MAGNETIC METHODS 237 



It may be further noted that, by compensating for Hq by superimposing 

 a magnetic field of a predetermined attitude at the location of the sensing 

 element, the term Hq sin /? would be reduced to a minimum, thus improv- 

 ing the apparent response of the detecting unit as well as the reliability. 



It is now apparent that there is excellent reason for designing airborne 

 magnetometers to measure the total intensity. 



Referring to Figure 116, the sensing element in this case is oriented 

 differently — such that its axis lies in the direction of the total intensity 

 To. For case (a) or (b), i.e., misalignment /? in any direction from the 

 vertical plane, the magnetic field intensity T acting on the sensing element is 



T = To cos /? 



.'. the % error due to a misalignment /? in any direction is 



% error = "^^ ~ "^ • 100 = (1 - cos /?) • 100 



Thus the error from misalignment in any direction of the instrument 

 oriented along To is the same as the minimum error (special case of E-W 

 misalignment) for the vertically oriented instrument. For example, a 

 misalignment of one degree causes an error of 



(l-cosl°) • 100 < 0.015%. 



This property is unique with respect to the direction of the total intensity, 

 and since the measuring element will be unavoidably deflected through 

 small angles of arbitrary orientation as a result of the motion of the aircraft, 

 the alignment along the total intensity vector is the most favorable one 

 possible. 



Airplane Operations 



Magnetometer operations with high speed aircraft unavoidably involve 

 both substantial investments and high operating costs. It is only the low 

 unit cost (cost per magnetometer profile mile) which makes this kind of 

 large scale operation economical. The economy rapidly vanishes with 

 decreasing size of the areas to be surveyed or when much idle time inter- 

 venes between surveys. It is therefore likely that mining and oil compan- 

 ies will conduct airborne magnetometer operations only if they have large 

 areas to be surveyed. f 



t Alvin W. Knoerr, "The airborne magnetometer: a new aid to geophysics", Engineering and 

 Mining Journal, Vol. 147, No. 6, pp. 70-75, June, 1946. 



Gordon B. Nicholson, "Airborne magnetometer expedites geophysical surveys", Oil Weekly, 

 Vol. 122 No. 1, pp. 29-32, June 3, 1946. 



"Seven groups to search Bahamas for petroleum", World Petroleum, Vol. 17, No. 11, pp. 

 44-46, Oct., 1946. 



Ray L. Dudley, "Flying magnetometer completing 80,000-square mile survey". World Oil, 

 Vol. 127, No. 7, pp. 247-251, Nov., 1947. 



Warren W. Burns, "Modern exploratory tools being used in five-company Bahama oil search", 

 Oil and Gas Jnl.. Vol. 46, No. 27, pp. 38-42, 129, Nov. 8, 1947. 



James Affleck, "Aeromagnetometer profile flown from Venezuela to Texas", World Oil, Vol. 

 128, No. 3, pp. 223-224, 227-228, July, 1948. 



Sylvain Pirson and Lloyal O. Bacon, "Airborne magnetometer survey in Central Pennsyl- 

 vania", Penn. State College Bull., Vol. 42, No. 10, March 5, 1948. 



