Return to Geophysical InstrumentsThe Soiltest R-40 earth resistivity system is powered by a 6-volt battery. Output ranges up to 120 volts, 60 milliamp maximum, as a 57 Hz square wave. The R-40 was designed to measure resistance through an internal bridge. However, our unit has been modified and is now used with digital volt-ohm meters (VOMs), increasing its flexibility as well as extending battery life. A pair of 15 meter cables is usually used to transmit current, and a pair of 5 meter cables used to measure potential drop. When necessary, the longer R-60 cables can be used.
Electrical resistivity is usually measured by transmitting an electrical current into the earth through a pair of electrodes and measuring the potential drop across a second pair of electrodes. The apparent resistivity of the ground is then calculated by dividing the voltage by the current and multiplying the result by a value determined by array geometry - arrangement of and spacing between electrodes.
Because the signal is "AC", electrode polarization presents no problem. Short steel pins driven a few cm into the surface are used for both transmitting current and measuring the potential drop. Schlumberger, Wenner, Lee-partitioning and dipole-dipole configurations have been used with the R-40. The choice of array depends on the nature of the study and the limits of the R-40: in some situations, R-40 power is insufficient for a successful Schlumberger, in which case we use the Wenner. Dipole-dipole with the R-40 has been successful only for very shallow imaging at archaeological sites.
The Soiltest R-60 earth resistivity system is powered by 3 45-volt and 3 225-volt batteries. Output ranges from 45 volts to 810 volts as a switch is turned, connecting batteries in series. The maximum current is about 300 milliamps. Because this unit transmits a "DC" signal, porous pot electrodes should be used for potential drop measurements. Its higher current output allows the R-60 to probe more deeply than the R-40, using either the Schlumberger or dipole-dipole configuration. The R-60 receiver is a potentiometer with 100 microvolt precision. A pair of 1000-foot cables allows for electrode spacings appropriate for probing depths on the order of 300 meters with a Schlumberger array.
Although its higher output frequently makes the R-60 an attractive choice for field projects, there are sometimes advantages to using the R-40. R-60 batteries are expensive and must be replaced when discharged. The 45-volt batteries around which R-60 was designed, as well as the mercury cell used as reference voltage in the potentiometer, are no longer available, and it is not clear how much longer substitutes will be available. The R-60 is much heavier than the R-40, and porous pots require more attention than steel pins.