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  Direct-Current Resistivity Methods, Physics tutorial

Direct Current Resistivity Methods:

The currents utilized in surveys explained as direct currents or DC is seldom really unidirectional. Reversing direction of flow permits effects of unidirectional natural currents to be eliminated by just summing and averaging results attained in two directions. DC surveys need current generators, voltmeters and electrical contact with ground. Cables and electrodes are cheap but very important parts of all systems, and it is with these that much of noise is associated.

Metal electrodes:

Electrodes utilized to inject current in ground are nearly always metal stakes that in dry ground may have to be hammered in depths of more than 50 cm and be watered to enhance contact. Where contact is very poor, salt water and multiple stakes may be utilized. In tremendous cases, holes may have to be blasted through highly resistive caliche or laterite surface layers. Metal stake electrodes come in several forms. Lengths of drill steel are excellent if ground is stony and heavy hammering essential.

Pointed lengths of angle-iron are only slightly less strong and have larger contact areas. If ground is soft and main consideration is speed, large numbers of metal tent pegs can be pushed in along traverse line by advance party. Problems can arise at voltage electrodes, as polarization voltages are produced wherever metals are in contact with groundwater. Though, reversal of current flow that is routine in conventional DC surveys normally attains acceptable levels of cancellation of these effects. Voltage magnitudes depend on metals concerned. They are, for example, small when electrodes are composed stainless steel.

Non-polarizing electrodes:

Polarization voltages are potentially serious sources of noise in SP surveys that involve measurement of natural potentials and in induced polarization (IP) surveys. In such cases, non-polarizing electrodes should be utilized. Their design depends on fact that one exception to rule that a metallic conductor in contact with electrolyte produces contact potential takes place when metal is in contact with saturated solution of one of its own salts. Most non-polarizing electrodes comprise of copper rods in contact with saturated solutions of copper sulphate. Rod is attached to lid of container or pot with porous base of wood, or, more usually, unglazed earthenware. Contact with ground is made via solution which leaks through base. Few solid copper sulphates must be kept in pot to make sure saturation and temptation to top up with fresh water should be resisted, as voltages will be produced if any part of solution is less than saturated. High resistance of the electrodes is not usually significant as currents must not flow in voltage-measuring circuits.

In induced polarization surveys it may very irregularly be desirable to employ non-polarizing current electrodes but not only does resistance then become problem but also electrodes deteriorate rapidly because of electrolytic dissolution and deposition of copper. Copper sulphate solution gets everywhere and decomposes everything and, despite some theoretical benifts, non-polarizing electrodes are seldom utilized in routine DC surveys.

Cables:

Cables utilized in DC and IP surveys are usually single core, multi-strand copper wires insulated by plastic or rubber coatings. Thickness is generally dictated by need for mechanical strength rather than low resistance, as contact resistances are almost always very much higher than cable resistance. Steel reinforcement may be required for long cables.

In virtually all surveys, at least two of four cables will be long, and good practice in cable handling is necessary if delays are to be avoided. Multicore cables which can be connected to multiple electrodes are becoming increasingly popular, as, once cable has been laid out and connected up, series of readings with diverse combinations of current and voltage electrodes can be prepared using selector switch. Power lines can be sources of noise, and it may be essential to keep survey cables well away from their obvious or suspected locations. 50 or 60 Hz power frequencies are very different from 2 to 0.5 Hz frequencies at which current is reversed in most DC and IP surveys but can affect very sensitive modern instruments, mainly in time-domain IP work.

Cables are generally connected to electrodes by crocodile clips, as screw connections can be hard to use and are simply damaged by careless hammer blows. Clips are, though, easily lost and every member of field crew must carry at least one spare, screwdriver and small pair of pliers.

Generators and transmitters:

The instruments which control and compute current in DC and IP surveys are called as transmitters. Most deliver square wave currents, reversing direction of flow with cycle times of between 0.5 and 2 seconds. Lower limit is set by need to minimize inductive (electromagnetic) and capacitative effects, upper by need to attain acceptable rate of coverage. Power sources for transmitters may be dry or rechargeable batteries or motor generators. Hand-cranked generators (Meggers) have been utilized for DC surveys but are now very unusual. Outputs of numerous kVA may be required if current electrodes are more than one or two hundred metres apart, and generators then utilized are not only not very portable but supply power at levels which can be lethal. Stringent precautions should then be observed, not only in handling electrodes but also in ensuring safety of passers-by and livestock along whole lengths of current cables.

Receivers:

The instruments which compute voltage in DC and IP surveys are called as receivers. Primary requirement is that negligible current be drawn from ground. High sensitivity moving-coil instruments and potentiometric (voltage balancing) circuits were once utilized but have been almost completely replaced by units based on field-effect transistors (FETs).

In most of low-power DC instruments now on market, transmitters and receivers are combined in single units on which readings are shown directly in ohms. To permit noise levels to be evaluated and SP surveys to be performed, voltages can be estimated even when no current is being supplied. In all other situations, current levels should be either predetermined or observed, as low currents may affect validity of results. In modern instruments desired current settings, cycle periods, numbers of cycles, read-out formats and, in few cases, voltage ranges are entered via front-panel key-pads or switches. Number of cycles utilized represents the compromise between speed of coverage and good signal-to-noise ratio. Reading is generally updated as each cycle is finished, and number of cycles chosen must be enough to permit reading to stabilize.

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