Geology Reference
In-Depth Information
9.1.6 The tipper
Magnetic-component results in most AMT surveys are presented in terms of
a measure of the tilt of the polarisation ellipse known as the
tipper
, formally
defined by the equation:
H
z
=
T
x
·
H
x
+
T
y
·
H
y
where
H
z
is the vertical magnetic field component at the selected frequency,
H
x
and
H
y
are the field components in any two horizontal directions at right
angles to each other, and
T
x
and
T
y
are the corresponding tipper components.
All the quantities in this equation are
complex
, involving the square root of
−
1 (see Section 5.2.3), and the expression above therefore represents two
equations rather than just one. They can be solved by squaring and adding.
Conventionally, the positive root is always taken so that, unlike the dip-angle,
the tipper is always positive.
Two steeply dipping conductors close to each other produce a tipper
anomaly similar to the sum of the anomalies that would have been produced
by each body individually. Where, however, one of the bodies is steeply
dipping and the other flat-lying, the results are more difficult to anticipate.
One important case of this type is provided by the presence of conductive
overburden, which influences, and can actually reverse, the phase of the
secondary field.
9.1.7 MT practicalities
Magnetotelluric and AMT surveys require only receivers, not transmitters,
and in many cases these are multi-purpose instruments that can equally well
be used for resistivity, IP, CWEM and TEM surveys. In principle, three
electrical and three magnetic field components could be measured at each
site, but the E2-H3 configuration (Figure 9.7) is more common. The vertical
electric field is the most difficult to measure and is frequently ignored, but the
Barringer E-Phase airborne VLF system, which was operated successfully
during the 1970s, used this component as a phase reference, and its use may
again become fashionable in the future. The comparatively long time needed
to collect stable AMT data has led to the use of multiple receivers, and the
cost of doing so can be kept down by 'slaving' several relatively simple units
with reduced numbers of recording channels to a single full-channel set.
It is often assumed that the vertical magnetic field is more stable than
the other components, and can therefore be measured at more widely
spaced points. Wherever electrical fields are to be measured, the sensors
are pairs of grounded porous-pot (non-polarising) electrodes (see Section
6.2.2) separated by distances of 50-100 m. Magnetic sensors are SQUID
