Chap. 8] 



MAGNETIC METHOD 



435 



able for building stone, such as granite, basalt, and the like. Fig. 8-76 shows 

 the magnetic anomalies of a lava flow above nonmagnetic Tertiarj' shales. 

 The magnetic anomalies are proportional to the thickness of the lava 

 cover; minima occur where the shales come through to the surface. 

 Ahrens and Schroeder and Reich ^ established bj'^ a detailed survey of 

 a basalt quarry that it was possible to differentiate between solid basalt 

 usable for construction purposes and unusable weathered rock, since the 

 anomalies over portions of the latter were much reduced. 



Fig. 8-77. Location of buried ammunition by magnetic and electrical measure- 

 ments. Anomalies are shown in profile view, together with equipotential lines of an 

 electrical survey. (After Ebert.) 



3. Dam site surveys. Magnetic surveys can be of much help in deter- 

 mining structural conditions and rock properties on dam sites since they 

 are much less expensive than drilling; at least, their use makes it possible 

 to limit drilling operations to the absolute minimum. How the use of 

 geophysics can reduce excavation and construction costs has been demon- 

 strated in the case of the Bonneville Dam. It is reported that in the 

 geological survej^ of that area the possibility of igneous intrusions was not 

 recognized and that, after the first excavations encountered basalt, a 

 geophysicist was called in to survey the basalt dikes. This subsequently 



"' Beitr. angew. Geophys., 2(4), 320 (1932). 



"» Beitr. angew. Geophys., 1(4), 432^36 (1931). 



