ADVANCES IN OIL PROSPECTING I51 
vantage in eastern Colorado, in close conjunction with geologic data, 
for the purpose of obtaining data about geologic structure. 
Figure 34 shows again some resistivity curves, obtained on four 
traverses in the San Fernando Valley in California. The results of 
this work were published by J. J. Jakosky (ref. list IIIs) in different 
form; it appears from the description of the work that some sort of a 
single-power-probe method was used. It may be possible that the 5- 
electrode system described by Jakosky! in a later paper was employed, 
on which no data are at present available, and which may be some sort 
of potential-drop-ratio method with an electrode arrangement similar 
to that shown in Figure 19 or may be the partition method (Fig. 7, Ja). 



= 
Hed a £ 
ee Aleve” Ne 
é eer Se Seen 
CREO i 
feet 
5 BP <e 
i ae a ecar a 
(eretacs 
tee 
q 4 ee a . bee si = j 
int ' a a AV fm ¢.] 0 # lls : 
‘ J fertarnmda - S SS #r/es (P7llecen o/ 
Fic. 34.—Results of four resistivity traverses (single-probe method) and corre- 
sponding geologic section in San Fernando Valley, California. (Plotted from data pub- 
lished by J. J. Jakosky.) 
As stated by Lundberg and Zuschlag (ref. list No. IV;) apparent re- 
sistivities may also be readily computed from the potential-drop ratios. 
In Figure 34, the results obtained by Jakosky are shown, plotted 
in millivolt units which are directly proportional to the apparent re- 
sistivity. The geologic conditions are as follows. The larger part of 
the area is covered by a blanket of alluvial strata with a maximum 
thickness of nearly 80 feet. Below, in the eastern part of the area, are 
inclined strata of the Fernando group of the Pliocene. These forma- 
tions are cut off by a fault in the western part of the area which has 
brought strata of the Cretaceous Chico formation in contact with the 
Pliocene in the east. These Cretaceous strata do not contain water, 
but the upper parts of the Pliocene are filled with highly conductive 
waters down to a depth where the impervious strata indicated as 
of de Jakosky, “Geophysical Examination of Meteor Crater, Arizona,’ Geophyst- 
cal Prospecting (Amer. Inst. Min. Met. Eng., 1932), pp. 63-98. 
481 
