Geophysical Fieldwork

Geophysical instruments vary widely in size and complexity but all are used
to make physical measurements, of the sort commonly made in laboratories, at
temporary sites in sometimes hostile conditions. They should be economical
in power use, portable, rugged, reliable and simple. These criteria are satisfied
to varying extents by the commercial equipment currently available.
 Choosing geophysical instruments
Few instrument designers can have tried using their own products for long
periods in the field, since operator comfort seldom seems to have been
considered. Moreover, although many real improvements have been made
in the last 30 years, design features have been introduced during the same
period, for no obvious reasons, that have actually made fieldwork more difficult.
The proton magnetometer staff, discussed below, is a case in point.
If different instruments can, in principle, do the same job to the same
standards, practical considerations become paramount. Some of these are
listed below.
Serviceability: Is the manual comprehensive and comprehensible? Is a
breakdown likely to be repairable in the field? Are there facilities for repairing
major failures in the country of use or would the instrument have to be sent
overseas, risking long delays en route and in customs? Reliability is vital but
some manufacturers seem to use their customers to evaluate prototypes.
Power supplies: If dry batteries are used, are they of types easy to replace
or will they be impossible to find outside major cities? If rechargeable batteries
are used, how heavy are they? In either case, how long will the batteries
last at the temperatures expected in the field? Note that battery life is reduced
in cold climates. The reduction can be dramatic if one of the functions of the
battery is to keep the instrument at a constant temperature.
Data displays: Are these clearly legible under all circumstances? A torch
is needed to read some in poor light and others are almost invisible in
bright sunlight. Large displays used to show continuous traces or profiles
can exhaust power supplies very quickly.
Hard copy: If hard copy records can be produced directly from the field
instrument, are they of adequate quality? Are they truly permanent, or will
they become illegible if they get wet, are abraded or are exposed to sunlight?
Comfort: Is prolonged use likely to cripple the operator? Some instruments
are designed to be suspended on a strap passing across the back of
the neck. This is tiring under any circumstances and can cause serious medical
problems if the instrument has to be levelled by bracing it against the
strap. Passing the strap over one shoulder and under the other arm may
reduce the strain but not all instruments are easy to use when carried in
this way.
Convenience: If the instrument is placed on the ground, will it stand
upright? Is the cable then long enough to reach the sensor in its normal
operating position? If the sensor is mounted on a tripod or pole, is this strong
enough? The traditional proton magnetometer poles, in sections that screwed
together and ended in spikes that could be stuck into soft ground, have now
been largely replaced by unspiked hinged rods that are more awkward to
stow away, much more fragile (the hinges can twist and break), can only be
used if fully extended and must be supported at all times.
Fieldworthiness: Are the control knobs and connectors protected from
accidental impact? Is the casing truly waterproof? Does protection from damp
grass depend on the instrument being set down in a certain way? Are there
depressions on the console where moisture will collect and then inevitably
seep inside?
Automation: Computer control has been introduced into almost all the
instruments in current production (although older, less sophisticated models
are still in common use). Switches have almost vanished, and every instruction
has to be entered via a keypad. This has reduced the problems that
used to be caused by electrical spikes generated by switches but, because the
settings are often not permanently visible, unsuitable values may be repeatedly
used in error. Moreover, simple operations have sometimes been made
unduly complicated by the need to access nested menus. Some instruments
do not allow readings to be taken until line and station numbers have been
entered and some even demand to know the distance to the next station and
to the next line!
The computer revolution has produced real advances in field geophysics,
but it has its drawbacks. Most notably, the ability to store data digitally in
data loggers has discouraged the making of notes on field conditions where
these, however important, do not fall within the restricted range of options
the logger provides. This problem is further discussed in Section 1.3.2. 
 Cables
Almost all geophysical work involves cables, which may be short, linking
instruments to sensors or batteries, or hundreds of metres long. Electrical
induction between cables (electromagnetic coupling, also known as crosstalk
) can be a serious source of noise (see also Section 11.3.5).
Efficiency in cable handling is an absolute necessity. Long cables always
tend to become tangled, often because of well-intentioned attempts to make
neat coils using hand and elbow. Figures of eight are better than simple loops,
but even so it takes an expert to construct a coil from which cable can be
run freely once it has been removed from the arm. On the other hand, a
seemingly chaotic pile of wire spread loosely on the ground can be quite
trouble-free. The basic rule is that cable must be fed on and off the pile in
opposite directions, i.e. the last bit of cable fed on must be the first to be
pulled off. Any attempts to pull cable from the bottom will almost certainly
end in disaster.
Cable piles are also unlikely to cause the permanent kinks which are often
features of neat and tidy coils and which may have to be removed by allowing
the cable to hang freely and untwist naturally. Places where this is possible
with 100-metre lengths are rare.
Piles can be made portable by feeding cables into open boxes, and on
many seismic surveys the shot-firers carried their firing lines in this way in
old gelignite boxes. Ideally, however, if cables are to be carried from place
to place, they should be wound on properly designed drums. Even then,
problems can occur. If cable is unwound by pulling on its free end, the drum
will not stop simply because the pull stops, and a free-running drum is an
effective, but untidy, knitting machine.
A drum carried as a back-pack should have an efficient brake and should
be reversible so that it can be carried across the chest and be wound from
a standing position. Some drums sold with geophysical instruments combine
total impracticality with inordinate expense and are inferior to home-made or
garden-centre versions.
Geophysical lines exert an almost hypnotic influence on livestock. Cattle
have been known to desert lush pastures in favour of midnight treks through
hedges and across ditches in search of juicy cables. Not only can a survey be
delayed but a valuable animal may be killed by biting into a live conductor,
and constant vigilance is essential.
 Connections
Crocodile clips are usually adequate for electrical connections between single
conductors. Heavy plugs must be used for multi-conductor connections and
are usually the weakest links in the entire field system. They should be
placed on the ground very gently and as seldom as possible and, if they do
not have screw-on caps, be protected with plastic bags or ‘clingfilm’. They
must be shielded from grit as well as moisture. Faults are often caused by dirt
increasing wear on the contacts in socket units, which are almost impossible
to clean.
Plugs should be clamped to their cables, since any strain will otherwise
be borne by the weak soldered connections to the individual pins. Inevitably,
the cables are flexed repeatedly just beyond the clamps, and wires may break
within the insulated sleeving at these points. Any break there, or a broken or
dry joint inside the plug, means work with a soldering iron. This is never easy
when connector pins are clotted with old solder, and is especially difficult if
many wires crowd into a single plug.
Problems with plugs can be minimized by ensuring that, when moving,
they are always carried, never dragged along the ground. Two hands should
always be used, one holding the cable to take the strain of any sudden pull,
the other to support the plug itself. The rate at which cable is reeled in should
never exceed a comfortable walking pace, and especial care is needed when
the last few metres are being wound on to a drum. Drums should be fitted
with clips or sockets where the plugs can be secured when not in use.
 Geophysics in the rain
A geophysicist, huddled over his instruments, is a sitting target for rain, hail,
snow and dust, as well as mosquitoes, snakes and dogs. His most useful piece

