Metallic minerals are relatively good conductors of electricity, whereas most of the common rock forming minerals are generally poor conductors.  This fact is the basis for geophysical exploration methods which measure conductivity to evaluate the metal content, and to some extent the geometry, of subsurface mineral deposits.  These methods are generally most useful at shallow depths (<500 feet), but are occasionally used for applications at greater depths.   Electrical methods can be used either on the surface or down drill holes.  Some electrical methods, called “active” methods,  introduce electricity into the ground, creating an artificial electrical field in which charges in the electrical current between electrodes can be measured.  “Passive” methods measure current flow related to naturally occurring electrical currents.  These methods measure the electric potentials, which develop due to the electrochemical action between minerals and pore fluids.

Metallic sulfide minerals or graphite are the most efficient mineral conductors.  Pore waters contained in underground formations also conduct electricity very well, and it is the very presence of these waters which makes electrical prospecting methods possible.  In most rock materials, the amount of porosity and the chemistry of pore waters have a greater influence on conductivity than do metallic mineral grains.  Where the pore waters contain salts (such as sodium chloride) in solution, the methods work especially well.  Clay minerals containing only a slight amount of moisture are also easily ionized. 

 

Resistivity

Measuring the conductivity of a rock involves measuring its resistance to the flow of electricity, or its “resistivity”.  Conductivity and resistivity are inversely related: high conductivity equates to low resistivity.   At a constant voltage, the relationship between resistance and current are expressed mathematically by Ohm’s Law: 

where V is the voltage ( in volts), I is the current (in amps), and R is the resistance (in ohms).

The amount of resistance is a function of the composition and physical condition of the rock.  In order to measure the amount of resistance of the rock we must specify two other factors, including the length and cross-sectional area of the region where the electrical current is being conducted, specifically the region of a cylinder where the current is passed.  When these factors are specified, the amount of resistance is referred to as the “resistivity”. The formula for resistivity is:

where r is the resistivity (in ohm-meters), S is the unit area of the cylinder cross-section, and l is the unit length of the cylinder.

In practice, several different pairs of electrodes are set up at different spacings.  As the spacing between the electrode pairs increases, the detection depth increases.  In this manner, changes in resistivity with depth can be plotted.  This sequential testing technique also detects lateral changes in resistivity along the survey line.  This information can be used to pinpoint zones which have strong resistivity contrasts.  Zones with distinctly high resistivity (low conductivity) may correspond to areas containing abundant quartz or silicification, such as vein deposits.  Zones with distinctly low resistivity may correspond to zones containing abundant metallic sulfides.

Although the range of resistivities for rocks and minerals is quite large, normal values for common rocks and minerals are fairly well established (Table T8).   Normally, resistivity does not vary as the frequency is varied.  However, if native metals or other metallic minerals are present, the resistivity changes dramatically as the frequency is varied. 

Common Rocks/Materials	Resistivity (ohm meters)	Ore Minerals	Resistivity (ohm meters)
Clay	1 – 100	Pyrrhotite	0.001 – 0.01
Graphitic Schist	10 – 500	Galena	0.001 – 100
Topsoil	50 – 100	Cassiterite	0.001 – 10,000
Gravel	100 – 600	Chalcopyrite	0.005 – 0.1
Weathered Bedrock	100 – 1000	Pyrite	0.01 – 100
Gabbro	100 – 500,000	Magnetite	0.01 – 1,000
Sandstone	200 – 8,000	Hematite	0.01 – 1,000,000
Granite	200 – 100,000	Sphalerite	1000 – 1,000,000
Basalt	200 – 100,000
Limestone	500 – 10,000
Slate	500 – 500,000
Quartzite	500 – 800,000
Greenstone	500 – 200,000

Induced Polarization

Current flowing through the ground causes some rocks to become electrically polarized or “charged” the way a car battery is charged by an alternator when the car is running.  Chargeability is measured by sending a pulsating current through the ground. 

Most modern electrical surveys measure both the resistivity and the chargeability (or “I.P. effect”).  The same equipment and electrode configuration can be used, however, there are at least eight different electrode configurations or arrays in use. High chargeability is usually caused by the presence of metallic or conductive minerals.  Low resistivity in rocks having a low chargeability is more likely due to conductive pore waters or moist clays instead of metallic minerals.

Induced Polarization Surveys:   The results of an I.P. survey generally plot chargeability values in a special type of profile, or cross-section, known as a “psuedo-section” (Figure F12).   A psuedo-section shows changes in electrical values with respect to depth below the surface and distance along a survey line.  The values are plotted at the intersections of lines sloping at 45 degrees from the dipole centers.  The values are plotted at regular depth intervals which correspond to the dipole spacing intervals.  Color coding and/or contour of the values is normally used to help accentuate the geometry of a conductor, for example the dip of a vein or mineralized strata.  A small, shallow conductor tends to produce what is called a “pant’s leg” anomaly as shown in Figure F12.

Figure F12:   Psuedo-section showing chargeability and “pants leg” anomaly associated with a shallow conductor (from Milsom, 1996).

 

Self  Potential

Self potential is natural electrical potential in rocks.  It is caused by electrochemical action between minerals and groundwater solutions.  When this action occurs in the oxidizing zone above the water table, current is generated (Figure F13)  An ore body containing metallic minerals, acting as a conductor, carries the current downward towards the reducing zone below the water table.  The overall effect is to create a negative potential in the rocks around the ore body as the electrons move downward.  Ore deposits containing pyrite, an iron sulfide mineral which oxidizes readily to hematite (iron oxide), develop the strongest negative self potentials.  Other minerals which are known to generate this type of pattern are pyrrhotite and magnetite.  Self potential fields associated with lead and zinc minerals are not very significant, so measuring self potential for these types of mineralization has not been very useful.

Figure F13:   Current flow and natural self potential field developed around a sulfide ore body (from Dobrin, 1976).