Love‐type Seam‐waves in Washout Models of Coal Seams*

Abstract

The propagation of Love seam-waves across washouts of coal seams was studied by calculating synthetic seismograms with a finite-difference method. Seam interruption, seam end and seam thinning models were investigated. The horizontal offset, the dip of the discontinuities and the degree of erosion served as variable parameters. Maximum displacement amplitudes, relative spectral amplitudes and phase and group slowness curves were extracted from the synthetic seismograms.Both seam interruption and seam thinning reduce the maximum displacement amplitudes of the transmitted Love seam-waves. The degree of amplitude reduction depends on the horizontal offset and the degree of erosion. It is four times greater for a total seam interruption than for an equivalent seam thinning with a horizontal offset of four times the seam thickness. In a seam cut vertically, the impedance contrast between the coal and the washout filling determines the maximum displacement amplitudes of the reflected Love seam-waves. They diminish by a maximum factor of four in oblique interruption zone discontinuities with a dip of maximum 27", and by a maximum factor of ten in a seam thinning with a degree of erosion of at least 22%.The analysis of the relative spectral amplitudes indicates a preferential transmission of those phases with frequencies below, and a preferential reflection of those phases with frequencies above the first mode Airy-phase. The relative spectral amplitudes of the reflected Love seam-waves show a distinct interference pattern of the waves reflected at both interruption zone discontinuities.The dispersion analysis reveals a flattening of the phase and group slowness curves with increasing frequencies, horizontal offset and degrees of erosion.These results imply that a detection of washouts in-mine will be possible in a frequency range including at least the first mode Airy-phase. An interference pattern and a flattening of the dispersion curve indicate a washout rather than other seam obstructions and leads to an estimate of the washout dimension.

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

“…Lower frequency signals are transmitted either into the neighbouring rock or into the seam part beyond the disturbance. For instance, Korn & Stöckl (1982) and Breitzke & Dresen (1986) found analogue results for Love‐type seam waves in seam interruption, seam end and seam thinning models. This property has to be considered when planning in‐seam measurements and choosing an adequate source to generate seismic data.…”

“…To compare the seam end model to the thinning models we define a frequency dependent reflectivity by the ratio of the amplitude spectrum of the reflected Rayleigh seam wave calculated at a definite distance to the amplitude spectrum of the incident Rayleigh seam wave calculated at the same distance in the undisturbed seam model (e.g. Breitzke & Dresen 1986; Kerner & Dresen 1985). Reflectivity curves are averaged over 26 seismograms calculated at source–receiver distance of 10–22.5 times the seam thickness (Fig.…”

“…For the different model types amplitudes and frequencies of the reflected waves depend on the properties of the disturbance as for example the fault throw and the degree of thinning or washout. Also other parameters, for example, the dip angle of seam endings and faults can affect the wave propagation (Freystätter & Dresen 1978; Breitzke & Dresen 1986). In some cases, mode conversion can occur.…”

“…Franssens et al (1985) studied leaking channel modes and analysed theoretical synthetic seismograms for three‐layer models. Kerner & Dresen (1985) and Breitzke & Dresen (1986) presented studies on the propagation of Love‐type seam waves using a FD method. They also investigated 2‐D heterogeneous geological structures like seam ends, faults and washouts (seam interruptions and seam thinning).…”

Parameter Estimation and Fault Detection by Three‐component Seismic and Geoelectrical Surveys in a Coal Mine*

“…Between about 115 m and 165 m on the x-and about 80 m-165 m on the z-component its amplitudes disappear almost completely. As shown in a former theoretical study on the propagation of Love channel waves in coal seams, including washout zones (Breitzke and Dresen 1986), this amplitude decay may be related to an inhomogeneity in the coal seam. The amplitudes of the normal mode wavegroup diminish significantly, if the detector is located in the inhomogeneous part of the coal seam.…”