Vessel-mounted current profiling
Currents close to the coast and in river estuaries vary in terms of strength and direction. They vary both across a given area and over time depending on the tide, the wind, the atmospheric pressure and the impact of man-made structures and natural formations. Measuring currents accurately is vital for our understanding of beach erosion, transport of materials, forces acting on marine structures and for safe navigation. Vessel-mounted current profiling is one of several tools that help us develop this understanding.
- Gathers data as the ship moves through an area
- Provides a geographical view of current variations
- Uses bottom-tracking to correct for the ship’s velocity
- Integrated package that cuts down on mobilization time
- No license required
What and why
What is current profiling and why is it useful?
Taking readings from a current profiler mounted on a moving vessel allows a picture of complex current patterns over a selected area to be drawn up accurately and rapidly. It is useful in areas such as harbors, estuaries and other coastal areas where currents can be unpredictable. Good knowledge of their variability is vital for safe navigation, scientific studies or construction work.
In the past, profiling currents across an area was a time-consuming business involving a number of instruments, which often produced poor data from inadequate software. The equipment used typically came from multiple suppliers and was not designed to work together. The instrumentation frequently produced inaccurate or confusing data, not least because of a poor ability to measure vessel velocity via bottom-tracking. Errors also occurred due to electrical or data-connection problems. A specialist operator was required to handle the equipment and data presentation, adding to personnel costs.
The best vessel-mounted systems in the market today allow you to carry out a survey rapidly and accurately using a small, state-of-the-art Acoustic Doppler Current Profiler (ADCP) attached to the side or bottom of a vessel.
Today’s ADCPs can profile currents using small cells (sampling areas) at multiple depths right down to the seabed. They produce highly accurate and robust data, transferred speedily to a dedicated computer terminal designed for use by non-specialists.
Who it’s for
Vessel-mounted ADCPs may be employed by a wide range of users who need comprehensive current data. In the scientific community, these include researchers into coastal morphology and marine biology. 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.
The 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.
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. A small vessel may be suitable for use in calm conditions, such as in an estuary. In conditions where the waves are more significant, a wide hydrographic survey vessel with a small draft may be the best choice.
In the past, data presentation from current profilers required a specialist that could process the data obtained. To reduce the effective time needed to report the survey results, Nortek has developed an integrated suite of products to enable a non-specialist to operate the system and access the data on board. This 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.
Choice of instrument
Nortek’s vessel-mounted ADCP current survey package, known as Signature VM, offers high data accuracy, operational convenience and reduced complexity. Data accuracy is substantially improved and installation time is reduced by using an integrated system, in which each module has been thoroughly tested beforehand.
Users have a choice of two Signature instruments:
The Signature1000 is designed for operations down to 30 meters’ depth. This would be a good choice for a small boat in a shallow estuary.
The Signature500 is required for depths up to 70 meters, usually found offshore.
Both versions have five beams to measure current and depth, including one vertical beam capable of taking depth measurements. The vertical beam also has high-resolution echo-sounding functionality.
Because these ADCPs are small and can be enclosed in streamlined housings, they produce reduced drag. This allows faster surveys and smaller mounting hardware.
Vessel-mounted current profilerThe Signature VM package delivers vessel-mounted AD2CP capabilities based on present-day technology