Geology Reference
In-Depth Information
arrivals. To aid recognition of weak coherent phases,
the individual seismograms are compiled into an overall
record section on which the various seismic phases can
be correlated from seismogram to seismogram. The
optimal type of display is achieved using a
reduced time
scale in which any event at time
t
and offset distance
x
is plotted at the reduced time
T
where
A
v
w
B
v
1
Tt
=-
xv
R
and
v
R
is a scaling factor known as the
reduction velocity
.
Thus, for example, a seismic arrival from deep in the
Earth's crust with an overall travel time of 30 s to an offset
distance of 150 km would, with a reduction velocity of 6
km s
-1
, have a reduced time of 5 s.
Plotting in reduced time has the effect of pro-
gressively reducing travel-time as a function of offset
and, therefore, rotating the associated time-distance
curves towards the horizontal. For example, a time-
distance curve with a reciprocal slope of 6 km s
-1
on a
t-x
graph would plot as a horizontal line on a
T-x
graph
using a reduction velocity of 6 km s
-1
. By appropriate
choice of reduction velocity, seismic arrivals from a par-
ticular refractor of interest can be arranged to plot about
a horizontal datum, so that relief on the refractor will
show up directly as departures of the arrivals from a hor-
izontal line.The use of reduced time also enables the dis-
play of complete seismograms with an expanded time
scale appropriate for the analysis of later arriving phases.
An example of a record section from a crustal seismic ex-
periment, plotted in reduced time, is illustrated in Fig.
5.20.
C
D
Fig. 5.19
The principle of the weathering correction in
refraction seismology.
common datum plane. The elevation correction
t
e
for
rays critically refracted in the
n
th layer is given by
{
}
)
12
)
(
(
t
=-
h
+
h
v
2
-
v
2
v v
e
s
d
n
1
1
n
where
h
s
and
h
d
are the heights above datum of the shot
point and detector location respectively. It is worth not-
ing that these corrections are more complex than those
used for seismic reflection surveys. The difference arises
since the assumption of vertical ray paths through the
weathered layer used in the reflection case cannot be
maintained.
In shallow water marine refraction surveying the
water layer is conventionally treated as a weathered
layer and a correction applied to replace the water layer
by material of velocity equal to the velocity of the sea
bed.
5.9 Other methods of refraction surveying
Although the vast bulk of refraction surveying is carried
out along profile lines, other spatial arrangements of
shots and detectors may be used for particular purposes.
Such arrangements include
fan-shooting
and irregularly
distributed shots and recorders as used in the
time term
method.
Fan-shooting
(Fig. 5.21) is a convenient method of ac-
curately delineating a subsurface zone of anomalous ve-
locity whose approximate position and size are already
known. Detectors are distributed around a segment of
arc approximately centred on one or more shot points,
and travel-times of refracted rays are measured to each
detector. Through a homogeneous medium the travel-
times to detectors would be linearly related to range, but
5.8.4 Display of refraction seismograms
In small-scale refraction surveys the individual seismo-
grams are conventionally plotted out in their true time
relationships in a format similar to that employed to dis-
play seismic traces from land reflection spreads (see Fig.
4.8). From such displays, arrival times of refracted waves
may be picked and, after suitable correction, used to
make the time-distance plots that form the basis of
refraction interpretation.
Interpretation of large-scale refraction surveys is often
as much concerned with later arriving phases, such as
wide-angle reflections or S-wave arrivals, as with first
