Principles Of Magnetics - Accurate Locators XL16 IMAGER User Manual

Principles of Magnetics

The earth's geomagnetic field has three principal components: the main field itself, an external field and local
perturbations superimposed on the main field. Caused by processes in the interior of the earth, the main field has a
large magnitude which varies slowly over time. At present, the earth's field amplitude (T) ranges from a low of
about 25,000 nanoTeslas (formerly gamma) (nT) near the geomagnetic equator to almost 70,000nT at the
geomagnetic poles. The field inclination is horizontal at the equator and vertical at the poles. The external field
originates outside the earth's crust and is associated mainly with electric currents in the ionized layers of the outer
atmosphere because of interaction with the solar winds. Traveling along magnetic flux lines solar winds are ionized
plasma or hot-charged particles, which transmit energy by wave motion. Local variation in the rock and mineral
assemblage of the near-surface crust produce local perturbations that are the anomalies of exploration interest.
The earth's magnetic field (a vector field having both amplitude and direction) is described by an intensity (total
field intensity, T), an inclination (I) and a declination (D), Figure 1.1. For Specific applications, horizontal and
vertical field components can be derived from T, D. and I
Rock units in the crust acquire a magnetization in the direction of the earth's field, which is referred to as induced
magnetization or magnetic polarization. The resulting induced field of a typical finite source body is dipolar: that
is, it contains positive and negative elements. In the middle-to-high magnetic latitudes of the Northern Hemisphere,
the dipolar nature of an induced anomaly is typified by a positive (high) and related, but subdued, negative (low)
on its north side. Anomalies of interest range in amplitude from a few nano Teslas (nT) for deep basement or
sedimentary anomalies to 1,000's of nT for near-surface mafic rocks or iron formation to 10,000's nT for magnetite
iron ore deposits. Several time-variant or temporal variations occur in the geomagnetic ambient field. A long
period, or secular change, occurs slowly over many decades or centuries and modifies inclination, declination and
intensity. Such a change can be observed as a change in the magnetic declination as noted on old maps when
compared to today's version. A complete reversal of the total field direction, occurring over tens-to-hundreds of
thousands of years, is a more dramatic effect of the secular change. More important to prospecting are the diurnal
variations. A diurnal variation of 10 – 100+ nT occurs regularly on a daily basis, (Breiner, 1973). This diurnal is
also related to solar winds, the small effects of which vary with the level of the ionosphere and intensity of solar
winds. Micro pulsations of 0.001 to 10s nT are random effects lasting from 0.02 to tens of minutes. Magnetic storms
also produce a short period random "noise" which may vary up to many 100s nT over periods of a few minutes to
hours. Storm effects are unpredictable but are related to and follow solar flaring, commencing abruptly and
decreasing slowly over hours or days.
Principles of Magnetics are an excerpt from:
Practical Geophysics II for the Exploration Geologist
© 1992 Northwest Mining Association
ISBN: 0-931986-05-2
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