ABSTRACT

Understanding rock fractures and their development in a varying stress environment is important for strata control and risk management in mining and civil engineering. A study on seismic transmission in different fractures zones was carried out a longwall mine to investigate the relationship between the fractures and seismic velocity and dominant frequency. Twenty triaxial geophones with five deployed in each of four deep boreholes formed a survey network. The seismic transmissions were conducted through repeatedly firing shots in three shot holes into the network on different dates. These shots generated clear seismograms for the velocity and frequency analysis. This study has shown that rock fracturing associated with mining can significantly change the seismic velocity and dominant frequency. At the experiment site, the roof caving processes induced extensive fractures in the roof rocks. The changes in the seismic velocity and frequency from 0% to more than 60% in relation to intact, partially and fully fractured fracture stages were obtained. The results have demonstrated that both seismic velocity and frequency changes can be used as indicators for diagnosing rock fracture condition. The seismic transmission method can be further developed as an efficient tool for mapping fracture development for engineering stability assessment and production control.

1 INTRODUCTION

Underground excavation can significantly change the virtual stress conditions and induce fractures to the surrounding rocks. The existence of the fractures may reduce the strength of the rocks and cause stability problems to engineered structures. The fractures can also form pathways for fluid flow, causing damages to the underground water system.

Fractures can also be used favorably in a number of ways. Thermal fracturing was used for rock breakage in drilling and tunneling. Block caving method induces fractures in ore/rock bodies to mine massive underground metalliferous deposits.

Understanding the development in fracture patterns and extent under dynamically changing stress environment is extremely helpful in solving fracture related stability problems and enhancing production. The first step for improving our understanding is the detection and characterization of underground fractures.

In laboratory scale, micro-fracturing in rock samples and its associated acoustic emissions have been studied extensively (Einstein and Dershowitz 1990; Lockner et al., 1992). It has been found that fractures present anomalous zones of physical properties which can be detected using seismic methods. A study on P-wave propagation through different oriented fractured rocks showed that the seismic velocity and amplitude change with different fracture conditions (King et al., 1986). An analysis of travel times, amplitudes and waveform shapes of seismic waves should enable us to relate these observations to the recent theory of wave propagation through fractured rock developed by Hudson et al (1997) and Pointer et al. (2000). However, the efficiency and economy of seismic methods for fracture detection have not been well studied in 4D sense and at a large scale (National Research Council 1996).

In this paper, an investigation on transmission seismic techniques for detecting rock fracture processes is presented. The objective of this project was to explore how significant of the seismic attributes.

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