Sonar Studies Reveal New Mediterranean Seabed Features
Unusual seabed features have been revealed in the Mediterranean just beyond the Rhône mouth by a team from the Southampton Oceanography Centre (SOC).
Working with a range of sonar equipment from Marine Electronics Ltd of Guernsey, the EU funded team obtained clear images of what they believe to be gas escape features on the delta front. The equipment also revealed deep channels cut into the delta, suggesting the activity of hyperpycnal flows - high density sediment currents capable of scouring the seabed.
The survey was part of the Eurostrataform project which is being coordinated by SOC and has been created to collect data relating to the movement of sediment on European continental margins. The aim is to understand the transport of sediment from its source at the river mouth to its deposition on the continental shelf and on the abyssal plain. It is hoped that the survey will make it possible to establish the pathways along which the sediment is carried, whether by surface plume or along the bottom. The accumulated data will be analysed at SOC and used for calibration purposes in sediment transport models being developed jointly by SOC and the
Institute of Arctic and Alpine Research (INSTARR) in Colorado.
A Marine Electronics 1640 Sonar operated in its side scan mode was used for a 300 km line survey of the lower reaches of the Rhône and its delta top and front areas. The team was working in an area that had never been surveyed and was pleased to observe the unexpected features. Unfortunately these were subsequently destroyed by a recent major flood event from the River Rhône when the huge volumes of water, in places, removed up to 6 metres of sediment.
A Sand Ripple Imaging sonar, also from Marine Electronics, was used to examine seabed forms within the mouth of the Rhône. The equipment was mounted on a benthic frame and used to scan horizontally over 360 degrees.
This made it possible to obtain high definition images of sand waves, including active sand ripples. By measuring their length and height the SOC team has been able to obtain valuable information about the behaviour of currents and sediment in the area. The sector scanning sonar shared the benthic frame with a sediment imaging sonar, also from Marine Electronics, and scanned a 1 metre square area of seabed below the tripod to provide more detailed images of ripple and sand wave formation. Data were collected from each location over periods of up to two hours and relayed to the survey boat above, where they were viewed in real time and stored for analysis.
The Sand Ripple Imaging Sonar supplied by Marine Electronics Ltd has a narrow horizontal beam and a wide vertical beam pattern. Typically the sonar is mounted near to the seabed and used to scan a circular area up to 5m in range. The transducer is rotated in the horizontal plane and data gathered for every 0.9° of the revolution. The sonar operates as a slave to a host computer which controls the sonar via an RS232 serial data link. The acoustic receive signal is amplified and sent to the host computer as an analogue signal allowing free control over the digitisation of the data at the surface.
