Geology Reference
In-Depth Information
(b)
(a)
t
t
x
x
(c)
t
x
Fig. 5.8
Various types of profile geometry used in refraction surveying. (a) Conventional reversed profile with end shots. (b) Split-profile
with central shot. (c) Single-ended profile with repeated shots.
that for a conventional reversed profile, it is based on the
same general travel-time equation (5.8).
The single-ended profile method (Fig. 5.8(c)) was
developed to derive interpretations of low-velocity
surface layers represented by refracted arrivals in single-
ended reflection spread data, for use in the calculation
of static corrections. A simplified treatment is given
below.
To obtain a value of refractor dip, estimates of apparent
velocity are required in both the forward and reverse
directions.The repeated forward shooting of the single-
ended profile method enables an apparent velocity in the
forward direction to be computed from the gradient of
the travel-time curves. For the method of computing the
apparent velocity in the reverse direction, consider two
refracted ray paths from surface sources S
1
and S
2
to sur-
face detectors D
1
and D
2
, respectively (Fig. 5.9).The off-
set distance is
x
in both cases, the separation
D
x
of S
1
and
S
2
being the same as that of D
1
and D
2
.
Since D
1
is on the downdip side of S
1
, the travel time
of a refracted ray from S
1
to D
1
is given by equation
(5.9), and omitting subscripts to
q
and
g
in this two-layer
case,
5.3 Profile geometries for studying
planar layer problems
The conventional field geometry for a refraction profile
involves shooting at each end of the profile line and
recording seismic arrivals along the line from both shots.
As will be seen with reference to Fig. 5.5(a), only the
central portion of the refractor (from B to C ) is sampled
by refracted rays detected along the line length. Inter-
preted depths to the refractor under the endpoints of a
profile line, using equations given above, are thus not
directly measured but are inferred on the basis of the
refractor geometry over the shorter length of refractor
sampled (BC). Where continuous cover of refractor
geometry is required along a series of reversed profiles,
individual profile lines should be arranged to overlap in
order that all parts of the refractor are directly sampled
by critically refracted rays.
In addition to the conventional reversed profile, illus-
trated schematically in Fig. 5.8(a), other methods of
deriving full planar layer interpretations in the presence
of dip include the
split-profile
method ( Johnson 1976)
and the
single-ended profile
method (Cunningham 1974).
The split-profile method (Fig. 5.8(b)) involves recording
outwards in both directions from a central shot point.Al-
though the interpretation method differs in detail from
x
sin
+
2
z
cos
(
qg
)
+
q
1
t
=
(5.13)
1
v
v
1
1
