168 BELL SYSTEM TECHNICAL JOURNAL 



It then became apparent that consideration of the areal geology 

 alone was not sufficient; instead, that the earth's structure to depths 

 ranging from several hundred to several thousand feet must be taken 

 into account. Data on this structure and the effective resistivities 

 indicated by mutual impedance measurements at a large number of 

 test sites have now been assembled. Analysis of these data shows a 

 more or less consistent relation between the resistivity at any given 

 point and the age and physical characteristics of the geological forma- 

 tions involved. This relation is such that, in general, decreasing 

 effective resistivity corresponds roughly to decreasing age of the 

 formations, as the earlier studv^ seemed to indicate. However, there 

 are certain exceptions to this rule. 



The principal correlation data derived from the tests are summarized 

 in the following tabulation. This is the result of grouping the tests 

 in accordance with the geological periods to which the principal strata 

 comprising the structure in each case belong and noting the ranges 

 within which the resistivities determined by the greater part of the 

 tests of each group lie. 



Pre-Cambrian and combinations of Pre-Cambrian and Cam- 

 brian 1000-10,000 m.-ohms 



Cambrian and Ordovician combinations 100-1000 m.-ohms 



Ordovician to Devonian, inclusive, and combinations of these 



periods 50-600 m.-ohms 



Carboniferous, Triassic, and combinations of Carboniferous and 



earlier periods 10-300 m.-ohms 



Cretaceous, Tertiary, Quaternary and combinations of these 



periods 2-30 m.-ohms 



It would be well to examine briefly the meaning of this summary and 

 to consider its limitations. The geologists tell us that underlying the 

 entire continent are extremely old rocks, extending downward to great 

 depths. Little is known of the relative ages of different parts of this 

 underlying structure. They are here grouped under the general term 

 pre-Cambrian. In some areas pre-Cambrian rocks appear at or near 

 the surface, the only covering being clays, soils, and other loose 

 materials. In other areas they are overlain by rocks and sediments 

 formed during later periods, the total thickness of which ranges up to 

 many thousands of feet. The ages, arrangement, and characteristics 

 of these upper strata are much better known. They are assigned by 

 geologists to various periods in accordance with the ages during which 

 they were formed. These periods appear in the tabulation in order 

 from the oldest to the youngest. 



In the case of tests made in areas where the pre-Cambrian rocks are 

 overlain by younger strata it becomes necessary to consider just what 

 portion of the structure probably influenced the test results. For 



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