A DESCRIPTION OF INSTRUMENTATION AND THE 

 MEASUREMENT TECHNIQUES USED FOR THE DETECTION OF 

 EXTREMELY LOW FREQUENCY ELECTROMAGNETIC 

 ENERGY IN THE SEA 



by 



James F. Orr 

 U. S. Navy Underwater Sound Laboratory, New London, Conn. 06321 



INTRODUCTION 



In the past decade there has been a renewed interest in the Extremely Low Fre- 

 quency (ELF) region. Reflecting this interest, the U. S. Navy Underwater Sound 

 Laboratory has extended its oceanographic program to include a determination of 

 the electromagnetic characteristics of the sea. Previously very little ELF atmos- 

 pheric noise data was available, and virtually none had been taken in the sea. No 

 background of information existed, except in the power and acoustical region. The 

 extremely low electromagnetic levels to be measured demanded careful design con- 

 sideration, especially in the preamplifiers and antennas. 



The ultimate purpose of this project will be to obtain absolute ELF measure- 

 ments of atnaospheric and sea-generated noise and to relate them to some of the 

 physical parameters of the sea, such as sea state and electrical conductivity. With 

 these goals in mind, a series of trips on the Navy's General Oceanographic Survey 

 Vessels (AGOR's) was planned. 



,, DESCRIPTION OF INSTRUMENTATION 



The instrumentation system installed on the USNS GILLISS (AGOR-4) for the 

 measurement of atmospheric and ambient electromagnetic noise in the sea consisted 

 of preamplifiers, 60-cycle reject filters, level adjusting attenuators, amplifiers, 

 FM modulators, and a tape recorder (Fig. 1). In addition, general-purpose test 

 equipment was included for on-the-spot analysis, observation of wave forms, meas- 

 urement of antenna impedance, and other experiments. This equipment was installed 

 in three standard 6-foot instrumentation racks. 



The frequency range of interest is approximately from 5 to 1000 cycles (Fig. 2). 

 This frequency range includes the l/f noise region of common amplifying devices. 



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