GEOPHYSICAL METHODS OF INVESTIGATING 233 



only is investigated because the high metalUc conductivity 

 of the casing prevents electrical resistivity readings in the 

 lined parts of the hole. Given ample geological data, 

 lithological sequences can be established fairly accurately 

 by this method. This has been done in Europe and South 

 America, while in Oklahoma and Kansas electrical key 

 horizons have been fixed by it. By slightly altering the 

 technique the dip of the strata can be got in favorable cases, 

 i.e., by noting the point of surface emergence of an electric 

 current sent into a relatively conductive stratum. Since 

 oil and gas offer high resistance to the flow of current, the 

 conductivity of which we have seen depends on the amount 

 of water present, we may be able to trace oil wells with the 

 basal salt water and the other wells in the oil proper. 

 Discontinued cores can also be completed by an electrical 

 log so as to determine all the beds traversed and get 

 their thicknesses. 



The cost increases as the log length (uncased part of the 

 hole) increases, and the method is useful where no cores 

 are yielded, as in churn drilling. Electrical key horizons 

 will make up for any lack of geological ones, thus permitting 

 of more precise correlation. It also enables us to get the 

 data of faulting, since these markedly affect the con- 

 ductivity range. Enough has been said to show that this 

 infant method is pretty vigorous and appears to have a 

 hopeful future. 



Seismic Methods. — This method of investigating the 

 earth's substructural conditions has been adopted for 

 places where the overburden wholly or partially hides the 

 solid geological structure of the region. It depends on the 

 propagation of waves in the earth, the passage of which 

 are affected by the physical characteristics of the rocks 

 traversed; they are therefore subject to the laws of the 

 elastic theory. Consequently the velocity of an elastic 

 wave is determined by the modulus of elasticity of the rocks, 

 the density, and Poisson's transverse contraction coefficient 

 for the various media. Elastic waves in air or water are 

 known as sound waves and those in the solid mass of the 



