Theoretical and practical aspects of absorption in the application of in‐seam seismic coal exploration

Abstract

During the last two decades, the detection of coal seam discontinuities by seismic waves guided by the seam has become a special branch of exploration seismics in Europe. Waves consisting purely of SH motion (so‐called waves of Love type) are of special interest, and the rather high frequencies of the Airy phase, in thin seams, are most important because they present very high reflectivity at seam interruptions. Absorption increases with frequency in most layers, and therefore attenuates the high Airy‐phase frequencies more severely than the earlier low‐frequency part of the guided waves. Another fact additionally impairs the Airy‐phase signal: the quality factor Q is much lower in coal than in the schists and sandstones of the Carboniferous country rock. Unfortunately, most of the energy of the Airy phase is transferred by the coal, whereas the lower frequencies have their main energy conveyed by the country rock above and below the seam. In order to allow a better understanding of the influence of absorption on Love‐type seam waves, several simplified computations were carried out for the fundamental mode of a seam typical for the northwest‐German Ruhr area. The assumptions are as follows: The quality factors [Formula: see text] for coal and [Formula: see text] for the country rock do not depend upon frequency; higher powers of [Formula: see text] and [Formula: see text] can be neglected; and the distance from the source is large enough to allow the two‐dimensional plane‐wave case to be considered. The mathematics resulting from these assumptions and adequate data processing of transmission records provides the possibility to determine the quality factor [Formula: see text] of coal in‐situ, although the thickness of the seam may be much smaller than the wavelengths involved. [Formula: see text] may become of interest for practical mining problems.

Modified image algorithm to simulate seismic channel waves in 3D tunnel model with rugged free surfaces

“…More importantly, the viscosity of both the surrounding rocks and the coal seams cannot be ignored due to the sedimentary rock properties. Theoretical, numerical, laboratory, and field data indicate that seismic waves travelling through a coal seam is significantly damped in viscoelastic media (Buchanan , ; Krey, Arnetzl, and Knecht ). In particular, the complicated coupling mechanisms of seismic waves between the surrounding rocks and coal seams vary with the changes in the quality factors as well, which is an indispensable parameter to characterize the attenuation and dispersion of viscoelastic media.…”

“…More importantly, the viscosity of both the surrounding rocks and the coal seams cannot be ignored due to the sedimentary rock properties. Theoretical, numerical, laboratory, and field data indicate that seismic waves travelling through a coal seam is significantly damped in viscoelastic media (Buchanan 1978(Buchanan , 1987Krey, Arnetzl, and Knecht 1982).…”

Imaging mining hazards within coalbeds using prestack wave equation migration of in-seam seismic survey data: A feasibility study with synthetic data

“…Channel waves are the most commonly used (Krey, 1963;Krey et al, 1982;Dresen et al, 1985). Relatively little has been done using body waves, although some Kirchhoff style approaches have been used to image faults (Mason et al, 1980;Buchanan et al, 1981;Millahn and Arnetzl, 1986) and other anomalies (Dickmann and Awasthi, 1999;Borm et al, 2001;Brückl et al, 2001;Ashida et al, 2002;Lüth et al, 2004).…”

Modelling of Rayleigh-type seam waves in disturbed coal seams and around a coal mine roadway

“…Krajewski et al (1987) studied dispersion and polarization characteristics of field data from two‐component in‐seam seismic measurements and the interaction of reflected seam waves with roadway modes. Attenuation, anisotropy and absorption of channel waves were also aspects of interest, for example, Buchanan (1978), Krey et al (1982), Buchanan et al (1983) and Emmerich & Korn (1987). Numerical modelling: Apart from the influence of different layering on channel wave characteristics, the propagation of seam waves in 2‐D horizontally inhomogeneous models was investigated with different methods to obtain information on the reflection and transmission of seam waves at different types of seam disturbances. Love‐type seam wave propagation in models with seam interruptions and seam ends were examined by Korn & Stöckl (1982).…”

“…Krajewski et al (1987) studied dispersion and polarization characteristics of field data from two-component in-seam seismic measurements and the interaction of reflected seam waves with roadway modes. Attenuation, anisotropy and absorption of channel waves were also aspects of interest, for example, Buchanan (1978), Krey et al (1982), Buchanan et al (1983) and Emmerich & Korn (1987).…”