Geology Reference
In-Depth Information
In a time-domain IP survey, the cycle period of the transmitter must be
manually entered into the receiver so that it can lock on to the transmis-
sions without use of a reference cable (which could carry inductive noise).
Synchronisation can also be achieved using GPS timing signals (see Sec-
tion 15.2.5). Cycle times of 4, 8 or 16 seconds are now generally favoured.
Changing the cycle time can produce quite large differences in apparent
chargeability, even for similar delay times, and chargeabilities recorded by
different instruments are only vaguely related.
7.3.3 Decay-curve analysis
With readings taken at several different delay times, curve analysis can be
attempted. A method first suggested for use with Huntec receivers assumed
that each decay curve was a combination of two exponential decays, corre-
sponding to electrode and membrane polarisations, which could be isolated
mathematically. This is far too drastic a simplification of the actual physical
processes, and the separation, using a limited number of readings, of two
exponential functions that have been added together is in any case virtually
impossible in the presence of even small amounts of noise. Nonetheless,
research continues into the controls on decay-curve shapes, and charge-
abilities should be recorded at as many decay times as are conveniently
possible in areas of interesting anomaly. In non-anomalous areas a single
value generally suffices.
7.4 Frequency-Domain Surveys
Quite small currents and voltages can be used for resistivity measure-
ments, and frequency-domain transmitters can therefore be lighter and more
portable than their time-domain equivalents. Special care has to be taken
when positioning cables, to minimise electromagnetic coupling. Coupling is
increased by increasing the spacing within or between dipoles, by increas-
ing frequency and by conductive overburden. Unfortunately, field crews may
have very limited control over this final factor (although swampy ground can
sometimes be avoided), and are also forced to use large electrode separations
if deep targets are being sought.
7.4.1 Frequency-domain transmitters
Square waves are commonly used for work in the frequency domain as
well as in the time domain, and most modern IP transmitters can be used
for both. Measuring resistivity at two frequencies in separate operations is
time-consuming and does not allow precise cancellation of random noise,
and complex waveforms are therefore used to take effectively simultaneous
readings at two different frequencies. Simple square waves may be used if
