Porosity Estimation of a Porous Goaf Area Based on Seismic Wave Attenuation

Abstract

In underground coalmines, the porosity of the goaf area is a significant indicator for evaluating the working face stability and subsidence of the ground surface. However, porosity is difficult to measure directly in a closed goaf. To overcome this, we investigated seismic wave attenuation and elastic velocity changes through porous rock by laboratory measurements using compressed porous rock and coal samples consisting of crushed coal and rock particles. The attenuation of seismic waves through porous rock is strongly influenced by porosity. However, for equal porosity, compressed porous media comprising large particles shows lower seismic attenuation than media with smaller particles. To evaluate seismic attenuation and elastic velocity changes in rock/coal with different porosities, we investigated the effect of particle size (D) and seismic wavelength (λ). We developed a new seismic attenuation model (the Revision model) based on the multi-fractured rock attenuation model by Boadu and Long (Geophysical Journal International 127(1):86–110, 1996). The Revision model accounts for (i) greater attenuation through fragmented rock due to increased contact points between the particles, and (ii) decreased attenuation caused by network propagating through the porous media. The Revision model was applied to the laboratory measurement results of ultrasonic wave attenuation using compressed porous rock and coal samples, and good agreement was found for various porosities, particle sizes and wavelength. Thus, the Revision model may be a useful tool for estimating the porosity in a goaf based on measurements of seismic attenuation and velocity change.