Geology Reference
In-Depth Information
5
Seismic refraction surveying
gation. A consequence of this requirement is that large
seismic sources are needed for the detection of deep re-
fractors in order that sufficient energy is transmitted over
the long range necessary for the recording of deep re-
fracted phases as first arrivals.The profile length required
in any particular survey depends upon the distribution of
velocities with depth at that location.The requirement in
refraction surveying for an increase in profile length with
increase in the depth of investigation contrasts with the
situation in conventional reflection surveying, where
near-normal incidence reflections from deep interfaces
are recorded at small offset distances.
Refraction seismology is applied to a very wide range
of scientific and technical problems, from engineering
site investigation surveys to large-scale experiments
designed to study the structure of the entire crust or
lithosphere. Refraction measurements can provide valu-
able velocity information for use in the interpretation of
reflection surveys, and refracted arrivals recorded during
land reflection surveys are used to map the weathered
layer, as discussed in Chapter 4.This wide variety of ap-
plications leads to an equally wide variety of field survey
methods and associated interpretation techniques.
In many geological situations, subsurface refractors
may approximate planar surfaces over the linear extent of
a refraction line. In such cases the observed travel-time
plots are commonly assumed to be derived from a set of
planar layers and are analysed to determine depths to, and
dips of, individual planar refractors.The geometry of re-
fracted ray paths through planar layer models of the sub-
surface is considered first, after which consideration is
given to methods of dealing with refraction at irregular
(non-planar) interfaces.
5.1 Introduction
The seismic refraction surveying method uses seismic
energy that returns to the surface after travelling through
the ground along refracted ray paths.As briefly discussed
in Chapter 3, the first arrival of seismic energy at a detec-
tor offset from a seismic source always represents either a
direct ray or a refracted ray.This fact allows simple refrac-
tion surveys to be performed in which attention is con-
centrated solely on the first arrival (or
onset
) of seismic
energy, and time-distance plots of these first arrivals
are interpreted to derive information on the depth to
refracting interfaces. As is seen later in the chapter, this
simple approach does not always yield a full or accurate
picture of the subsurface. In such circumstances more
complex interpretations may be applied. The method is
normally used to locate refracting interfaces (refractors)
separating layers of different seismic velocity, but the
method is also applicable in cases where velocity varies
smoothly as a function of depth or laterally.
Refraction seismograms may also contain reflection
events as subsequent arrivals, though generally no special
attempt is made to enhance reflected arrivals in refrac-
tion surveys. Nevertheless, the relatively high reflection
coefficients associated with rays incident on an interface
at angles near to the critical angle often lead to strong
wide-angle reflections
which are quite commonly detected
at the greater recording ranges that characterize large-
scale refraction surveys. These wide-angle reflections
often provide valuable additional information on sub-
surface structure such as, for example, indicating the
presence of a low-velocity layer which would not be
revealed by refracted arrivals alone.
The vast majority of refraction surveying is carried out
along profile lines which are arranged to be sufficiently
long to ensure that refracted arrivals from target layers are
recorded as first arrivals for at least half the length of the
line. Refraction profiles typically need to be between
five and ten times as long as the required depth of investi-
5.2 Geometry of refracted ray paths:
planar interfaces
The general assumptions relating to the ray path
