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.
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.
No comments:
Post a Comment