Successful But Eventful Start to New Zealand's Largest Coastal Survey

31 October 2008: Media release

The first phase of a survey of the Bay of Islands and eastern Northland seabed is making good progress – even after stopping to assist in the rescue of three troubled boats in the area.

Since mid-October, two National Institute of Water and Atmospheric Research (NIWA) vessels – the deepwater research vessel Tangaroa and the survey launch Pelorus – have been mapping physical features of the marine environment using multibeam mapping to build up a picture of the seabed.

The survey, coordinated by LINZ, is the largest of its kind to take place in New Zealand coastal waters and is part of the government’s Ocean Survey 20/20 programme.

NIWA ocean geologist Anne-Laure Verdier, (the surveyor in charge), said the team was grateful to the people of Northland for their support and interest in the project, which will greatly improve the understanding of marine biodiversity and habitats and help with future decision-making.

“We’re also pleased to have been able to give back to the community. The survey launch Pelorus has been able to assist in three marine rescues since arriving in the area,” Ms Verdier said.

Pelorus provided assistance to a yacht that had run aground at The Brothers off Kerikeri Inlet, responded to another mayday and also towed a small catamaran which had been washed off its mooring by wind to safety.

Good weather and sea conditions mean Tangaroa is ahead of schedule, mapping seafloor depths between 50 and 200 metres. While criss-crossing the Bay of Islands, Pelorus also made a successful run through the Hole in the Rock, recording depths greater than 80 metres at times.

“This is the first time the seafloor under the Hole in the Rock has been surveyed using multibeam mapping and it’s providing some exciting new information for us,” Ms Verdier said.

A third NIWA vessel, Bella Vita, joins the Bay of Islands survey this weekend to map areas close to shore between 2 and 10 metres.

Boat operators are being asked not to venture closer than 200 metres to Bella Vita. The sidescan sonar being used is extremely sensitive to air bubbles in the water caused by boat wakes which can last in the water for up to 10 minutes and completely block the seabed image.

Background

The main focus of this project is to map the Bay of Islands' marine biodiversity as well as its seabed habitats. Sediment and water quality data will also be collected, and possibly information on the geology under the seabed.

The first phase of the project will map the entire Bay of Islands and 3,230 square kilometres of coastline between North Cape and Mimiwhangata to a depth of 200 metres. This work is expected to be completed at the end of November.

The project's second phase, a biological survey, is expected to begin around May 2009.

Data collected during the project can be used by government agencies, regional and district councils, and local groups to develop and manage the Bay of Islands coastal resources in a more effective and sustainable way.

Planning for the project has been done by LINZ, the Ministry of Fisheries, the Department of Conservation and the Northland Regional Council. Other government agencies and local groups from the Bay of Islands, such as the Bay of Islands Maritime Park society and tangata whenua, will also be involved as the project progresses.

Read more about :

• the Bay of Islands Coastal Survey Project
• Ocean Survey 20/20.

Multibeam mapping & sidescan sonar explained (courtesy of NIWA)

Multibeam mapping uses acoustic beams to measure water depth and information about the seafloor.

RV Tangaroa and the survey launch Pelorus are fitted with multibeam systems on their hulls which measure how long sound waves take to travel to the seafloor and back. The time is adjusted for water velocity, ship movements, and tides to provide an accurate calculation of the water depth.

By using several measurements it is then possible to build up a detailed map of the seafloor.

Also the intensity of the returning sound is collected to provide information on the geological make up of the seafloor.

The multibeam system on Tangaroa has 135 separate acoustic beams and each swath of the seafloor is up to 5 km wide depending on water depth. Pelorus has a high resolution system which has 254 separate acoustic beams.

Bella Vita will survey the inshore areas with sidescan sonar. Sidescan sonar emits fan-shaped pulses down towards the seafloor and records the amplitude of the returning energy.

With each pulse a 200 metre strip of the seafloor is mapped. By putting the strips together a map of the seafloor is created, similar to an aerial photograph.