Geology Reference
In-Depth Information
lower frequencies, and will still contain information on ground resistivity,
but the equations that have to be solved to extract the resistivity information
become much more complex.
The CSAMT depth of investigation (
d
inv
in Figure 9.10) is closely related
to the far-field limit (
d
). They are both linked to the skin-depth and pro-
portional to the square root of resistivity divided by frequency. The greatest
depth of investigation is achieved at the frequency at which the far-field
limit and the range maximum (
D
) coincide. Its actual value does not depend
explicitly on the frequency or resistivity but is, if the widely quoted far-field
limit of three times the skin depth is correct, a little less than a quarter of the
power-related system range (Figure 9.10). This provides a quick way to as-
sess system capability. Thus, the range maxima quoted for the 'normal' and
high-power versions of the Geometrics Stratagem are 400 m and 800 m re-
spectively, implying maximum investigation depths of about 100 and 200 m.
It would, of course, be logistically very difficult to carry out surveys that
achieve this theoretical maximum penetration, because the source would
have to be moved in concert with the receivers. As an alternative, CSAMT
and MT/AMT surveys can be combined, with CSAMT used principally to
fill in the frequency 'gaps' where natural signal strengths are inadequate.
CSAMT surveys require transmitters capable of providing signals over
a wide range of frequencies, and time is needed to record data at each
frequency in turn. An alternative approach is to use the inherently multi-
frequency characteristics of transient signals (see Section 5.2.4). This tech-
nique, known as LOTEM (
Long Offset Transient Electro-Magnetics
)isbeing
used to a limited extent in a variety of applications.
