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Scientists and Navy join forces to map the floor of Lake Rotorua - 01/12/2017

The Royal New Zealand Navy’s Military Hydrographic Group, in collaboration with GNS Science and the Te Arawa Lakes Trust, has gathered high resolution multibeam sonar data to build a new map of the floor of Lake Rotorua.  The surveys spanned 15 weeks over two years and included 29 Navy personnel.

View looking northwards over the hydrothermal eruption craters off Sulphur Point.  Red colours are 5m water depth and purple colours greater than 30m deep. Image – Royal NZ Navy

View looking northwards over the hydrothermal eruption craters off Sulphur Point. Red colours are 5m water depth and purple colours greater than 30m deep. Image – Royal NZ Navy

The new map will replace existing maps of Lake Rotorua that date to the 1970s and were made using older single beam sonar technology. 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 40 km2, or about 80% 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, adjacent to the city’s eastern flank.  

In addition to the sonar measurements, GNS Science used a magnetometer which helps locate hot spring activity on and under the lakefloor.

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.

Unusual, circular pockmarks up to 100m in diameter were seen west of Mokoia Island surrounded by flat lake floor. Image – Royal NZ Navy

Unusual, circular pockmarks up to 100m in diameter were seen west of Mokoia Island surrounded by flat lake floor. Image – Royal NZ Navy

This allowed scientists to estimate that the lake could have up to 200MW of energy being discharged into it from geothermal systems at depth. Scientists emphasised that this was a very preliminary figure and a lot more work would be needed to determine if any of this energy could be harnessed for electricity.

The survey also showed more than 1000 pockmarks on the lakefloor, up to 100 of which were venting gas into the lake.  

Lieutenant Commander Daniel Wierenga said the Military Hydrographic Group enjoyed working with GNS Science and the Te Arawa Lakes Trust in surveying the Lake.  

“The experience has been really worthwhile for the Military Hydrographic Group as 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.”

Lieutenant Commander Wierenga said the Lake was obviously an active geothermal area, which provided an interesting and challenging environment in which 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."

The Navy vessel Adventure undertaking lakefloor survey work in Lake Rotorua. Photo – Royal NZ Navy

The Navy vessel Adventure undertaking lakefloor survey work in Lake Rotorua. Photo – Royal NZ Navy

Lead scientist, Cornel de Ronde of GNS Science, said the relationship with the Navy had been a positive and fruitful one, as it had enabled scientists to survey Lake Rotorua which would have been a struggle to achieve if GNS Science worked on its own.   

“The equipment the Navy has brought to the project means we can increase capacity to map the volcanic lakes in the Rotorua district,” Dr 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.