Automatic measuring instruments provide information on the properties of the sea

Automatic measuring devices are practical. The results can be obtained continuously without anyone having to go to sea. Automated measurements can monitor certain oceanographic characteristics, such as currents, phytoplankton content, or water temperature and salinity. Water samples can also be taken automatically.


Measuring devices can be mounted on a variety of platforms

Some automated measuring devices are installed in fixed locations, such as on the shore or the seabed. Others are attached to buoys, which are either anchored or drift with the currents. The movement of some gauge platforms can be remotely controlled from the land. Merchant ships are also fitted with automatic measuring devices.

Regardless of the type of measuring device, the results need to be transferred to land somehow. This can occur via a fixed cable, by satellite or through a cellular network. A system of communication based on high frequency (HF) radio waves is also used near the coast.

However, not all automated measuring devices send their results to land in real-time but collect them in their internal memory instead. The memory is downloaded onshore after the meter has been collected.

The CTD gauge detects the properties of water

The properties of water masses are generally measured with a so-called CTD measuring device. A CTD probe measures the temperature, salinity, and depth of water, and can be mounted on a variety of measurement platforms. When equipped with additional sensors, the CTD device can also measure the oxygen and turbidity of the water, as well as the amount of phytoplankton, based on the chlorophyll-a content.

On oceanographic research vessels, a CTD probe is a basic instrument for measuring a water mass from the surface to the bottom. In newer applications, CTDs have also been installed on surface buoys, as well as those anchored to the seabed. A winch is attached to the buoys, which lifts the CTD probe from its off-bottom position to the surface at pre-programmed intervals. In this way, measurements can be taken from different depths.

 A photo about CTD on Aranda.
A CTD-measurement starting on Aranda. The measurement probes are below and the tubes will take water samples.

Gliders and Argo floats represent robot technology

The CTD probe is also a basic instrument on gliders (or AUV, autonomous underwater vehicle) and Argo-floats. These robotic technological measurement platforms also provide observations at different depths. An underwater route is programmed into the glider in advance, and after it has gathered water data, it rises to the surface to transmit its CTD measurements via satellite. Once on the surface it receives new commands.

The glider begins to dive. Video: Kimmo Tikka.
 

Argo floats also send measurement data and receive commands via satellite. They drift freely with the currents and occasionally rise to the surface, all while constantly measuring the properties of the water mass in which they move. Their drifting depth and the timing of their ascent to the surface climb are controlled manually from the land.

Simpler measuring devices can also be installed on the floats. For example, some only measure water temperature and barometric pressure.

 To further aid its visibility, the orange-coloured surface temperature buoy also has yellow reflectors glued to its surface
A buoy for measuring surface temperature.

The Imaging FlowCytobot takes photos of phytoplankton

Plankton research, in turn, is served by an imaging device known as Imaging FlowCytobot that can be mounted on a variety of platforms. This device automatically produces up to 30,000 accurate black and white photos of phytoplankton cells per hour. Thus, it can be used to monitor the development of the phytoplankton community at more frequent intervals than traditional methods.

With its continuous operation capability, the Imaging FlowCytobot device can be used on both research stations and vessels. This method produces so much image material that its analysis requires algorithms or other machine learning methods for character recognition.

Currents and wave heights are measured in many ways

A simple way to measure surface currents is to use free-floating surface buoys. Surface buoys can also be used to measure deeper currents when equipped with a flow-sail gauge mounted at the desired depth.

However, in most cases, currents are measured with acoustic current meters installed on the seabed or off-bottom. These meters function based on the change in the frequency of the ultrasound as it is reflected from the tiniest particles in the water. Such ultrasound gauges provide flow information either at specific depths or across the entire water column.

Although wave heights can also be measured with acoustic current meters, they are more commonly measured using special wave buoys. These measurements are based on three accelerometer sensors and a compass. The results can be used to calculate, among other things, the wave direction and the so-called significant wave height.

Most buoys can only be used during ice-free periods as they cannot withstand the movements of sea ice. Similarly, the sensitive measurement equipment of wave buoys does not tolerate freezing. Therefore, such buoys must be recovered to land well before the sea begins to freeze.

Sea ice and its movements are measured using buoys which float along with the ice. The motion of the ice can also be tracked using coastal radar imagery.

 3 people lifting a wave buoy from the sea in dark.
Lifting a wave buoy from the sea with the research vessel Aranda before winter. Ice has already collected on the antenna.

Sea levels have been measured by mareographs since the 19th century

Sea levels have traditionally been measured from wells connected by a pipe to the sea. The connecting tube dampens the effect of waves, and a float inside the well registers the fluctuations in water level. Such water level measuring stations are called mareographs or tide gauges. There are 14 such stations located on the Finnish coast. The oldest of these was built in 1887 in Hanko.

Water level can also be measured using pressure sensors, acoustic technology, and radar. However, for all methods, it is essential that the resulting sea level is compared to a specified elevation (fixed point) on dry land. Since the land is rising at different rates in different locations along the Finnish coast, these land-based reference points also need to be levelled regularly.

Hydrophones reveal underwater noise

Fixed underwater microphones or hydrophones have also been installed in Finnish sea areas. A total of nine hydrophones are anchored to the seabed and record various sound frequencies. The measurement results examine both continuous low-frequency noise, as well as momentary peaks in low- and medium-frequency noise.

There are also hydrophones in the Swedish and Estonian sea areas. Their results also benefit the Finnish underwater noise mapping programme.