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Chapter 11: Notes on AMT (natural field) versus CSAMT
(controlled source)
The difference between CSAMT and AMT acquisition is that in MT (magnetotellurics) or AMT
(audio magnetotellurics) only natural field signals are used. In CSAMT (controlled-source audio
magnetotellurics) only the transmitter signals are used. Details of the mathematical basis of
MT theory is given in Chapter 12 below. The question is when should AMT mode be used and
when CSAMT mode be used. There are advantages and disadvantages of both.
MT and AMT advantages:
1. You do not need a transmitter source other than signals naturally generated in the
atmosphere. These are mostly by distant lightning strikes, and by ionospheric currents in the
MT band.
2. The processing of MT and AMT signals requires the signals to be both far-field (plane wave)
and un-polarized (i.e. multiple polarizations). This is the nature of atmospheric signals. That
means it does not require the processing software to included transmitter terms, i.e. any
information about the transmitter can be ignored other than the assumption it is far field and
un-polarized.
3. Because there are natural field signals from as low as 0.0001 Hz to over 50,000 Hz a great
depth can be mapped using natural field MT and relatively shallow features detected in the AMT
band.
MT and AMT disadvantages:
1. Natural field signals vary in strength with the time of day, time of year, location on the earth,
and atmospheric conditions. The availability of good signal can be compromised during the
survey resulting in poor quality data.
2. For MT measurements at frequencies below 0.1 Hz acquisition time can be anywhere from 6
to 12 hours for a single sounding making low-frequency MT expensive and time consuming. This
is less of a problem with the higher-band AMT measurement.
3. Data quality is compromised from environmental noise can from power lines, cultural noise
from factories, water pumps, power generators, electrified fences, etc.
CSAMT advantages:
