A
Adrian Tuddenham
- Jan 1, 1970
- 0
I am trying to take 4-terminal ground resistance measurements in such a
way as to distinguish between shallow and deep anomalies. The basic
idea is shown at:
http://www.poppyrecords.co.uk/other/images/Electrodes3.gif
By spacing the electrodes further apart, the depth of penetration is
increased. In theory, by comparing the 'short' spacing reading with the
'long' one (proportionately weighted), the differences due to the deeper
penetration should be measurable. (There ought not to be a difference if
the ground is entirely homogenous, so anomalies will indicate the
presence of deeply buried objects.)
In practice the system is made a lot easier if five electrodes are used,
with the connections being changed as appropriate:
http://www.poppyrecords.co.uk/other/images/Electrodes4.gif
However, this raises a question: If the current path is the same for
both the 'short' and 'long' readings, the sum of the short readings
ought to equal the long one as long as the electrodes aren't disturbed
when changing over the connections. The current doesn't know where the
voltage is going to be measured and the rule of summation of voltages
should apply. In practice this does not happen and there is usually a
difference.
I have not yet tried exchanging the current and voltage leads to give
different short and long current paths with fixed voltage measurement.
One theory says this should give a different current penetration and
show up deeply buried objects - but the the theory of reprocity suggests
that there should be no overall difference between this and the
previous set-up.
I'm getting in a tangle with this, is there someone who can see the wood
for the trees and explain what is really going on?
[All measurements were made with A.C. at about 850c/s to avoid electrode
polarisation. The current was around 60mA with a bridge-type
measurement system to balance out variation in the current and a
synchronous detector to reject mains hum. Voltmeter input impedance was
around 9 Megohms with screened electrode leads using bootstrapped
screens to avoid capacitance effects]
way as to distinguish between shallow and deep anomalies. The basic
idea is shown at:
http://www.poppyrecords.co.uk/other/images/Electrodes3.gif
By spacing the electrodes further apart, the depth of penetration is
increased. In theory, by comparing the 'short' spacing reading with the
'long' one (proportionately weighted), the differences due to the deeper
penetration should be measurable. (There ought not to be a difference if
the ground is entirely homogenous, so anomalies will indicate the
presence of deeply buried objects.)
In practice the system is made a lot easier if five electrodes are used,
with the connections being changed as appropriate:
http://www.poppyrecords.co.uk/other/images/Electrodes4.gif
However, this raises a question: If the current path is the same for
both the 'short' and 'long' readings, the sum of the short readings
ought to equal the long one as long as the electrodes aren't disturbed
when changing over the connections. The current doesn't know where the
voltage is going to be measured and the rule of summation of voltages
should apply. In practice this does not happen and there is usually a
difference.
I have not yet tried exchanging the current and voltage leads to give
different short and long current paths with fixed voltage measurement.
One theory says this should give a different current penetration and
show up deeply buried objects - but the the theory of reprocity suggests
that there should be no overall difference between this and the
previous set-up.
I'm getting in a tangle with this, is there someone who can see the wood
for the trees and explain what is really going on?
[All measurements were made with A.C. at about 850c/s to avoid electrode
polarisation. The current was around 60mA with a bridge-type
measurement system to balance out variation in the current and a
synchronous detector to reject mains hum. Voltmeter input impedance was
around 9 Megohms with screened electrode leads using bootstrapped
screens to avoid capacitance effects]