Resistivity Probes, Installation and Readout Techniques

Resistivity gages discussed in this manual rely on the measurement of electrical resistance between conductors mounted along a cylindrical probe (see Figure 1) to determine where the soil is frozen and where it is thawed. This determination is based on the wide difference between the volume resistivity of frozen soil (from 500,000 up to several million ohms) and thawed soil (20,000 to 50,000 ohms normally). Frost penetration is determined by making sequential resistance measurements between adjacent pairs of electrodes down the resistivity probe and documenting at what depth the resistance goes from a high to a low value. It is not even necessary to actually read the resistance since a readout circuit may be used in which voltage measurements for each probe section may be ratioed to a fixed one megohm resistor located in the output circuit. This arrangement will work just as well as the actual resistance measurement since it is the shape of the curve that contains the frost depth information, not the absolute value of the measurements. Figure 1 is a sample resistivity installation. Typical curves are shown in Figure 2. For comparison, two curves of temperature vs depth are also shown, one at mid-winter conditions and one during spring thaw. The resistivity gage measurements were made by reading the resistance (or voltage drop) between solid copper rings spaced evenly on a buried PVC rod using an alternating current (ac) voltage source.