Geomara were contracted by our clients McMahon Design and Management to provide a cable route survey for a newly proposed cable system. The Tuskar fibre optic cable system is being planned to replace the now defunct ESAT 1 cable which runs onshore at Ballyteige Bay just west of Kilmore Quay. The survey included Multibeam Echo Sounder (MBES), Sidescan Sonar (SSS), Sub Bottom Profiler (SBP) and Magnetometer across a survey corridor running from the 12NM limit south of Kilmore Quay, west of the Saltee Islands and then east across Ballyteige Bay.
Geomara always adopt a partnership approach to new projects. In this case the partnership included our clients MDM, vessel providers Fasnet Shipping and survey equipment rentals Unique Group in Aberdeen.
Conducting offshore survey works in December is all about forward planning and then reacting quickly to suitable weather windows. The survey spread was mobilised by a team of Geomara’s International Hydrographic Organisation (IHO) certified surveyors on board the Fastnet Pelican from Fastnet Shipping’s base on the River Suir in Waterford City. The sonar winch and SSS including an Ultra Short Baseline System (USBL) for precise location of the towed arrays arrived promptly from Unique survey rentals.
Geomara’s own high resolution Teledyne T50R MBES with integrated Inertial Navigation System (INS) from Applanix was pole mounted on the starboard side of the Fastnet Pelican. Our Edgetetech 3100 Full Spectrum Chirp SBP was also pole mounted and deployed on the port side of the vessel. The SSS and Magnetometer were piggy backed and deployed using the sonar winch and A-Frame from the back deck of the vessel.
Following calibrations of the USBL and MBES survey operations were completed in 10km blocks running from the 12NM limit towards the shore. All of the survey data was reduced to Lowest Astronomical Tide (LAT) using the Vertical Offshore Reference Frame (VORF) from the UK Hydrographic Office.
Backscatter from the MBES was collected alongside the bathymetric data. Backscatter data is a measure of the intensity of the sound waves which are reflected back from the seabed. Different bottom types will have a different intensity with harder rockier bottom types reflecting more sound then softer bottom types. By collecting the backscatter data in conjunction with the bathymetry it allows us to further understand the characteristics of the sea floor.
The high resolution MBES, backscatter and SSS data allows a detailed view of the seafloor and its characteristics to be obtained, which is then used to plan the precise route of the proposed cable by avoiding any potential seafloor obstacles such as rocks or sharp geological or topographical features which may cause damage during installation or over the lifespan of the cable.
The SBP data will give some indication of the depth of deposits and geological layers underlying the seafloor. Information from the top few metres of the profiles will be very useful when planning the cable routes and potential cable burial depths.
Other seafloor obstacles which need to be avoided include ship wrecks. These are numerous in the area around the Saltee’s as was identified prior to the survey from the National Monuments Wreck Viewer which is based on the Wreck Inventory Database compiled by the Underwater Archaeology Unit (UAU) of the National Monuments Service. A detection device licence was in place from the UAU prior to the survey.
The datasets collected during the cable route survey will inform the final route of the proposed cable system within the route corridor and the assessments for ecology, cultural heritage and navigation that will be undertaken.
Acknowledgements
The Geomara team would like to thank Fastnet Shipping and the crew of the Pelican for their help and professionalism during the surveys. Thanks are also due to Unique survey rentals who so promptly provided some of the survey spread and gave valuable technical support during the mobilisation. Brian Smith of Hydromaster provided the loan of a MBES receiver, proof positive of a developing collegiality within the small but developing Irish commercial hydrographic survey community. Our clients MDM provided backend support during the surveys. Aaron Mair of Seagard provided Fishery Liaison services.
Finally thanks to the Harbour Master and fishing community in Kilmore Quay for their assistance in facilitating the surveys.