Appendix E

Appendix E                     Cables,  Electrodes and Reels

 

CABLES

 

The MiniRes provides laboratory levels of accuracy.  However, to take advantage of this high level of accuracy, considerable attention must be paid to auxiliary field equipment and field procedures.  For example, improper layout of cables or abused cables can degrade the acquired data to the point of worthlessness.  Attention must be paid to detail!

 

Both the transmitter and receiver cables should be of high quality and as free from nicks and abrasions as possible.  A cable will be severely degraded if a car or truck (or human) runs over it and, thereby, cuts small nicks into the insulation.  That damaged cable may work sufficiently well on dry ground, but if there is the slightest moisture due to dew or precipitation, the resistivity readings may be worthless.  A wise procedure to follow as you roll up or roll out the cable is to run the wire along your bare fingers and feel for any nick, abrasion or cut in the wire's insulation.  Any nick should be carefully covered with electrical tape or other insulating material.  A section of wire that has had a vehicle run over it may have to be cut out entirely.  The remaining wire should be spliced together with a good quality solder joint and insulated with self insulating heat shrink tubing.  Both the receiver and transmitter cables must have perfect insulating integrity.

 

Prime concern should be focused on the type and condition of the cables, both receiver and transmitter.  A set of poor cables may work well in a situation of short electrode spacings, dry surface conditions, and low earth resistivities.  But the same cables may be absolutely worthless if there is the slightest amount of moisture on the ground, the electrode spacings are large or the earth resistivity is large.  It is wisest to always use high quality cables that are in excellent condition.

 

Neither the transmitter nor receiver cables need to be of heavy gauge.  The gauge of the conductors can be as small as 24 AWG (American Wire Gauge).  A larger gauge wire will tend to be more durable in the field but heavier to carry around.  The conductor may be tinned or untinned.  Stranded conductor wire is best since it provides the greatest flexibility, but unstranded (solid) wire may also be used. The exposed wire ends (where the connections to the electrodes and MiniRes binding posts are made) tend to accumulate coatings of oxides and carbonates after exposure to field conditions.  The transmitter cables are more tolerant of this degradation, however, the receiver connections should always be clean, bright, tight and solid.  An occasional stripping of the insulation at the ends of the receiver cables to expose fresh conductor surface is recommended.  Pliers, wire cutters, wire strippers or a knife can be used to strip the insulation.

 

The ends of the cables may be fitted with “banana” plugs. The MiniRes cable terminals can accept either stripped wire or banana plug. The electrodes that are supplied with the MiniRes have a hole near the top designed to accept a banana plug. 

 

All types of cable insulation leak electricity to a certain degree.  Teflon leaks the least of all and PVC (PolyVinyl Chloride) is at the other extreme. However, there are other field considerations besides leakage.  PVC insulation, despite its leakage characteristics, is strong in many other aspects and is generally sufficient for high quality surveys.  Manufacturer representatives should be consulted about a type of cable and insulation that may be used in a particular situation.  Cable characteristic to consider are cost, brittleness at low temperatures, deterioration from ultra violet sunlight, weight, flexibility, abrasion resistance, and ease of stripping.

 

More important than the type of insulation, is the condition of the insulation.  A single small nick in the insulation of a Teflon cable can increase its leakage beyond the worst PVC cable. It is poor practice to leave cables on the ground overnight. Many animals find the cable insulation a delightful nighttime morsel!


ELECTRODES

Generally, any type of conductive metal may be used as an electrode.  However, it is advisable to use the same type of electrode for both receiver electrodes.  The transmitter electrodes may be made of different metals - steel, copper, stainless steel, brass, lead, etc., but both of the receiver electrodes should be of the same metal.   Copper sulfate or cadmium containing non-polarizing electrodes are neither required nor recommended.

The electrodes supplied with the MiniRes are stainless steel. They have a hole just below the top designed to accept a “banana” plug. The electrodes are or two types. The small electrodes are about 25 cm (10 inches) long and 3/8-inch in diameter. They are NOT designed to be driven into the ground. They have a large plastic knob on their top end to allow a good hand grip and allow them to be pushed and wiggled into the ground. If the ground is too hard for this, the ground can be wetted before pushing into the ground.

If this is not practical, cheap metal rods can be used. Half-inch or five-eighths-inch reinforcing rod is often used for this purpose. LRI also manufactures 60cm (2 foot) long and 7/16-inch diameter stainless steel electrodes for driving into the ground. Eight cm (3 inches) below the top is a hole for accepting a “banana” plug. At the opposite end, the rod is pointed.

ELECTRODES  IN  DRY  SOIL  See Appendix Q for details.

RECEIVER  ELECTRODE  CONTACT

If the receiver (potential) cables are connected to the MiniRes or MiniGPS receiver at one end and not connected to the earth at their other ends, they form an antenna. Stray electrical signals such as distant lightning noise can be detected by the instrument. The better the electrode ends of the receiver cables are connected to ground the more the antenna property of the receiver cables are “shorted” and the less the tendency to pick up stray atmospheric noise. So, it is always good practice to minimize receiver electrode ground resistance.

