Geology Reference
In-Depth Information
Figure 9.8
Comparison of the results of audio-magnetotelluric (AMT) and
controlled-source audio-magnetotelluric (CSAMT) soundings over a simple
layered Earth (overburden 100 m thick with a resistivity of 100
m over-
lying a 100-m thick low-resistivity layer resting on bedrock with 1000-
m
resistivity). The AMT signal generates a plane wavefront at all frequencies
but the CSAMT wavefront, from a grounded wire 2 km long and 8 km away,
ceases to be effectively planar at a frequency of between 100 and 200 Hz. A
similar transition from far-field to near-field conditions would be observed
if the distance between source and receiver were gradually decreased over
homogeneous ground and measurements were made at a single frequency.
Illustration adapted from an original drawing by Kenneth Zonge.
near-field interpretation, but quality control in the field is usually carried out
using only the far-field approximation.
9.2.2 CSAMT data
In addition to the ratios of
E
x
to
H
y
, their phase difference (the
impedance
phase
) is measured in most CSAMT surveys. Even in the field it is a simple
