MAGNETIC METHOD - Field Geophysics - page 69

gradient is likely to be significant but gives only approximate correction fac-
tors, because of the existence of very considerable local variations. Gradients
are roughly parallel to the local magnetic north arrow, so that corrections
have E-W as well as N-S components. In ground surveys, where anomalies
of many tens of nanotesla are commonplace, regional corrections, which
amount to only a few nanotesla per kilometre, are often neglected.

3.2.2 The International Geomagnetic Reference Field (IGRF)
The Earth's main field varies not only with latitude and longitude but also
with time. Between 1980 and 2002 the field strength in the central At-
lantic decreased by about 6%. These secular changes are described by the
empirical International Geomagnetic Reference Field (IGRF) equations,
which are defined to order N =

10 by 120 spherical harmonic coefficients,
supplemented to order N = 8 by a predictive secular variation model. The
shortest wavelength present is about 3000 km. IGRFs provide reasonable
representations of the actual fields in well-surveyed areas, where they can
be used to calculate regional corrections, but discrepancies of as much as
250 nT can occur in areas from which little information was available at
the time of formulation. Since 2000, the accuracies of the IGRFs have been
greatly improved by incorporating data from the Danish Oersted and German
CHAMP satellites.
Secular changes are adequately predicted by extrapolation from past ob-
servations for only a few years into the future, and IGRFs are updated every
five years and are also revised retrospectively to give definitive models
(DGRFs). These time-dependent corrections are vital for comparing air-
borne or marine surveys carried out years or even months apart but are less
important in ground surveys, where base stations can be reoccupied.

3.2.3 Diurnal variations
The Earth's field also varies because of changes in the strengths and direc-
tions of currents circulating in the ionosphere. The variations, with peak-to-
trough amplitudes of the order of a few tens of nanotesla in mid-latitudes,
tend to be directly related to local solar time, because the upper atmosphere
is ionised by solar radiation. In the normal solar-quiet (Sq) pattern, the back-
ground field is almost constant during the night but decreases between dawn
and about 11 a.m., increases again until about 4 p.m. and then slowly falls
back to the overnight value (Figure 3.4). However, at points up to a few hun-
dred km apart, amplitude differences of more than 20% due to differences in
crustal conductivity may be more important than time dependency. Shorter
period, horizontally polarised and roughly sinusoidal micropulsations are
significant only for surveys that are to be contoured at less than 5 nT.