ELECTRICAL PROSPECTING 19 
depth of the beds are determined. The conducting beds which are followed 
are called marker beds. 
Fig. 9° shows the results of a survey across a shallow salt dome. Three con- 
ducting beds were assumed, one at the surface, one at an intermediate depth, 
and the last at about 400 feet. The intermediate bed is arched over the dome. 
The deeper conductor is missing where it is pinched out by the salt, and 
locates the flanks of the dome. 
Fig. 10 gives the electrical indication of the Bruner fault zone, Texas. A 
clay bed at about 500 feet was used as a marker. When the survey was started 
there was only one producing well in the area. Later production has followed 
parallel to the electrically indicated fault. The electrical picture was not only 
qualitively right, but later drilling showed a constant interval between the 
clay bed and the Austin chalk, which is the producing horizon. Thus, a very 
good shallow picture of this fault was obtained electrically. 
IV. ConcLusIon 
Up to the present point we have discussed in a general way the problems 
encountered in electrical prospecting under very simple geological conditions. 
Experience in the field has shown that in general good marker beds can be 
found within the depth to which investigation can be carried electrically. 
However, in countries where the shallow sediments have been laid down in 
the form of lenses, these beds may not be continuous enough to give a struc- 
ture picture. 
On monoclinal slopes where successive beds are outcropping so that the 
surface material is changing in character, the lateral variation in conductivity 
of the surface material is large. This resistance variation of the near surface 
material makes the problem of interpreting electrical measurements some- 
what difficult. A condition of this kind was encountered in certain parts of the 
Balcones fault zone of Texas. The variation in surface resistivity resulted in 
many apparent fault indications and it was difficult to distinguish these ap- 
parent indications from the true ones. 
The depth to which geophysical investigations can be carried is of con- 
siderable importance in regions where the lower beds which contain the oil 
are not laid down conformably with the shallow beds. If the beds in which oil 
must be sought lie below an uncomformity which occurs at a depth of 5000 
feet, then it is obvious that a prospecting scheme cannot be used which does 
not give information below 5000 feet. 
Many extravagant claims have been made for the possible depths to 
which it is possible to work by electrical methods. In direct current cases, if 
the earth were homogeneous except for two or three sharp breaks in conduc- 
tivity, it would be possible to map to a large depth. However, even in this 
ideal case as the depth of investigation is increased, there would be a gradual 
loss in detail. Actually conditions are much more complicated. As the depth 
of investigation is increased, more and more discontinuities enter both later- 
5 Surveys shown by Figs. 9 and 10 have previously been published by Zuschlag in 
A.I.M.M.E. Technical Publication No. 313. 
163 
