Geoscience Reference
In-Depth Information
and some of the older magnetometry data are displayed in these units. However, more recently,
the common geophysical unit of measure for a magnetic field is called a tesla, where one tesla is
equal to 10
4
oersteds, or 10
9
gammas. The Earth's magnetic field intensity varies between 0.25 and
0.65 oersteds (25,000 to 65,000 nanoteslas [nT], or 25,000 to 65,000 gammas).
Magnetometers are passive remote sensing devices that measure the magnitude of Earth's local
magnetic field at the location of the sensor. Magnetometers may be placed on the ground surface,
in the air (airborne magnetometer), in satellites (satellite magnetometry), or beneath the surface of
Earth (borehole magnetometry). In practice, for agricultural purposes, handheld magnetometers are
used with the sensor positioned within a couple meters of the ground surface. Commonly used mag-
netometers measure The Earth's local magnetic field with a precision between 0.1 nT and 1.0 nT.
To put this sensitivity into context, a magnetometer operator cannot wear glasses, zippers, or other
objects containing ferromagnetic metals (Clark, 1996; Gaffney and Gater, 2003), because these
objects are likely to interfere with measurements.
8.1.2
t
y P e s
o f
M
a g n e t o M e t e R s
Fluxgate and optically pumped magnetometers are the two most commonly used instruments. Pro-
ton precession magnetometers, a third type, are rarely used due to long data acquisition times.
Fluxgate magnetometers use a nickel-iron alloy core surrounded by a primary and secondary wire
wrapped around the core. The core is magnetized by the component of Earth's magnetic field paral-
lel to the core and by an alternating current in the primary winding. The alternating current in the
primary winding creates an alternating magnetic field, which induces a current in the secondary
winding that is proportional to Earth's local magnetic field. Fluxgate magnetometers have a 0.1 nT
precision and fast acquisition times but require careful alignment of the sensor to avoid introducing
anomalies (Scollar et al., 1990).
Optically pumped magnetometers commonly use cesium vapor and the Zeeman-effect to measure
the magnitude of the Earth's local magnetic field. The Zeeman-effect arises when atoms containing a
magnetic moment are placed in an external magnetic field. An oversimplified model of such an atom
is a bar magnet with a north-south dipole. In the absence of an external magnetic field, the bar mag-
net has the same energy regardless of its alignment. In the presence of an external field, a bar magnet
will become reoriented to align with the external magnetic field. The aligned state of the magnet has
a lower energy than the nonaligned state. The difference in energy between these two states is related
to the magnitude of the external magnetic field. Optically pumped magnetometers have a 0.1 nT pre-
cision, acquisition times of ten readings per second, and do not have the fluxgate alignment issues.
Because both instruments use different methods of measuring Earth's magnetic field, they have the
potential to record different data when used at the same site (Scollar et al., 1990).
Both fluxgate and optically pumped magnetometers can be used in single mode or gradient
mode. In single mode, a single sensor is used to record the magnetic field. Because the background
magnetic field varies with time, some means of compensating for these background changes (called
temporal variations) must be incorporated into the design of each magnetometry field survey. Com-
pensation is achieved by removing the temporal magnetic field variations from the data collected
during a magnetometry field survey, and the temporal magnetic field variations are determined by
maintaining a separate continuously recording stationary magnetometer at a base station or by reoc-
cupying a fixed location periodically (e.g., every few minutes) throughout the time of the survey. An
alternative is to use two magnetometers that are horizontally or vertically separated (Figure 8.1) in
a setup called a gradiometer. The difference found between the magnetic fields recorded by each
sensor is divided by the sensor separation resulting in the gradient of the magnetic field. Because
the gradient is a spatial measurement that can be considered to be independent of time, there is no
need to compensate for temporal variations in the Earth's magnetic field. Additionally, the gradient
