Geology Reference
In-Depth Information
the receiver can analyse the voltage waveform to extract the high-frequency
effects.
7.4.2 Frequency/phase receivers
Sophisticated receivers are needed to analyse waveforms and extract fre-
quency effects from either single- or dual-frequency transmissions, but this
sophistication is normally not apparent to an operator recording PFEs from
a front panel display.
To measure phase differences for multi-frequency (spectral) IP surveys, a
common time reference for transmitter and receiver is essential. Reference
cables are operationally inconvenient and could increase inductive coupling,
and crystal clocks that could be synchronised at the start of a day's work
were used to avoid this problem. These were designed to drift no more than
a fraction of a millisecond in 24 hours, but the continual updating possible
using GPS signals is now generally preferred.
7.4.3 Phase measurements
A typical spectral IP plot is shown in Figure 7.4. The frequency at which
the maximum phase shift occurs is dependent on grain size, being higher
for fine-grained conductors. The sharper the peak, the more uniform the
grain size. Most attempts to distinguish between different types of IP source
are now based on analysis of these spectral curves, since grain size may be
correlated with mineral type. However, exploration programmes soon reach
the point at which further theoretical analysis of IP curves is less effective
than drilling a few holes.
At high frequencies, increases in phase shifts are caused by electromag-
netic coupling. Simple
decoupling
calculations involve readings at three dif-
ferent frequencies and assume a quadratic relationship (i.e. ϕ
=
A
+
Bf
+
Cf
2
) between phase-shift and frequency. The three readings allow this equa-
tion to be solved for A, the
zero-frequency
phase shift value. At most survey
points only the value of A will be worth recording, but at points that are
clearly anomalous an entire phase spectrum, using many more than the three
basic frequencies, may be stored for further processing.
7.4.4 Comparison of time- and frequency-domain methods
The relationship between polarisation and current is not precisely linear. This
not only limits the extent to which time, frequency and phase measurements
can be interrelated, but can also affect comparisons between different surveys
of the same type. The effects generally do not exceed a small percentage,
but provide yet another reason for the very qualitative nature of most IP
interpretation.
