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the arrivals. If the difference is less than this, a single
arrival, albeit with an increased period/wavelength,
a)
is
Pretoria
Gp
1
6.7.1.1
Horizontal resolution
arrival can be constructed from the Fresnel volumes of
two direct raypaths located to account for the incident
Since these are oblique to the re
ector, their intersection
with the re
ector is elliptical with its long axis in the plane
containing the source and detector. For the case of zero-
offset (normal incidence) data the intersection is circular
and is a Fresnel zone. Recall the representation of a reflect-
ing interface in zero-offset data as a series of adjacent
diffractors (see Post-stack time migration in
Section
Chun'
Gp
BR U/C
2
Vent'
Super
Gp
VCR U/C
2
C'Rand
Gp
-
detector location comprises all the
that return
seismic waves whose travel times differ from the normal-
incidence travel time by less than half the dominant period
of the seismic signal.
A clear description of calculating Fresnel-zone dimen-
geometry in
Fig. 6.42b
, the normal-incidence raypath rep-
resents the shortest path with which the longer lengths of
the oblique paths are compared. The diameter of the
normal-incidence Fresnel zone (D
Fresnel
) and its area
(A
Fresnel
) at a speci
c wavelength (
'
diffractors
'
W'Rand
Gp
0
1
Kilometres
b)
λ
) for a planar re
ector
at depth (Z) are given by:
q
ð
2
Z
2
D
Fresnel
¼
2
Z
+
λ=
4
Þ
ð
6
:
22
Þ
DF
2
Z
2
A
Fresnel
¼ π½ð
Z
+
λ=
4
Þ
ð
6
:
23
Þ
S4F
D
Fresnel
is simply twice the length of one side of a right-
angled triangle formed by the normal-incidence and
oblique raypaths. Note that the term
S2F
/4 is used because
the seismic waves travels the path twice, i.e. down from
λ
banded iron formation and lacustrine deposits. The Ventersdorp
Supergroup (Vent
WRF
Group) comprises bimodal volcanics
including tuffs and lavas. The Central Rand Group (C
'
S
'
Rand Gp)
comprises quartz arenite and quartz wacke, conglomerate and
shale. The West Rand Group (W
'
0
2
Kilometres
Rand Gp) comprises
predominantly shale and arenite. BLR
'
Figure 6.40
Seismic re
ection sections data from the Witwatersrand
Basin. (a) Correlation between seismic data and stratigraphy, and (b)
examples of faults. The Pretoria Group comprises mudstone-
sandstone, andesitic lavas, conglomerates, diamicite and carbonate
units. The Chunleepoort Group (Chun
Black Reef Formation, the
basal unit in the Chunleepoort Group, DF
-
-
Danies Fault, S2F
-
Shaft 2 Fault, S4F
-
Shaft 4 Fault, VCR
-
Ventersdorp Contact Reef,
-
WRF
'
Gp) comprises dolomite,
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