of field clothing is often a large waterproof cape which he can not only wrap
around himself but into which he can retreat, along with his instruments, to
continue work .
Electrical methods that rely on direct or close contact with the ground
generally do not work in the rain, and heavy rain can be a source of seismic
noise. Other types of survey can continue, since most geophysical instruments
are supposed to be waterproof and some actually are. However, unless
dry weather can be guaranteed, a field party should be plentifully supplied
with plastic bags and sheeting to protect instruments, and paper towels for
drying them. Large transparent plastic bags can often be used to enclose
instruments completely while they are being used, but even then condensation
may create new conductive paths, leading to drift and erratic behaviour.
Silica gel within instruments can absorb minor traces of moisture but cannot
cope with large amounts, and a portable hair-drier held at the base camp may
be invaluable.
 A geophysical toolkit
Regardless of the specific type of geophysical survey, similar tools are likely
to be needed. A field toolkit should include the following:
• Long-nose pliers (the longer and thinner the better)
• Slot-head screwdrivers (one very fine, one normal)
• Phillips screwdriver
• Allen keys (metric and imperial)
• Scalpels (light, expendable types are best)
• Wire cutters/strippers
• Electrical contact cleaner (spray)
• Fine-point 12V soldering iron
• Solder and ‘Solder-sucker’
• Multimeter (mainly for continuity and battery checks, so small size and
durability are more important than high sensitivity)
• Torch (preferably of a type that will stand unsupported and double as a
table lamp. A ‘head torch’ can be very useful)
• Hand lens
• Insulating tape, preferably self-amalgamating
• Strong epoxy glue/‘super-glue’
• Silicone grease
• Waterproof sealing compound
• Spare insulated and bare wire, and connectors
• Spare insulating sleeving
• Kitchen cloths and paper towels
• Plastic bags and ‘clingfilm’
A comprehensive first-aid kit is equally vital.

