THE SMOOTH SOUNDING GRAPH. A Manual for Field Work ... - BGR

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2.1. How to carry out a field measurement
Now we drive on a measuring car into the field to record a sounding
curve. For this we choose a suitable point in the considered measuring
area, which is the centre point of the measuring lay-out. One shall pay
attention to the potential electrodes. Their site should always be on natu-
rally grown earth.
The direction of the lay-out can be determined by an angle reflector. Then
the measuring car will drive into the right position to the site of the po-
tential electrodes.
In Fig.24 we show a wrong placing of the car. The distance between
measuring car and the potential electrodes is too small, so that already
low leakage currents can more or less influence the potential electrode
voltage. Those leakage currents flow into the earth either via the car or
the cable drums, depending on the resistivity of the first layer.
In Fig.25 the right placing of the measuring car is to be seen. The dis-
tance to the potential electrodes should amount to at least 20m and the
car as well as the cable drums should stand on the “0”-line between the
potential electrodes, running vertically to the potential electrode extent.
This placing of the measuring car should be desirable in every case.
The following processing is schematically demonstrated in Fig.26. When
the car has the right position, the measuring lay-out can be built up. The
cable drums are taken out of the car and set to ground. In order to
enlarge the insulation between the cable drums and the ground, it is rec-
ommended to pose a rubber-mat below each cable drum (Fig.27). On
each drum there is in addition to the electrode cable a 100m long meas-
uring tape (wire of insulating material), on which points are marked for
usual electrode distances according to the table with the geometric factors
K (on page 36). The assistants are walking (or better running) in opposed
directions pulling the measuring tapes. By a stick they fix these measur-
ing tapes at both ends.