$2 million to dig up city secrets

The scientists who measured the ground moving at Eden Park during the All Blacks' Rugby World Cup triumph want to drill two more holes under Auckland to measure seismic activity from volcanoes and earthquakes.

The University of Auckland's Institute of Earth Science and Engineering buried two seismometers in a borehole at 25 metres and 450 metres below Auckland's Eden Park in 2008 during the stadium's reconstruction.

During the World Cup the seismometers - which usually measure seismic waves generated by earthquakes and volcanic eruptions - measured ground movements caused by fans.

Data showed the ground really did move during the All Blacks nail-bitting 8-7 victory over France and painted a picture of crowd tension and jubilation as the game unfolded. Not surprisingly, the biggest recorded surge came when captain Ritchie MaCaw hoisted the Webb Ellis Cup.

The institute is now investigating locations and funding options to drill another two boreholes beneath Auckland and hopes to put a proposal before the Auckland City Council within six months. It believes the holes will cost $2 million.

''This is something we are passionate about so we certainly want to advance the understanding of what's beneath us,'' spokeswoman Rachel Fenton says.

She says the institute has no set locations in mind and will work with the council to find sites that are accessible,drillable and useful for collecting data. Existing boreholes are also being considered, as it would be cheaper to extend those than start afresh.

''It depends on the type of rock because obviously drilling through rock is very expensive and difficult, and how accessible the area is,'' Fenton says.

Scientist Kate Kenedi said by digging the borehole deep into the earth all human noises are eliminated. Human noises can block out those that indicate volcanic activity, she said.

"if you put a seisosometer near the surface  it picks up noise from trucks, cars, electric signals... that noise can block out earthquake noises. The goal is to go down deeper, where you don't have alot of noise."

Plans are also dependent on funding which the institute is currently seeking.

It's hoped the boreholes will be drilled to a depth of at least 1km which would cost about $800,000 a hole to dig. Recording instruments for the holes cost between $100,000 to $200,000 each, making the boreholes worth about $1m each.

Data from the seismometers won't ''predict earthquakes, that's the holy grail'', but will help the institute map the ''geographical profile of the region'' and better understand the implications of a natural disaster.

The plans come following the discovery of four new volcanic craters in south Auckland.

The most significant of the craters is one in Favona, Mangere, which has a 300m diameter crater. The other three craters, each with a diameter of about 200m, are all in the Puhinui Reserve, in Wiri.

The volcanoes hadn't been recognised before because they are not major landforms.

The discovery bring Auckland's volcanic total to 55.

- © Fairfax NZ News