Geology Reference
In-Depth Information
that can both transmit and receive. Topographic effects are cancelled by
measuring and averaging the receiver coil dip angles with first one and then
the other coil held horizontal and used as the transmitter.
8.1.2 Slingram
Most ground EM systems use horizontal co-planar coils ('horizontal loops'),
usually with a shielded cable carrying a phase-reference signal from trans-
mitter to receiver. The sight of two operators, loaded with bulky apparatus
and linked by an umbilical cord, struggling across rough ground and through
thick scrub, has provided light entertainment on many surveys. Very sen-
sibly, some instruments allow the reference cable to be also used for voice
communication. Fortunately, memory units have not (yet) been added to
record the conversations.
The Swedish term
Slingram
is often applied to horizontal-loop systems
but without any general agreement as to whether it is the fact that there are
two mobile coils, or that they are horizontal and co-planar, or that they are
linked by a reference cable, which makes the term applicable.
8.1.3 Response functions
In a Slingram survey, the electromagnetic
response
of a body,
s
, is propor-
tional to its mutual inductances (coupling) with the transmitter and receiver
coils (indicated respectively by
M
ts
and
M
sr
) and inversely proportional to its
self-inductance (
L
), which limits eddy current flow. Anomalies are generally
expressed as percentages of the primary field and are therefore also inversely
proportional to the mutual inductance (
M
tr
) between transmitter and receiver,
which determines the primary field strength. These four parameters can be
combined in a single
coupling factor
,
M
ts
M
sr
/
M
tr
L
.
Anomalies also depend on a
response parameter
that involves frequency,
self-inductance (always closely related to the linear dimensions of the body)
and resistance. Response curves (Figure 8.2) illustrate both how the in-phase
and quadrature responses would vary over targets of different resistivity
using a fixed-frequency system, and over a single target as frequency is
varied. The quadrature field dominates at low frequencies but is very small at
high frequencies, where the distinction between good and merely moderate
conductors tends to disappear.
Most single-frequency systems (except, as discussed in Section 8.2, those
used for conductivity mapping) operate below 1000 Hz, and even the multi-
frequency systems that are now the norm generally work entirely below
5000 Hz. Narrow, poor-quality conductors may produce measurable anoma-
lies only at the highest frequency or not at all.
