signals displayed by Figure 2(b) and (c) 
having periods of the order of 15 to 30 
seconds are characteristic of a recog- 
nized type whose occurrence and frequency 
distribution are decidedly not random. 
PROBLEMS IN MEASUREMENT 
1. The small percentage fluctuation: 
A large disturbance at 100 seconds period 
is only about 10-3 of the earth's main 
field while small signals at a 8" cycles 
per second may be as low as 107*¥, The 
amplitudes of some typical micropulsa- 
tions are shown by Figure 2. Signal 
levels such as those recorded in part (a) 
dictate the use of sensitive detecting 
systems. 
Re The amplitude-frequency relation: 
As a general rule the mean amplitude 
level increases with increasing signal 
period. Figure 2 also illustrates this 
point. Part (a) shows a typical signal 
of about 1 cps while parts (b) and ie) 
show that much greater amplitudes char- 
acterize the lower frequencies. Further, 
the longest period signals of part (c), 
technically outside the scope of this 
discussion, are of even greater ampli- 
tude because they have been attenuated 
by 20 to 30 db below the indicated scale. 
Until recently little has been known 
about the continuous background between 
0.1 and 3 cps because of the low signal 
levels and the inadequate receiving 
equipment in this frequency range. 
with the detector systems which are 
sensitive to rate of change of flux it 
is normally necessary to filter out the 
low frequencies sharply in order to 
receive these elusive signals. 
3. Wide dynamic range: The received 
signal levels may change abruptly by 
orders of magnitude. Thus, as Figure 2 
shows, the amplifiers and recording 
system must have a wide dynamic range. 
4. Man-made interference: The 
receiving site must be chosen to mini- 
mize power line, radio and especially 
teletype interference. Surprisingly low 
frequencies can occur in such interfer- 
ence and in populated areas these signals 
can be larger than the geomagnetic 
activity even when care is taken to 
choose a site as far removed from power 
lines, radio transmitters, roads, etc. 
as possible. Filters must be used to 
reduce the level of this interference. 
Moving vehicles within several hundred 
feet are of course intolerable. 
Se Effects of the site: The noise 
observed is greatly influenced by the 
conductivity and permeability structure 
near the site and it will be different 
Even 
323 
at sea and on land. Structural discon- 
tinuities can determine the total signal 
strength and its partition among field 
components, It will also materially 
affect the region of coherence of the 
signals, The site must be removed from 
trees or other objects likely to cause 
wind interference. 
DETECTORS 
It has been established that al- 
though the flux density of micropulsation 
activity is low compared to the earth's 
main field it is coherent over a large 
area, especially over regions of uniform 
sub-surface conductivity. Thus antennas 
which gather signal energy over a large 
area can do so without an appreciable 
loss of resolution, and therefore possess 
an inherent advantage over devices which 
utilize only a small area or volume for 
their activation. This paper is res- 
tricted to discuss equipment which 
utilizes large effective area fixed loop 
antennas, Within limits set by other 
considerations the most economical 
method of increasing sensitivity is to 
increase the effective area of the 
antenna, It can be shown that for a 
circular air-core coil of constant avail- 
able power the inductance-to-resistance 
ratio is approximately inversely pro- 
portional to the cube of the diameter of 
the coil, and the mass of the copper con- 
ductor to the inverse square. Hence 
large diameter coils such as those to 
be described are desirable. 
On a two-station operation near 
Ralston, Alberta, low impedance air- 
cored coils were used to detect the 
vertical component, which in that region 
is considerably smaller than the horiz- 
ontal components. The two coils were 
wound 30 miles apart, on level ground, 
electrostatically shielded and buried. 
Each was 400 feet in diameter and con- 
tained 40 turns of PVC-insulated No. 12 
wire giving a resistance of 80 ohms and 
an inductance of 1.1 henries. Another 
air-cored antenna which has been suc- 
cessfully used consists of two-conductor 
No. 16 shielded cable arrayed in a circle 
4000 feet in circumference. One advan- 
tage in such a configuration is apparent 
in that for 32 ohms resistance, with the 
conductors in series, the inductive reac- 
tance at 40 cps is only 2.8 ohms. On 
the other hand detectors which depend on 
a much smailer |e Ae of the avail- 
able flux for their activation possess 
great advantages in portability and con- 
venience. Thus coils with high permea- 
bility cores have been used in lieu of 
large rigid air-core coils for detecting 
the horizontal components. 
