Subsurface void detection using seismic tomographic imaging Page: 3 of 9
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interference of phases created under various angles of reflection leads to dispersion of the channel
waves. Dispersion occurs when the propagation velocity of a wave in a medium is a function of
frequency, which means that the different frequencies of the channel waves propagate at different
speeds. As a result, the longer the travel distance of the wave the more its phases get separated and
the wave train is recorded as an elongated arrival on the seismogram (Gritto and Dresen, 1992).
Both wave types, the refracted interface waves and the channel waves, can be used for tomographic
imaging of coal properties. Although the use of channel waves in tomographic applications to
image coal properties is more reliable than that of refracted waves, the former is less commonly
used, because the processing of the data has to account for the dispersion of the waves. In the
following two examples of P- and channel wave tomography in low velocity channels will be
Tomographic Imaging of Subsidence in Old-Mine Workings
The current example is reported by Mason (1981), where a tomographic survey was performed in
the High Hazles seam of Thorseby colliery near Nottingham, England. The objective was to
delineate old abandoned mine workings
Re onsina Velocity 1nnonlog.neity 299
sub arroy sub array below a currently mined coal seam. Old mine
B_ _C - workings can cause problems beyond the
1 23 6 55 e 1f 2 ont
threat of water flooding of active mine areas,
by increasing the stress in the panels above or
1.25m below the old workings. Zones of increased
stress can interfere with long-wall mining
activities or produce outburst of rock mass
a " " " from the coalface.
950m The imaged area included a 950 x 425 m
block of coal, shown in Figure 1, where
Figure 1: Plan view of the surveyed panel. Shot point and access to three sides was provided b tunnels
receiver locations are indicated by black dots and ellipses,
respectively. (Mason, 1981). in the mine. An array of shot locations and
two sub-arrays of receivers were set up along
opposing panels, as shown in Figure 1. Shots consisted of 227g charges located in 1-m long
horizontal boreholes separated by 20 m along the tunnel wall, while two groups of geophones
where positioned along the opposite tunnel at
30 m spacing. The resulting ray coverage is B--- C
shown in Figure 2. First arrival times were
determined from refracted P-waves
propagating along the interface between the
coal and bedrock, and an algebraic
reconstruction technique was used to estimate
the velocity distribution in the coal. The
result of the inversion process is given in Al D
Figure 3, where the velocity field is given by Figure 2: Straight raypaths linking shots to geophones
contour lines. Estimates of the highest and showing the variation in ray density underlying the final
velocity map. (Mason, 1981).
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Gritto, Roland. Subsurface void detection using seismic tomographic imaging, article, June 26, 2003; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc738282/m1/3/: accessed September 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.