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It's a vision thing

New three-dimensional imaging software for interpreting seismic data offers geotechnical engineers a much clearer picture of the ground. Dan Simpson reports.

Geophysical technology that can 'see through' the ground is being pioneered in Europe after its development in the US.

Cementation Skanska has negotiated exclusive European rights to the system, called 3D Tomographics (3dT), and has set up a division of that name to implement it.

Where earlier software took weeks of processing raw seismic data to present two-dimensional results, 3dT can produce detailed three-dimensional images in a couple of days.

Data is collected using conventional cross-hole or surface methods. Any ground that shows a significant change in velocity or attenuation can be imaged, such as old mine workings, slope failure planes, soil-rock interfaces, karst ground voids, saturated zones and fault planes.

3dT project manager Mark Kirkbride says existing data processing software, which produces twodimensional tomograms, uses only 100 to 1000 ray paths from the seismic source and took weeks to present the results in a usable graphical format.

The new RockVision 3D seismic tomography technology typically uses more than 5000 ray paths and presents detailed results in a couple of days, with preliminary results often available within 24 hours.

'The new thing about this software is the way it processes and presents the data, ' he says. 'The geophysics is still doing what it did 20 years ago, but the new software shows it in a form that the end user can understand. '

During development Skanska carried out a lot of research to find out what products were already available. The system was originally developed by the now defunct US Bureau of Mines.

US firm NSA Engineering bought the rights to the system and software and developed it into a far more powerful tool.

'We came across NSA Engineering and found that they were five years ahead of anyone else, ' says Cementation Skanska technical manager Alun Price Jones.

Although the software has been available commercially since 1999, it is constantly improving as the technology is developing and as more powerful computers become available, he says.

Three-dimensional images are produced as the seismic waves refract around objects with different velocity characteristics.

Kirkbride explains: 'The software concentrates mainly on P-wave [primary wave] energy and how this travels between two points. Thousands of signals are processed and the software creates a map of ray path coverage.

'The software goes through the records and automatically picks the first arrivals from the seismic waves. '

It maps the path of the seismic signals to ensure that large areas of ground have not been missed. It then converts the data into a three-dimensional coloured contoured image based on a velocity scale.

The cooler colours represent low velocity seismic waves, such as cavities, and the warmer colours represent higher velocities, such as strong dense material.

Once created the 3D model can be rotated in any direction and sliced at any level horizontally or vertically.

Different colours or velocities can then turned on or off from the model to highlight a certain velocity.

Price Jones says: 'The software has been developed from an end user's point of view, so engineers can understand the final image.

'The end image is a plot of different coloured zones, which clearly show areas such as cavities or boulders. '

He adds that the model can be split up into x, y and z planes to investigate the appropriate contour plots in detail.

The program was used to process seismic data to assess the stability of Silbury Hill (Ground Engineering November 2001) using a combination of cross- hole and surface systems.

Here a borehole source powered by compressed gas was used at different levels to obtain a full data set for the hill. Surface geophones were placed over the hill to pick up waves travelling outwards from locations.

'Using well-spaced boreholes, objects about 1m 3in size can be found. '

Future software developments could potentially determine design parameters from the seismic data, says Price Jones.

'Although we are not at this stage yet, there is research being carried out to determine a correlation between the seismic signal with parameters such as elasticity, rock mass properties and stress, ' he says.

Kirkbride adds: '3dT has value to various people in the industry. Contractors will use it as a troubleshooting tool, to solve problems encountered during construction and save time and money.

'Consultants will use it to supplement site investigations and clients will use it to reduce uncertainties and risks in the ground. '

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