Geology Reference
In-Depth Information
x
x
Mid-point
S (Source)
Detector G
y
y
Velocity =
V
d
A
α
P
d
S
′
(Source image)
′
′
2
= AG
2
+ AS
2
SG
y
′
2
= 4
d
2
+ 4(
x
.cos
α
)
2
SG
y
G
′
(Detector image)
Figure 12.7
Effect of dip in common mid-point (CMP) shooting. In contrast
to single-fold shooting (see Figure 12.3), the shot-points as well as the
geophone locations are different for the different traces. Shot-points and
detector locations are interchangeable and the 'depth point', P, on the
reflector moves updip as the offset increases. The moveout equation is most
easily derived by noting that the path from source to detector is equal in
length to the path SG
from the source to the detector 'image point', and that
the geometric relationships between similar triangles imply the equality of
all the lengths marked 'y'. Thus AG
=
2
d
. The Pythagoras relationship can
be applied to the triangle SG
A to obtain SG
, and the times can be obtained
by dividing the distances by
V
. Thus,
T
0
=
2
d
/
V
and T
=
SG
/
V
.
12.2.5 Depth conversion
Reflection events are recorded not in depth but in
two-way time
(TWT).
Velocities are needed to convert times into depths, but the Dix velocities
obtained from NMO curves (see Section 12.1.3) may be 10-20% in error,
even for horizontal reflectors, and by still more where dip is involved.
Interpretations should be calibrated against borehole data wherever possible,