Fields

Although there are many different geophysical methods, small-scale surveys
all tend to be rather alike 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, unfortunately,
many are also of fields. Field theory is fundamental to gravity,
magnetic and electromagnetic work, and even particle fluxes and seismic
wavefronts can be described in terms of radiation fields. Sometimes ambiguity
is unimportant, and sometimes both meanings are appropriate (and
intended), but there are occasions when it is necessary to make clear distinctions.
In particular, the term field reading is almost always used to identify
readings made in the field, i.e. not at a base station.
The fields used in geophysical surveys may be natural ones (e.g. the
Earth’s magnetic or gravity fields) but may be created artificially, as when
alternating currents are used to generate electromagnetic fields. This leads to
the broad classification of geophysical methods into passive and active types,
respectively.
Physical fields can be illustrated by lines of force that show the field
direction at any point. Intensity can also be indicated, by using more closely
spaced lines for strong fields, but it is difficult to do this quantitatively where
three-dimensional situations are being illustrated on two-dimensional media.
 Vector addition
Vector addition (Figure 1.1) must be used when combining fields from different
sources. In passive methods, knowledge of the principles of vector
addition is needed to understand the ways in which measurements of local
anomalies are affected by regional backgrounds. In active methods, a local
anomaly (secondary field) is often superimposed on a primary field produced
by a transmitter. In either case, if the local field is much the weaker of the two
(in practice, less than one-tenth the strength of the primary or background
field), then the measurement will, to a first approximation, be made in the
direction of the stronger field and only the component in this direction of
the secondary field (ca in Figure 1.1) will be measured. In most surveys the
slight difference in direction between the resultant and the background or
primary field can be ignored.

If the two fields are similar in
strength, there will be no simple
relationship between the magnitude
of the anomalous field and the
magnitude of the observed anomaly.
However, variations in any given
component of the secondary field
can be estimated by taking all
measurements in an appropriate
direction and assuming that the
component of the background or
primary field in this direction is
constant over the survey area.
Measurements of vertical rather than
total fields are sometimes preferred
in magnetic and electromagnetic
surveys for this reason.
The fields due to multiple sources
are not necessarily equal to the
vector sums of the fields that would
have existed had those sources
been present in isolation. A strong
magnetic field from one body can
affect the magnetization in another,
or even in itself (demagnetization
effect), and the interactions between fields and currents in electrical and
electromagnetic surveys can be very complex.
 The inverse-square law
Inverse-square law attenuation of signal strength occurs in most branches of
applied geophysics. It is at its simplest in gravity work, where the field due
to a point mass is inversely proportional to the square of the distance from
the mass, and the constant of proportionality (the gravitational constant G)
is invariant. Magnetic fields also obey an inverse-square law. The fact that
their strength is, in principle, modified by the permeability of the medium
is irrelevant in most geophysical work, where measurements are made in
either air or water. Magnetic sources are, however, essentially bipolar, and
the modifications to the simple inverse-square law due to this fact are much
more important (Section 1.1.5).
Electric current flowing from an isolated point electrode embedded in
a continuous homogeneous ground provides a physical illustration of the

significance of the inverse-square law. All of the current leaving the electrode
must cross any closed surface that surrounds it. If this surface is a sphere
concentric with the electrode, the same fraction of the total current will cross
each unit area on the surface of the sphere. The current per unit area will
therefore be inversely proportional to the total surface area, which is in turn
proportional to the square of the radius. Current flow in the real Earth is, of
course, drastically modified by conductivity variations.
1.1.3 Two-dimensional sources
Rates of decrease in field strengths depend on source shapes as well as on
the inverse-square law. Infinitely long sources of constant cross-section are
termed two-dimensional (2D) and are often used in computer modelling to
approximate bodies of large strike extent. If the source ‘point’ in Figure 1.2
represents an infinite line source seen end on, the area of the enclosing (cylindrical)
surface is proportional to the radius. The argument applied in the
previous section to a point source implies that in this case the field strength
is inversely proportional to distance and not to its square. In 2D situations,
lines of force drawn on pieces of paper illustrate field magnitude (by their
separation) as well as direction.

The lines of force or radiation intensity from a source consisting of a homogeneous layer of
constant thickness diverge only near its edges (Figure 1.3). The Bouguer plate of gravity reductions (Section 2.5.1) and the radioactive source with 2π geometry (Section 4.3.3) are examples of infinitely extended layer sources, for which field strengths are independent
of distance. This condition is approximately achieved if a detector is only a short distance
above an extended source and a long way from its edges.

