Geology Reference
In-Depth Information
Velocity (m s
-1
)
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Topsoil
Clay
Boulders
Shale
Sandstone
Gneiss
Limestone
Granite
Breccia
Caliche
Conglomerate
Slate
Fig. 5.25
Table showing the variation of
rippability with seismic P-wave velocity
for a range of lithologies. (After Bell
1993.)
Could be ripped
using D-9 tractor
Could not be ripped
Marginal zone
sediments. Here the observation scheme specified a 2 m
geophone spacing, and a 30 m shot spacing. The data
were recorded with a 48-channel seismograph, with
shot points re-fired as the 48 geophones were moved
down the profile.The source was a sledgehammer.
The P-wave seismic velocity is related to the elastic
constants and the density of the material. It is possible
to derive an empirical relationship between the seismic
velocity and the 'hardness' of the rock. In engineering
usage, an important parameter of rock lithology is its
resistance to excavation. If the rock can be removed by
mechanical excavation it is termed 'rippable', rather
than requiring fracturing by explosives. Empirical tables
have been derived relating the 'rippability' of rock units
by particular earthmoving equipment to the P-wave
seismic velocity. Figure 5.25 shows a typical example
of such a table. The range of velocities considered as
rippable varies for different lithologies based on empiri-
cal averages of such relevant factors as their typical degree
of cementation and frequency of jointing. Simple re-
versed P-wave refraction surveys are sufficient to provide
critical information to construction and quarrying
operations.
For surveys of near-surface geology, the data collec-
tion and interpretation must be efficient and rapid, to
make the survey cost-effective against the alternative of
direct excavation. The interpretation of seismic refrac-
tion profile data is most conveniently carried out using
commercial software packages on personal computers.A
wide range of good software is available for the plotting,
automatic event picking and interpretation of such data.
In some situations the option of excavation instead of
geophysical survey is very undesirable. Seismic surveys
may be used to define the extent and depth of unrecord-
ed landfill sites, or structures on 'brown-field'redevelop-
ments. Commonly seismic and resistivity surveys may be
used together to attempt to 'characterize' the nature of
the landfill materials. There is an increasing demand for
this sort of investigation in many parts of the world.
5.11.2 Hydrological surveys
The large difference in velocity between dry and wet
sediments renders the water table a very effective refrac-
tor. Hence, refraction surveys find wide application in
exploration programmes for underground water sup-
plies in sedimentary sequences, often employed in con-
junction with electrical resistivity methods (see Chapter
8). There can, however, be an ambiguity in interpreta-
tion of P-wave refraction data since a layer at depth with
a velocity in excess of 1500 m s
-1
could be either the
water table, or a layer of more consolidated rock.
Recording both P- and S-wave data overcomes this
problem, since the water table will affect the P-wave
velocity, but not that of the S-waves (Fig. 5.26).
5.11.3 Crustal seismology
The refraction method produces generalized models of
subsurface structure with good velocity information,
