Geology Reference
In-Depth Information
Data Processing
Analysis of in-seam wave dispersion can help determine whether a
propagation path of a transmitted wave is disturbed by geologic or old mine
features. The seismogram is transformed into a velocity-frequency diagram,
where the dispersed channel waves appear as curved-amplitude plots. Because
the seismic velocities and densities of both coal and bedrock can be measured
easily in coal mines, accurate theoretical dispersion curves can be calculated for
the rock-coal-rock geology and plotted for comparison with the seismic
velocity-frequency data (Raeder et al., 1985). If the theoretical dispersion curve
and field data velocity-frequency plots match closely, an undisturbed
propagation path is indicated. However, if the channel waves are reflected from
an obstruction back to the panel containing the seismic source, the match may
be poor. If a disturbance is inferred, a seismic reflection survey conducted with
both the seismic source and geophones at the same seam face may help to
estimate the location of the disturbance. Because the seismic-wave velocity in
the coal is known, the two-way travel time of any waves reflected from the
disturbance back to the geophones can be transformed into distance to indicate
the location of the reflector.
NUCLEAR MAGNETIC RESONANCE
Nuclear magnetic resonance measurements were initially performed by
physicists investigating molecular-scale phenomena. A radio-frequency pulse
excites nuclei to a higher energy state and their return to the original state is
monitored, modeled as a sum of exponential decays, and recorded as two
relaxation-time constants, T 1 associated with the longitudinal component of the
magnetization, and T 2 with the transverse component. Nuclear magnetic
resonance can be used to study any nuclei that have an intrinsic magnetic
moment, such as hydrogen or carbon-13. (See McMurray's 1984 review of
nuclear magnetic resonance theory.)
Surface geophysical nuclear magnetic resonance was pioneered by a
Russian team (Semenov et al., 1988) who developed the “hydroscope”
consisting of a transmitter and receiver in which antennas approach 330 feet in
diameter (a lower limit on horizontal spatial resolution). The total volume of
water present as a function of depth is proportional to the number of hydrogen
nuclei in the sample which is proportional to the amplitude of the initial
magnetization. When the transmitter is turned off, the resulting relaxation time
contains information about the grain size of the water-saturated rock. If the rock
or soil contains water, the relaxation time is a
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