Geology Reference
In-Depth Information
Table 1.2
Geophysical surveying applications.
Application
Appropriate survey methods*
Exploration for fossil fuels (oil, gas, coal)
S, G, M, (EM)
Exploration for metalliferous mineral deposits
M, EM, E, SP, IP, R
Exploration for bulk mineral deposits (sand and gravel)
S, (E), (G)
Exploration for underground water supplies
E, S, (G), (Rd)
Engineering/construction site investigation
E, S, Rd. (G), (M)
Archaeological investigations
Rd, E, EM, M, (S)
* G, gravity; M, magnetic; S, seismic; E, electrical resistivity; SP, self-potential; IP, induced polarization; EM, electromagnetic; R,
radiometric; Rd, ground-penetrating radar. Subsidiary methods in brackets.
distinctive physical property and possible geological
importance. A local variation of this type is known as a
geophysical anomaly
. For example, the Earth's gravitation-
al field, after the application of certain corrections,
would everywhere be constant if the subsurface were of
uniform density. Any lateral density variation associated
with a change of subsurface geology results in a local
deviation in the gravitational field. This local deviation
from the otherwise constant gravitational field is referred
to as a gravity anomaly.
Although many of the geophysical methods require
complex methodology and relatively advanced mathe-
matical treatment in interpretation, much information
may be derived from a simple assessment of the survey
data.This is illustrated in the following paragraphs where
a number of geophysical surveying methods are applied
to the problem of detecting and delineating a specific
geological feature, namely a salt dome. No terms or units
are defined here, but the examples serve to illustrate the
way in which geophysical surveys can be applied to the
solution of a particular geological problem.
Salt domes are emplaced when a buried salt layer,
because of its low density and ability to flow, rises
through overlying denser strata in a series of approxi-
mately cylindrical bodies. The rising columns of salt
pierce the overlying strata or arch them into a domed
form. A salt dome has physical properties that are differ-
ent from the surrounding sediments and which enable its
detection by geophysical methods.These properties are:
(1) a relatively low density; (2) a negative magnetic sus-
ceptibility; (3) a relatively high propagation velocity for
seismic waves; and (4) a high electrical resistivity (specif-
ic resistance).
1.
The relatively low density of salt with respect to its
surroundings renders the salt dome a zone of anom-
alously low mass. The Earth's gravitational field is per-
turbed by subsurface mass distributions and the salt
dome therefore gives rise to a gravity anomaly that is
negative with respect to surrounding areas. Figure 1.1
presents a contour map of gravity anomalies measured
over the Grand Saline Salt Dome in east Texas, USA.The
gravitational readings have been corrected for effects
which result from the Earth's rotation, irregular surface
relief and regional geology so that the contours reflect
only variations in the shallow density structure of the
area resulting from the local geology.The location of the
salt dome is known from both drilling and mining oper-
ations and its subcrop is indicated. It is readily apparent
that there is a well-defined negative gravity anomaly
centred over the salt dome and the circular gravity con-
tours reflect the circular outline of the dome. Clearly,
gravity surveys provide a powerful method for the loca-
tion of features of this type.
2.
A less familiar characteristic of salt is its negative mag-
netic susceptibility, full details of which must be deferred
to Chapter 7.This property of salt causes a local decrease
in the strength of the Earth's magnetic field in the vicin-
ity of a salt dome. Figure 1.2 presents a contour map of
the strength of the magnetic field over the Grand Saline
Salt Dome covering the same area as Fig. 1.1. Readings
have been corrected for the large-scale variations of the
magnetic field with latitude, longitude and time so that,
again, the contours reflect only those variations resulting
from variations in the magnetic properties of the subsur-
face. As expected, the salt dome is associated with a
negative magnetic anomaly, although the magnetic low
is displaced slightly from the centre of the dome. This
example illustrates that salt domes may be located by
magnetic surveying but the technique is not widely used
as the associated anomalies are usually very small and
therefore difficult to detect.
3.
Seismic rays normally propagate through salt at a
higher velocity than through the surrounding sedi-
ments. A consequence of this velocity difference is that
