Geology Reference
In-Depth Information
SIDEBAR 5.1 CASE HISTORY: COAL VOID SCHEMATIC
Seismic reflection methods have been used successfully in a few
cases to detect old coal mine workings. This diagram depicts schematic
seismic-reflection data superimposed on a hypothetical geological cross-
section containing a coal mine void. A coal seam usually produces a
strong seismic reflection from its top and bottom interfaces with
surrounding rock, such as shale. The reflections from the top and bottom
of the coal are represented by waves with blackened peaks that can be
followed by eye in a coherent fashion from one seismic trace to another.
While the coal represents a strong seismic reflector, the absence of coal
(i.e. the mine void) results in the absence of the strong reflection as
illustrated by the two seismic traces that pass through the void without
producing a reflection.
The seismic data are processed and displayed such that each
seismic trace represents seismic-wave motion as a function of time, as if
the seismic source (such as a small explosion) and the seismic receiver (a
geophone) were located at the same point on the Earth's surface. Hence,
the reflections with blackened peaks occur at a time on the seismogram
that represents travel from the Earth's surface downward to the coal seam
and then back to the Earth's surface.
Schematic seismic-reflection response to a coal-mine void.
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