Abstract

It is of economic importance to locate zones of geological disturbance in longwall coal panels prior to the start of mining operations. This can be done with electromagnetic (EM) longwall attenuation tomography in which the amplitude of a single frequency wave in the 300 kHz range is measured over a number of raypaths. The main component of the signal is a fundamental waveguide mode. Zones with high attenuation rates have been observed to be zones of geological disturbance.A two-dimensional TM (magnetic field transverse to the plane of propagation) mode, time domain finite difference solution was developed and used to determine the excess attenuation (EA) that occurs in disturbed zones. EA is defined as the actual attenuation going through a disturbed zone less the attenuation for an equal length of the normal, undisturbed waveguide. A typical model has a coal seam bordered above and below by rock with a disturbed zone; various types of disturbances such as sand channels and faults were modeled. Synthetic magnetograms are computed at points along the seam and are Fourier transformed to get spectral amplitudes.Model results show that high attenuation can occur in disturbed zones. However the EA that occurs going through a zone is highly dependent on the properties of the zone and waveguide. Models with high conductivity (.05 S/m) roof and floor rock gave larger EA than low conductivity models (.001 S/m). However, low conductivity roof and floor models can still give 5 to 10 db of attenuation, which is consistent with observed values. With high conductivity, simple roof and floor faults can cause significant attenuation.

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