Geology Reference
In-Depth Information
T
is
not
a function of the position only, even
if we consider
F
as a time independent vector
quantity, because it is obtained by projecting
F
onto a time-varying field direction.
Magnetic anomalies are scalar fields that can
be considered as harmonic functions under some
simplifying assumptions. In general, the expres-
sion (
5.2
) does not warrant the harmonicity of
T
in any region. However, if F is approximately
constant over the survey area, then taking the
Laplacian of (
5.3
)gives:
magnetometers
, measurements devices that fur-
nish the magnitude of the ambient magnetic field.
Today there are essentially three kinds of scalar
magnetometers:
proton precession magnetome-
ters
,
Overhauser effect magnetometers
,and
cae-
sium vapor magnetometers
. Proton precession
magnetometers are commonly used in land-based
magnetic surveys. The operation of these mea-
suring devices is based on the Larmor preces-
sion of magnetic moments in an external field
magnetic moments are that of hydrogen nuclei
(i.e., protons). Therefore, basically a proton pre-
cession magnetometer is formed by a cylindrical
container (
bottle
) filled with a liquid rich in hy-
drogen atoms. The bottle is surrounded by a coil,
as shown in Fig.
5.2
.
The liquid in the bottle can be distilled water,
kerosene, alcohol, or any other hydrocarbon
fluid rich in hydrogen atoms. The measurement
proceeds in two steps. During phase 1, a DC
current is delivered by a battery through the
circuit, generating a relatively strong magnetic
field
B
in the solenoid. This field causes the
dipole moments to align with the solenoid axis
in the direction of
B
. The objective of this
polarization is to start a coherent precession
of the protons about the external field
T
and
2
T
D
F
r
2
F
D
0
r
(5.4)
In fact, the components of the crustal field
F
are harmonic functions. Therefore, under
the assumptions that
F
<<
j
F
j
and that F is
approximately constant over the survey area, a
magnetic anomaly field can be considered as a
harmonic function.
5.2
Acquisition and
Pre-processing of Raw
Magnetic Data
The acquisition of magnetic data that can be used
in studies of crustal magnetization, in particular
in plate tectonics, is done mainly through
scalar
Fig. 5.2
Operation of proton precession magnetometers.
During phase 1, the current flowing through the coil
generates a strong magnetic field
B
in the bottle, which de-
termines the alignment of atomic spins. When the circuit
is open (phase 2), the magnetic moments start precessing
about the ambient geomagnetic field
T
. The Larmor pre-
eters and the field strength
T
. This precession induces a
small alternating current in the coil, with frequency equal
to the Larmor frequency of precession, which is measured
through the frequency counter. Finally, the field intensity
T
is calculated scaling the observed Larmor frequency
