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Fracture analysis using High Resolution Acoustic Televiewers

A number of boreholes of up to 30m deep on an embankment in the central district of Hong Kong was logged as ground investigation for the Shatin to Central Link to investigate slope stability. Weathered granite posed a risk to slopes in the area and poor core recovery made it difficult to assess the same.

The Robertson Geo High Resolution Acoustic Televiewer (HRAT) was used to determine the lithology and fracture analysis of the boreholes. The probe provides a continuous high resolution oriented ultrasound image of the borehole wall and can be used to characterise features that intersect the wall. This includes bedding and drilling-induced natural fractures and faults.

Integrated orientation measurements allow the inclination and direction of features to be measured relative to true north, magnetic north or the borehole dip direction.

The HRAT was able to make 100% continuous 360 degree images of the boreholes, including areas where no intact core recovery could be made. The dip and direction of the fractures and bedding in highly fractured zones could be clearly identified and measured using the HRAT; this could not be seen on the recovered core samples.

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