104 C.A. HETLAND 
the cathode is larger than the anode, reducing polarization to a great 
extent. For this purpose, the electrodes have a large ratio of areas, as 
seen in the figure. 
For more accurate work on the surface, the Megger and McCol- 
lum’s earth-resistivity testers, or the 4-terminal Gish-Rooney equip- 
ment is used, which will be described later. 
J. Koenigsberger (ref. list No. I;) uses two circular iron electrodes 
about 25 cm. in diameter, and about 5 mm. in thickness, provided 
with a contact substance which is either hematite powder or a clayey 
paste with an aqueous solution of FeCl;, or FeSO,, or of 10 per cent 
NaCl. Where the necessity arises of using larger electrodes, iron 
screens may be employed. As a source of energy, a buzzer is used at a 
frequency from 100-400 cycles; the electrodes are placed in a Wheat- 
stone bridge with a telephone as indicator. Koenigsberger gives the 
formulae to be used with this arrangement of the electrodes. 
Cc. DETERMINATION OF RESISTIVITY IN WELLS 
This is the only method for obtaining accurate resistivity values 
for any formation below the surface. The equipment and technique 
has been perfected by the Schlumberger Company (ref. list No. 
Ig), and excellent results have been obtained with it. The arrange- 
ment is shown in Figure 6. It consists of four electrodes, two of which 
are supplied with current, while the potential difference between the 
two intermediate electrodes is observed; one current electrode is 
grounded at the surface. The arrangement of the three other elec- 
trodes always remains fixed at any depth. With the symbols used in 
the figure, the resistivity at any depth is given by the relation 
/ 
4mAV~ rr 


‘ i r—fr 
if 7 is the current read on a milliammeter in the current circuit and 
AV is the potential difference read on a potentiometer between the 
electrodes 2 and 3. Excellent results have been obtained by this 
method, and have occasionally resulted in the detection of oil and 
coal horizons which had been overlooked in drilling. The results, 
together with some outstanding examples, will be discussed later. 
1 See: S. Ewing: “Soil Survey Methods,” Oil and Gas Journal, April 21, 1932, Pp. 42. 
C. R. Weidner and L. E. Davis: ‘‘Relation of Pipe Line Currents and Soil Resis- 
tivity to Corrosion,” The Oil Weekly, December 4, 1931, p. 26. 
434 
