Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 116

spacing. For ground surveys a single tie line could be
suf cient for the typically smaller area surveyed.

In the case of airborne systems, it is continuously measured
by a dedicated on-board magnetic sensor and a heading
correction is automatically applied, as the data are

Survey specifications
Based on the characteristics described above for each par-
ameter, and considering the cost of conducting a survey, a
range of line/height combinations has evolved that have, by
experience, been found to satisfy the requirements of the
various classes of magnetic surveys.
For wide-ranging regional airborne surveys aimed at
providing the

recorded, to compensate for it.
Note that the various corrections also remove the back-
ground value of the magnetic field, the reason that the
reduced data sometimes have unexpectedly low absolute
values. This is not a problem since the mapping of spatial
variations in the field is the object of the survey. Sometimes
an average field strength value from the survey area is
added to return the data to a realistic absolute value.
We briefly describe the various corrections in the order
that they are applied, and after which the corrected data
are levelled to remove residual errors (see Section 2.7.1 ) .

of the large-scale geological envir-
onment, and useful for targeting areas for more detailed
work, the lines are often spaced 400 m apart and the survey
conducted at 80 to 120 m above the terrain. In highly
prospective areas, the line spacing and height are often
reduced to 200 m and 50 to 60 m, respectively. Surveys
for prospect targeting require the lines to be spaced 40 to
100 m apart, and the survey conducted at 20 to 50 m above
the ground.
Of course, there are exceptions to these relationships.
For example, the very long and thin ilmenite-rich heavy
mineral sand strandlines, which typically are many kilo-
metres in length, are surveyed with lines spaced about
400 m apart, but at a height of 20 m above the ground,
and even lower when helicopter systems are available
(Mudge and Teakle, 2003 ). The exceptionally low survey
height is essential for detecting the very weak responses,
typically less than 1 nT, associated with these near-surface
mineral deposits. Similarly low survey heights, but with
closer lines, are typical in kimberlite exploration where the
laterally confined targets can be weakly magnetic. For
highly magnetic banded iron formations, low survey height
improves resolution of the thin magnetic bands.

' 'first view '

3.6.1 Temporal variations in field strength

Very short-period variations in the strength of the mag-
netic field, such as micropulsations (see Micropulsations in

Section 3.5.1.1 ) and noise , are identi ed and

excised from both the survey data and the base station
record. This is based primarily on manual inspection and
editing of the data. The edited base station record is then
synchronised in time with the survey data, and diurnal
variations removed by simply subtracting the base station
record from the observations. It is worth noting here that
any noise in the base station record, owing to magnetic
disturbances local to the base station, needs to be identi ed
and removed from the recording first; otherwise the noise
will propagate via the diurnal correction and contaminate
the survey data.
Continuous base station recordings (see Section 3.5.3 )
are usually quite effective for removing diurnal variations
in surveys conducted close to the base station. The process
is imperfect, however, because in the case of airborne
surveys diurnal variations at the survey level are often not
accurately represented by a base station located at ground
level. Also, features, of all frequencies, in the diurnal vari-
ation will have different amplitudes at different locations.
Short-wavelength variations (micropulsations) are a par-
ticular problem and the diurnal data must be low-pass
filtered (see Frequency/wavelength in Section 2.7.4.4 )
before subtraction. Deploying a network of base stations
around a large survey area usually presents major logistical
challenges and is rarely undertaken. Errors related to the
use of a single base station are the generally largest source
of error in modern airborne TMI data. Diurnal variation
can often be removed without the base station record, in

' spikes '

3.6 Reduction of magnetic data

The reduction of magnetic survey data is principally aimed
at removing the effects of temporal variations in the Earth s

magnetic field that occur during the course of the survey
(see Section 3.5.1.1 ). Like gravity reduction, planetary-scale
spatial variations in the field are compensated for, but in
contrast, elevation-related variations in the magnetic eld
are minor. Corrections for variations in survey ground
clearance may also be required if the terrain is rugged.
With moving platform systems, there is a small residual
magnetic response related to the platform

'

'

s magnetism
that varies with its attitude and heading with respect to
the Earth

' s magnetic field and known as the heading error.