View looking northwards over the hydrothermal eruption craters off Sulphur Point. Image/supplied
View looking northwards over the hydrothermal eruption craters off Sulphur Point. Image/supplied
The floor of Lake Rotorua has been mapped for the first time since the 1970s, through a collaboration between the Royal New Zealand Navy, GNS Science and Te Arawa Lakes Trust.
The surveys spanned 15 weeks over two years and included 29 navy personnel from the Military Hydrographic Group.
The new map will replace existing maps of Lake Rotorua made using single beam sonar technology in the 1970s.
The process involved gathering 500 million soundings during the navy surveys.
The map, which is expected to be useful to a number of groups, was produced as part of a multi-year GNS Science-led project to map many of the lakes in the Rotorua region.
The navy used its survey vessel Adventure and a multibeam echo sounder to map all parts of the lake greater than 5m deep covering about 40sq km (80 per cent) of the lake.
Although much of the lake is reasonably shallow at about 20m, the new map shows the deepest point is about 34m deep and located off Sulphur Point.
Unusual, circular pockmarks up to 100m in diameter were seen west of Mokoia Island surrounded by flat lake floor. Image/supplied
In addition to the sonar measurements, GNS Science used a magnetometer to help locate hot spring activity on and under the lake floor.
GNS Science also deployed heat-flow devices in the deeper parts of the lake to measure the amount of geothermal heat coming into the lake from below.
This allowed scientists to estimate that the lake could have up to 200MW of energy being discharged into it from geothermal systems at depth.
But the figure is preliminary and scientists have stressed more work needs to be done to determine if any of this energy could be harnessed for electricity.
The survey also showed more than 1000 pockmarks on the lake floor, up to 100 of which were venting gas into the lake.
Lieutenant Commander Daniel Wierenga said the survey had been worthwhile.
"It meant we could use our skills and equipment to produce a very high resolution bathymetric map of the lake floor which is valuable to a number of different groups."
The Navy vessel Adventure undertaking lakefloor survey work in Lake Rotorua. Photo/supplied
Wierenga said the geothermally active lake provided an interesting and challenging environment to survey – something seldom experienced when the navy operated in coastal areas.
"The lake floor itself is mostly flat, however there are some complex craters off Sulphur Point and pockmarks strewn all over the lake."
GNS Science lead scientist Cornel de Ronde said the relationship with the navy had been a positive and fruitful one.
"The equipment the navy has brought to the project means we can increase capacity to map the volcanic lakes in the Rotorua district," de Ronde said.
"The detailed swath mapping of Lake Rotorua done by the navy, when combined with the magnetic survey of the same area and deployment of heat flow blankets, has meant we have a much clearer picture of the geology and geothermal activity occurring on the floor of Lake Rotorua."
De Ronde said heat was coming up through the lake, especially off Sulphur Point, but also in other parts of the lake.
Fellow scientist and marine geophysicist Fabio Caratori Tontini said the successful collaboration between different organisations was the key to the multidisciplinary approach needed to understand complex volcanic and geothermal systems such as Lake Rotorua.
