence of two or more waves in the same position, resulting in a new wave pattern.
- reflection: 1. The change in direction, or bouncing back of a wave when it strikes a surface. 2. Mirroring. 3. Casting back, as in light or heat.
- star: A self-luminous celestial body consisting of a mass of gas held together by its own gravity.
- density: How tightly a certain amount of matter (atoms or molecules) of a substance is compacted in a given volume. Density is commonly measured in grams per millilitre (g/ml) or cubic centimetre (g/cm3).
- water column: The vertical section of water between the freshwater or ocean floor and the surface.
- sediments: Material that settles to the bottom of a liquid. In geology, it describes the solid fragments of inorganic or organic material that come from the weathering of rock and are carried and deposited by wind, water or ice.
- plankton: A group of marine organisms including single-celled and multi-celled organisms.
- velocity: Speed in a particular direction.
- infrared: Invisible electromagnetic radiation with a wavelength between approximately 0.75 micrometres and 1 millimetre. Infrared occurs between the red end of the visible light spectrum and microwaves. All things over a certain temperature (absolute zero) absorb and emit infrared radiation. Infrared radiation and observing technologies are used in many industries from medicine to finding people buried under rubble and by the military and others in night-vision goggles.
- sediments: Material that settles to the bottom of a liquid. In geology, it describes the solid fragments of inorganic or organic material that come from the weathering of rock and are carried and deposited by wind, water or ice.
- silt: A granular material of a size somewhere between sand and clay. Its mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in water. It may also exist at the bottom of a water body.
- radiation: Energy that is transmitted (radiates) from a source in the form of rays or waves or particles.