Understanding vessel-mounted current measurement

Acoustic Doppler current profilers (ADCPs) can measure the complex movements of currents in oceans, rivers and lakes with great accuracy. But to take those measurements, you need to attach your device to something – and in the middle of a river or at sea, away from the shore, the easiest place to put it is often the underside of a vessel.
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11 minutes

1. The case for choosing vessel-mounted current measurement

It’s not just a matter of convenience. Vessel-mounted ADCPs open up whole new ways in which to take measurements. The speed and accuracy with which vessel-mounted ADCPs can make measurements mean we can respond more effectively to impacts created by our maritime environment.

Currents along coasts and in river estuaries are frequently complex, varying in space and time depending on the tide, wind, atmospheric pressure and the impact of man-made structures and natural formations. The accurate measurement of their speed and direction using ADCPs is crucial for our understanding of factors such as beach erosion, transport of materials, forces acting on marine structures and for safe navigation.

Port authorities use current profiling to draw up detailed maps of currents during different phases of the tide to enable vessels to enter and leave harbor safely.

Dredging and marine construction firms need accurate current profiling to assess sediment transport and enable vessels to operate effectively.

Tidal-energy firms benefit from detailed current profiles to ensure their turbines are located on the optimal site to take advantage of current flows.

1.1 How do vessel-mounted ADCPs work?

Using the acoustic Doppler technique, ADCPs measure water movement by interpreting sound waves transmitted by and reflected back to the instrument from particles in the water, marine life, etc. A vessel-mounted ADCP can produce a profile of the currents between the vessel and the seabed while the vessel is moving. An ADCP can also calculate the speed of the vessel over the seabed. This enables a single ship to accurately measure currents over a large area in a matter of minutes.

1.2 Advantages of vessel-mounted current measurement

So, vessel-mounted ADCPs are now a vital part of the toolset for maritime researchers. They are rapid to deploy, as they don’t require the time-consuming and potentially costly construction of permanent infrastructure to hold them – they just need a boat to house them for the duration of a project. So, if time is short, vessel-mounted ADCPs make sense.

Vessel mounting also makes sense if you want to take measurements at more than one time, or in more than one place. For example, a device mounted on a vessel can be taken to the middle of a river to measure currents during periods of high and low water discharge.

In a port, you may want measurements at several locations along a route where large ships are experiencing difficulties navigating, to check how current patterns vary. In a coastal inlet, you may want to check out how currents between two islands behave as the tide changes.

Finally, vessel mounting is indispensable if you want to measure currents over an entire area, rather than a few discrete places. Today’s ADCPs are able to profile currents as a vessel moves through the water, providing digitized information that allows you to compile a detailed 3D map of currents across a zone.

Inside the vessel
The best systems are coherent and quite easy to use. It is an advantage to be able to operate vessel-mounted current measurement technology yourself.

1.3 Technical considerations for vessel-mounted measurements

Water depth: A vessel-mounted ADCP may operate down to water depths of around 70 meters. Different water depths require a different acoustic frequency, so users need to be aware of the depth range in which they will be operating.

Type of vessel: The vessel should be large enough to provide a stable platform in the conditions where it will be used. An unstable ship will result in reduced data accuracy. On a river, a small, maneuverable vessel is needed. At sea, a hydrographic survey vessel, specifically designed to perform ocean measurements, is best. Many hydrographic survey vessels are catamarans, as these are wide, with a shallow draft, and the central section between the two hulls is ideal for mounting instruments.

Maneuvering: The boat needs to be maneuvered properly. Making sharp turns and increasing speed too quickly won’t give you very good data. So, stay calm, be in control and go with the flow.

Orientation: So that you know where you are and how the instruments are oriented, you need a global navigation satellite system (GNSS), such as GPS, for positioning, an electronic compass and a pitch and roll sensor. Many vessels have these systems on board, and you can also get combined units for all signals.

Speed of sound: Sound travels at different speeds through water, depending on the water’s make-up, and you need to know that speed to ensure ADCP readings are accurate. Sound speed is affected by the salinity and temperature of the water, so you will need those measurements.

Accurate time-keeping: This is essential because you need to know how the timing of your measurements relates to changes in the water level or the tide. In addition, you must know what the time lag is between the ADCP and any auxiliary sensors such as the GNSS – modern systems can close the gap to milliseconds or even less.

Vessel mounted instrument
You need a global navigation satellite system (GNSS), such as GPS, for positioning, an electronic compass and a pitch and roll sensor.

1.4 Ensuring high-quality data from vessel-mounted ADCPs

The best ADCPs on the market provide great data. Both the bottom-track data and the measured currents are accurate, as well as robust.

They often combine this with a modern GNSS sensor, which also gives you heading, pitch and roll and – on top of that – has the most accurate timing option, known as Precision Time Protocol (PTP).

