Geology Reference
In-Depth Information
Figure 12.3
Effect of dip on a single-fold record. Rays are reflected from
the dipping interface as if derived from the image point S
at depth 2
d
.cos(
α
)
below the surface, where
d
is the perpendicular distance from the shot-point
to the interface. The normal incidence travel time is 2
d
/
V
but the shortest
travel time is for the ray that is vertical after reflection. An identical moveout
hyperbola would be produced by a shot at point G and a horizontal interface
at depth
d
.cos(
α
).
may well be less than the errors caused by using the NMO equation in the
first place. Dix conversion may not help very much in these cases.
12.1.4 Effect of dip
If the source is placed at the centre of the geophone spread, the curves
obtained over horizontal interfaces will be symmetrical about the source
point. If, however, the reflector has a uniform dip,
, the reduction in travel
path on the updip side of the shot compensates to some extent for the offset,
and some travel times will be less than the normal-incidence time (Figure
12.3). The minimum time 2
d
.cos(
α
)
/
V
is recorded at a distance 2
d
.sin(
α
)
from the shot, on the updip side. The reflected ray rises vertically to this
point, about which the moveout curve is symmetrical. Dip effects in shallow
reflection surveys are detectable only for very large dips or very long spreads.
α
12.1.5 Multiple reflections
A wave reflected upwards with high amplitude from a subsurface interface
can be reflected down again from the ground surface and then back from the
same interface. This is a simple
multiple
. Two strong reflectors can generate
peg-leg
and
intraformational
multiples (Figure 12.4).
Multiples are difficult to identify with certainty on single traces. They
can sometimes be recognised on multi-channel records because they have
