80 beers. GEOPHYSICAL PROBLEMS [Ch. 4 



distribution of rocks at depth. The method has a high resolving power 

 when all possible corrections are made. These include corrections for 

 sea-level elevation of each station, distribution of surface rock densi- 

 ties and their elevation, topographic corrections, and instrumental 

 drift corrections. When properly gathered and interpreted, gravity 

 data are of great value in detecting the presence of buried structures 

 and mineral deposits. Interpretation of the data requires an intimate 

 knowledge of the rock characteristics in the layered sequence, as well 

 as of the general geologic setting in which the gravity survey is con- 

 ducted. The method is of value to the sedimentologist in locating 

 placer deposits such as buried river channels, filled basins, faults and 

 dikes, and many geologic anomalies based upon density contrasts. 



Variations in the magnetic susceptibility of rocks are almost en- 

 tirely determined by the magnetite content. Although there are many 

 magnetic minerals in sediments, none have such widespread occurrence 

 as magnetite. If the geological and mineralogical associations in sedi- 

 mentary rocks are well understood, it is possible to map the distribu- 

 tion and attitude of such formations. For this purpose, measurements 

 of the earth's magnetic field in a bore hole, at the surface, or from 

 aircraft may reveal valuable information. The magnetometer is the 

 oldest prospecting device used by man which has continued to the 

 present day. Like other geophysical instruments, it does not lead 

 directly to the discovery of treasure, excepting for magnetic minerals; 

 therefore, its usefulness is confined to the indications which it gives 

 on structure, stratigraphy, and the distribution of magnetic materials. 



In applied sedimentation the magnetometer may be used for the 

 broad scope of mapping the distribution of magnetite-bearing rocks 

 or for local surveys for ore bodies. Its use has been most widespread 

 in prospecting for ore, relatively less for petroleum surveys than 

 other methods. Because of its low cost and speed of operations it is 

 well suited for broad reconnaissance surveys immediately after geo- 

 logic mapping. Favorable areas isolated by the magnetometer are 

 usually explored in further detail by other methods. 



The electrical conductivity of rocks in a place is determined by two 

 principal factors: (a) their water content, and (b) concentration of 

 mobile ions. If the presence of conducting ions is systematically re- 

 lated to the occurrence of oil, gas, or minerals, electric resistivity 

 surveys will be of value. These have been successfully conducted 

 from the surface of the earth by employing a profile method which 

 traverses lines of exploration related to the area under investigation. 

 The presence of a conducting layer, such as a sand formation saturated 

 with conducting water at moderate depths (not over 5,000 feet), may 



