Geology Reference
In-Depth Information
ϕ
ϕ
ϕ
ϕ
Figure 8.7
Slope corrections for a two-coil system calibrated for use in
co-planar modes. Readings should be multiplied by the appropriate factors.
need to carry lists of the coil tilts required at each station. Instruments that
incorporate tilt meters and communication circuits are virtually essential,
but even so errors are depressingly common and noise levels tend to be
high. A simpler option is to use the vertical co-planar configuration, which
does not rely on a horizontal reference, but penetration is reduced and less
interpretational material is available. Moreover, not all systems can be easily
used in this way.
8.1.7 Applying the corrections
For any coupling error, whether caused by distance or tilt, the in-phase field
that would be observed with no conductors present can be expressed as a
percentage of the maximum-coupled field,
F
o
.
A field calculated to be 92% of
F
o
because of non-maximum coupling
can be converted to 100%
either
by adding 8%
or
by multiplying the actual
reading by 100/92. If the reading obtained actually were 92%, these two
operations would produce identical results of 100%. If, however, there were a
superimposed secondary field (e.g. if the actual reading were 80%), addition
would correct only the primary field (converting 80 to 88% and indicating the
presence of a 12% anomaly). Multiplication would apply a correction to the
secondary field also and would indicate a 13% anomaly. Neither procedure
