220 



EXPLORATION GEOPHYSICS 



To quote again from Jenny§ : "The basic ideas prevalent in former years, that the 

 magnetic effect is due almost exclusively to the chemical properties or the tectonic 

 dislocations of the basement rocks underlying the oil areas, has led to many misinter- 

 pretations and has in many places discouraged magnetic prospecting, first, because 

 under this assumption the station net was laid out on too wide a scale, and therefore 

 would not permit the location of anomalies due to shallow local influence, and secondly, 

 because the field measurements were not sufficiently accurate. Magnetic anomalies may 

 be produced by lateral variation in the ferruginous content of a sedimentary bed, by 

 lateral differentiations of igneous rocks, by thickening or thinning, or by the normal 

 dip of a certain magnetic horizon. Besides these anomalies of a petrographic or strati- 

 graphic origin, magnetic anomalies may be due to structural features. A magnetic shale 

 bed may be brought closer to the surface on the up-throw side of a fault. Practically, 

 non-magnetic dolomites may be uplifted along an anticlinal axis and thus displace 

 stronger magnetic shales and sandstones. A nephelite syenite plug may intrude into 

 much less magnetic sedimentary beds, or old igneous ridges may be covered by sedi- 

 mentary deposits of much smaller magnetic susceptibility." 



The following simplification is often valid : if the magnetic results in an 

 area are picturing basement topography and the sedimentary members above 

 the basement are conformable thereto, magnetic highs and lows should cor- 

 respond with structural highs and lows. (See Figure 105 A.) If, on the 







Fig. 105. — A shows magnetic high (a) over a magnetic sediment (b) lying conform- 

 able to a magnetic basement high (c). B shows magnetic low (a) over a magnetic sedi- 

 ment (b), thinning over a basement high (c) of low magnetic effect. 



other hand, the basement is relatively non-magnetic or uniform in its 

 magnetic efifects, the magnetometer results may show merely the relative 

 thickening or thinning of the overlying sedimentary section, or of the more 

 magnetic members thereof. Magnetic highs then may correspond to struc- 

 tural lows and magnetic lows to structural highs, as shown in Figure 

 105B. 



Oftentimes it is difficult to determine which of these two conditions 

 exists. Previous exploration in a nearby geologically similar area may fur- 

 nish a clue. Another geophysical method may supply the necessary data. 

 For instance, the area in the immediate region of the magnetic anomaly 

 might be checked with the reflection seismograph to map the structure of 

 the sediments. Another somewhat less certain guide would be to determine 



§ W. p. Jenny, op. cit. 



