CHAPTER III
Geophysical Techniques of Mineral Exploration
By
G.P. Mathur, Geophysicist.
Geophysics as a science is more than three centuries old, but as an
exploration tool it is munch younger, although not as recent a development
as most people believe.
The word, geophysics' means the physics or nature of the earth. It embraces
the study of the earth, which evolves investigation of the materials and
transformation, and their changes in energy states ; it involves speculation
on the origin and age of the earth, as well as studies of its present appearance
and its reaction of forces, external and internal, acting upon it. some of the
knowledge thus gained, especially from these studies, when applied to the
search for mineral deposits become the art of geophysical exploration.
The exploration may be defined main as prospecting for mineral deposits
and geologic structure by surface measurement of physical quantities. The
methods used in geophysical quantities. The methods used in of physical
exploration are the techniques of geophysical exploration are the techniques
of exact science, but the interpretation, or translation of the observations thus
made into terms of geology and mineralogy is an art in which the
knowledge, the experience and the ability of the geophysicist are fully as
important as the physical properties he measure .
Application of geophysical methods:Though, the mineral resources are bountiful but they are always irregularly
distributed within the earth's crust. These mineral deposits and the host rocks
in which they occur have very often different physical properties.
This difference in physical property when detectable through instruments at
the surface forms the the basic of geophysical exploration techniques.
The main geophysical methods that are employed in the search of minerals
are listed in the following tables:S.No. Geophysical method
Physical property
1
Specific
Gravitational
geologic application
gravity Anticlinal structures,
contrast.
buried
ridges,
salt
domes,
faults,
intrusions.
major
structural trends
2
Magnetic
Magnetic
Anticlinal
susceptibility
burried
structure,
ridges,
instruction, faults iron
ore,
phyrrhotite,
associated
sulphide
ores, gold placers
3
Electrical
Natural
(a) Self potential
field
potential Sulphide ore bodies
flow of underground
water
b
Resistively
Conductivity or its sulphide ore bodies,
converse
bed
rock
resistively
ground water, Engg.
geology
depth,
problems,
etc.
c
Electromagnetic
Inductance
Conductive
ore
bodies sulphide ore
Disseminated
metallic ores
d
Induced polarization
Changeability
Disseminated
metallic
4(a)
Seismic (a) Refraction
Acrostic
Salt
Domes,
Impedance
anticlines, structures,
faults foundation and
highway
problems,
etc.
(b)
Refraction
Do
Low dip Structure,
buried rides faults.
5
Radioactive
Radioactive
Radioactive
ores,
well- logging
Metallic- minerals
The Metallic minerals are about 1000- fold more Conductive than the rock
of their environment. Also the metallic sulphides have 30% to 100% more
specific gravity and if these are associated with magnetic mineral like the
magnetite and pyrrhotite they have much higher magnetic susceptibility
compared to that host rocks. Thus, the metallic deposits are normally
distinguishable physically from the Precambrian rock topes in which they
occur by means of one or more of the following physical properties:1.
Electrical properties mainly conductivity contrast,
2.
Magnetic susceptibility, and
3.
Specific gravity
Non- Metallic Minerals
The Conventional geophysical methods comprising gravitation, magnetic,
seismic, electrical and radiometric have been already helpful; in discovering
important deposits of asbestos in Quebee & johns Manville in Canada the
phosphate deposits of Pennsylvania, and the coal seam in Australia to name
a few. However, the geophysical methods have been mostly useful as an
indirect in locating methods in locating the non- metallic minerals, since the
physical property contracts between these minerals and the host rocks is not
large. The following table shows the applicability of geophysical method for
non metallic minerals.
Table-2
S.No. Mineral
Geophysical parameter
Applicability
1
Electrical conductivity
Mica
Mica
Zones
in
pegmatite affected by
Post ore tectonics are
indicated by apparent
resistively zone due
to presence of ground
water.
2
Quartzand
felspar piezoelectric effect.
nepheline
Direct detection of
piezoelectric zones.
bearing rocks.
3
Asbestos
Magnetic
Chrysotile asbestos is
associate
magnetite
with
resulting
from
the
serpentinisation
of
ultra basic rocks.
4
Barite
and Mercury vapour
fluorite
High
mercury
anomalies over barite
and fluorite deposit
occurring
in
sedimentary
or
in
slightly
metamorphosed
rocks.
5
Barite
Gravitational
Direct application.
6
Rock Phosphate
Radioactive
Rock
Phosphate
deposits
associated
with uranium.
7
Coal
Gravity, electrical
Though
deciphering
suitable structure.
8
Graphite
Electrical S.P.
Direct application
The geophysical exploration techniques are if added use in the areas which
are covered by sand and solid. Over 7307% areas of Rajasthan is covered,
the remaining area being exposed by rock of pre-Cambrian age (19.6% area)
, Maspzoic (4%), Tertiary (1.3%), and by volcanic eruptives & batholiths
(1.3%). this means that the geophysical surveys have greater role to play in
the search for minerals in Rajasthan.