Geology Reference
In-Depth Information
investigation of a Slingram system is often quoted as being roughly equal to
twice the coil separation, provided that this is less than the skin depth (see
Figure 5.5), but this ignores the effects of target size and conductivity and
may be unduly optimistic.
Because signals in Slingram surveys are referenced to primary field
strengths, the
100% level
must be verified at the start of each day by reading
at the standard survey spacing on ground that is level and believed to be
non-anomalous. This check has to be carried out even with instruments that
have fixed settings for allowable separations, because drift is a continual
problem. Finding such ground can be a major problem.
A check must also be made for any leakage of the primary signal into the
quadrature channel (
phase-mixing
). Instrument manuals describe how to test
for this condition and how to make any necessary adjustments. Receivers
and transmitters must, of course, be tuned to the same frequency for sensible
readings to be obtained. Care is needed, because a receiver can be seriously
damaged if a transmitter tuned to its frequency is operated close by.
Figure 8.4 shows the horizontal-loop anomaly over a thin, steeply dip-
ping conductor. No anomaly is detected by a horizontal receiving coil im-
mediately above the body, because the secondary field there is horizontal.
Similarly, there will be no anomaly when the transmitter coil is vertically
above the body because no significant eddy currents will be induced. The
strongest (negative) secondary fields will be observed when the conductor
lies mid-way between the two coils. Coupling depends on target orien-
tation, and lines should be laid out across the expected strike. Oblique
intersections produce poorly defined anomalies that may be difficult to
interpret.
Readings obtained with mobile transmitter and receiver coils are plotted at
the mid-points. This is reasonable because in most cases where relative coil-
orientations are fixed, anomaly profiles are symmetrical over symmetrical
bodies and are not affected by interchanging receiver and transmitter. Even
where this is not completely true, recording mid-points is less likely to lead
to confusion than recording either transmitter or receiver coil positions.
In all EM work, care must be taken to record any environmental variations
that might affect the results. These include obvious actual conductors and
also features such as roads, alongside which artificial conductors are often
buried. Power and telephone lines cause special problems because they
broadcast noise that, although different in fundamental frequency, may have
harmonics that pass through the rejection (
notch
) filters. It is important to
check that the rejection frequencies are appropriate (i.e. 60 Hz in most of
the Americas and 50 Hz nearly everywhere else).
Ground conditions should also be noted, since variations in overbur-
den conductivity can drastically affect anomaly shapes as well as signal
