Geology Reference
In-Depth Information
Magnetic Method
Magnetic methods are based on the observation of anomalies in the magnetic
field of the earth that are caused by magnetic susceptibility of different
rocks. As dolerite dykes are very common features in hard rock and important
for groundwater flow, these methods are very useful to delineate even the
buried dykes. Magnetic surveys are classified as grid and profile survey
depending upon the density and geometrical distribution of the points of
observation. Measurements with a magnetometer are taken 2 to 4 times at
a station and their average value is considered to that station. The time of
measurement is also noted for every observation since the magnetic field
does not remain constant with time and hence necessary correction is applied
for each station value by having repeat observations at base station.
Different types of instruments are available for magnetic surveys e.g.
Schmidt type or by compensation as in Torsion magnetometer, Induction
type of instrument, Fluxgate magnetometer, Proton precession magnetometer,
Optical absorption magnetometer or the high sensitivity atomic resonance
magnetometer etc. Magnetic systems consist of a permanent magnet, which
can be deflected under the influence of geomagnetic field. The value of the
magnetic field is determined by the deflection as in Schmidt type or by
compensation as in Torsion magnetometer. Induction types of instruments
consist essentially of an induction coil operated by a motor in the earth's
magnetic field. The electromotive force developed in the coil due to
intersection of force of the earth's magnetic field is a measure of the field.
Instruments with sensitive fluxgate elements consist of a coil with core made
of an alloy whose magnetic permeability strongly depends on the minor
changes in the external magnetic field. The change in the electromagnetic
parametres of the sensitive element determines the intensity of the earth's
magnetic field.
Proton precession magnetometer consists of a container of water with a
coil wound around it. When a strong magnetic field is applied in a direction
approximately perpendicular to the earth's field, the protons will align parallel
to the applied field. At this stage if the field is suddenly cut off, the protons
start processing, inducing a small e.m.f. in the coil which is a measure of the
earth's field. Optical absorption magnetometer or the high sensitivity atomic
resonance magnetometer is the latest type. There are three types namely, the
metastable helium, rubidium or cesium magnetometer. All these
magnetometres make use of optical pumping technique. They are highly
sensitive and also enable the measurement of the vertical gradient of the
earth's magnetic field employing two magnetometres kept separated vertically
apart from each other. One example of a magnetic profile carried out across
a dyke in Maheswaram watershed is shown in Fig. 6.
