From surface to seafloor, Valeport innovative profilers are shaping the future of surveying
“TIME is money,” said 18th Century scientist and inventor Benjamin Franklin, words that will strike a chord with anyone involved in underwater survey work.
With survey vessels costing up to £100,000 a day on the water, anything that reduces time surveying is going to save money.
Over the last decade there have been huge strides forward in positioning and motion referencing technology, reducing errors in multibeam date acquisition considerably. Performance of multibeam sonar systems has increased and processing power has resulted in greatly enhanced resolution, accuracy and detection capabilities.
Taking measurements can still be a time consuming and therefore costly business, which is where Valeport’s new profilers are a major step forward.
Using the latest Bluetooth technology, state-of-the-art winches and new formulae developed by the British company, accuracy has been improved and downloading times reduced, eliminating the survey downtime normally associated with profile gathering.
Multibeam systems are improving all the time, but there remains the unknown of the environmental variability of the water column and the resultant sound speed field. This unknown factor can have a significant and measurable effect on data collected.
Significant errors in depth and positional accuracy of ‘ping’ locations will occur if an incorrect sound speed profile is applied to correct for refraction effects.
If more measurements are taken, is there a greater likelihood of reducing data inaccuracies?
Valeport examined that question during a series of tests in one of the most surveyed stretches of water in the world. The Barn Pool area of Plymouth Sound has arguably more pings per square metre than any other area of seabed.
Barn Pool was used as one of the data collection areas for the Shallow Survey 2015 Conference and, prior to that, the Shallow Survey 2005 Conference.
The University of Plymouth, the Royal Navy Survey School and commercial providers carry out hydrography training there. The popularity of the area can be attributed to the varied conditions found in a relatively small area.
Barn Pool sits at the mouth of the Hamoaze, the combined lower reaches of the Tamar and Lyner rivers on the border between Cornwall and Devon in the United Kingdom.
A flooded ria, the exchange between the Hamoaze and outer Plymouth Sound, occurs through the narrows between Devil’s Point and Wilderness Point.
Tidal currents can reach up to three knots on the Spring ebb tide.
Over a week in November 2015 a joint exercise between Valeport, Teledyne OceanScience and the Fugro Academy used Barn Pool to train in the deployment of the recently launched rapidCAST winch using two Valeport profilers the SWiFT SVP and the rapidCTD. Fugro Academy provided the MV Bruyn and access to their waterside facilities in Plymouth Sound.
The rapidCAST winch was developed by Teledyne OceanScience as an evolution of the Underway SV system. Fully automated and able to gather vertical profiles down to 500 metres while surveying at speeds up to eight knots, the rapidCast winch uses an advanced, active line payout system with precise tension control. This configuration allows the effects of vessel speed and heave to be eliminated and allows the profiler to maintain a plus or minus five per cent depth accuracy.
The minimal deck footprint of the rapidCAST winch means it can be installed quickly and easily on most survey vessels. The mobilisation work on-board MV Bruyn took around three hours. A repeat installation would take approximately an hour.
MV Bruyn is usually used for training company staff in multibeam and underwater positioning. For the purpose of the Barn Pool trial the underwater positioning system was removed and the deployment mount repurposed to mount the rapidCAST winch. The existing multibeam system was retained alongside the rapidCAST.
Two Valeport profilers were used for the trials, both incorporating their world-leading sound velocity technology. A SWiFT SVP, the latest sound speed profiler from Valeport with built-in GPS, Bluetooth (comms) and a rechargeable battery. This measures sound speed, temperature and depth and calculates salinity and density using a new formula developed by Valeport. The second probe was a rapidCTD, which measures conductivity, temperature, and pressure to calculate salinity, density and sound speed.
While training on the rapidCAST winch, the Barn Pool area was used extensively as it offers one of the deepest areas in the Sound — up to 35 metres — along the flooded river channel.
The variation in sound speed in Barn Pool was both significant spatially across the pool but also had a distinct temporal variation, most likely due to tidal effects.
Two vertical transects through the Barn Pool Area are used as illustrations. Both were collected on the same day, approximately three hours apart. The first was collected at high tide with the rapidCTD as part of a longer transect from the breakwater to the upper reaches of the Tamar above the Tamar Bridge.
The second transect was collected on a dropping tide with the SWiFT SVP on the return leg. Tides were neap.
Figure four shows the locations where the profiles were taken with transect one (rapidCTD) in green and transect two (SWiFT SVP) in red. The positions recorded for transect one were extracted by matching the timestamp of the instrument with the GPS log from MV Bryn. The positions for transect two were recorded using integral GPS. Both transects took around 15 minutes to complete. As the first profile of transect one and the last profile of transect two were taken within five metres, data could be aligned for relatively easy comparison. All data processing was carried out with Python and the SciPy library.
The data presented a series of vertical transects showing contoured plots of salinity (fig 5), temperature (fig 6), sound speed (fig 7) and finally the difference between the sound speed and the two transects (fig 8). They are plotted with the western end of the transect on the left-hand end of the plot and the eastern end on the right.
Below 15 metres the salinity, temperature and sound speed can be seen to be relatively stable. In transect one a 5-10mm thick layer of colder and fresher water spreads across the transect towards the riverine end. In transect two, as the ebb tide is peaking, the fresher water has been displaced from the eastern end of the transect but the fresher water influence at the western end of the transect has intensified and deepened.
The effect this has on the sound speed field is dramatic (see fig with a spatial variation in sound speed along transect in the order of and a temporal variation between transects of ±7m/s.
The impact of this variability on the propagation of sound has not been assessed and will be the focus of the second paper by Valeport to be released shortly.
Marine instrumentation technology continues to advance and Valeport’s newest profilers bring direct benefits of better, more accurate data and improved ship time utilization. New communications technologies and instrument automation offer exciting opportunities across a wide range of marine markets.
As Benjamin Franklin also said: ‘an investment in knowledge pays the best interest’ and customers can be confident Valeport is dedicated to pushing the boundaries of marine instrumentation to deliver the ultimate accuracy. Research and development coupled with collaborative partnerships will continue to lead Valeport’s focus on growing its portfolio of innovative marine instrumentation and its role in supporting environmental measurement.
For further information: please contact: Kevin Edwards, Valeport sales & marketing manager, email@example.com or call +44 (0)1803 869292
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