EARTH RESISTIVITY AND GEOLOGICAL STRUCTURE 



171 



degree of complexity of the earth's structure at the test site. The 

 maps show, for each test, roughly the average of the resistivities 

 indicated by the different measurements. They also indicate for each 

 test the relative extent of the lines involved in the test. 



Within the heavy boundary lines of Fig. 1 the rocks are largely pre- 

 Cambrian. It will be noted that most of the tests in these areas 

 indicated very high earth resistivities — from 1400 to 10,000 meter- 



Fig. 2 — Area! geology and effective resistivities — Gulf Coastal Plain areas. 



ohms. The single exception, one test in New York which showed a 

 value of 200 meter-ohms, is illustrative of the effect of alluvial deposits. 

 The lines involved in this test were located in a narrow valley, the 

 floor of which was partly covered with alluvium. By contrast, the 

 structures of the areas shown in Fig. 2 are of the three latest periods, 

 the Quaternary, Tertiary and Cretaceous, and the effective resistivities 

 are very^low — from 4 to 32 meter-ohms. 



