ABSTRACT:

The distributed optic fiber sensing (DOFS) technique can record seismic signals at 2-5 m spatial resolution along a single fiber cable over 5-10 km in length. This is particularly useful for accurate mapping of rock fractures associated with longwall coal mining. In this paper, DOFS was evaluated at a coal mine site for monitoring microseismicity induced by the longwall caving process. The sensor network was constructed using a single 3 km long fiber cable installed in a 250 m deep hole and buried in shallow trenches at the ground surface, around the longwall panel that was being mined. The results of this experiment have successfully demonstrated that DOFS can acquire high quality seismic signals associated with longwall mining and that highly accurate event location can be achieved using the fiber data. Several challenges have been identified in this experiment and they must be addressed before DOFS can be practically useful for sensing in longwall coal mining.

1. Introduction

Microseismic monitoring has been recognized as an efficient tool to map rock fracture network development associated with mining progress, for assessment and management of geodynamic risks, and it has been widely used around the world at different types of mines [1,2]. A microseismic monitoring system normally consists of a network of individual geophones that are distributed at isolated locations around a mining area, sparsely spaced at 50–200 m. The sensitivity of microseismic event detection and accuracy of the event location are both highly dependent on the geometry of the geophone network and the number of the sensors used. To capture broadband seismic signals and achieve high accuracy in locating events, a densely spaced geophone network is required. This is a challenge to state of the art microseismic systems due to the high costs of instruments and cables, geophone installation and relocation, and instrument decommission and maintenance.

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