range of I. For a world-wide range of I, the residual vertical field of 

 the aircraft may be somewhat larger. 



c. Navigational accuracy 



Under ideal conditions it is probable that the aircraft position 

 is determined to about ±5 miles when far at sea. Adverse weather 

 conditions can degrade this accuracy severely. 



d. Magnetic anomalies 



The presence of magnetic anomalies can often be deduced 

 from the VAM magnetic profiles, but the effect on survey accuracy 

 may remain uncertain. 



e. Temporal variations 



Time variations in the geomagnetic field sometimes can be 

 deduced from magnetic observatory records, but the effect on surveys 

 is uncertain. 



D„ MARINE GEOMAGNETIC SURVEYS 



The Hydrographic Office currently is conducting extensive marine 

 magnetic surveys. The magnetometers used in these surveys are 

 designated XN-4901 and are modifications for marine use of Varian 

 V-4910 airborne nuclear precession instruments. The XN-4901*s are 

 installed aboard three survey vessels. 



The Varian XN-4901 Marine Navigational Aid Magnetometer is 

 a nuclear precession instrument. The sensing head, with its liquid 

 hydrocarbon sample is towed sufficiently aft of the ship to remove 

 it from the magnetic field of the vessel. The instrument measures 

 absolute magnetic total intensity and is drift-free. The principle 

 of operation is based upon the gyromagnetic characteristics of protons, 

 which will precess like tiny spinning tops when in an ambient magnetic 

 field at a frequency proportional to the total intensity value of that 

 magnetic field. Equipment sensitivity is limited for practical purposes 

 to the inherent accuracy of the counting circuitry. 



The counter uncertainty is in the order of ± 1 to 2 gammas. The 

 instrument field range is 25,000 gammas to 73,000 gammas, with the 

 range being selected by 15 plug-in units. The ultimate sensitivity of 

 nuclear precession magnetometers that use the most refined com- 

 ponents and techniques under ideal static conditions appears to be 

 about 0.1 gamma. 



XIV- 7 



