Humboldt H-4385 Resistivity Meter - Manual (Page 5)

So, for example, if the H-4385 produces a resistance value of 15 ohms for an electrode
(pin) spacing of 20 feet, the earth’s average resistivity value to a depth of 20 feet would
be:
ρ = 191.5 x 20 x 15 = 57,450 ohm.cm
If the resistivity value is required to be expressed in ohm.m, rather than ohm.cm, the
ohm.cm value is divided by 100. In the above example, the resistivity would be 574.5
ohm.m
Metric Example (electrode spacing measured in meters):
Since there are 100 centimeters in 1 meter, the above formula can be written as:
ρ (Ω.cm) = 2π x 100 x (electrode spacing in meters) x R (ohms)
or, ρ (Ω.cm) = 628.32 x (electrode spacing in meters) x R (ohms)
So, for example, if the H-4385 produces a resistance value of 15 ohms for an electrode
(pin) spacing of 7 meters, the earth’s average resistivity value to a depth of 7
meters would be:
ρ = 628.32 x 7 x 15 = 65,973.6 ohm.cm
If the resistivity value is required to be expressed in ohm.m, rather than ohm.cm, the
ohm.cm value is divided by 100. In the above example, the resistivity would be
659.73 ohm.m
Note: The above formula is accurate only if the electrode depth (d) is small relative to
the electrode spacing (S). An “S” value equal to, or greater than, 20 times the “d”
value is recommended. This means that if d is 1 foot, for example, then S has to be
at least 20 feet (or, if d is 0.3 meter, then S has to be at least 6 meters)
The Wenner Method is the basis of an ASTM Standard Test Method
(ASTM G57-78).
Figure 1