Geology Reference
In-Depth Information
refracted ray
travels obliquely down to the interface at ve-
locity
v
1
, along a segment of the interface at the higher
velocity
v
2
, and back up through the upper layer at
v
1
.
The travel time of a direct ray is given simply by
fracted rays travel down to the interface at the critical
angle there is a certain distance, known as the
critical dis-
tance x
crit
, within which refracted energy will not be re-
turned to the surface. At the critical distance, the travel
times of reflected rays and refracted rays coincide because
they follow effectively the same path. Reflected rays are
never first arrivals; they are always preceded by direct rays
and, beyond the critical distance, by refracted rays also.
The above characteristics of the travel-time curves
determine the methodology of refraction and reflection
surveying. In refraction surveying, recording ranges are
chosen to be sufficiently large to ensure that the cross-
over distance is well exceeded in order that refracted rays
may be detected as first arrivals of seismic energy.
Indeed, some types of refraction survey consider only
these first arrivals, which can be detected with unsophis-
ticated field recording systems. In general, this approach
means that the deeper a refractor, the greater is the range
over which recordings of refracted arrivals need to be
taken.
In reflection surveying, by contrast, reflected phases
are sought that are never first arrivals and are normally of
very low amplitude because geological reflectors tend to
have small reflection coefficients. Consequently, reflec-
tions are normally concealed in seismic records by
higher amplitude events such as direct or refracted body
waves, and surface waves.
Reflection surveying methods therefore have to be
capable of discriminating between reflected energy and
many types of synchronous noise. Recordings are nor-
mally restricted to small offset distances, well within the
critical distance for the reflecting interfaces of main
interest. However, in multichannel reflection surveying
recordings are conventionally taken over a significant
range of offset distances, for reasons that are discussed
fully in Chapter 4.
t
=
x v
dir
1
which defines a straight line of slope l/
v
1
passing through
the time-distance origin.
The travel time of a reflected ray is given by
12
(
2
2
)
x
+
4
z
t
refl
=
v
1
which, as discussed in Chapter 4, is the equation of an
hyperbola.
The travel time of a refracted ray (for derivation see
Chapter 5) is given by
2 cos
q
z
x
v
c
t
=+
2
refr
v
1
which is the equation of a straight line having a slope of
l/
v
2
and an intercept on the time axis of
2
z
cos
q
c
v
1
Travel-time curves, or time-distance curves, for
direct, refracted and reflected rays are illustrated in Fig.
3.13. By suitable analysis of the travel-time curve for
reflected or refracted rays it is possible to compute the
depth to the underlying layer. This provides two inde-
pendent seismic surveying methods for locating and
mapping subsurface interfaces,
reflection surveying
and
refraction surveying
. These have their own distinctive
methodologies and fields of application and they are dis-
cussed separately in detail in Chapters 4 and 5. However,
some general remarks about the two methods may be
made here with reference to the travel-time curves and
seismogram of Fig. 3.13. The curves are more compli-
cated in the case of a multilayered model, but the follow-
ing remarks still apply.
The first arrival of seismic energy at a surface detector
offset from a surface source is always a direct ray or a re-
fracted ray. The direct ray is overtaken by a refracted ray
at the
crossover distance x
cros
. Beyond this offset distance
the first arrival is always a refracted ray. Since critically re-
3.8 Seismic data acquisition systems
The fundamental purpose of seismic surveys is accur-
ately to record the ground motion caused by a known
source in a known location. The record of ground
motion with time constitutes a
seismogram
and is the
basic information used for interpretation through either
modelling or imaging (see Chapter 2). The essential
instrumental requirements are to
• generate a seismic pulse with a suitable
source
• detect the seismic waves in the ground with a suitable
transducer
