Geology Reference
In-Depth Information
Table 1.1 Geophysical methods.
Method
Measured parameter
Operative physical property
Seismic
Travel times of reflected/refracted
Density and elastic moduli, which
seismic waves
determine the propagation velocity of
seismic waves
Gravity
Spatial variations in the strength of
Density
the gravitational field of the Earth
Magnetic
Spatial variations in the strength of
Magnetic susceptibility and
the geomagnetic field
remanence
Electrical
Resistivity
Earth resistance
Electrical conductivity
Induced polarization
Polarization voltages or frequency-
Electrical capacitance
dependent ground resistance
Self-potential
Electrical potentials
Electrical conductivity
Electromagnetic
Response to electromagnetic radiation
Electrical conductivity and inductance
Radar
Travel times of reflected radar pulses
Dielectric constant
being able to survey areas where ground access is difficult
or impossible.
A wide range of geophysical surveying methods
exists, for each of which there is an 'operative' physical
property to which the method is sensitive.The methods
are listed in Table 1.1.
The type of physical property to which a method
responds clearly determines its range of applications.
Thus, for example, the magnetic method is very suitable
for locating buried magnetite ore bodies because of their
high magnetic susceptibility. Similarly, seismic or elec-
trical methods are suitable for the location of a buried
water table because saturated rock may be distinguished
from dry rock by its higher seismic velocity and higher
electrical conductivity.
Other considerations also determine the type of
methods employed in a geophysical exploration pro-
gramme. For example, reconnaissance surveys are often
carried out from the air because of the high speed of
operation. In such cases the electrical or seismic methods
are not applicable, since these require physical contact
with the ground for the direct input of energy.
Geophysical methods are often used in combination.
Thus, the initial search for metalliferous mineral deposits
often utilizes airborne magnetic and electromagnetic
surveying. Similarly, routine reconnaissance of conti-
nental shelf areas often includes simultaneous gravity,
magnetic and seismic surveying. At the interpretation
stage, ambiguity arising from the results of one survey
method may often be removed by consideration of
results from a second survey method.
Geophysical exploration commonly takes place in a
number of stages. For example, in the offshore search for
oil and gas, an initial gravity reconnaissance survey may
reveal the presence of a large sedimentary basin that is
subsequently explored using seismic methods. A first
round of seismic exploration may highlight areas of
particular interest where further detailed seismic work
needs to be carried out.
The main fields of application of geophysical survey-
ing, together with an indication of the most appropriate
surveying methods for each application, are listed in
Table 1.2.
Exploration for hydrocarbons, for metalliferous
minerals and environmental applications represents
the main uses of geophysical surveying. In terms of the
amount of money expended annually, seismic methods
are the most important techniques because of their
routine and widespread use in the exploration for hydro-
carbons. Seismic methods are particularly well suited to
the investigation of the layered sequences in sedimentary
basins that are the primary targets for oil or gas. On the
other hand, seismic methods are quite unsuited to the
exploration of igneous and metamorphic terrains for
the near-surface, irregular ore bodies that represent the
main source of metalliferous minerals. Exploration for
ore bodies is mainly carried out using electromagnetic
and magnetic surveying methods.
In several geophysical survey methods it is the local
variation in a measured parameter, relative to some nor-
mal background value, that is of primary interest. Such
variation is attributable to a localized subsurface zone of
 
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