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logic structure is concerned, the dip of surface rocks is of course most impor-
tant; faulting and other anomalous conditions are an object of search as well.
Naturally those dips which delineate anticlinal relationships are de rigueur. The
stratigraphy of any area can be determined in general terms, and in most in-
stances specific formations can be identified and traced on photographs more
easily and quickly than by field methods. Obviously, some exceptions may be
made to that statement particularly in areas of heavy vegetative cover or soil
mantle but just as obviously, you can’t map what you can’t see by any surface
method. Yet stratigraphy, and its attending lithologic differences is a prime fac-
tor in the development of topography, and this combination with structure un-
ravels many geologic knots.
At the present time reconnaissance and detailed photogeology are being
done daily by about 100 geologists in the United States alone, who have made
the stereoscopic determination of structure and lithologic types their special
skill. Perhaps half the number are employed directly by an oil company; the
remainder are those individuals who offer their services as independents and
those who are employed by consulting firms. These geologists have contributed
to geologic knowledge by mapping well over a half million square miles here in
the United States. The mapped areas in other countries are perhaps not so large,
but the potential is even greater.
Photogeology has been the responsible tool in several recent petroleum dis-
coveries, and will be for further discoveries. Photogeology has been the respon-
sible exploration tool in pointing the way for a more detailed examination of
promising areas. It has been possible to eliminate more expensive methods in
some areas and to focus attention to those prospects that merit detailed photo
work, detailed surface work, or seismic methods. In many areas drilling has yet
to be done to fully evaluate the photogeology already completed.
As a general rule, in all types of photogeology the technique is much the
same. A geologist merely changes his viewpoint from the horizontal to the ver-
tical; in no case is the transition a difficult one for the geologist well grounded
in the fundamentals of tectonics and sedimentation, and who has had sufficient
field experience to appreciate fully the natural forces and conditions necessary
to create the geologic situations encountered. Naturally, the ability to see a
stereo model easily and without strain for a considerable period of time is essen-
tial. However, the basic consideration must be a thoroughly trained geologist.
The tools of the trade are otherwise simple. A simple folding stereoscope of the
two-power lens type is used in nearly all cases.
As in the case of any new idea or method, constant experimentation and
research is necessary. The benefits of study and research of photogrammetric
problems has been of considerable magnitude as you all know. Naturally per-
tinent results have been applied in the geological interpretation as well. In ad-
dition, problems peculiar to geologic studies have been undertaken at various
times by inquiring individuals. Although still not voluminous, the literature is
growing richer due to fine contributions by Melton, Desjardins, Nugent, Smith,
Elliott, Wengerd and many others.
The determination of the true degree of dip of exposed rocks on air photo-
graphs has been a problem which has had a lot of thought. There are, of course,
the several methods of measurement of parallactic displacement, and resolution
of three-point determinations. But a disadvantage common to all is a general
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