Geology Reference
In-Depth Information
(a)
(b)
t
1
v
2
1
v
1
x
crit
x
cros
x
(c)
Fig. 3.13
(a) Seismogram showing the
output traces of 24 geophones distributed
along the Earth's surface as a function of
time. (b) Travel-time curves for direct,
reflected and refracted rays in the case of a
simple two-layer model. (c) Direct,
reflected and refracted ray paths from a
near surface source to a surface detector
in the case of a simple two-layer model.
D
S
X
z
v
1
v
2
Diffracted phases are commonly observed in seismic
recordings and are sometimes difficult to discriminate
from reflected and refracted phases, as discussed in
Chapter 4.
depth
z
, the compressional wave velocity being higher in
the underlying layer (i.e.
v
2
>
v
1
).
From a near-surface seismic source S there are three
types of ray path by which energy reaches the surface at a
distance from the source, where it may be recorded by a
suitable detector as at D, a horizontal distance
x
from S.
The
direct ray
travels along a straight line through the top
layer from source to detector at velocity
v
1
. The
reflected
ray
is obliquely incident on the interface and is reflected
back through the top layer to the detector, travelling
along its entire path at the top layer velocity
v
1
. The
3.7 Reflection and refraction surveying
Consider the simple geological section shown in Fig.
3.13 involving two homogeneous layers of seismic ve-
locities
v
1
and
v
2
separated by a horizontal interface at a
