Seismic investigation of natural coal fires: A pre-fieldwork synthetic feasibility study.

Elastic modeling

We have used the code e3d, described by Larsen and Grieger (1998), to simulate a seismic survey at the coal fire field site. E3d is an explicit 2D/3D elastic finite-difference wave-propagation code that is widely used (e.g., Martin et al., 2006). It is accurate to fourth order in space and second order accurate in time.

Vmodel
Figure 1. Subsurface structure; a dipping, partially burned (grey) coal (black) layer. P and S wave velocities are given in Table 1. [NR]

Figure 1 shows the layout of our 2D model space, with layer properties shown in Table 1. A 5m thick layer of coal, half burned, dips 10$^{\circ}$ across a background of sandstone. We synthesized a seismic survey across the model with shots and receivers at 1-m spacings. The source is a vertical force with the waveform of a 125-Hz Ricker wavelet. This is intended to simulate a better-than-average sledgehammer impact. The frequency of a real sledgehammer impact is likely to be considerably lower and the waveform completely dependant on the local conditions. The receivers record both vertical, $V_z$, and horizontal, $V_x$, (in-plane) components of particle velocity. In this report we focus on the vertical (PP) component but include the horizontal component for interested readers. The model neglects the major near-surface heterogeneity like soil layer, shale layers and fissures known to exist at the site.

Table 1: Seismic P and S wave velocities and velocity gradients.

Unit	$V_p$ [m/s]	$\partial_zV_p[1/s]$	$V_s$ [m/s]	$\partial_zV_s$[1/s]
Sandstone	2000	.2	1200	.154
Coal	1200	0.0	800	0.0
Ash	300	0.0	200	0.0

Seismic investigation of natural coal fires: A pre-fieldwork synthetic feasibility study.