A dipole consists of equal-strength positive and negative point sources a very small distance apart. Field strength decreases as the inverse cube of distance and both strength and direction change with ‘latitude’ (Figure 1.4). The intensity of the field at a point on a dipole
axis is double the intensity at a point the same distance away on the dipole ‘equator’, and in the opposite direction.

Electrodes are used in some
electrical surveys in approximately
dipolar pairs and magnetization is
fundamentally dipolar. Electric currents
circulating in small loops are
dipolar sources of magnetic field.





Exponential decay
Radioactive particle fluxes and seismic and electromagnetic waves are subject
to absorption as well as geometrical attenuation, and the energy crossing

closed surfaces is then less than the energy emitted by the sources they
enclose. In homogeneous media, the percentage loss of signal is determined
by the path length and the attenuation constant. The absolute loss is proportional
also to the signal strength. A similar exponential law (Figure 1.5),
governed by a decay constant, determines the rate of loss of mass by a
radioactive substance.
Attenuation rates are alternatively characterized by skin depths, which
are the reciprocals of attenuation constants. For each skin depth travelled, the
signal strength decreases to 1/e of its original value, where e (= 2.718) is the
base of natural logarithms. Radioactivity decay rates are normally described in
terms of the half-lives, equal to loge2 (= 0.693) divided by the decay constant.
During each half-life period, one half of the material present at its start is lost.

The Second Law and Molecular Behavior

At the present time we are familiar enough with molecules to formulate the Second
Law entirely in relation to an intuitive perception of their behavior. It is easy to see that the
Second Law, as expressed in terms of heat flow in section 11, could be violated with some
cooperation from molecules.
Consider two systems each consisting of a fixed quantity of gas. At the boundary of
each system are rigid, impenetrable, and well insulated walls except for a metal plate made
of a good thermal conductor and located between the systems as shown in Figure 1. The gas
in one system is at a low temperature T1 and in the other at a higher temperature T2. In
terms of molecular behavior the

temperature difference is produced by different distributions of molecules among the
velocities in the molecular states of each system. The number of different velocities,
however, is so large that the high temperature system contains some molecules with lower

speeds than some molecules in the low temperature system and vice versa.
Now suppose we prepare some instructions for the molecules in each system as
shown on the signs in Figure 1. When following these instructions, only the high speed
molecules in the low temperature gas collide with molecules on the surface of the plate, give
up energy to them, and thus create on this surface a higher temperature than in the gas.
The boundary between the gas and the plate then has a temperature difference across it and,
according to our definition, the energy thus transported is heat. In the plate this becomes
thermal energy which is conducted through it because the molecules on its other surface are
at a lower temperature as a consequence of their energy exchanges only with the low speed
molecules in the adjacent high temperature gas system. This constitutes likewise a heat flow
into this system. The overall result is then the continuous unaided transfer of heat from a
low temperature region to one at a higher temperature, clearly the wrong direction and a
Second Law violation. The Second Law therefore, is related to the fact that in completely
isolated systems molecules will never of their own accord obey any sort of instructions such
as these.
                          Microstates in Isolated Systems

To explain why molecules always behave as though instructions of this type are
completely ignored, imagine that we have a fantastic camera capable of making a
multidimensional picture which could show at any instant where all the ultimate particles
in the system are located and reveal every type of motion taking place, indicating its
location, speed, and direction. Every type of distinguishably different action at any moment,
the vibration, twisting, or stretching within molecules as well as their translational and
rotational movements, would be identified in this manner for every molecule in the system.
This picture would thus be a photograph of what we have defined as an instantaneous
microstate of the system.
Now, instead of the instructions on the signs in Figure I, suppose we ask the
molecules to do everything they can do by themselves in a rigid walled and isolated
container where no external arranging or directing operations are possible. We will say,
"Molecules, please begin now and arrange yourselves in a sequence of poses for pictures
which will show every possible microstate which can exist in your system under the
restrictions imposed by your own nature and the conditions of isolation in the container".
If we expressed these restrictions as a list of rules to be followed in assigning molecules to various positions and motions, the list would appear as follows:
1. In distributing yourselves among the various positions and motions for each microstate
picture, do not violate any energy conservation laws. Consequently, because you are in
an isolated system the sum of all your individual translational, vibrational, and
rotational kinetic energies plus all your intermolecular potential energies must always
be the same and equal to the fixed total internal energy of the system.
2. Likewise, do not violate any mass conservation laws. There are to be no chemical
reactions among you so that the total number of individual molecules assigned must
always remain the same.
3. All of you must, of course, remain at all times within the container so the total volume
in which you distribute yourselves must be constant.
4. Do not violate any laws of physics applicable to your particular molecular species. You
must remember that no two of you can have all of your microstate position and motion
characteristics exactly the same otherwise you would have to occupy the same space at
the same time. Furthermore, do not be concerned that there might not be enough
different microstates available for each of you to have a different one. Although you are
numerous, the number of different possible position and motion values is even more
numerous, so that there will never be enough of you to fill all of them and many
possible values will be left unoccupied by a molecule in each picture for which you pose.

