Geophysics
J K Gascoyne and A S Eriksen, Zetica,
Witney, UK
ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction
Geophysics can be defined as the study of the Earth
through the measurement of its physical properties.
Use of the discipline dates back to ancient times,
but only since the advent of modern-day instrumen-
tation has its application become widespread. The
development of modern geophysical techniques and
equipment was initially driven by oil and mineral
exploration during the early to middle parts of the
twentieth century, and many of the instruments used
today in engineering geophysics owe their evolution
to the field of exploration geophysics.
Engineering geophysics involves using geophysical
techniques to investigate subsurface structures and
materials that may be of significance to the design
and safety of an engineered structure. Unlike the
deeper investigations associated with exploration geo-
physics (up to 2–3 km), engineering surveys are usu-
ally concerned with investigation of the near-surface,
at depths in the range of 1–100 m.
The key advantages of geophysics over intrusive
site-investigation techniques, such as digging trial pits
or drilling boreholes, are that geophysical methods are
comprehensive and non-invasive. Large areas can be
evaluated rapidly without direct access to the subsur-
face. One class of engineering geophysics, borehole
geophysics, is an exception in that it makes use of
boreholes already drilled to sample the local area
around the borehole.
When combined with intrusive methods, geo-
physics provides a cost-effective means of analys-
ing the undisturbed subsurface to aid selection of,
and interpolation between, widely spaced sampling
locations.
Engineering geophysics can be applied throughout
the life cycle of an engineered structure, starting with
the initial ground investigation to determine the suit-
ability of a particular site and provide design-sensitive
and critical safety information. This may be followed
by materials testing during the various stages of con-
struction, monitoring the impact of construction on
surrounding structures, on-going monitoring of the
integrity of structures after completion, and helping
to determine when to schedule essential maintenance
tasks, such as pavement or ballast renewal on a road
or railway, respectively.
The success of all geophysical methods relies on
there being a measurable contrast between the phys-
ical properties of the target and those of the surround-
ing medium. The properties used are typically density,
elasticity, magnetic susceptibility, electrical conduct-
ivity, and radioactivity. Knowledge of the material
properties likely to be associated with a target is thus
essential to guide the selection of the correct method
to be used and to interpret the results obtained. Often
a combination of methods provides the best means
of solving complex problems. It is sometimes the
case that, if a target does not provide a measurable
physical contrast, the association of the target with
other measurable conditions may indirectly lead to
detection.
Methods
Engineering geophysical methods can be split into
two main categories – passive and active.
With passive methods, naturally occurring sources,
such as the Earth’s magnetic field, over which the
observer has no control, are used to detect abnormal
variations in background caused by the presence of
the target. Interpretation of this data is non-unique
and relies heavily on the knowledge of the interpreter.
Active methods involve generating signals in order
to induce a measurable response associated with a
target. The observer can control the level of energy
input to the ground and measure variations in energy
transmissibility over distance and time. Interpretation
of this data can be more quantitative with improved
depth control compared with passive methods, but
ease of interpretation is not guaranteed.
Table 1 lists of some of the techniques most com-
monly used in engineering geophysics.
Measurements are commonly taken at the surface
and from boreholes, underground mineworkings,
over or under water, or from aircraft platforms. The
advent of powerful computer-aided modelling has led
to the development of a number of sophisticated im-
aging techniques, such as cross-hole seismic and re-
sistivity tomography and reflective tomography,
which are capable of imaging the properties of the
ground in three dimensions between the surface and
two or more boreholes or beyond the face of a tunnel.
Armed with a knowledge of the physical properties
of a target (see Table 2), its burial setting, and the
requirements of the survey, a feasibility assessment is
carried out by a geophysicist to determine the likely
deliverables of a geophysical survey. Based on the
results of this assessment, an appropriate geophysical
482 ENGINEERING GEOLOGY/Geophysics