Geology Reference
In-Depth Information
ticularly with large velocity variations, it can produce
greatly improved seismic images.
The information derived from seismic tomography
may be used to predict spatial variations in, for example,
lithology, pore fluids, or rock fracturing, and the method
is therefore of potential value in a wide range of explo-
ration and engineering applications. As with many geo-
physical methods, it can also be applied on a variety of
spatial scales, from ranges of hundreds of metres, down to
engineering or archaeological investigations of single
columns in ancient buildings (Cardarelli & de Nardis
2001).
shear wave arrivals, and the combination of P- and S-
wave velocity information enables calculation of Pois-
son's ratio (Section 3.3.1). If an estimate of density is
available, the bulk modulus and shear modulus can also
be calculated from P- and S-wave velocities. Such esti-
mates of the elastic constants, based on the propagation
of seismic waves, are referred to as dynamic, in contrast
to the static estimates derived from load-testing of rock
samples in the laboratory. Dynamic estimates tend to
yield slightly higher values than loading tests.
5.11.1 Engineering and
environmental surveys
On the local scale, refraction surveys are widely used in
foundation studies on construction sites to derive esti-
mates of depth to rockhead beneath a cover of superficial
material. Use of the plus-minus method or the general-
ized reciprocal method (Section 5.4) allows irregular
rockhead geometries to be mapped in detail and thus re-
duces the need for test drilling with its associated high
costs. Figure 5.24 shows a typical profile across fluvial
5.11 Applications of seismic
refraction surveying
Exploration using refraction methods covers a very wide
range of applications. Refraction surveys can provide es-
timates of the elastic constants of local rock types, which
have important engineering applications: use of special
sources and geophones allows the separate recording of
Fig. 5.24
T
-
x
graph of a seismic refraction profile recorded over Holocene fluvial sediments overlying Palaeozoic rocks.The geophone
separation was 2 m and the shot point separation 30 m.The multiple, overlapping, reversed data allow a continuous plus-minus
interpretation of the rockhead interface.
