424 MAGNETIC METHOD [Chap. 8 



salt dome anomalies seldom exceeded 20 or SOy in any of the above 

 surveys. On the salt dome of Hettenschlag, in Alsace/^^ the negative 

 anomalies were of the same order. When magnetometers were first in- 

 troduced on the Gulf coast, experimental surveys were made on a number 

 of known domes. The author ran a number of traverses on the Barbers 

 Hill and Esperson domes in 1925. In the following years the magnetom- 

 eter found more application for the mapping of basement topography, and 

 it was not until its accuracy was increased that it was used again in salt 

 dome work on the Gulf coast. Surveys have been published by Barret^^^ 

 for the Simmsboro area and for the Anse La Butte and Port Barre domes,"* 

 and by Jenny"^ and Clark and Eby"^ for the Fannet dome. The latter, 

 comparatively near the surface, produces an anomaly of —257. The 

 Port Barre dome, at much greater depth, has an anomaly of only — ISy 

 (see Fig. 8-69). 



2. Determination of structure by the mapping of magnetic sedimentary 

 formations. While most magnetic anomalies in producing or prospective 

 oil territory are due to changes in topography or composition of the base- 

 ment rocks or to intrusions of igneous rocks, there are certain areas, 

 particularly those distinguished by great thicknesses of sedimentary forma- 

 tions, where the effect of sedimentary beds predominates. The effective 

 magnetic members may be ferruginous shales, sandstones, volcanic tuffs, 

 iron conglomerates, and the like. Uplifts of such formations are charac- 

 terized by positive anomalies except where magnetic members thin out 

 over the crest or have been eroded away. Such conditions occur in 

 California*'^ where there is a marked variation in susceptibilities of sedi- 

 mentary rocks. In the Tertiary formation, susceptibilities vary from 

 14 • 10~^ in the Saugus of the Upper Pliocene to 4000 • 10"^ in the vivianitic 

 sandstone of the McKittrick group in the Pliocene. 



The Cretaceous is, on an average, more magnetic than the Tertiary, and 

 the Jurassic (Franciscan) is more magnetic than either Tertiary or Cretace- 

 ous. The picture is further complicated by interbedded volcanics, and 

 metamorphic and intrusive rocks. At the Raven Pass anticline with flank 

 dips of 40" to 50°, Cretaceous beds in the center are more magnetic than 

 the Miocene on the flanks, giving rise to positive anomalies of IIO7 on 

 the crest. On the other hand, on the White Creek syncline 20 miles 

 northwest of Coalinga with strongly magnetic cretaceous beds on the 



1" P. Geoffrey, Ann. Off. Comb. Liqu., 6, 1015-1021 (1929). 

 "' Barret, Mapping Geologic Structure with the Magnetometric Methods 

 (Shreveport, Feb., 1937). 



"^A.A.P.G. Bull., 19(7), 1O7O-1071 (July, 1935). 



"6 Oil Weekly, April 27, 1936. 



"« A.A.P.G. Bull., 19(3), 363 (Mar., 1935). 



»" E. D. Lynton, A.A.P.G. Bull., 16(11), 1351-1370 (Nov., 1931). 



