Measuring instruments

Collecting data 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.

Rights: Dr Rob Bell

Rob Bell and tsunami buoy

Dr Rob Bell beside a DART® tsunami buoy. The buoy transmits pressure (depth) data from a tethered sea-floor pressure gauge. There is a network of DART® buoys around the Pacific Ocean.

Measuring instruments are becoming more complex and refined as technology advances, so we can measure with greater precision. 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 pressure 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 tsunamis because they measure sea level at short intervals (1 minute).

Rights: Dr Willem de Lange

Water-level gauge

Servicing a clockwork water-level gauge in Tauranga Harbour in the 1980s. Information from harbour gauges is important for modelling harbour circulation. Bubbler pressure gauges have now largely replaced clockwork gauges.

Measuring waves

Buoys can be used to measure the height, period and direction of waves. The buoy can even measure its own acceleration – this can tell scientists whether it is falling from the top of a high wave into a trough.

Tsunami 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.

Rights: NOAA

Sensing a tsunami

These sensors are attached to buoys anchored in the oceans. Data is gathered from sensors and passed via satellite to ground stations that then send the information to tsunami warning centres. This buoy can detect tsunamis as small as 1cm. DART stands for Deep-ocean Assessment and Reporting of Tsunamis.

Acknowledgement: National Oceanic and Atmospheric Administration/Department of Commerce

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 sonar 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.

Rights: Dr Willem de Lange

Side-scan sonar

Side-scan sonar is an important technique for mapping the texture of the seabed. The side-scan (the narrow torpedo-like instrument) emits sound pulses that are reflected by the seabed. Here, researchers are mapping the seabed of Tauranga Harbour.

The choice of sound frequency matters because low-frequency sounds penetrate further but higher frequencies give greater resolution. 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 ultrasound waves) don’t penetrate well at all.

Interference patterns caused by the reflection 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 star 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 density 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 column between the sound detector and the seabed in terms of suspended sediment, plankton density and so on. It enables measurements of the size and velocity of these particles.

Light systems using infrared light are also used to measure concentrations of suspended sediments, especially silt and mud. Infrared light is useful because there are no natural sources of that radiation 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.