146 CWA. -HELEAND 
phase curves. It will be noticed that the potential decreases above a 
good conductor, while the phase angle has a maximum, and vice versa. 
b. LOCATION OF STRUCTURE 
Both resistivity and potential-drop-ratio methods have found 
already several remarkable applications in locating geologic structure 
favorable for the accumulation of oil, such as anticlines, salt domes, and 
the like. They have also been applied very successfully in extend- 
ing known geologic information into unknown territory, which is an 
application always recommended for any kind of geophysical method. 
20p00 
upeo 
Ohms per cm? 
3 
fre 7065 ge ais 
Miss. & Penn. 
| Shakes RSS. 


Elev Feet 
o 2200 qooo S000 fe. 
Fic. 30.—Resistivity profile across buried anticline, taken with electrode spacing @ 
equal to 200 feet (after Hubbert). 
I. BY RESISTIVITY MAPPING 
The Schlumberger Company has located and mapped geologic 
structure of several types by the resistivity method. As examples, the 
anticline illustrated in Figure 28 and the fault illustrated in Figure 29 
have been selected. The first figure shows an equi-resistivity map of the 
Aricesti anticline in Roumania which appears in the resistivity map 
as a body of lower resistivity. Very instructive is the location of a 
fault in Alsace-Lorraine by the method of resistivity mapping, as it 
illustrates how definite resistivity values may be identified with 
definite geologic horizons. The marls have the lowest resistivity, then 
follow the limestones, and then the sandstones in this area. The re- 
sistivity contours follow truly the trace of the fault which has dis- 
placed the formations under discussion in a horizontal direction. 
