88 beers. GEOPHYSICAL PROBLEMS [Ch. 4 



which exhibit density contrasts. The resolving power of the method 

 depends primarily upon the numerical magnitude of the gravity con- 

 trasts and the size and distance of the anomalous formation from the 

 surface. It gives its best results on flat terrains; uneven topography 

 introduces errors which cannot always be accounted for. If the gravity 

 contrasts are associated with structural features, these may often 

 be correctly inferred by the intelligent interpretation of the data and 

 their correlation with geologic data on the prospect. The successful 

 application of the gravitymeter extends to density contrasts in excess 

 of 0.1. 



Ground magnetometer surveys. These surveys show a high degree 

 of resolving power for buried deposits of magnetic rocks and min- 

 erals, if the effective magnetite content of such minerals is at least 

 0.1 percent. Masses of such formations can be detected and outlined 

 satisfactorily. Negative anomalies may also prove significant, be- 

 cause they show the presence of formations of low susceptibility in 

 a region of normally higher values. Estimates of depth based upon 

 magnetic data are, like those of gravity data, not entirely satisfactory 

 unless the limits of depth estimates are supported by good geologic 

 data. 



Electrical and electromagnetic methods. Such methods are of im- 

 mediate value in the determination of depth of overburden, thickness 

 of soil, and depth to ground water, and in the differentiation of types 

 of rock and sediments. The accuracy of depth and thickness measure- 

 ments is good, depending upon the amount of detail applied in the 

 field. In locating water supply, buried gravel deposits, placers, and 

 geologic discontinuities involving changes in conductivity, these meth- 

 ods are cheap, rapid, and effective. The tools are readily available, 

 awaiting only their application to problems in applied sedimentation. 



REFERENCES 



Texts and Handbooks 



Berry, F. A., Bollay, E., and Beers, N. R. (editors) (1945). Handbook of meteor- 

 ology: McGraw-Hill Book Co., New York, 1068 pages. A valuable reference 

 text for data, fundamental theory, weather forecasting, and analysis. Empha- 

 sizes scientific and engineering aspects of meteorology. 



Bullen, K. E. (1947). Introduction to the theory of seismology : Cambridge Uni- 

 versity Press, 276 pages. A concise treatise of the theoretical phases of 

 seismic-wave propagation over long distances of the earth's crust and interior. 

 Contains valuable list of published works on the subject. 



Byerly, Perry (1942). Seismology : Prentice-Hall, New York, 256 pages. An ad- 



