HT GEO416, User Manual

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GEO416 - GEO416GS
EN - 27
The measuring method allows defining the specific resistivity of a ground layer up to the
depth corresponding approximately to the distance “
a ” between the rods. If you increase
the distance “ a ” you can reach deeper ground layers and check the ground homogeneity.
After several measurements you can trace a profile according to which the most suitable
rod is chosen.
Curve 1 : as


decreases only in depth, it’s
advisable to use a very deep rod
Curve 2 : as

E
decreases only until the depth
a, it’s not useful to increase the
depth of the rod beyond a
Curve 3 : the ground resistivity is quite
constant, so increasing depth does
not make

E
decrease, therefore a
ring rod must be used
Fig. 10: Measurement of ground resistivity
11.3.1. Approximate evaluation of intentional rods' contribution
The resistance of a rod Rd can be calculated with the following formulas (

= average
resistivity of the ground).
a) resistance of a vertical rod
Rd =

/ L
where L = length of the element touching the ground
b) resistance of an horizontal rod
Rd = 2

/ L
where L = length of the element touching the ground
c) resistance of linked elements
The resistance of a complex system made of more elements in parallel is always higher
than the resistance, which could result from a simple calculation of single elements in
parallel, especially if those elements are close to each other and therefore interactive. For
this reason, in case of a linked system the following formula is quicker and more effective
than the calculation of the single horizontal and vertical elements:
Rd =

/ 4r
where r = radius of the circle which circumscribes the link