EQUIPMENT FOR OBSERVATION OF THE NATURAL ELECTRO- 
MAGNETIC BACKGROUND IN THE FREQUENCY RANGE 0.01-30 
CYCLES PER SECOND 
by W.N. ENGLISH, D. J. EVANS, J.E. LOKKEN, J. A. SHAND, 
and C.S. WRIGHT 
Pacific Naval Laboratory 
Defense Research Board of Canada 
Esquimalt, British Columbia 
ABSTRACT 
The instruments designed and con- 
structed for measuring the geomagnetic 
background between 0.01 and 30 cps, 
which has a great dynamic range in fre- 
quency and time, are described. Large 
effective area detector loops combined 
with high gain, very low noise ampli- 
fiers form an effective receiving sys- 
tem. Methods of absolute calibration 
are discussed. 
INTRODUCTION 
Because sea water is an electrical 
conductor, electromagnetic radiation is 
attenuated appreciably as it penetrates 
the water. The attenuation is a func- 
tion of frequency and is about 55 db per 
wavelength. Figure 1 shows the wave- 
length as a function of frequency in sea 
water. At 100 cps the wavelength is 
150 meters, and the skin depth about 
253 meters. At 1 cps the wavelength is 
1500 meters and the skin depth 250 
meters, a very useful penetration. 
Since PNL is primarily ‘interested in 
frequencies which penetrate sea water 
321 
to an appreciable extent, we have chosen 
100 cps as the upper limit of our region 
of interest and have so far made measure- 
ments up to 30 cps. Our lower limit is 
set by convenience and probable appli- 
cation at about 0.01 cps. The magnetic 
disturbances at lower frequencies have 
been extensively studied by magnetic 
observatories and many of their proper- 
ties are known. The upper end of our 
region of interest overlaps the lower end 
of the ELF range. 
Antenna and noise problems at very 
low frequencies make the magnetic com- 
ponent of an electromagnetic wave much 
more accessible to measurement than the 
electric component, hence we have res- 
tricted our observations to the former. 
Furthermore, possible applications tend 
to use the magnetic rather than the 
electric field. The naturally occurring 
geomagnetic fluctuations are often 
referred to as background noise, but 
their character is very different from 
that usually associated with "noise"; for 
example, the distribution of energy with 
frequency is not random over any con- 
venient time interval. The prominent 
