GEOPHYSICAL INTERPRETATION
Better geophysical maps will be made by geophysicists, and
better use of them by geologists will come about, when the two
sciences attain more complete coordination. We may even hope
for the day when a really competent combination of geologist
and physicist will emerge in the same man who will be truly a
"geo-physicist" in all that that combined title implies.
THE JUDGMENT AND WEIGHING OF GEOPHYSICAL RESULTS
Let us now look at the making of a geophysical map with a
view toward its final utility, i.e., its economic value in the finding
of oil reserves.
Test Surveys over Known Structure.—One of the most useful
aids in the interpretation of geophysical results is a test survey
of an area where subsurface structure is more or less accurately
known. Such a test gives a reasonable basis for estimating the
expected geophysical effect over another similar geologic situa-
tion. It may go even further and give definite values for the
physical constants of the earth materials that produce the
geophysical effect. For example, a gravity survey over a known
structure may provide the basis for a quantitative estimate of the
density contrasts that are responsible for the gravity effect. A
seismograph survey will give definite information on the velocity
distribution and may, in favorable circumstances, serve definitely
to correlate a given reflection pattern or reflection character with
certain geologic horizons. A test survey in the case of the
magnetometric method requires more careful choice, since com-
mercial exploitation of an oil field seriously interferes with the
determination of the true magnetic effect because of the magnetic
disturbance resulting from buried casing as well as steel derricks,
pipe, and other magnetic materials on the surface. Thus, if a
reliable magnetic picture of a known structure is desired, it
should precede and anticipate the extensive development by
which the structure will become known. The single wildcat
discovery well, which proves the existence of a structure or the
first few wells following it, will not greatly disturb the picture,
so that in the very early stages a reasonably satisfactory magnetic
survey can be obtained. Then as the field development pro-
ceeds and the structural picture develops, a reliable basis for
determining the magnetic effect of that particular structure will