204 ? MAGNETIC METHOD

magnetite content of sediments as 0.07 per cent1 which, by Eq.
(132), indicates a corresponding average susceptibility of about
200 • 10"6. Barrett2 claims that susceptibilities of sediments
can cause measurable magnetic anomalies; but his susceptibility
curves are in arbitrary units. Collingwood3 gives an average
value of 49.6 • 10~6 from measurements on 376 samples of sedi-
mentary rocks, measured in a field of 18 gauss. This paper also
gives magnetite content for some samples, from which values of k

calculated by Eq. (132) are of

the same order of magnitude as

those measured.
If we apply Fig. 84 and Eq.

(133) and calculate the minimum

' i- detectable polarization contrast

FIG. ss.-Magnetic anomaly for for, say, a IQy magnetic effect,
a fault with displacement equal to it COmeS Out 0.000016, and
ept ' the corresponding susceptibility
contrast (for H = 0.6) would be about 25 • 10~6. However, only
under very ideal geologic conditions (such as a fault with a dis-
placement large compared with its depth) would such a condition
be realized. More specifically, if we consider a fault with a
displacement equal to the minimum depth of the magnetic
contrast (Fig. 85), the required polarization contrast to give
AF = 10y would be 0.00007, and the corresponding susceptibility
contrast (for H = 0.6) would be 120 • 10~6. Prom these con-
siderations it is probable that in the type of structure more
commonly of interest in oil prospecting, a susceptibility contrast
of the order of 100 • 10""6 or more would be required before it
would produce a measurable magnetic anomaly. From such
data as are available, it seems unlikely that susceptibilities as
high as 100 -10~6 are at all .common in sediments, although there
are some definite exceptions.4 Therefore, in most cases of
1 Steam, loc. tit., p. 330.
2 Barrett, 1937.
3 Collingwood, 1930a.
4 Some claims are made (e.g., Barrett, loc. cit.) that magnetic anomalies
result directly from structure of magnetized sedimentary rocks. Calcu-
lations indicate that such effects should be extremely small in magnetic
relief, and there is doubt that the examples given are not partly caused by
contributions from underlying igneous rocks. Also, in some examples of
magnetic effects of structure known from drilling, the measured anomalies