Geology Reference
In-Depth Information
a 'bird' to remove the instrument from the magnetic
effects of the aircraft or fixed in a 'stinger'in the tail of the
aircraft, in which case inboard coil installations compen-
sate for the aircraft's magnetic field.
Aeromagnetic surveying is rapid and cost-effective,
typically costing some 40% less per line kilometre than a
ground survey.Vast areas can be surveyed rapidly without
the cost of sending a field party into the survey area and
data can be obtained from areas inaccessible to ground
survey.
The most difficult problem in airborne surveys used
to be position fixing. Nowadays, however, the availabil-
ity of GPS obviates the positioning problem.
Marine magnetic surveying techniques are similar to
those of airborne surveying. The sensor is towed in a
'fish' at least two ships' lengths behind the vessel to re-
move its magnetic effects. Marine surveying is obviously
slower than aeromagnetic surveying, but is frequently
carried out in conjunction with several other geo-
physical methods, such as gravity surveying and con-
tinuous seismic profiling, which cannot be employed in
the air.
Fig. 7.12
A typical flight plan for an aeromagnetic survey.
7.9 Reduction of magnetic observations
The reduction of magnetic data is necessary to remove all
causes of magnetic variation from the observations other
than those arising from the magnetic effects of the
subsurface.
continuous-reading instrument which records magnetic
variations at a fixed location within or close to the survey
area.This method is preferable on land as the survey pro-
ceeds faster and the diurnal variations are fully charted.
Where the survey is of regional extent the records of a
magnetic observatory may be used. Such observatories
continuously record changes in all the geomagnetic
elements. However, diurnal variations differ quite
markedly from place to place and so the observatory used
should not be more than about 100 km from the survey
area.
Diurnal variation during an aeromagnetic survey
may alternatively be assessed by arranging numerous
crossover points in the survey plan (Fig. 7.12).Analysis of
the differences in readings at each crossover, representing
the field change over a series of different time periods, al-
lows the whole survey to be corrected for diurnal varia-
tion by a process of network adjustment, without the
necessity of a base instrument.
Diurnal variations, however recorded, must be exam-
ined carefully. If large, high-frequency variations are
apparent, resulting from a magnetic storm, the survey
results should be discarded.
7.9.1 Diurnal variation correction
The effects of diurnal variation may be removed in sev-
eral ways. On land a method similar to gravimeter drift
monitoring may be employed in which the magnetome-
ter is read at a fixed base station periodically throughout
the day. The differences observed in base readings are
then distributed among the readings at stations occupied
during the day according to the time of observation. It
should be remembered that base readings taken during a
gravity survey are made to correct for both the drift of
the gravimeter and tidal effects; magnetometers do not
drift and base readings are taken solely to correct for
temporal variation in the measured field. Such a
procedure is inefficient as the instrument has to be
returned periodically to a base location and is not practi-
cal in marine or airborne surveys. These problems
may be overcome by use of a base magnetometer, a
