Ultrasonic characteristics of a rock mass

Abstract

A programme aimed at the in-situ assessment of rock properties and rock condition has been undertaken at the Julius Kruttschnitt Mineral Research Centre of the University of Queensland. The project aims to quantify rock mass parameters and to use them in a model of blasting to predict and control fragmentation. A technique for characterization of a rock mass utilizing acoustic data obtained from test or production boreholes was investigated as part of the programme. The system is a cross-hole acoustic system which uses wave propagation velocities and the attenuation parameter, Q (as estimated from a measurement of the change in pulse shape during propagation) to asses the rock mass condition. This system is a modification of the ultrasonic stress monitoring device (USMD) developed by Gladwin [5] in which a magnetostrictive transducer, mechanically clamped in an NX borehole is used to produced ultrasonic pulses of approximately 37.5 kHz. Pulse transmission times to detectors clamped at measured depths are used to produce a velocity profile for distances of up to 15 m from the source hole. The viability of the technique and the required instrument specifications, have been studied in a series of field experiments, which indicate good agreement between structural core log data and pulse rise time. Characterization of the state of fracture of a rock mass is being carried out at several sites with markedly different geological environments. These sites include a high grade open cut hematite mine, an open cut coal mine, an underground dolomitic copper ore body, and an underground lead-zinc ore body. The ultrasonic technique is being supplemented by studies in the sonic frequency range using small explosive charges as a acoustic sources. A comparison between the two sets of data will be made for both velocity and Q measurements for the preliminary field experimentation.