GNS surveys Christchurch

An aerial view of the city of Christchruch. Photo / Nasa

GNS Science is carrying out a gravity survey of Christchurch and the Canterbury Plains to get a clearer picture of the subsurface structure of the region following February's deadly earthquake.

The two-week project is part of a multi-pronged effort to learn more about the geological structure under Canterbury to help with Christchurch's earthquake recovery and rebuilding decisions.

Information collected during the gravity survey, which measures the small variations in the pull of gravity and helps determine the geology beneath the surface, will give scientists a clearer view of the subsurface and help locate any faults.

Project leader, Vaughan Stagpoole of GNS Science, said the main aim was to achieve more uniform coverage of gravity measurements and therefore more uniform information about subsurface geology.

``We hope to collect gravity data at up to 200 locations in Canterbury during the next two weeks,'' Dr Stagpoole said.

The scientists will use a highly sensitive gravity meter, about the size of a brief case, which is placed on the ground at each location for a few minutes and the measurement recorded visually from the instrument.

``Typically we place the instrument beside a road or a farm track and it takes just a few minutes at each location to record the gravity,'' Dr Stagpoole said.

The instrument is sensitive to subtle changes in gravity that can be associated with the density of different rock types.

Measurement locations will cover the region where earthquakes have happened since September 2010, including some locations in the Port Hills.

The gravity data will augment information from a seismic survey of Christchurch being done by the University of Canterbury and its Canadian associates. The seismic data will enable scientists to map faults that may be hidden under hundreds of metres of gravels and sediments.

A third leg of the research will involve an aerial survey of Christchurch to measure magnetic variations in the subsurface.

The final leg of the project will be detailed analysis of all the aftershocks, which GNS Science has been undertaking since September last year.

The analysis helps scientists understand the size and orientation of the faults under Canterbury.

The work is taking place under contract to the Canterbury Earthquake Recovery Authority and the Ministry of Civil Defence and Emergency Management.

- NZPA