Geology Reference
In-Depth Information
Figure 7.4 Typical plot of induced polarisation (IP) phase and amplitude
against frequency.
The relative merits of time- and frequency-domain IP have long been
argued, especially by rival instrument manufacturers. Time-domain surveys
are essentially multi-frequency and the shapes of decay curves provide
information equivalent to that obtained by measurements at several different
frequencies in frequency-domain or phase work. It is, moreover, generally
conceded that PFEs and phase shifts are more vulnerable to electromagnetic
interference than are time-domain chargeabilities, and that the additional
readings needed if correction factors are to be calculated take additional time
and demand more sophisticated instruments. However, frequency-domain
surveys require smaller currents and voltages and may be preferred as safer
and using more portable instruments. The final choice between the two
usually depends on personal preference and instrument availability.
7.5 IP Data
The methods used to display IP data vary with the array. Profiles or contour
maps are used for gradient arrays, while dipole-dipole and pole-dipole data
are almost always presented as pseudo-sections. In all surveys, the spacing
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