In addition to an ADCP, it would also be advantageous to add an altimeter, distance measurement on the vertical beam and a super-resolution echosounder for echo profiling. Combining all that with a mounting bracket, a connection box and a computer, you will have a coherent, comprehensive and easy-to-use package.

2. Processing and interpreting data from vessel-mounted ADCPs

In the past, current profiling required a specialist to process and interpret data from the instrument. Now, an integrated suite of products enables non-specialists to operate the system and access the data on board. This saves time and money and can be done after only a few hours of training.

The system includes everything needed, from the ADCP instrument and a GNSS antenna to the specially selected on-board computer and its software, with transmission via high-speed, reliable Ethernet connections. The data-acquisition software allows users to easily discover the instruments, store data and report on the functioning of the entire system.

The acquired data can be fed into a post-processing package such as the US Geological Survey (USGS) Velocity Mapping Toolbox to show current vectors on a map, display current cross-sections, etc.

There will be a lot of data – and it may well be very colorful! Typically, you will view your data as current vectors (arrows) on a map or in Google Earth.

You will be able to look at the distribution of current speeds as a function of along-track distance and depth below the water surface. There is also the option to display along-shore currents in a graph etc. Finally, you can use a series of these graphs over time to create an animation.

Others may need to access your data too – perhaps your boss wants to analyze it for a model he or she is developing, or a port authority may need a current map. So, you need to be able to deliver the data in a format that is accessible to others, such as geographic information system (GIS) mapping.

Helder Vector
The image shows the currents on an almost 2500 m long transect along the main channel while crossing a smaller, secondary channel. The arrows show the direction and magnitude of the current. These currents are averaged from the bed all the way up to the water surface. It is helpful to see how much the currents change along the transect. You can see the outflow of the secondary channel in the center and the flow along the main channel at the top.
Den Helder Flow Direction
The image shows the direction of the currents. Note how much the direction can differ between the bed and the surface – for example, the section around 500 m.
Den Helder Flow Magnitude
The image depicts the magnitude (speed) of the current with different colors. Blue indicates weak currents, and red, stronger currents. The part of the graph between 0 and 500 m matches the smaller flow vectors on the lower left of the transect in the first picture.

3. What’s next for vessel-mounted current measurement?

The potential to use unmanned vessels, or even autonomous vehicles, offers the possibility of lower-cost measurements and the opportunity to harvest even more data.

Improvements in data processing techniques should make analysis easier and further improve data quality. Ever-increasing computer power should allow more tasks that once had to be done on shore to be carried out on the vessel.

More sophisticated use of the Internet should allow more people, whether on the vessel or on shore, to monitor measurements in real time.

Measuring currents using acoustic Doppler equipment is already highly accurate and much easier to do than it was a few years back. Expect it to get faster, simpler and even more accurate in the future.

4. Can I operate vessel-mounted current measurement technology myself?

Yes, you can. The best systems are coherent and quite easy to use.

The acquisition software is designed to automatically communicate with the ADCP and the GNSS. The software will store all data, continuously provide updates on the project and will ask you to make a decision, if one is needed.

You can transfer the data into a flexible data processing package with just a few mouse clicks.

5. Looking back: A brief history of vessel-mounted ADCPs

In the 19th century, mechanical devices were developed that could interact with the currents, including current meters based on small propellers that could be lowered from a vessel into the water – the faster the propeller rotation, the stronger the current.

One pioneer of mechanical current meters, in the 18th century, was Christiaan Brunings, the multi-talented inventor of a meter to measure flow in the delta of the River Rhine. Vagn Walfrid Ekman was another important figure, creating an ingenious device used mainly for scientific studies in oceans.

Over time, mechanical devices became more sophisticated – and, indeed, similar instruments are sometimes still used today. But deploying them was far from straightforward. They had to be lowered accurately to the relevant depth – and if the current was strong, there was a risk of damage and potential safety concerns.

And if you needed to do anything that required multiple measurements in a short period of time, such as measuring current profiles at different depths regularly throughout a tidal cycle, then you needed more than one vessel. Sometimes more than ten vessels – and their crews, of course – were needed over a very long day.

Current measurement really began to change in the early 1990s, as computers became more portable, enabling the development of a new wave of digital instruments, including ADCPs.

Initially, profiling currents across an area remained a time-consuming business, requiring a number of instruments. These often produced poor data from inadequate software, with results made worse by poor bottom-tracking ability. The equipment typically came from multiple suppliers, whose products were not designed to work together. Electrical and data-connection problems were frequent. A specialist operator was required to handle the equipment and data presentation, adding to personnel costs.

That has all changed. Today, the best vessel-mounted systems on the market allow you to carry out a survey rapidly and accurately using a small, state-of-the-art ADCP, easily attached to the side or bottom of a vessel.

See the video: Simplifying vessel-mounted current measurement with Signature VM Coastal


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