244 EXPLORATION GEOPHYSICS 



After each day, or flight, the records are transcribed into proper scale 

 using an automatic transcriber. The results after transcription are then 

 plotted on the map in proper scale and contours of isointensity are drawn, 

 as in magnetic surveys on the ground. 



Shipborne Magnetometer 



For small areas the shipborne magnetometer has proven an econom- 

 ical and accurate reconnaissance method.f A standard airborne magneto- 

 meter is used with the magnetically-sensitive detector mounted in a water- 

 tight housing or fish, and towed behind the ship to minimize the ship's 

 magnetic effects. With this arrangement the speed of the ship is slow 

 enough to use inexpensive means for determining the ship's position. Radar 

 can be effectively employed if suitable targets are available for ranging. 

 Another method involves the use of buoys anchored at strategic points in 

 the area and surveyed-in by conventional means. With the buoys in position, 

 the ship runs from one buoy to another at a uniform rate of speed. By 

 knowing the course, speed, and time, the ship's position can be determined 

 readily. If the ship is equipped with a gyroscopic compass with automatic 

 steering, its course is accurately and easily maintained. To further simplify 

 navigation, a course recorder is available which continuously records the 

 ship's heading. 



Magnetic surveys in the Gulf Coast have shown that salt domes, which 

 are the oil producers in this area, have only small anomalies associated with 

 them. Since the magnetic anomaly of a structure varies as the square or 

 higher power of the distance to the point where the measurements are 

 made, it is important that the magnetometer be as close to the structure as 

 possible. For this reason the shipborne magnetometer has a distinct advan- 

 tage over the airborne magnetometer. If a structure located at a depth 

 of 1,000 feet gives an anomaly of 10 gammas at the surface, the same 

 anomaly mapped when flying at 1,000 feet altitude would be about 2.5 

 gammas. 



MAGNETIC METHODS 



UNITED STATES PATENTS 

 400,661 Issued Apr. 2, 1889. Aclolphus Gipperich. "Apparatus for Determining 



Electric or Magnetic Forces." 

 961,298 Issued June 14, 1910. Dell W. Jewell. "Prospector's Needle." 

 1,574,350 Issued Feb. 23, 1926. J. B. Johnson. "Electrical Testing." 

 1,675,121 Issued June 26, 1928. Burton McCollum. "Method and Apparatus for 



Studying Geologic Contours." 

 1,676,619 Issued July 10, 1928. Burton McCollum. "Method and Apparatus for 



Studying Geologic Contours." 

 1,724,495 Issued Aug. 13, 1929. Burton McCollum. "Method and Apparatus for 



Determining the Slope of Subsurface Rock Boundaries." 

 1,724,720 Issued Aug. 13, 1929. Burton McCollum. "Method and Apparatus for 



Studying Subsurface CouvOurs." 



t Eugene Frowe, "A total field magnetometer for mobile operation", Geophysics, Vol. 13, pp. 

 209-214, 1948. 



