Geology Reference
In-Depth Information
NW
SE
S1-Section
1000ft
sec.
WELL D
S1
S2
1.5
Line 1
Line 1
1.5
Austin
chalk
1.5
2.5
T/GR
Z
2.0
3.5
TN1
2.0
M.B.
S2-Section
sec.
1000ft
1.5
2.5
2.5
Austin
chalk
Figure 7.37
Travel time differences of fast (S1) shear wave (left
hand section) and slow (S2) shear wave (right hand section) are
evidence for the presence of fractures in the Green River Formation,
N E Utah (after Lynn et al.,
1995
). Note the top of the fractured unit is
close to the Z marker on the sections.
3.5
Figure 7.38
Fast (S1) and slow (S2) shear wave sections in the
Austin Chalk (after Mueller,
1992
). Note the zone of decreased
amplitude on the slow shear wave section associated with the
presence of fractures.
birefringence (
Chapter 5
). In the presence of frac-
tures, shear waves are polarised in two directions,
parallel to the fractures (S1 or fast) and perpendicular
to the fractures (S2 or slow) (
Chapter 5
).
Figure 7.37
shows an example of the differential timing of fast
and slow shear wave sections derived from multi-
component seismic data through a well which inter-
sects fractures in the Green River formation of NE
Utah. Fast (S1) shear waves carry information about
the rock matrix, whereas slow shear waves carry
information on relative fracture density and other
fracture parameters (Lynn et al.,
1995
). Thus, the
amplitudes of fast shear waves at the top of the frac-
tured unit tend to be insensitive to the presence of
fractures whereas the slow shear wave will show amp-
litude dimming. An example from the Austin Chalk is
shown in
Fig. 7.38
.
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