Geoscience Reference
In-Depth Information
The density and velocity of coal is signi
cantly less than
those of the clastic and carbonate lithotypes with which it
is associated (
Fig. 6.35
). This means that large acoustic
impedance contrasts and re
ection coef
cients can be
expected at both the top and base of a coal seam. The
re
ection coef
cient at the top will be negative and that
at the base will be positive. Coal seams as thin as 1/50 of
the dominant wavelength have been reported to produce
recognisable reflections as a result of the very large acoustic
impedance contrast (up to ~0.5).
Figure 6.45
shows a density log through part of the
Carbondale Formation incorporating the S5, I6 and Rider
seams. As expected, the coal seams have very low density
relative to the rest of the sequence. A reflectivity series has
been computed based on this log, the lack of a sonic log
being overcome by assuming a simple linear relationship
between density and velocity (which is not ideal, but
should allow the major velocity contrasts to be identi
between non-zero re
ection coef
cients are smaller than
the duration of the wavelet, so all re
ections are necessarily
composite
responses
to multiple
acoustic-impedance
contrasts.
Figure 6.46a
shows a time-migrated normal-incidence
seismic section from the study area. The section extends to
a depth of about 400 m, which is comparatively shallow, but
the requirement to image the seams in detail necessitated
using a source with a central frequency of about 150 Hz. The
source frequency spectrum extended from about 60 to
250 Hz. The general character of the seismic section is
typical of a sedimentary terrain having little deformation.
Distinct sub-horizontal reflections with good lateral con-
tinuity are prominent across the section. The reflection at
0.1 s TWT is from a major shale
-
limestone interface.
ed geological model of the
area interpolated from drillhole intersections. This has
been used, along with density data, to compute the syn-
thetic seismic data shown in
Fig. 6.46b
. A 150 Hz Ricker
wavelet was used as the source. The change from a simple
layered form at the western end of the section to a more
complex situation further east is of particular importance
from a mining perspective. The change is due to a
Figure 6.46c
shows a simpli
ed).
The large re
ection coef
cients of opposite polarity at the
top and base of the coal seams are evident. A synthetic
seismogram has been computed using a 150 Hz Ricker
wavelet. The three seams (I6, Rider, S5) give rise to high-
amplitude negative de
ections in the trace. Note how the
maxima (the shaded peaks) coincide with the upper
boundary of their respective seam, where the
rst major
acoustic impedance contrast for the seam occurs. Note also
the width of the seismic wavelet compared with the separ-
ation of features in the reflectivity series. The spacings
'
between 120 and 150 m related to differential compaction
associated with the sandstone lenses between 125 and
140 m. Across the roll, depth to the I6 seam decreases by
about 9 m; it is thinner in the centre of the roll and to the east
splits to form the Rider seam. Also, depth to the underlying
S5 seam increases significantly to about 18 m, but it retains a
near-constant thickness of 1.2 m. The
'
roll
represents a
significant impediment to mining, mostly owing to the
changes it causes in the roof and roof conditions.
The synthetic and observed data show good agreement,
with each coal seam associated with a distinct negative
feature on the traces. The thickness of the seams, and the
surrounding units, varies between 1/5 and 1/15 of the
dominant seismic wavelength and is insuf
cient to pro-
duce separate re
ections from their top and bottom sur-
faces. Instead, changes in the geology cause changes to the
interference between re
ections from the top and base of
the seams and produce subtle variations in the wavelength
and amplitude of
'
roll
'
Synthetic
seismogram
Density
(g/cm
3
)
Reflection
coefficient
Two-way
time (s)
1.0
3.0
-0.5
0.5
-
+
0.130
I6
I6
0.140
Rider
Rider
S5
0.150
S5
Coal
Shale
from the seams. The laterally
changing response from the S5 seam is due to changes in
the geology above and below it. The response from the I6
seam gets broader towards the east and, eventually, forms a
doublet (a pair of peaks or troughs in the trace) created by
the response of the Rider seam interacting with the
responses of the various thin beds in the vicinity. Although
'
re
ections
'
Source
wavelet
Sandstone
Limestone
Sandy shale
Figure 6.45
Lithology and density log from the southern Illinois coal
basin and the computed re
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