430 MAGNETIC METHOD [Chap. 8 



highest point, where the magnetic anomaly is some 3OO7. In the eastward 

 extension of the Amarillo Ridge, the Wichita and Arbuckle Mountains 

 and their buried forelands have been surveyed in detail by the Shell Oil 

 Co. A portion of their magnetic map is reproduced in Fig. 8-75. The 

 geophysical surveys and subsequent drilling brought out the fact that the 

 Arbuckles form a separate folding system, are not connected with the 

 Wichita Mountains, and are separated from them by the Ardmore basin. ^^^ 

 The Wichita Mountains continue uninterruptedly as the Walters Arch 

 into the Muenster Arch. The interval of the magnetic isanomalics (ob- 

 tained with magnetometers) is IOO7. The Arbuckle, Criner, Walters, and 

 Muenster Arches are indicated by magnetic highs approximately cor- 

 responding to gravity highs, while the intervening Ardmore and Marietta 

 synclines are magnetic lows. While the magnetic anomalies are, in the 

 main, caused by the contact of the sediments with the pre-Cambrian 

 gneisses, granites, granite porphyries, and the like, changes in the com- 

 position of the basement rocks also affect the picture. Sediments appear 

 to have little or no effect on anomalies in this area. 



For Alabama, similar regional surveys and interpretations are discussed 

 by Eby and Nicar,^ and for the coastal plains of North and South Caro- 

 lina, by McCarthy. Regular trends of highs and lows of considerable 

 amplitude (up to I2OO7) were observed. The actual number of detailed 

 magnetometer surveys made for oil exploration in Texas, Louisiana, Mis- 

 sissippi, Alabama, Oklahoma, Kansas, Colorado. New Mexico, California, 

 Wyoming, and the Dakotas is much greater than these few examples 

 would indicate. The general conclusion to be derived from this work is 

 that the magnetic method can be exceedingly useful in structural oil 

 prospecting of such areas where the basement rocks are of uniform com- 

 position and uniformly magnetized and where magnetic sediments, if 

 present, are continuous and conformable to basement topography^ Where 

 there are rapid changes of igneous rocks in the basement, intrusions of 

 irregular character, erosion of magnetic sediments, or irregular distribu- 

 tions of magnetic materials within them, magnetic exploration should be 

 replaced by a different geophysical method. 



As previously mentioned, faults may be located magnetically where they 

 have affected the basement rocks. In the Amarillo field (N. H. Stearn"®) 

 the Potter County fault of 1200-1500 foot throw showed by an abrupt 

 drop in Hotchkiss superdip readings. Over the Beckham County fault 

 with a throw of 300-500 feet the intensity dropped about 4OO7. The 



193 World Petrol. Congr. Proc, B(I), 174 (1933). See also Fig. 7-516. 



"■> Geol. Survey Ala. Bull., 43 (June, 1936). 



"8 Jour. Geol., 44(3), 396-406 (April-May 1936). 



"« A.I.M.E. Geophysical Prospecting, 189-191 (1932). 



