Seabed detail ‘as if the water had been drained away’

A109 Light Utility Helicopter flight with mayor Gisborne City from the air in November 2023.

Toitū Te Whenua Land Information New Zealand is holding an open day to explain how data captured through seabed surveys helps to improve nautical charts and knowledge of the marine environment.

The open day is on Tuesday, November 7 at 11.30am at the Gisborne District Council chambers and will explain a hydrographic survey being undertaken to map the seabed in Tūranganui-a-Kiwa/Poverty Bay to help make navigation safer.

Toitū Te Whenua Land Information New Zealand is the government agency responsible for providing navigational products and services that support safe shipping. The information underpinning these products is captured through hydrographic surveys that map the seafloor.

The survey will take about eight weeks and will include Te Tapuwae o Rongokako Marine Reserve, which has never been mapped in such detail.

“The technology we’re using will give us 100 percent coverage of the seafloor, which we haven’t had in the Poverty Bay area before,” Toitū Te Whenua hydrographic survey manager Stuart Caie said.

“Technology has vastly improved since the last survey was done in the 1950s by the Navy. We’ll be able to see the full detail of the seabed as if the water had been drained away.”

While the survey will focus on data needed to update the charts, hydrographic surveys also gather information that can help with marine habitat management, aquaculture developments, flood planning and research.

The survey will use multi-beam echosounders mounted on a boat that slowly criss-crosses the area to build detailed 3D images of the seafloor.

Echosounders send out a fan of sound waves that bounce off objects with the echoes returning information about the shape and nature of the seafloor. They are set at a frequency that does not harm marine mammals.

Coverage will extend up to four nautical miles from the coastline and up to 80 metres deep in some places. The survey, which is being done by Discovery Marine Limited on behalf of Toitū Te Whenua, may identify previously unknown features like reefs, shoals and wrecks, and will recheck depths of known objects.

The 3D images will be published on the LINZ data service  website and updated nautical charts will be available from the free NZ Electronic Navigational Chart Service.