The construction of the Botafoc dam in 2003 created the opportunity for development of the infrastructure of Ibiza port on the Balearic island of Ibiza, Spain. The dam created sheltered conditions in the inner harbour and allowed construction of port infrastructure along a seafront of 500 meter length. The construction was proposed in order to reduce traffic in the south docks, located close to the city’s busy commercial and entertainment neighbourhoods. The Balearic Island Port Authority, Port de Balears, commissioned the development of docks for simultaneous berthing of five large vessels and 75,000 m2 of port area, observing stringent environmental and landscaping considerations. The temporary consortium UTE Botafoc, consisting of Ferrovial, Cyes and LlullSastre, won the contract to carry out the work. Prosermar Ingenieria was contracted by UTE to carry out the reconnaissance hydrographic survey for the dredging operations subsequently completed by Rhode Nielsen A/S.
Hydrographic Survey
A high resolution hydrographic survey of an area of approximately 250,000 m2 was carried out in September 2010 with the aim of determining the pre-dredge surface and in particular the boundaries of the shore slopes. The GeoSwath Plus shallow water multibeam was chosen, ensuring the specified accuracies, while offering the possibility of a portable installation on a vessel supplied by the client. The system has a seafloor coverage of up to 12 times water depth making it a time-efficient tool to survey in this shallow water environment. The survey including the system calibration was completed within a day, while the mobilisation and demobilisation were carried out in the afternoons of the previous day and of the survey day respectively. The wide swath has the additional advantage of being able to survey under moored vessels, pontoons and rigged platforms, which are common in a construction site. Also through the wide beam opening angle of 240° of this dual head system it was possible to survey up to the waterline close to the shore. Hence a full coverage of the area was achieved, without the need of moving equipment, which would have been impossible with a single beam survey.
Quick, easy and cost effective
The installation consisted of an adjustable metal framework, developed by Spanish company Prosermar (specialists in work control and positioning solutions in the civil engineering and marine construction sector) for mounting the sonar head over the side of the vessel while holding all ancillary sensors. This way the relative positions of the sensors are fixed, making it easy to calibrate the system, while facilitating a fast installation on any given vessel. It is based on a modular cross beam with 120 mm cross section that can be fixed to the gunwales of any boat between 2 and 4 m wide. At one end of the beam a crosshead is mounted, holding a sliding tube that supports the GeoSwath Plus sonar head over the side. It is possible to raise the head for fast transfers to the working area. The entire system fits into an SUV and was driven to the site from the office location on the Spanish peninsula via ferry to the survey site by the two operators. Ancillary sensors used were the Kongsberg Seatex MRU-H, the Hemisphere VS110 and a Leica GPS system using Leica RTK corrections. A Tritech altimeter was deployed on the sonar head for quality control of the multibeam data and for navigation safety. Also a Valeport SVP was located on the head that supplied online sound velocity measurements to the GeoSwath Plus system. The same sensor was also used to take several sound velocity profile measurements throughout the survey day to correct for sound speed variations in the water column. Tidal information was logged and used for corrections with a local tide gauge. The GS+ software package, supplied with the system and running on the deck unit installed in the cabin was used for line planning and helmsman display, data acquisition, calibration and data post processing. Final data cleaning was done with QPS Qinsy data post processing package. The data collection of the 250,000 m2 area was completed in four hours, while the collection of data for patch test calibration took another hour including transfer to a suitable site. Benchmark tests were carried out by surveying lines over two large concrete blocks of known position and depth from different directions in the presence of a clients’ representative as well as a representative of the dredging contractor. After completion of the hydrographic survey a total of 500,000 m3 were dredged by the 96 m long Gefion R multi-purpose trailer suction hopper dredger operated by Rhode Nielsen. The uncontaminated material was dumped in an area chosen for its environmental suitability twelve miles offshore at c. 100 m depth.
Conclusion
This application shows that it is possible to carry out a high resolution multibeam hydrographic survey from a vessel in a time and cost efficient manner using an adaptable mounting arrangement for a truly portable system.
For more information contact Dr Martin Gutowski, Product Sales Manager, GeoAcoustics martin.gutowski@geoacoustics.com, www.geoacoustics.com