The Total Energy Transfer

Because thermodynamic systems are conventionally defined so that no bulk
quantities of matter are transported across their boundaries by stream flow, no energy
crosses the system boundary in the form of internal energy carried by a flowing fluid. With
the system defined in this way the only energy to cross its boundary because of the flow
process is that of work measured by the product of the pressure external to a fixed mass
system in a stream conduit and the volume change it induces in this system. In the case of
diffusion mass transport, as discussed in section 15, the system does not have a fixed mass
but the entire change associated with the diffusion mass transport is given by work
evaluated by computing the product of an external chemical potential and a specific
transported mass change within the system. As a result the combination of heat, work, and
any energy transport by non-thermodynamic carriers includes all the energy in transition
between a system and its surroundings. Energy by non-thermodynamic carriers is that
transported by radiant heat transfer, X-rays, gamma radiation, nuclear particles, cosmic
rays, sonic vibration, etc. Energy of this type is not usually considered as either heat or
work and must be evaluated separately in system where it is involved. Energy transport by
nuclear particles into a system ultimately appears as an increase in thermal energy within
the system and is important in thermodynamic applications to nuclear engineering.
In every application of thermodynamics, however, it is essential that we account for
all the energy in transition across the system boundary and it is only when this is done that
the laws of thermodynamics can relate this transported energy to changes in properties
within the system. In the processes we will discuss, heat and work together include all the
transported energy.

The Chemical Potential

For processes involving diffusion mass transport we can, however, define a
thermodynamic intensive driving force responsible specifically for the total energy change
accompanying the diffusion mass transfer of molecules from one region to another. This
driving force can be defined simply as a partial derivative representing the variation of the
total internal energy of a region with respect to an increment in the number of moles of one
particular species in the region when no other extensive properties are altered. This partial
derivative is an important intensive property called the chemical potential. By reason of its
definition the chemical potential is an intensive property because whenever it is multiplied
by the extensive property change in moles of a particular molecular species within a system
the result is identically the internal energy change of the system resulting only from this
change in moles, and not from the change in any other extensive property.
In elementary physics the energy per unit mass, per mole, or per particle involved in
moving the mass, mole, or particle from one region to another is generally defined as a
potential. Table I lists several types of potentials (driving forces) which are important in
thermodynamic applications. A potential therefore can always be regarded as a driving
force for a mass change. The chemical potential is a driving force of this type. Physically the
driving force represented by the chemical potential results from the same molecular actions
which give rise to a partial vapor pressure in a liquid or a partial pressure in a gas. Each of
these has the ability to expel molecules of a given type out of a multi-component phase.
The energy change within a system accompanying a change in the number of moles
of a given component of the system by molecular processes can now be defined as a type of
work which results from a difference in a chemical potential driving force between the
system and its surroundings. As is the case of other types of work, in order to evaluate
quantitatively the work of a chemical potential driving force it is first necessary to define a
system. In accordance with the principles discussed in section 14,this work is then defined
as the product of a chemical potential of a component outside the system on its external
boundary and a change in the number of moles of this component inside the system. When
the number of moles of a molecular species increases in a system, work must be done on the
system to overcome the molecular forces tending to expel molecules of this species.
Consequently, in accordance with the sign convention, the work relative to the system
receiving this increase in moles within it must be a negative number.
Although we have discussed the chemical potential as a thermodynamic driving force
for the diffusion mass transport, its utility is not confined to this particular process alone.
Because of its definition the chemical potential is a driving force for changes in moles of a
molecular species in a system not only by means of diffusion mass transfer but by any other
molecular process as well. The most important example is the role of the chemical potential
within a system as the driving force for changes in moles brought about in the system by
chemical reactions.