Research adds new perspective to underwater measurements

Ben Higgs’s spent six weeks on a NIWA research vessel to investigate a recently discovered underwater field of methane seeps off the coast of Gisborne.

Ben Higgs

“The sonar technology needed to discover methane seeps has been around for only about 10 years, so we’re only starting to find out about them. Quite a few have been found around New Zealand, but this one was particularly shallow—just 200 metres down—causing concern about whether the methane was able to escape and get into the atmosphere.”

The first step for Ben was figuring out what tools could be used to measure the amount of methane escaping from the seep.

“We used an underwater camera with two lasers fixed 20 centimetres apart, and pointed at the sea floor, to give us a scale reference. One of the challenges we needed to overcome was taking into account the perspective for bubbles at different distances from the camera. We knew we couldn’t just estimate the bubble size, because that wouldn’t give us enough accuracy.”

Ben says that, as a Master’s student, he had to think creatively to solve problems. Taking into account the angle of the camera, the lasers and the view, Ben developed a perspective grid that mapped the sea floor and could automate the measurements that would have previously been done manually by drawing lines on pictures.

“A lot of time went into perspective geometry and coding, but what we ended up with was a program we could run the video through, and by freezing on stills and selecting the edges of the bubbles we could get accurate measurements faster.

“Now that this program has been developed it can be used to measure other seeps, but also anything else underwater—such as taking regular measurements of the size of fish or molluscs to track the health of a marine environment.”