Geology Reference
In-Depth Information
profile line increase as the required line length increases.
Further, the problems of surveying on land are quite dif-
ferent from those encountered at sea. A consequence of
these logistic differences is a very wide variety of survey
arrangements for the implementation of refraction
profile lines and these differences are illustrated by three
examples.
For a small-scale refraction survey of a construction
site to locate the water table or rockhead (both of which
surfaces are generally good refractors), recordings out to
an offset distance of about 100 m normally suffice. Geo-
phones are connected via a multicore cable to a portable
24- or 48-channel seismic recorder. A simple weight-
dropping device (even a sledge hammer impacted on to a
steel base plate) provides sufficient energy to traverse the
short recording range. The dominant frequency of such
a source exceeds 100 Hz and the required accuracy of
seismic travel times is about 0.5 ms. Such a survey can be
easily accomplished by two operators.
The logistic difficulties associated with the cable con-
nection between a detector spread and a recording unit
normally limit conventional refraction surveys to maxi-
mum shot-detector offsets of about 1 km and, hence, to
depths of investigation of a few hundred metres. For
larger scale refraction surveys it is necessary to dispense
with a cable connection. At sea, such surveys can be
carried out by a single vessel in conjunction with
free-floating radio-transmitting sonobuoys (Fig. 5.16).
Having deployed the sonobuoys, the vessel proceeds
along the profile line repeatedly firing explosive charges
or an air-gun array. Seismic signals travelling back to
the surface through the water layer are detected by a
hydrophone suspended beneath each sonobuoy, ampli-
fied and transmitted back to the survey vessel where
they are recorded along with the shot instant. By this
means, refraction lines up to a few tens of kilometres may
be implemented.
For large-scale marine surveys, ocean bottom seis-
mographs (OBSs) are deployed on the sea bed. These
contain a digital recorder together with a high-precision
clock unit to provide an accurate time base for the seis-
mic recordings. Such instruments may be deployed
for periods of up to a few days at a time. For the pur-
poses of recovery, the OBSs are 'popped-up' to the
surface by remotely triggering a release mechanism. Sea-
bed recording systems provide a better signal-to-noise
ratio than hydrophones suspended in the water column
and, in deep water, recording on the sea bed allows much
better definition of shallow structures. In this type of
survey the dominant frequency is typically in the range
5.7 Refraction in layers of continuous
velocity change
In some geological situations, velocity varies gradually as
a function of depth rather than discontinuously at dis-
crete interfaces of lithological change. In thick clastic se-
quences, for example, especially clay sequences, velocity
increases downwards due to the progressive compaction
effects associated with increasing depth of burial. A seis-
mic ray propagating through a layer of gradual velocity
change is continuously refracted to follow a curved ray
path. For example, in the special case where velocity in-
creases linearly with depth, the seismic ray paths describe
arcs of circles.The deepest point reached by a ray travel-
ling on a curved path is known as its turning point .
In such cases of continuous velocity change with
depth, the travel-time plot for refracted rays that return
to the surface along curved ray paths is itself curved, and
the geometrical form of the curve may be analysed to
derive information on the distribution of velocity as a
function of depth (see e.g. Dobrin & Savit 1988).
Velocity increase with depth may be significant in
thick surface layers of clay due to progressive compaction
and dewatering, but may also be significant in deeply
buried layers. Refracted arrivals from such buried layers
are not true head waves since the associated rays do not
travel along the top surface of the layer but along a curved
path in the layer with a turning point at some depth
below the interface. Such refracted waves are referred to
as diving waves (Cerveny & Ravindra 1971). Methods of
interpreting refraction data in terms of diving waves are
generally complex, but include ray-tracing techniques.
Indeed, some ray-tracing programmes require velocity
gradients to be introduced into all layers of an interpreta-
tion model in order to generate diving waves rather than
true head waves.
5.8 Methodology of refraction profiling
Many of the basic principles of refraction surveying have
been covered in the preceding sections but in this section
several aspects of the design of refraction profile lines are
brought together in relation to the particular objectives
of a refraction survey.
5.8.1 Field survey arrangements
Although the same principles apply to all scales of refrac-
tion profiling, the logistic problems of implementing a
 
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