Geology Reference
In-Depth Information
The small alternating currents induced in the Earth by variations in the
ionospheric component of the magnetic field and by thunderstorms are
discussed in Chapter 9. Only the long-period components of the associated
voltages, seldom amounting to more than 5 mV, can be detected by the DC
voltmeters used in SP surveys. If, as is very occasionally the case, such
voltages are significant, the survey should be repeated at different times of
day so that results can be averaged.
7.1.2 SP surveys
Voltmeters used for SP work must have millivolt sensitivity and very high
impedance so that the currents drawn from the ground are negligible. Copper/
copper-sulphate 'pot' electrodes (see Section 6.2.2) are generally used,
linked to the meters by lengths of insulated copper wire.
An SP survey can be carried out by using two electrodes separated by
a small constant distance, commonly 5 or 10 m, to measure average field
gradients. The method is useful if cable is limited, but errors tend to accumu-
late and coverage is slow because the voltmeter and both electrodes must be
moved for each reading. More commonly, voltages are measured in relation
to a fixed base. One electrode and the meter remain at this point and only
the second electrode is moved. Sub-bases must be established if the cable is
about to run out or if distances become too great for easy communication.
Voltages measured from a base and a sub-base can be related provided that
the potential difference between the two bases is accurately known.
Figure 7.2 shows how a secondary base can be established. The cable
is almost fully extended at field point B, but it is still possible to obtain a
reading at the next point, C, using the original base at A. After differences
have been measured between A and both B and C, the field electrode is left
at C and the base electrode is moved to B. The potential difference between
A and B is thus estimated both by direct measurement and by subtracting
the B to C voltage from the directly measured A to C voltage. The average
difference can be added to values obtained with the base at B to obtain values
relative to A.
Figure 7.2
Moving base in an SP survey. The value relative to A at the new
base (B) is measured directly and also indirectly by measurements of the
voltage at the field point C relative to both bases.
