Hi-tech survey for Rotomahana lakebed

Marine geophysicist Fabio Caratori Tontini (left) and Woods Hole Oceanographic Institution senior scientist Maurice Tivey with a "blanket" they have been using to survey the lakebed of Lake Rotomahana.

From the steaming banks of Lake Rotomahana, watching a man hanging off the side of a boat and dropping a strange yellow object into the water might seem an odd sight.

But for scientists trying to understand the complex engine that heats the central North Island lake, the work couldn't be more serious.

Over the past few days, US geologist Dr Maurice Tivey has been working alongside the same GNS Science team who discovered remnants of the long-lost Pink and White Terraces beneath the lake two years ago.

This time, the scientists are carrying out a full geothermal survey of the lakebed, using technology never seen before in New Zealand.

For years, scientists have known there are two active geothermal systems under the 800ha lake, considered the warmest of the Rotorua lakes with year-long temperatures of 11C to 14C.

One of the systems is believed to be related to the 1886 Mt Tarawera eruption that destroyed the famed terraces.

The surrounding Okataina Caldera gives rise to a substantial amount of volcanic and geothermal activity east of Rotorua, and was the source of the massive eruption.

To gain a detailed picture of these separate systems, scientists are dropping state-of-the-art heat measuring devices at more than 100 different points across the lakebed.

The heavy instruments - called "blankets" which look more like ski biscuits - are dropped from a boat and lie on the lakebed for 12 hours collecting measurements before being moved to a new spot.

Scientists are also using a camera to take high resolution photos of geothermal features on the lake floor while collecting water samples for analysis.

Preliminary results from the first batch of measurements have shown a couple of hot spots where heat output is about five times higher than similar measurements at ocean vents in the Pacific.

Results for the entire lake will represent another piece in the puzzle to help scientists understand the size and the state of the magma body that underlies this part of the Bay of Plenty.

"Once we have the information about lake's heat energy output, we will be able to put together a comprehensive story on the evolution of the system since the Tarawera eruption of 1886," said project leader Cornel de Ronde of GNS Science.

Dr Tivey, whose Massachusetts-based Woods Hole Oceanographic Institution pioneered the technology, said the rate of watts coming out of the lake was "ridiculously high".

The results would be compared with a 1992 survey in 1992 could be followed up in five year's time to check whether the lake had cooled or was heating up, he said.