ADVANCES IN OIL PROSPECTING 161 
toward the east, the resistivities become smaller than the values 
obtained toward the west. 
This is the only place at which, to the writer’s knowledge, it has 
been definitely established that the oil can be located directly by 
surface resistivity measurements. 
After this article was written, Swartz (ret. list No. IIIs7) pub- 
lished the results of further resistivity measurements in Kentucky. 
In the recent observations, the depth of the oil-bearing sands was 
much greater than in the determinations discussed above; it varied 
between 500 and 800 feet. It was found that generally the resistivity 
curves rose where oil and gas were present and fell where dry terri- 
tory was encountered. One case is cited where predictions as to the oc- 
currence of hydrocarbons made on the basis of resistivity determina- 
tions were checked later by drilling with striking accuracy. Further- 
more, in the area of the Legrande pool, the boundary of the dry and 
‘producing area could be outlined from the resistivity data and checked 
the information obtained from wells. The striking fact about this 
boundary is that it is not marked by edge water, but by a decrease in 
porosity with corresponding decrease in the oil and gas content: 
another fact worth mentioning is that the best indications were ob- 
tained from a gas sand only 4 feet thick in a depth of about 640 feet; 
that is, the sand detected was only 0.6 per cent of the depth. 
The author concludes that “for the partitioning method the shield- 
ing effect of such low resistivity beds as are encountered in oil and gas 
fields are negligible as far as the detectability of underlying gas and 
oil horizons is concerned,” and continues as follows: ‘‘It is feasible to 
locate oil and gas zones, as well as salt water zones, directly by re- 
sistivity measurements on the surface of the ground. Whether such 
beds can be detected under all conditions and at all depths is, of 
course, not at present known. So far it is safe to say only that for 
depths up to 1,000 feet and under the physical and stratigraphic con- 
ditions here obtaining, such beds may be determined with excellent 
accuracy.” 
G. SUMMARY AND CONCLUSIONS 
Resistivity-and seismic refraction methods have several physical 
characteristics in common. They both involve the advantage of con- 
trolling the depth of penetration and thus a greater definiteness of 
interpretation than obtained in other geophysical methods. On the 
other hand, they are largely subject to the same limitations as other 
491 
