Acoustic current meter (ADCP) and speedlog | Flanders Marine Institute

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# Acoustic current meter (ADCP) and speedlog

An Acoustic Doppler Current Profiler (ADCP) measures the vertical distribution of the water speed as well as the direction of the current. As the name suggests, the device’s working principle is based on the Doppler effect. Measuring currents is an important concept in the oceanography. By determining how water masses move, scientists are able to relate this to how organisms, nutrients and other biological and chemical components are transported across the sea.

The speedlog is a device used for measuring the speed of the ship.

Both instruments are located in the hull of the RV Simon Stevin.

## How does an Acoustic Doppler Current Profiler (ADCP) and speedlog work?

### Acoustic doppler current profiler

The ADCP transmits ‘pings’ of sound at a constant frequency through the water. As the sound waves travel, they bounce off particles suspended in the moving water, and reflect back to the instrument. Due to the Doppler effect, the frequency of the reflected wave will be high or low depending on the particles moving either to or from the profiler. The sound wave reflected by particles moving away from the instrument will have a lower frequency return and vice versa. The difference in frequency between the waves the profiler sends out and the frequency of the returning waves is called the Doppler shift. Since the particles move at the same speed as the water that carries them, the Doppler shift is proportional to the speed of the water or current.

The ADCP has four acoustic transducers that emit and receive acoustic pulses from four different directions. By using trigonometric relationships, the current direction can be converted to earth coordinates. Because the emitted sound extends from the ship down to the seafloor, ADCP can measure the current at different depths simultaneously. As a consequence, the velocity and current direction can be determined from the surface to the seafloor.

The ADCP is located in the keel of the RV Simon Stevin. The results of the magnitude and direction of the current throughout the water column can be computed and presented on a computer on board. In this way, scientist can observe changes in ocean currents nearly continuously while the vessel is in motion.

### Speedlog

Just like the ADCP, the principle of the speedlog is based on the Doppler effect. The speedlog that VLIZ has put at the disposal is a JLN550 Doppler log. This transducer is designed for dual frequency measurements. Using a low ultrasonic frequency, the velocity of the vessel is measured relative to the bottom. Using higher ultrasonic frequencies, the velocity of the ship is measured compared to the water.

### Acoustic doppler current profiler (ADCP) and speedlog

The speedlog is used to subtract the movements of the vessel from the data acquired by the ADCP. This allows more accurate measurements of the velocities of the ocean currents.

## Usefulness of the ADCP and speedlog?

• An ADCP can measure the absolute velocity of water
• The combination of the ADCP and speedlog is an efficient method for sampling a large part of the ocean in a short period of time

## Some studies:

• ##### Shoreface nourishments as a measure for coastal safety at the Belgian Coast:

Hydrodynamic information (ADCP) is analyzed by the Flanders Hydraulics Research. This project is executed in the framework of the Gïntegreerd Kustveiligheidsplan (GKVP)

## Technical details

• Acoustic doppler current profiler (ADCP):
• Max range: up 165 m
• Frequeny: 600 kHz
• Speedlog:
• Depth range: > 3m (below hull bottom)
• Frequency: 2 MHz