Geology Reference
In-Depth Information
may have to be applied in those archaeological surveys in which only a
single sensor is used.
3.2.5 Geological effects
The Curie temperatures (in the range 500-600
◦
C) of the geologically impor-
tant magnetic materials are reached in the lower parts of normal continental
crust but below the Moho beneath the oceans. The upper mantle is only
weakly magnetic, so that the effective lower limit of rock magnetic sources is
the Curie isotherm beneath the continents and the Moho beneath the oceans.
Massive magnetite deposits can produce magnetic fields of as much as
200 000 nT, which is several times the magnitude of the Earth's normal
field. Because of the dipolar nature of magnetic sources, these and all other
magnetic anomalies have positive and negative parts, and directional mag-
netometers may actually record negative fields in extreme cases. Anomalies
of this size are unusual, but basalt dykes and flows and some larger basic
intrusions can produce fields of thousands and occasionally tens of thou-
sands of nT. Anomalous fields of more than 1000 nT are otherwise rare, even
in areas of outcropping crystalline basement. Sedimentary rocks generally
produce changes of less than 10 nT.
In some tropical areas, occasionally large magnetic fields are produced
by maghemite formed as nodular growths in laterites. The nodules may later
weather out to form ironstone gravels, which give rise to high noise levels in
ground surveys. The factors that control the formation of maghemite rather
than the commoner, non-magnetic form of hematite are not yet fully under-
stood. Human activity can also change iron oxidation states and produce
small (
<
10 nT) anomalies in natural materials, for example by deliberate
heating of clays in brick and pottery manufacture or simply by disturbing
soil horizons.
3.2.6 Man-made sources
Iron and steel are ferromagnetic and are usually strongly magnetised. Even
quite small steel objects can produce fields of hundreds of nT, although for
small objects these fields decrease rapidly with distance. In investigations of
'brownfield' sites and in the search for unexploded ordnance (UXO), man-
made objects may be the targets and their associated magnetic fields may
be very useful, but in geological work they are not, and high-cut filters may
have to be applied to survey results. In extreme cases, meaningful results
may be impossible to obtain.
Most power lines transmit electricity as alternating current, producing
alternating magnetic fields that may interfere with instrument operation but
are unlikely to give rise to readings that are both repeatable and spurious.
However, where the distances involved are very large, electric power is often
