11. Artificial disturbances 



Man-made disturbances are generated by accumulations of 

 ferromagnetic materials such as structures, ships, submarines, mines, 

 etc. In general, these artificial anomalies are considerably smaller 

 than naturally occurring anomalies. Artificial disturbances also can 

 be caused by electric currents. 



C. AIRBORNE GEOMAGNETIC SURVEYS 



Since 1952 the Hydrographic Office has been conducting airborne 

 geomagnetic surveys using an NOL Vector Airborne Magnetometer, 

 Type 2A (VAM-2A). This instrument was developed and two proto- 

 types constructed by NOL under joint sponsorship of this Office 

 and Office of Naval Research. After a period of testing and evaluation, 

 two additional instruments, designated VAM-2B, were procured by 

 the Hydrographic Office under commercial contract. 



Currently one VAM-2A is installed in an R5D aircraft and one 

 VAM-2B in a WV-2 aircraft. Both aircraft have been modified exten- 

 sively to facilitate accurate airborne measurements of the geomagnetic 

 field. Both aircraft are provided with a variety of special navigational 

 equipment. The WV-2 aircraft is provided with a Kollsman KS-124 

 photoelectric sextant to supplement the VAM system. A cosmic ray 

 neutron monitor also is installed in the WV-2. 



1. Vector Airborne Magnetometer 



The VAM system is one whereby four parameters are measured 

 directly: (1) Magnetic heading (MH), (2) inclination (I), (3) celestial 

 relative bearing (RB), and (4) total magnetic intensity (F). The VAM 

 utilizes three mutually perpendicular saturable inductors, two for 

 orienting the servomotor control and the third for measuring the total 

 field. A modified Kollsman periscopic sextant, manually operated, is 

 used to measure celestial relative bearing (RB). 



The VAM measurement of total field intensity (F) is accomplished 

 by passing a precisely controlled electric current through the detector 

 inductor which is servo-oriented parallel to the magnetic field vector. 

 This "neutralizing" current is manually adjusted in precise 50-gamma 

 steps (1 gamma = 10- 5 oersteds) to establish base lines. The detector 

 inductor a.c. output voltage is an amplitude analog of the difference 

 between the ambient field and a base line value. This analog value is 

 detected and recorded on an Esterline- Angus recorder to an accuracy 

 of about ±3, ±9, or ± 15 gammas depending upon the sensitivity selected. 



XIV- 4 



