Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 114

The strength of the magnetic field in a particular direc-
tion can be measured with a ux-gate sensor, whose oper-
ation depends upon the magnetic properties of iron

In addition to the TMI measurements, station location,
obtained from the GPS, and aircraft height, obtained from
a radio altimeter, are measured and recorded, typically,
every 0.1 s (10 hertz sampling). As far as is possible, the
aircraft maintains constant survey speed, typically 70 m/s,
50 m/s for low- ying crop-duster aircraft and as low as
30 m/s for helicopters. The constantly sampled data are
then spaced along-line at intervals of approximately 7, 5
and 3 m respectively. Obviously, the sampling interval is
determined by the survey speed, but these very small
sampling intervals are usually more than adequate for
nearly every geological environment.
For ground magnetic surveys, readings are taken at
specified locations. Usually a station interval of 10 m, or
even 5 m, is suitable for most purposes, but for environ-
ments with near-surface occurrences of highly magnetic
materials, a closer interval of 1 m or less is desirable. These
highly magnetic and disrupted features produce very large
amplitude variations in the magnetic field over small dis-
tances that require a very close sampling interval in order
to avoid aliasing (see Section 2.6.1 ) . When properly
sampled, the high-frequency noise can be removed with
low-pass filtering (see Frequency/wavelength in Section
2.7.4.4 ) . A better and logistically more convenient solution
in these environments is to use a continuously time-
sampling magnetometer (see Section 3.5.2.1 ) con gured
like an airborne system with a portable data acquisition
computer and a positioning system. At normal walking
speed, the data interval can be small as 10 cm, producing
very dense sampling of the high-frequency noise which
allows its removal with filtering.
The key variables when designing magnetic surveys are
survey height, line direction and spacing.

nickel
alloys. It measures the strength of the magnetic field in the
direction of its sensor, the alloy cores, which need to be
oriented in the direction of the field component being
measured. Orientation errors are a source of measurement
noise. The flux-gate magnetometer is a vector magnetom-
eter and finds application where the direction to a mag-
netic source is required, such as downhole magnetometery
and downhole electromagnetics (see Section 5.8 ) .

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3.5.3 Magnetic survey practice

Comprehensive descriptions of general aspects of geophys-
ical data acquisition and survey design are given in Section
2.6 . Details pertaining speci cally to magnetic surveying

are described here.
Strategies for conducting magnetic surveys are princi-
pally determined by the need to record temporal changes
in the geomagnetic field occurring during the survey so
that they can be removed during data reduction. The
problem is addressed by a combination of repeat readings
and continuously monitoring the field. The latter is done
by establishing a second, base station, magnetometer at a
fixed location close to or central to the survey area to
record the changing geomagnetic field, usually at a time
interval of several minutes. For large surveys spatial vari-
ations in diurnal changes require multiple base stations.
The base station also allows the rate of field change to be
monitored, which is the operational basis for identifying a
magnetic storm (see Magnetic storms in Section 3.5.1.1 ) .
The survey data acquired during storm periods will be
dominated by storm events, so they are rejected and the
survey repeated when the field is deemed to be stable. Survey height

The sensor height is a key variable in magnetic survey
design affecting both the amplitude and wavelength of
the anomalies detected (see Sections 3.10.1.1 and 2.3 ).
Reducing the height places the sensor closer to the mag-
netic sources, increasing the measured amplitude of any
anomalies, i.e. survey sensitivity is increased. It also means
that there will be more shorter-wavelength, i.e. high-
frequency, information in the field, increasing the risk of
aliasing (see Section 2.6.1 ) , a problem mainly with ground
surveys. For example, in areas where highly magnetic
material occurs near-surface, decreasing the survey height
causes the measurement to be dominated by the effects of
the near-surface material. This may not be a bad thing if

3.5.3.1 Ground and airborne surveys
Ground and airborne magnetic surveys usually comprise a
series of parallel traverses and tie lines as appropriate (cf.
Fig. 2.10a ) . The line spacing and line direction, and survey
height in the case of airborne surveys, are the main param-
eters to consider in survey design. When the survey is
conducted with a moving vehicle, usually an aircraft, the
along-line data interval is determined by the speed of the
vehicle and the sampling rate of the magnetometer.
Modern instruments sample the field rapidly, resulting in
a very small along-line data interval compared with the
survey line spacing.