Chap. 2] METHODS OF GEOPHYSICAL EXPLORATION 19 



of curves for profiles at right angles to the assumed strike (see Fig. 2-8). 

 Interpretation of magnetic anomalies is usually qualitative. Depth de- 

 terminations are the exception rather than the rule, because magnetic 

 anomalies may be due not only to variations in the relief of a magnetic 

 formation but also to changes in magnetization; moreover, the ratio of 

 induced and remanent magnetization is frequently subject to unpredict- 

 able variations. In the interpretation of magnetic data in oil exploration, 

 magnetic anomalies ranging from fifty to several hundred gammas may be 

 assumed to result from variations in topography and composition of 

 igneous or metamorphic basement rocks or from igneous intrusions. 

 Anomalies of lesser magnitude are usually due to variations in the mag- 

 netization and structural arrangement of sedimentary rocks. Magnetic 

 anomalies observed in mine exploration are of large magnitude and result 

 in most cases from igneous rocks or magnetic ore bodies. 



In quantitative interpretation magnetic effects of assumed bodies are 

 calculated, compared with the field curves, and assumptions changed until 

 a geologically reasonable agreement is obtained. Direct methods of in- 

 terpretation 'are applicable when the magnetic anomaly is simple and 

 arises ^rom one geologic body only; in that case, approximate calculations 

 of depth may be made directly from the anomaly curves by assuming that 

 the magnetic bodies are equivalent to single poles, magnetic doublets, 

 single magnetized lines, and line doublets. 



The pole and line theories make no assumptions regarding the origin of 

 the magnetic poles and may, therefore, be applied irrespective of whether 

 geologic bodies are normally or abnormally polarized. On the other hand, 

 where the magnetization is sufficiently homogeneous and the remanent 

 magnetization small, the magnetic anomalies may be attributed to induc- 

 tion in the earth's magnetic field. In that case the so-called "induction 

 theory" is applied. This theory relates the magnetic effects to the strength 

 and direction of the earth's magnetic field and therefore to the magnetic 

 latitude in which geologic bodies occur. 



Considerable help may be derived in the interpretation of magnetic 

 anomalies from the use of model experiments. In underground magnetic 

 exploration it is necessary to measure both horizontal and vertical intensi- 

 ties, since magnetized bodies may occur not only below but also above the 

 plane of observation. Depths of magnetized bodies may be determined 

 by observations on scaffolds, in balloons, and in airplanes. 



IV. SEISMIC METHODS 



General. Seismic methods are in the category of "indirect" geophysi- 

 cal methods, in which the reactions of geologic bodies to physical fields 

 are measured. Since the depth of penetration of such fields depends upon 

 the spacing of transmission and receiving points, variations of physical 



