A Mineral Storehouse
103
investigation due to the overlying conductivity of the sea. Of the
three remaining categories, seismic techniques are probably the
most widely used but, before considering these, a word or two is
needed about gravitational and magnetic surveying.
Greettætional methods are based on the way in which the ‘gravitational pull’ of the Earth on an object suspended above it varies from
place to place according to the density of the underlying material
and its distance away from the object. ln other words, the weight
of an object at a given latitude and altitude depends on the nature
of the rocks lying beneath the surface in that locality. In general,
the older rocks which have become compacted with time tend to
exert the greatest inuence. By plotting lines of equal attraction the
gravitational contours of the underlying strata can be drawn.
Magnetic techniques also favour the study of older rocks; igneous
(volcanic) rocks and metamorphic rock (sediments converted into
compact rock by pressure and heat) contain sufcient quantities of
magnetic minerals such as ilmenite, magnetite and pyrrhotite to
produce detectable changes in the Earth’s local magnetic eld. The
more recent sedimentary rocks generally contain relatively few of
these magnetic minerals. However, being of geologically recent
origin, the sedimentary rocks lie on top of the igneous_and meta—
morphic layers so that changes detected in the older, basement
rocks will inuence the overlying sedimentary rock.
Oil companies, as a result of investigations on the land, know the
types of geological formations which favour the accumulation of oil
and natural gas. Where possible, it helps to know from the outset
the form of the basement rocks in the prospective areas. In the
case of the North Sea search ten companies banded together to
share the cost (over £3 50,000) of producing a magnetic map of the
entire area from southern Norway to the Straits of Dover, a total of
144,000 square miles. The survey
was carried out from an aircraft
using a magnetometer to measure the strength of the magnetic
eld.
Setsmtc methods give a picture of the underlying rocks irrespective
of their origin. The name comes from seismology, the study of
earthquakes. ln seismic surveying an articial earthquake is created,
sending out waves of energy which surge downwards through the
Earth. The velocity at which these shock waves pass through rock
depends upon the latter’s elasticity, density and composition.
Whenever these properties change—as, for example, from one rock
103
investigation due to the overlying conductivity of the sea. Of the
three remaining categories, seismic techniques are probably the
most widely used but, before considering these, a word or two is
needed about gravitational and magnetic surveying.
Greettætional methods are based on the way in which the ‘gravitational pull’ of the Earth on an object suspended above it varies from
place to place according to the density of the underlying material
and its distance away from the object. ln other words, the weight
of an object at a given latitude and altitude depends on the nature
of the rocks lying beneath the surface in that locality. In general,
the older rocks which have become compacted with time tend to
exert the greatest inuence. By plotting lines of equal attraction the
gravitational contours of the underlying strata can be drawn.
Magnetic techniques also favour the study of older rocks; igneous
(volcanic) rocks and metamorphic rock (sediments converted into
compact rock by pressure and heat) contain sufcient quantities of
magnetic minerals such as ilmenite, magnetite and pyrrhotite to
produce detectable changes in the Earth’s local magnetic eld. The
more recent sedimentary rocks generally contain relatively few of
these magnetic minerals. However, being of geologically recent
origin, the sedimentary rocks lie on top of the igneous_and meta—
morphic layers so that changes detected in the older, basement
rocks will inuence the overlying sedimentary rock.
Oil companies, as a result of investigations on the land, know the
types of geological formations which favour the accumulation of oil
and natural gas. Where possible, it helps to know from the outset
the form of the basement rocks in the prospective areas. In the
case of the North Sea search ten companies banded together to
share the cost (over £3 50,000) of producing a magnetic map of the
entire area from southern Norway to the Straits of Dover, a total of
144,000 square miles. The survey
was carried out from an aircraft
using a magnetometer to measure the strength of the magnetic
eld.
Setsmtc methods give a picture of the underlying rocks irrespective
of their origin. The name comes from seismology, the study of
earthquakes. ln seismic surveying an articial earthquake is created,
sending out waves of energy which surge downwards through the
Earth. The velocity at which these shock waves pass through rock
depends upon the latter’s elasticity, density and composition.
Whenever these properties change—as, for example, from one rock