CABLE  REELS

A precarious situation arises when wire reels (spools) are used for deploying the transmitter cables.  This condition can cause extreme instability, wandering readings, and unreasonably high or low value readings.  Very little information exists in the literature describing this condition, but, if reels are used for the transmitter cable deployment, eventually this frustrating behavior will manifest itself.

Nature of Problem

This problem typically shows up as:

1) wandering readings or time varying readings

2) unusually high or unusually low readings (but constant in time)

3) readings that seems stable for a time - then change erratically

4) readings that seem to change quite drastically depending on battery voltage

5) readings that change quite drastically when the cable on the transmitter spool is retracted or extended

Cause of Problem

This problem is caused by the constant current generator trying to drive a precise constant current into the ground through the transmitter reeled cables.  These transmitter reeled cables can be viewed in terms of their "equivalent circuit" which appears as a high "Q" resonant circuit.  The cable, which is coiled around the spool, acts as an inductance and the capacitance between the adjacent windings on the reel act as distributed capacitance and that combination of inductance

and capacitance forms a resonant circuit.  It is very difficult for the constant current generator to remain stable while feeding a resonant circuit.  In fact, the constant current generator, in particular instances, will not be able to maintain the current constant or, if it maintains it constant, it may be completely inaccurate and quite different than the value of current that is expected. 

The constant current generator, in such instances, will break into high frequency "parasitic" oscillation - typically in the hundreds of kilohertz or megahertz.  This high frequency oscillation may be transparent to the operator unless he happens to be monitoring the transmitter current by means of a high frequency oscilloscope. The oscillation may be constant, in which case, the readings may be constant, or the oscillation may start and stop at a slow rate, in which case, the readings will fluctuate at the same rate that the oscillations start and stop.  In either case the readings may be greatly in error.

This tendency to oscillate does not indicate a deficiency in the design of the constant-current-generator.  This tendency is a physical phenomena which is inherent to the problem of trying to drive a constant current into a high "Q" resonant circuit.  Every resistivity or IP instrument (independent of manufacture) faces this problem.  Although each instrument will be selective in the number of turns or wire which will cause the oscillation.  Even same model instruments from a single manufacturer will show differences in the sensitivity to oscillate.  And, complicating matters further, that tendency to oscillate may arise or disappear with changing temperature or battery voltage.  So clearly, this is not a well-behaved problem, difficult to replicate, nearly impossible to compensate for, and exhibits dependence on many different variables.

Receiver Cable Reels - NOT a Problem

Due to the nature of the problem, high "Q" cable reels can be used in the receiver portion of the circuit without any of the above mentioned abnormal behavior. The equivalent circuits of the transmitter and receiver portions of the instrument are very different.  High "Q" resonant circuits can be added in series with the receiver circuit without any deleterious effects on the measurement.  The same cannot be said about the transmitter circuit.  This difference in behavior of the transmitter and receiver circuits may be used to advantage in attempting to diagnose an instability problem.

 

Possible Solutions to the Problem 

There are a few methods that may minimize the tendency to oscillate:

1) Use NO reels for the transmitter cables. Even an unspooled length of wire has inductance and capacitance but those values are so small and the equivalent "Q" value is so low that the constant current generator can function properly.  The constant current generator was designed and tested to drive these types of "well behaved" loads. 

2) Have only small amounts of cable on reels. Likewise, if there are very few turns of wire on the reel and/or the diameter of those turns is small then there will be little tendency for the constant current generator to break into oscillation.  There is a limit of turns of wire and spool

diameter, beyond which, the constant current generator will break into oscillation. The problem is knowing where that limit is. That limit is sensitive to battery voltage and instrument temperature.

3) Add damping mechanism to the transmitter reels. A way to minimize the tendency for constant-current-generator oscillation is to lower the "Q" of the equivalent resonant circuit.  This concept can be best understood by viewing the reel as a transformer rather than a simple coil.  If a single shorted turn is added to that transformer than the transmitter coils of wire will couple their energy (inductively) into that shorted turn.  The reel may still show some resonance phenomena but the "Q" of that resonance will be drastically reduced by the added shorted turn of wire.  And the constant-current-generator will be able to operate properly when driving such a low "Q" device.

Adding Damping Mechanism to Reels

There is a simple method for adding "damping" (the same as lowering the "Q") to existing reels of cable: 

a) Remove all wire from the cable so that the inner spool surface is exposed

b) Obtain some copper foil or thin sheet whose width is nearly as wide as the width of the inner spool surface

c) Cut the copper foil or sheet so that it fits over the inner spool surface and can overlap itself so that it becomes like a single shorted turn of wire. (A lot of overlap is not detrimental and may make the soldering operation easier.)

d) With a heavy-duty soldering iron, solder the exposed copper foil seam.  Try to solder along the entire length of the seam.

e) Protect the transmitter cable from nicks by winding electrical tape around the soldered copper foil.  Ideally, no exposed copper will show after winding with electrical tape.

f) Rewind the original cable back onto the reel. The revised cable now has had its effective "Q" lowered drastically.

 

 


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