Geology Reference
In-Depth Information
1
The principles and limitations of
geophysical exploration methods
ration programmes by maximizing the rate of ground
coverage and minimizing the drilling requirement. The
importance of geophysical exploration as a means of
deriving subsurface geological information is so great
that the basic principles and scope of the methods and
their main fields of application should be appreciated by
any practising Earth scientist.This topic provides a gen-
eral introduction to the main geophysical methods in
widespread use.
1.1 Introduction
This chapter is provided for readers with no prior
knowledge of geophysical exploration methods and is
pitched at an elementary level. It may be passed over
by readers already familiar with the basic principles and
limitations of geophysical surveying.
The science of geophysics applies the principles of
physics to the study of the Earth. Geophysical investiga-
tions of the interior of the Earth involve taking measure-
ments at or near the Earth's surface that are influenced by
the internal distribution of physical properties. Analysis
of these measurements can reveal how the physical
properties of the Earth's interior vary vertically and
laterally.
By working at different scales, geophysical methods
may be applied to a wide range of investigations from
studies of the entire Earth (global geophysics; e.g. Kearey
& Vine 1996) to exploration of a localized region of
the upper crust for engineering or other purposes (e.g.
Vogelsang 1995, McCann
et al.
1997). In the geophysical
exploration methods (also referred to as geophysical sur-
veying) discussed in this topic, measurements within
geographically restricted areas are used to determine the
distributions of physical properties at depths that reflect
the local subsurface geology.
An alternative method of investigating subsurface
geology is, of course, by drilling boreholes, but these
are expensive and provide information only at discrete
locations. Geophysical surveying, although sometimes
prone to major ambiguities or uncertainties of interpre-
tation, provides a relatively rapid and cost-effective
means of deriving areally distributed information on
subsurface geology. In the exploration for subsurface
resources the methods are capable of detecting and
delineating local features of potential interest that could
not be discovered by any realistic drilling programme.
Geophysical surveying does not dispense with the need
for drilling but, properly applied, it can optimize explo-
1.2 The survey methods
There is a broad division of geophysical surveying meth-
ods into those that make use of natural fields of the Earth
and those that require the input into the ground of artifi-
cially generated energy.The natural field methods utilize
the gravitational, magnetic, electrical and electromag-
netic fields of the Earth, searching for local perturbations
in these naturally occurring fields that may be caused by
concealed geological features of economic or other
interest. Artificial source methods involve the genera-
tion of local electrical or electromagnetic fields that may
be used analogously to natural fields, or, in the most im-
portant single group of geophysical surveying methods,
the generation of seismic waves whose propagation ve-
locities and transmission paths through the subsurface
are mapped to provide information on the distribution
of geological boundaries at depth. Generally, natural
field methods can provide information on Earth proper-
ties to significantly greater depths and are logistically
more simple to carry out than artificial source methods.
The latter, however, are capable of producing a more
detailed and better resolved picture of the subsurface
geology.
Several geophysical surveying methods can be used at
sea or in the air. The higher capital and operating costs
associated with marine or airborne work are offset by
the increased speed of operation and the benefit of
