Watch: Shipwrecks, WWII history revealed in complex underwater survey of Marlborough Sounds seafloor

The most complex underwater coastal survey of the seafloor ever undertaken in New Zealand has been carried out in the Marlborough Sounds.

The survey revealed amongst other things previously undiscovered natural features, shipwrecks and man made features from World War II.

The Queen Charlotte Sound/Tōtaranui and Tory Channel/Kura Te Au mapping project was undertaken by NIWA and Discovery Marine Ltd in partnership with Land Information New Zealand (LINZ) and the Marlborough District Council.

It was completed using the latest multibeam echo sounder technology to gather more than five billion data points.

A statement released by NIWA says the survey was undertaken from two research vessels and included 280 days on the water mapping more than 43,000 hectares.

Multibeam echo sounders produce a fan of acoustic beams or sound waves directed downwards from the bottom of a boat.

These beams reflect off the seafloor, enabling the surveyors to calculate the depth of the seafloor, and map the seafloor habitat in extraordinary detail.

NIWA says the data has been used to produce an extensive new catalogue of navigational charts which were last updated in the 1940s, seabed maps and 3D images.

NIWA General Manager Operations and marine geologist Dr Helen Neil said the seafloor survey revealed natural features such as complex pockmark structures, scouring, sand waves, sediment braids and freshwater seeps not previously known about, as well as an extensive survey of the kelp habitats in the sounds.

The survey also revealed a number of man-made features and structures on the seafloor, including the marine farms of the region, a trench that remains from the 1940s, dug for a war-time communications cable between islands in the Sounds, once considered for use as a base for the Pacific Fleet.

Several shipwrecks and sunken boats were also located, some known to divers and locals, and some unknown. One of those was the wreck of the Hippolas, a barque which struck Walker Rock in 1909 and was abandoned with no loss of life.

It was completed using the latest multibeam echo sounder technology. Source: Supplied