Gravity Survey Included In Christchurch Subsurface Study

MEDIA RELEASE from GNS Science 12 APRIL 2011

Gravity Survey Included In Christchurch Subsurface Study

GNS Science will spend the next two weeks undertaking a gravity survey of Christchurch and the Canterbury Plains to get a clearer picture of the subsurface structure of the region.

The project is part of a multi-pronged effort to learn more about the geological structure under Canterbury to help with earthquake recovery and rebuilding decisions in the wake of the devastating magnitude 6.3 earthquake on 22 February 2011.

Information collected during the gravity survey will be combined with seismic reflection data and aeromagnetic data to give scientists a clearer view of the subsurface and help locate any faults.

Several hundred gravity measurements already exist in Canterbury, some of them dating back several decades.

However, the spacing between the measurement points is uneven so the knowledge of the subsurface geology in some areas is good and quite poor in other areas.

Project leader, Vaughan Stagpoole of GNS Science, said the main aim of this initiative 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 be using a highly sensitive gravity meter, which is about the size of a brief case. They place it on the ground at each location for a few minutes and record the measurement 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.

Small changes in gravity measurements over a region help scientists determine geological structures in the subsurface. The instrument is sensitive to subtle changes in gravity that can be associated with the density of different rock types.

The measurement locations will cover the region where earthquake activity has occurred since September 2010, including some locations in the Port Hills.

The gravity data will augment information from a seismic survey of Christchurch currently being undertaken 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 meters of gravels and sediments.

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

This is likely to involve a light aircraft flying on a grid pattern and carrying sophisticated magnetic measuring equipment.

The fourth and final leg of the project is detailed analysis of all the aftershocks. GNS Science has been undertaking this work since September 2010. 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. Both organisations will use the data, along with other information, to help with planning and rebuilding decisions.

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