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Typically, a soil probing system is outfitted with four electrodes (metal bands) near its point. An electric current is connected to one or two of the electrodes (depending on the measurement method used), and the voltage difference is then measured between the electrodes. The greater the voltage difference, the lower the electrical conductivity of the soil which is in contact with the probe.
Soil conductivity can be influenced by many factors. However, high conductivities are usually associated with clay-rich soils, and low conductivities are usually associated with sandy and gravelly soils. This is a result of the shape and physical properties of the particles which make up the soils.
Clay particles tend to be pancake-shaped and lie flat against each other, giving them a high degree of surface-to-surface contact and thus allowing electricity to pass more easy from grain to grain. Sand grains tend to be more spherical and thus they have a lower degree of grain-to-grain contact. Changes in conductivity can also be caused by changes in water content of the soil and by soil or groundwater contamination.
Geoprobe Systems, Inc. is widely known for the development of percussion soil probing equipment for the collection of soil vapors, soil cores, and groundwater samples. A new probing system was recently developed to enable the direct sensing of soil electrical conductivity.
The primary purpose of the conductivity logging system is for the determination of variation in soil conditions with depth, which can then be used to aid in the interpretation of subsurface geology and groundwater or contaminant movement. Soil conductivity measurements have been used in the past by agricultural scientists for the purpose of determining soil salinity. Cone penetrometer systems also have been used to measure soil resistivity.
The sensing portion of the probe consists of a steel shaft running through the center of four stainless steel contact rings. An engineering grade plastic electrically isolates the rings and the shaft from each other. This portion of the probe is approximately eight inches long and has a diameter the tapers from 1-1/8 inch at the top to 1 inch at the point. The taper provides better ground-to-probe contact than would be possible in a straight configuration. The probe assembly threads directly to standard Geoprobe probe rods.
A signal cable is threaded through the inside of the rod string and into a PC-based data acquisition system housed in a ruggedized case. Depth measurements are obtained by a stringpot system configured to measure the distance from the driving mechanism to the ground surface. The stringpot signal is used both to determine probe position in the ground and speed at which the probe is advancing.
Two different electrode arrays can be used with the system -- the Schlumberger array and the dipole array. The Schlumberger array reacts linearly to variations in formation conductivity and yields good vertical resolution. The dipole array does not react linearly but has improved vertical resolution due to the more closely-spaced electrodes and has the added benefit of only requiring two of the four electrodes, thus allowing the second pair to be used as backup.
Repeatability and calibration tests have been published by Geoprobe, and there may be evidence that the system can detect liquid phase resistive organic contaminants in the ground.
On-site demonstration by contractor
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Last Modified: 1 January 2002 by dave eckels
Expedited Site Characterization: etd/technologies/projects/esc/technologies/elecconductivity.html
