Sub-basalt coal seam structure imaging - results from numerical modelling

Abstract

Tertiary volcanic basalts (high velocity layers) exist in both the Bowen and Sydney Basins of Eastern Australia. The use of conventional seismic for coal mine planning is often compromised and produces sub-surface images of variable quality because the wave propagation through these heterogeneous basalt layers becomes complex. The difficulty of exploring beneath the Tertiary volcanics makes these areas less attractive for coal mining exploration. Workers involved in sub-basalt imaging in the petroleum sector, mainly give the following reasons for the difficulties in imaging beneath high-speed surface layers: 1. The strong impendance contrast between the basalts and the underlying sedimentary rocks prevents the penetration of seismic energy into target zones (Fruehn et al., 1998; Behera, 2006). 2. The roughness of the basalt boundaries causes significant disruption and scattering of the transmitted wavefield (Behera, 2006). 3. The scattering of heterogeneity of the basalt (Ziolkowski et al., 2001; Hobbs, 2002). To improve seismic imaging in these cases, modifications to data acquisition and data processing procedures which improve data quality and increase S/N were proposed (Fliedner and White, 1999; Hu et al., 2003). Commonly used techniques involve long offset, low frequency and converted wave data acquisition (Ryu, 1997; Wombell et al., 1999; Hanssen et al., 2003; Lau et al., 2007; Spitzer et al., 2008). In data processing, procedures such as pre-stack depth migration, have improved the quality of sub-basalt images (Fruehn et al., 1999; Reshef et al., 2003; Gallagher and Dromgoole, 2008). Wave-equation modelling techniques have also been used to investigate the influence of high speed layers on seismic wave propagation (Battig and Hearn, 2001; Hanssen et al., 2003). In this paper, we use a 2D finite-difference-based full elastic wave-equation forward modelling algorithm to further investigate the effects and characteristics of seismic wave propagation under different basalt settings. Our objective is to provide some insights into the seismic wave propagation and guidance for improving the quality of seismic sections for coal mine investigations.