Geoscience Reference
In-Depth Information
fIGURe 8.1
Magnetometry field survey employing the gradient mode for data collection.
is a measure of how rapidly the magnetic field is changing, resulting in an enhanced view of the
shallow subsurface.
8.1.3 M
a g n e t o M e t R y
f
i e l d
P
R i n c i P l e s
Data are collected discreetly or continuously while moving along transects. In discreet mode, the
magnetometer is held stationary over a position along the transect while the magnitude of Earth's
local magnetic field is recorded. In continuous mode, the magnetometer moves at a normal walk-
ing pace along the transect taking approximately ten readings every second. At a normal walking
speed, this corresponds to a reading approximately every 10 cm. Transects are commonly separated
by 25 to 50 cm when looking for human alteration of the landscape (Figure 8.1), and larger transect
spacing when looking at geologic features. Data can be acquired in a unidirectional or bidirectional
format. Unidirectional is where one travels along the first line from south to north, walks back to
the start of the second line while not gathering data, and then gathers data along the second line
while walking south to north. During a bidirectional survey, one collects data south to north along
the first transect, turns around, and moves over to the second transect and gathers data while walk-
ing north to south.
8.1.4 M
a g n e t o M e t R y
d
a t a
P
R o c e s s i n g
a n d
a
in a l y is i is
Magnetometer surveys produce
x
,
y
, and
z
data, where
x
and
y
are horizontal locations, and
z
is
the magnitude of the vertical component (fluxgate) or the total field (optical pumping) of Earth's
local magnetic field. The first step in postsurvey analysis of magnetic data is to examine the data
using mapping software such as Golden Software's Surfer, Geoscan's Geoplot, or Geosoft's Oasis
Montaj. These programs allow one to present the data as contour, image, shaded-relief, and surface
plots. Large changes in the magnetic field caused by highly magnetic materials should be identified
and “despiked.” Large magnetic changes create scaling problems that obscure smaller changes of
interest, and despiking removes any magnetic features above or below a selected value (Figure 8.2).
Another filter can aid in removing large geologic trends.
