Geology Reference
In-Depth Information
Figure 9.1
The low-frequency natural radiation spectrum. The peaks in the
spectrum above 1 Hz are the Schumann resonances in the Earth-ionosphere
waveguide. Their exact location varies in a daily cycle controlled by the
height of the base of the ionosphere. The abbreviations VLF, ELF and ULF
stand for, respectively, very, extra and ultra low frequency.
about 1 kHz (Figure 9.2). There is thus a second minimum in the natural
radiation spectrum centred at around 2 kHz (Figure 9.1), due in part to the
strong attenuation of these frequencies in the Earth-ionosphere waveguide.
The time between the peaks increases with distance from the source event
and, inevitably, the amplitude decreases. In some cases the VLF component
is lacking and the signals are then virtually invisible on long-period plots of
the sort shown in Figure 9.2a.
Figure 9.2
Sferics. (a) Typical natural radiation recorded over a period of
two seconds. The individual sferics appear as simple spikes at this scale.
(b) A single sferic signal, consisting of an initial very low frequency (VLF)
oscillation and a longer extra low frequency (ELF) 'tail'.
