Application of borehole imagery in iron ore: examples from Mount Whaleback, Western Australia

Abstract

The borehole televiewer is a down-hole geophysical logging device that produces continuous, oriented, 360° images of the drill hole wall. Two types of borehole images are currently being generated at the Mount Whaleback iron ore deposit in Western Australia. The optical televiewer (OTV) tool captures a 'real' image of the borehole wall whereas an acoustic televiewer (ATV) generates an image through variations in physical borehole size, scattering the energy from an acoustic beam. Orientation of the resulting 'virtual core' is derived through magnetometers and accelerometers, providing values within one degree for both inclination and azimuth. Down-hole structures such as fractures, joints, foliations and bedding planes are selected directly off the borehole image, recorded and displayed in a three-dimensional modelling package alongside adjacent drill holes. Measurements are corrected from apparent to true dip and magnetic to true north. Strata-bound magnetic anomalies are identified and corresponding measurements excluded. The importance of borehole imaging increases substantially with its ability to integrate the OTV and/or ATV data with other forms of lithological and structural data. At Mount Whaleback, the borehole televiewer has been implemented to provide information for geotechnical exploration including structural analysis, lithological characterisation and rock-mass conditions. Within the large area in and around Mount Whaleback, drilling may intersect a number of different geological formations with various rock types and conditions. With the application of the borehole televiewer, reverse circulation drilling can provide information traditionally only obtained by diamond drilling.