Sources of Noise
Even given the simple experiment outline on the previous page, there are a number of sources of noise that can affect our measurements of voltage and current from which we will compute apparent resistivities.
- Electrode Polarization - A metallic electrode, like a copper or steel rod, in contact with
an electrolyte other than a saturated solution of one of its own salts, like ground water,
will generate a measurable contact potential.
In applications such as SP, these contact potentials can be larger than
the natural potential that you are trying to record.
Even for the DC methods described here, these potentials can be a significant fraction of the total
potential measured.
For DC work, there are two possible solutions.
- Use nonpolarizing electrodes. These are electrodes that contain a metallic conducting rod in contact with
a saturated solution of its own salt. Copper and copper sulfate solution are commonly used.
The rod and solution are placed in a porous ceramic container that allows the saturated solution to slowly
leak out and make contact with the ground.
Because these solutions are rather environmentally unfriendly, and because the method described below is
easy to employ, these so-called porous pot electrodes are rarely used in DC work.
They are, however, commonly used in SP and IP surveys.
- A simple method to avoid the influence of these contact potentials is to periodically reverse the current flow in the current electrodes or use a slowly varying, a few cycles per second, AC current. As the current reverses, the polarizations at each electrode break down and begin to reverse. By measuring over several cycles, robust current and voltage measurements can be made with negligible polarization effects.
- Use nonpolarizing electrodes. These are electrodes that contain a metallic conducting rod in contact with
a saturated solution of its own salt. Copper and copper sulfate solution are commonly used.
The rod and solution are placed in a porous ceramic container that allows the saturated solution to slowly
leak out and make contact with the ground.
Because these solutions are rather environmentally unfriendly, and because the method described below is
easy to employ, these so-called porous pot electrodes are rarely used in DC work.
They are, however, commonly used in SP and IP surveys.
- Telluric Currents -As described previously, naturally existing currents
flow within the earth. These currents are referred to as telluric currents.
The existance of these currents can generate a measurable voltage across the potential electrodes even when
no current is flowing through the current electrodes.
By periodically reversing the current from the current electrodes, or by employing a slowly varying AC current,
the effects of telluric currents on the measured voltage can be cancelled.
- Presence of Nearby Conductors -Electrical surveys can not be performed around conductors that make
contact with the ground. For example, the presence of buried pipes or chain-linked fences will act as
current sinks. Because of their low resistivity, current will preferentially flow along these structures
rather than flowing through the earth.
The presence of these nearby conductors essentially acts as electrical shorts in the system.
- Low Resistivity at the Near Surface -Just as nearby conductors can act as current sinks that short out
an electrical resistivity experiment, if the very near surface has a low resistivity, it is difficult to get
current to flow more deeply within the earth.
Thus, a highly conductive* near-surface layer such as a perched water table can prevent current from flowing
more deeply within the earth.
- Near-Electrode Geology and Topography - Any variations in geology or water content localized
around an electrode that produce near-surface variations in resistivity can greatly influence
resistivity measurements.
In addition, rugged topography will act to concentrate current flow in valleys and disperse current flow
on hills.
- Current Induction in Measurement Cables - Current flowing through the cables connecting the current source to the current electrodes can produce an induced current in the cables connecting the voltmeter to the voltage electrodes, thereby generating a spurious voltage reading. This source of noise can be minimized by keeping the current cables physically away from, a meter or two, the voltage cables.
*Conductivity is the opposite of resistivity. Highly conductive media transmit electrical current with great ease and thus have a low resistivity. Mathematically, conductivity is the reciprical of resisitivity and is measured in the units of 1 over Ohm meters. One over Ohm is referred to as a siemen (S). Thus, the units of conductivity are siemens per meter.
