Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 403

Seam waves are a useful source of information about the
coal seam and its environment, in particular the nature of
the roof and roof and the seam thickness and continuity

a)

S N

320 340 360 380 400 SP

-
all factors of great importance for longwall mining. In-
seam seismic surveys have greater resolving power than
conventional surface surveys, which usually cannot detect
features of the size that seriously affect mining, for example
faults with throws of less than the seam thickness. Most in-
seam seismic surveys access the seam where it is exposed in
underground drives etc., although surveys from drillholes
intersecting the seam are also possible. There are two basic
types of in-seam survey: transmission surveys and reflec-
tion surveys ( Fig. 6.50a ), analogous to the two types of in-
mine radar survey described in online Appendix 5 .
Transmission surveys record seam waves that have
passed through an area of interest. Various characteristics
of the seam wave can be analysed in terms of geological
structure: for example, discontinuities such as faults and
dykes can be identi ed because they are highly attenuating.
Re ection surveys allow particular geological discontinu-
ities to be mapped in more detail. They are fundamentally
the same as surface re ection surveys, except that the
sources, detectors, waves and re ectors all lie in the plane
of the seam, with the discontinuity detected from the
re ection and diffraction of the seam waves. Processing
techniques can be applied to resolve re ections in the
dispersed waves, after which interpretation is essentially
the same as that for conventional surface surveys. In
Fig. 6.50b , note the complex waves caused by dispersion
of the seam waves and, for the transmission survey, note
that the slower seam-wave arrives after the faster direct P-
wave.
A concise summary of seam waves and their application
to coal mining is provided by Regueiros ( 1990a , 1990b ).

500

0.2

0.3

1000

0.4

1500

Two-way
time (s)

Approximate
depth (m)

0 500

Metres

b)

320 340 360 380 400 SP

500

0.2

0.3

1000

0.4

1500

Two-way
time (s)

Approximate
depth (m)

Sulphide Rhyolite Felsic tuff or dyke

Gabbro Basalt Mafic-intermediate dyke

Figure 6.49 Detail of Fig. 6.48 in the vicinity of the Bell Allard VMS
deposit. (a) Uninterpreted data, and (b) with simpli

ed geology
superimposed. Redrawn, with permission, from Calvert and Li ( 1999 ) .

means that very large acoustic impedance contrasts, and
therefore very large re ection coef cients, occur at the
margins of the seams. Consequently, seismic waves gener-
ated by a source located in a coal seam tend to travel within
the seam, repeatedly re ecting into the seam from its upper
and lower surfaces, i.e. the seam acts as a waveguide (see
Section 6.3.4.2 ). Seismic waves propagating within the
seam are known as channel waves or seam waves. They
are a form of surface waves and so they are dispersive, i.e.
different frequency components travel at different veloci-
ties. Dispersion causes the arrivals to have longer and more
complex waveforms than the more compact wavelets of a
conventional seismic recording.

6.8.2 Tomographic surveys

Seismic tomography (see Tomography in Section 2.11.2.1 )
applied in the mining industry usually involves cross-hole
surveys, where a series of sources in one drillhole is
recorded by detectors located in another (see Fig. 6.1 ) .
The intention is to map the geology in the region between
the drillholes. The target must have signi cantly different
seismic properties to its surrounds. The property measured
is usually seismic travel time, resulting in a velocity (or
slowness) tomogram. Normally, only first arrivals are used
in the analysis, but more sophisticated approaches are
currently being developed to obtain more information