Geology Reference
In-Depth Information
9
REMOTE-SOURCE ELECTROMAGNETICS
Some electromagnetic surveys use sources that are at very large distances
from the detectors. The actual separation then becomes irrelevant, and the
penetration achieved is limited by frequency and conductivity/resistivity, via
the skin-depth (see Section 5.2.5).
Natural electromagnetic radiation covers a broad range of frequencies.
Longer wavelengths (lower frequencies) are associated with ionospheric
micropulsations, but most of the radiation used in audio-frequency mag-
netotelluric (AMT or AFMAG surveys, at above 1 Hz) comes from dis-
tant thunderstorms. The strength of these waves varies considerably with
both time and frequency, and natural radiation surveys may be supple-
mented or replaced by methods using signals produced by distant artificial
sources.
Not all the artificial signals used are transmitted with geophysics in mind.
The construction, from about 1960 onwards, of large numbers of high-power
military transmitters working in the 15-25-kHz
very low frequency
(VLF)
band gave rise to an entire subset of exploration geophysics, and also led to
interest in the geophysical use of more conventional (i.e. higher frequency)
radio signals. Many of the military transmitters have now been decommis-
sioned and only the radio-frequency systems continue in geophysical use,
for applications that require high precision and little penetration. The VLF
band is still very important, but is observed within the wider context of the
entire natural frequency spectrum.
9.1 Natural Electromagnetic Radiation
Currents produced in the ionosphere by the impact of the protons of the
'solar wind' dominate the terrestrial electromagnetic spectrum at frequencies
below 1 Hz. There is a 'dead band' between about 0.5 and 5 Hz (Figure 9.1),
beyond which the signals (
sferics
), at frequencies up to about 20 kHz, are
mainly generated by thunderstorms in the tropical rainbelts of Africa, South
America and Indonesia. A single sferic typically consists of an initial higher
frequency (VLF) oscillatory portion with maximum spectral power in the
4-10 kHz range, followed by a long ELF tail with power concentrated below
