Geology Reference
In-Depth Information
Fig. 5.18
A possible observational scheme to
obtain shallow and deeper refraction coverage
along a survey line.The inclined lines indicate the
range of coverage from the individual shots
shown.
Fig. 5.18. Such a scheme might include off-end shots
into individual reversed profile lines, since off-end shots
extend the length of refractor traversed by recorded head
waves and provide insight into the structural causes of
any observed complexities in the travel-time curves.
Selection of detector spacing along the individual profile
lines is determined by the required detail of the refractor
geometry, the sampling interval of interpretation points
on the refractor being approximately equal to the de-
tector spacing. Thus, the horizontal resolution of the
method is equivalent to the detector spacing.
It is often the case that there are insufficient detectors
available to cover the full length of the profile with the
desired detector spacing. In this case the procedure is to
deploy the detectors to cover one segment of the line at
the required spacing, then to fire shots at all shot points.
The detectors are then moved to another segment of the
line and all shot points fired again.The process can be re-
peated until full data are compiled for the complete pro-
file. At the price of repeating the shots, a profile can thus
be recorded of any length with a limited supply of equip-
ment.The same principle is equally applicable to shallow
penetration, to detailed refraction surveys for engineer-
ing, to environmental and hydrological applications, and
to crustal studies.
corrected for the variable delay introduced by the layer.
This weathering correction is directly analogous to that
applied in reflection seismology (see Section 4.6). The
weathering correction is particularly important in shal-
low refraction surveying where the size of the correction
is often a substantial percentage of the overall travel time
of a refracted ray. In such cases, failure to apply an accu-
rate weathering correction can lead to major error in
interpreted depths to shallow refractors.
A weathering correction is applied by effectively re-
placing the weathered layer of velocity
v
w
with material
of velocity
v
1
equal to the velocity of the underlying
layer. For a ray critically refracted along the top of the
layer immediately underlying the weathered layer, the
weathering correction is simply the sum of the delay
times at the shot and detector ends of the ray path.Appli-
cation of this correction replaces the refracted ray path by
a direct path from shot to detector in a layer of velocity
v
1
.
For rays from a deeper refractor a different correction is
required. Referring to Fig. 5.19, this correction effec-
tively replaces ray path ABCD by ray path AD. For a ray
critically refracted in the
n
th layer the weathering cor-
rection
t
w
is given by
t
=-
(
z
+
z
)
w
s
d
{
}
12
12
)
)
¥-
(
vv
2
2
vv vv
--
(
2
2
vv
n
1
1
n
n
w
w
n
5.8.3 Weathering and elevation corrections
The type of observational scheme illustrated in Fig. 5.18
is often implemented for the specific purpose of map-
ping the surface zone of weathering and associated low
velocity across the length of a longer profile designed to
investigate deeper structure. The velocity and thickness
of the weathered layer are highly variable laterally and
travel times of rays from underlying refractors need to be
where
z
s
and
z
d
are the thicknesses of the weathered layer
beneath the shot and detector respectively, and
v
n
is the
velocity in the
n
th layer.
In addition to the weathering correction, a correction
is also needed to remove the effect of differences in ele-
vation of individual shots and detectors, and an elevation
correction is therefore applied to reduce travel times to a
