A new highly detailed three-dimensional geological map of Christchurch is one of the first of its kind in the world.

Thousands of pieces of underground data compiled after the September 4, 2010, earthquake have been integrated into the map, which shows soil and rock layers below the surface down to about 45 metres.

The GNS Science researchers who developed the map say it will be useful in the rebuild, especially for showing the kind of ground into which new foundations will be sunk and potential areas of liquefaction.

The map is about to go through an intensive peer-review process - by GNS Science and New Zealand and international scientists - before being made publicly available next year.

Over the next 15 years, GNS Science plans to build similar maps for the rest of the country's cities.

GNS Science geologist John Begg, who spent about 18 months working on the map with colleague Katie Jones, said interest was already coming from overseas in the mapping model.

"Just last week we had a guy from the United States who is very heavily into geotechnical data and I showed him what we were doing, and he was very interested," he said.

The map, which has been built from groundwater bore data, covers an area of about 2000 square kilometres across the Selwyn and Waimakariri districts and Christchurch City, but does not include the Port Hills or Banks Peninsula.

Environment Canterbury gave GNS Science data from 18,000 boreholes drilled across several decades, with an average depth of 30m. Some were several hundred metres.

Figures from 1000 cone penetrometer tests collected post-quake by environmental and engineering consultants Tonkin & Taylor were added to the mix. They provided an indication of soil strength.

The map, which received about $300,000 in core government funding, can be viewed from any angle and zoomed in on to see extra layer details.

Begg said soil material properties, combined with groundwater depth information, could be used to gauge liquefaction potential.

"In terms of the rebuild, it tells us something about not only the materials beneath the city," he said.

"In the CBD, the model goes down to about 20m to 25m and to about 40m in the New Brighton area. It provides information about the density of materials, and also the sediment type, such as sand, silt or clay or mud.

"The modelled data will be a strong indicator for the most desirable areas to put multi-storey buildings. It tells you quite a lot about the ground conditions and potentially it could save a lot of money in foundation design and construction.

"If the site you choose to put a building has poor soil conditions, it might be significantly more expensive just because of those foundation conditions."

The geological modelling work had been challenging, Begg said.

"If we were making a choice, we would have started off with something a bit simpler, but because of the quake, Christchurch is the obvious starting point," he said.

"We're looking for ways to help Christchurch recover, and we think this model will be potentially of great help with the recovery."

- © Fairfax NZ News

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