Geology Reference
In-Depth Information
1
INTRODUCTION
1.1 What Geophysics Measures
Applied or exploration geophysics can be defined as mapping the subsurface
through the remote measurement of its physical properties. The discipline
dates back to ancient times but only since the advent of modern-day instru-
mentation has its use become widespread. The development of geophysical
techniques and equipment during the early to middle parts of the twentieth
century was driven by oil and mineral exploration, for targets that could
be several kilometres deep. Many of the instruments used today in archae-
ological, environmental and engineering surveys owe their development
to this kind of geophysics, but have been adapted to investigations of the
near-surface, in the range of 0.5-100 m.
The success of any geophysical method relies on there being a measurable
contrast between the physical properties of the target and the surrounding
medium. The properties utilised are, typically, density, elasticity, magnetic
susceptibility, electrical conductivity and radioactivity (Table 1.1). Whether
a physical contrast is in practice measurable is inextricably linked to the
physics of the problem, the design of the geophysical survey and the se-
lection of suitable equipment. Not all equipment is fit for purpose. Often a
combination of methods provides the best means of solving a complex prob-
lem, and sometimes a target that does not provide a measurable physical
contrast can be detected indirectly by its association with conditions or ma-
terials that do. One of the aims of this handbook is to give the field observer
an appreciation of the notional detectability of targets and the influence of
burial setting, survey design, equipment selection and operating procedures
on actual detectability.
1.2 Fields
Although there are many different types of geophysical measurement, small-
scale surveys all tend to be rather similar and involve similar, and sometimes
ambiguous, jargon. For example, the word
base
has three different common
meanings, and
stacked
and
field
have two each.
Measurements in geophysical surveys are made
in the field
but, unfor-
tunately, many are also
of
fields. Field theory is fundamental to gravity,
