The application of VSP in the Pilbara

Abstract

The construction of geological and geotechnical models in typical Pilbara iron ore environments is vital to enable an optimized mine design for the life of the asset, while maintaining pit wall integrity and overall mine safety. Geotechnical assessments require the measurement of geomechanical properties, such as the triaxial shear, direct shear and unconfined compressive strength tests and pressure and shear wave velocities on diamond core samples. Ideally, these velocities would be measured in Reverse Circulation (RC) boreholes as their spatial density is far higher than diamond drilled holes. Unfortunately, despite its value, such data is seldom collected as a large proportion of the holes are above the water table, limiting the use of sonic-logging tools. Even if measurements are possible, damage to the borehole caused by drilling biases the resulting velocity measurements. This paper details the results of a trial using the vertical seismic profile method to directly measure in-situ seismic velocities in RC boreholes. The method was successful in determining the velocities of the formations through the entire length of the holes. The data in several boreholes was of sufficient quality for the application of more advanced processing methods, important for geological mapping and the processing and interpretation of surface seismic data. The success of this first trial has implications for future iron-ore developments in the Pilbara. The widespread acquisition of accurate seismic velocity data is likely to enable the creation of more accurate geotechnical models and could improve future development decisions.