Geology Reference
In-Depth Information
(see Section 5.2.4). The asymmetry of the voltage curve implies frequency-
dependent phase shifts between the applied current and the measured volt-
age. In
spectral
IP surveys, these shifts are measured, in milliradians, over
a range of frequencies.
7.3 Time-Domain IP Surveys
Large primary voltages are needed to produce measurable IP effects. Current
electrodes can be plain metal stakes but non-polarising electrodes must be
used to detect the few millivolts of transient signal.
7.3.1 Time-domain transmitters
A time-domain transmitter requires a power source, which may be a large
motor generator or a rechargeable battery. Voltage levels are usually se-
lectable within a range from 100 to 500 V. Current levels, which may be
controlled through a current limiter, must be recorded if apparent resistivi-
ties are to be calculated as well as chargeabilities
Current direction is alternated to minimise the effects of natural voltages,
and cycle times can generally be varied from 2 to 16 seconds. One second
each for polarisation and reading is not usually sufficient for reliable results,
while cycle times of more than 8 seconds unreasonably prolong the survey.
7.3.2 Time-domain receivers
A time-domain receiver measures primary voltage and one or more decay
voltages or integrations. It may also be possible to record the SP, so that
chargeability, resistivity and SP data can be gathered together.
Early
Newmont
receivers integrated from 0.45 to 1.1 seconds after current
termination. The SP was first balanced out manually and the primary voltage
was then
normalised
by adjusting an amplifier control until a galvanome-
ter needle swung between defined limits. This automatically ratioed
V
p
to
V
o
for the M (chargeability) values recorded by a second needle. Experi-
enced operators acquired a 'feel' for the shape of the decay curve from the
rates of needle movement and were often able to recognise electromagnetic
transients where these persisted into the period used for voltage sampling.
Instruments with dials have now been replaced by instruments with key-
boards and display screens, but with purely digital instruments the diagnostic
information provided by a moving needle is lost and enough cycles must
be observed for statistical reduction of noise effects. Digital systems allow
more parameters to be recorded and very short integration periods, equiva-
lent to instantaneous readings, to be used. Natural SPs are now compensated
(
backed-off
or
bucked-out
) automatically rather than manually. Memory cir-
cuits store data and minimise note-taking.
