Chap. 8] MAGNETIC METHOD 425 



flanks and moderately magnetic Etchegoin beds in the core, a drop in 

 vertical intensity was observed. The Kettleman Hills structure shows a 

 series of highs on the upturned edges of the fold, magnetic lows along the 

 axis of the north dome, and a series of highs on the axis of the middle dome, 

 indicating that the magnetic beds were eroded from the north dome but 

 are continuous across the middle dome. Similar conditions appear to 

 exist on the Dominguez dome. 



Erosion of magnetic formations from the crest of anticlines may also be 

 responsible for magnetic lows found above uplifts in other states. The 

 problem of their origin has not been settled and the following explanations 

 have been advanced: (a) effects of well casing and derricks, (6) abnormal 

 polarization, (c) erosion of magnetic formations from crests, and (d) ir- 

 regular basement magnetization. Negative anomalies have been found in 

 the Healdton, Oklahoma City, and Garber fields.^^^ Conversely, Som- 

 ers^^ beHeves that positive anomalies may be encountered on anticUnes 

 because of uplifts of magnetic sedimentaries as well as on synclines be- 

 cause of an increase in thickness of magnetic sediments. 



According to Jenny^^^ both effects are in evidence on the Gulf coast. 

 In the northern part of that area magnetic anomalies are presumably 

 caused by structural uplifts, while along the coastal portion they are 

 assumed to be due to changes in thickness of the magnetic strata. The 

 magnetically active beds are assumed to be of Eocene age in the north and 

 of lower Miocene age in the southern portion. In the former, the Conroe 

 field produces a distinct positive anomaly^^^ (15 to 2O7) (see Fig. 8-70). 

 When magnetic formations are uplifted by a salt dome, the negative salt 

 effect may be obliterated and replaced by a positive anomaly above the 

 dome (Jenny^ ). 



The presence of magnetic members in a sedimentary column gives an 

 opportunity for the location of faults. They may reveal themselves by 

 a transition of higher to lower magnetic values or by abrupt depressions 

 in the curve, all depending on thickness, depth and dip of the magnetic 

 beds, and hade and throw of the fault. Fig. 8-71 (from Lynton^**) shows 

 a magnetic survey on the San Andreas fault and Fig. 8-72 on the Walnut 

 Creek fault. The trace of the latter is indicated by a sharp drop in the 

 profile curves, high values to the northeast corresponding to the more 



"8 California Oil World, April 30, 1931. 

 "9 Oil and Gas J., Nov. 15, 1928. 



180 "Anomalies of Vertical Intensity," Colo. Sch. Mines Mag., Aug.-Dec, 1930. 

 Jan.-Feb., 1931. 



"1 Oil Weekly, July 16 and 23, 1934. 



"2 L. H. Williams, Oil Weekly, Aug. 21, 1934 



"» Oil Weekly, April 27, 1936. 



18* Loc. cit. 



