Equipment for Soil Resistivity Measurements
Radio, 1967, 1
Large heterogeneity or objects in the surface layer of soil can be found by detecting the changes in electrical resistance of the soil at the depth of their location. The figures 1 and 2 shows a diagram of the device for measures the relative change in resistance of the soil. This device may be useful in archaeological excavations.
Fig. 1
Fig. 2
The device consists of a measuring bridge (Fig. 1), the ac voltage generator (Fig. 2) and two probes, driven in the ground. Each arm of the measuring bridge circuit is composed of resistors and the resistance of the soil layer between the electrodes connected to the bridge leads.
Before measuring bridge must to be balanced by resistor 100 Ohm, i.e., the meter pointer 50μA must be set to the centre "zero".
Fig. 3
The design of the probe is shown in the Figure 3. Each of the probes is an isolated rod 15 mm in diameter, made of wood. On the surface of the rod along its axis is placed rigidly fixed electrodes, made in the form of six thin-walled tubes, isolated from each other.
Fig. 4
Each electrode probe with six-core cable is connected to the switch S1 (fig. 1), which allows you to connect to the bridge one of the six pairs of electrodes. In this case each pair of electrodes, and hence each position of the switch S1 corresponds to a certain depth of soil layer.
By inserting probes into the ground, as shown in Figure 4, it could be measured the electrical resistance of soil layers located at different depths.
Depth of buried items (the layer) could be found by comparing the achieved values of resistance.
The distance between the probes adjusted in almost every case. Sometimes, good results are obtained by a distance equal to 2.4 m.
Variable resistor 500 ohm of the circuit is used to adjust the sensitivity of the bridge depending on the type of soil.
"The Radio-Constructor", 1966, №8