Jamie Morton

Jamie Morton is science reporter at the NZ Herald.

Smart science: Understanding what lies beneath

Science reporter Jamie Morton writes how a 3D mapping project can read terrain to assist with development.

Scenes of liquefaction after the quake were some of the most gripping images. Photo / Brett Phibbs
Scenes of liquefaction after the quake were some of the most gripping images. Photo / Brett Phibbs

The liquefaction that swamped Christchurch East streets with tonnes of silt and sludge has become one of the most enduring images of the city's earthquakes.

Well-known Canterbury geologist Dr Mark Quigley has since used slides of red-zoned Bexley, his old suburb, when arguing how better science and research could make it harder for such poorly-founded areas to be opened up for development.

Now an ambitious project using state-of-the-art technology, being developed by GNS Science, may also prevent planners and developers from building another Bexley.

A new Crown-funded 3D mapping project, which has began with a nearly complete model of Christchurch, will boost our understanding of the geology beneath our main centres, presenting masses of data in an impressive interactive model.

The country's population is increasingly concentrated within a small number of urban centres - 72 per cent of Kiwis live in 15 major cities, 86 per cent in cities or towns of 40,000 or more residents.

These areas, said GNS Science's Dr John Begg, also have high population densities and are characterised by high financial investment from companies, ratepayers and taxpayers.

Mapping the centres over the next 15 years would provide a much better understanding of the engineering properties and possible hazards associated with the materials that make up the terrain, and allow councils to make better informed planning decisions, he said.

The highly-detailed digital maps, constructed with modern 3D computer software, would also assist the management of potentially valuable aggregate and groundwater resources in and around urban areas. Due to our location astride one of the Earth's more active tectonic plate boundaries, New Zealand has more than its fair share of geological hazards, including earthquakes, tsunami, volcanoes and landslides.

"But in addition to these obvious hazards, we have many areas with high erosion rates, high sedimentation rates and relatively steep slopes," Dr Begg said. "These provide special design challenges for building foundations. The value of identifying areas prone to these geological hazards has been made clear by the social and economic disaster wrought by the 2010-2012 Canterbury earthquake sequence."

The technology, when combined with considered specialist engineering advice and sound planning, had the potential to prevent large scale Bexley-type scenarios, he said, although there was always a possibility that localised sensitive soils might remain undetectable.

"Although it should not be the case, local authorities have caved in to pressures exerted by developers and governments to ease regulation for development of specific projects or sites," he said.

"The 3D geological maps and their expert geotechnical interpretation will make it easier for planners to determine the boundaries of land vulnerable to a number of natural hazards."

For many urban centres, surface geological data, soil maps and geological and geotechnical reports held by councils would provide information on distribution of materials close to the surface.

Three-dimensional geological mapping, a technology which had made rapid advances in recent years, further relied on information on subsurface materials and their properties from a variety of sources.

In mapping Christchurch's underground, scientists drew on subsurface geotechnical probe data, borehole log databases and geophysical data.

Although not yet complete, Christchurch's map has already contributed to the planning and reconstruction of stopbanks for the Avon and Heathcote rivers and is guiding the routing of sewer, water and drainage pipes and underground power cables in the rebuilding of Christchurch's infrastructure.

Auckland presented another huge challenge because of the amount of existing data to be compiled, and the nature of its geology, ranging from weak sedimentary rock to hard, difficult-to-drill volcanic rock.

"But probably the largest problem is that Auckland City is so large in area, meaning that even when these problems are taken out of the equation, the job itself will be huge and difficult to complete with available funding," said Dr Begg.

GNS expects to complete Christchurch's 3D map by mid next year.

What is Quality data?
With Crown funding, GNS Science plans to create highly detailed, digital 3D maps displaying the geological make-up beneath each of our main centres. It has nearly completed such a map for Christchurch and aims to do the same in other towns and cities after securing further funding.

What does it mean for me?
Such high-quality data will prove an invaluable tool for developers and councils, enabling better forward planning, ensuring that sensitive land is used for appropriate purposes and that the most appropriate land will be earmarked for urban development. The Christchurch suburb of Bexley, ravaged by liquefaction and now in the red zone, has proven a costly example of land poorly suited for development.

The series

Monday: The flu and us: The Shivers Project.
Today: What lies beneath: Mapping our underground.
Tomorrow: Secrets of the ice part one - life in Antarctica.
Thursday: Secrets of the ice part two - unlocking Antarctica's past.
Friday: Our drone future: Miniature air vehicles.

- NZ Herald

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