Subsurface void detection using seismic tomographic imaging Page: 2 of 9
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between the wells, tomographic methods yield reliable estimates about the physical parameters of
In the case of underground coal mines, access is given by panels and shafts, which allow sources
and receivers to be placed along the face of coal seams to image the properties of the coal in a
horizontal plane. Such a transmission survey will yield good estimates of the coal properties
between the source and receiver locations. If underground access is not available, an array of
boreholes that penetrate the coal seam from the surface can be set up. In this case the boreholes can
be used as source and receiver holes (sources and receivers would be fired at mid-seam depth).
However, this type of survey, which is often referred to as a cross-hole seismic survey, is limited to
transmission experiments, as the density of boreholes needed for sufficient tomographic ray
coverage is prohibitively expensive. A cross-hole seismic survey would yield information about the
existence of a disturbance between sources and receivers, but not about its location or nature.
Results, similar to those obtained for underground transmission surveys, can be produced from the
surface when horizontal boreholes are drilled within the coal seam.
During the past 30 years in-seam seismic methods have been applied in coal mines around the
world to find disturbances in the coal seams, which pose a risk to coal mining activities (Buchanan
et al., 1981; Mason, 1981; Dresen and Rueter, 1994). Disturbances may vary from faults that offset
coal seams and reduce production rates of longwall mining operations, to abandoned mine
workings that may be water-filled and pose a general risk to underground activities, to gas-filled
mylonized zones which may trigger explosions upon sudden stress release due to mining. In-seam
seismic techniques include transmission surveys (including tomographic methods) and reflection
surveys. In the current paper the application of tomographic, in-seam seismic, and scattering
techniques, will be discussed and evaluated with respect to subsurface void detection.
Principles of Seismic Wave Propagation in Coal
A coal seam embedded in bedrock constitutes a low velocity channel for elastic waves propagation.
If a source (e.g. an explosive charge) is fired in the middle of the coal seam, elastic waves
propagate from the source in all directions throughout the coal. Upon encountering the coal rock
interface along the roof and floor of the coal seam part of the energy is refracted along this interface
as compressional- (P) and shear waves (S) propagating with their respective velocities of the
bedrock medium. Because these waves refract along the interface they are not directly affected by
voids in the coal. However they are affected by secondary effects caused by the presence of a
disturbance in the coal seam (see below). In addition to being refracted along the interface upon
encountering the coal boundary, the waves are also reflected back into the seam under various
angles with different phase velocities and form a system of constructive interference. The created
interference system is a channel (or seam) wave propagating in two dimensions within the coal
seam without radiating energy into the surrounding bedrock. Because the propagation is only two
dimensional, long transmission distances of up to 2 km have been reported for channel waves in
coal seams in the past (Greenhalgh et al., 1986). There are two types of seam waves: Rayleigh
waves, which are comprised of body waves of the P and SV type that have a vertical elliptic
particle motion, and Love waves, comprised of SH waves only, which reveal a horizontal particle
motion. Both types are commonly excited and interpreted in in-seam seismic surveys. The
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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/m1/2/: accessed May 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.