LVl&TtlUL)
lower end of the pipe will have a breadth that will depend upon
the length of pipe but will, in general, be large compaied to the
spacing between wells. Therefore, the total magnetic effect of
an oil field should be a series of individual extremely sharp highs
at each wellhead, together with a broad negative effect which will
depend on the total number of wells and on their depth. Thus,
if we were making a magnetic profile toward a drilled field, we
should expect a negative anomaly on approaching the field.
The distance out at which the negative anomaly should begin
to be felt would be greater the deeper the drilling. A detailed
calculation of this effect for the Healdton field is given by Van
Weelden.1 For this field the actual magnetic survey shows a
maximum negative anomaly of approximately 2007. The
magnitude and form of the measured anomaly are quite con-
sistent with those expected from the calculation of the sums of
the magnetic effects of the wells in the field.
-^APPLICATIONS OF MAGNETIC PROSPECTING
The various formulas and calculation methods for estimating
the general form and magnitude of the magnetic anomaly that
may be expected from a given distribution and polarization of
magnetic rocks have been outlined. Such estimates, in turn,
can serve as a general guide to the selection of magnetic anomalies
that may be the expressions of reasonable geologic structure and
excluding those which must have other causes. To a limited
extent such calculations may indicate whether or not a given
geologic problem has a reasonable chance of being solved by
magnetic methods.
It has been shown (page 205) that the expected polarizations
of igneous rocks are so much greater than those of the sediments
that magnetic anomalies are primarily determined by the depth
and attitude of the basement rocks. For most oil prospecting,
the contribution of the sedimentary rocks to the magnetic pic-
ture is almost negligible.2
The oil geologist's interest in determining the configuration of
the basement rocks lies in the fact that in many cases sedimentary
structures are underlain by basement uplift. If a basement
1 Van Weelden, op. cit., pp, 88-90.
2 See calculations and footnote, p. 204.