Surface and Subsurface Methods of Mineral Exploration
major motive in exploring the earth’s surface and its interior is scientific
curiosity or the desire to understand better the nature of the Earth. Another
key motive is the prospect of economic profit. Improved standards of living have
increased the demand for water, fuel, and other materials, creating economic
incentives. Scientific knowledge has often been a by-product of profit-motivated
exploration. In equal measure
however, significant economic benefits have resulted from the quest for
Many surface and subsurface exploratory projects are undertaken with the aim of locating:
a) oil and gas accumulations and coal beds
b) concentrations of commercially important metallic minerals (ores of iron, copper, and uranium)
c) deposits of nom-metallic minerals (building materials - sand, gravel, etc.)
d) recoverable groundwater
e) various rock types at different depths for engineering planning
geothermal reserves; and
g) archaeological features.
exploration of the Earth's interior was confined to the near surface, and this
was largely a matter of following downward those discoveries made at the
surface. Most present-day scientific knowledge of the subject has been obtained
through geophysical research conducted since World War II, and the deep Earth
remains a major frontier in the late 20th century. With conventional approaches and tools, only a very limited
portion of the subsurface regions of the Earth can be studied. Investigators can
drill into only the uppermost crust, and the high cost of drilling
severely limits the number of holes that can be dug. The deepest borehole so far
drilled extends only to a depth of about 10 kilometres (6 miles). Because direct
exploration is so restricted, investigators are forced to rely extensively on
geophysical measurements (see below Methodology and instrumentation). In recent
years data returned by Earth satellites have led to several notable discoveries,
as, for example, drainage patterns in the Thar region, which are relics of a
period when this region was not arid.
main methods of mineral exploration and their applicability (whether surface or
subsurface are listed hereunder:
maps provide exploration agencies or companies with regional geological and
geophysical information so that target areas that are consided to have a better
prospect in terms of mineral deposits may be identified. The cost of undertaking
geological surveys, many of which will not prove to be prospective, is high.
Geological surveys provide exploration and mining companies with pre-competitive
geoscientific data that is designed to encourage the company to undertake
methods rely on the identification of rocks and minerals and an understanding of
the environment in which they formed. These surveys aim to find what rock types
occur at or close to the surface and how these rock types are related to each
other i.e. their boundaries, ages, and structure.
Based on known "environments for mineralisation" or models for
mineralisation, regional geological surveys can be used to define smaller areas
in which more detailed studies can be undertaken.
A geological survey can be undertaken
using a number of methods depending on the size of a region and the amount of
information that is required.
methods involve the measurement of the chemistry of the rock, soil, stream
sediments or plants to determine abnormal chemical patterns, which may point to
areas of mineralisation. When a
mineral deposit forms, the concentration of the ore "metals" and a
number of other elements in the surrounding rocks is usually higher than normal.
These patterns are known as primary chemical halos.
a mineral deposit is exposed to surface processes, such as weathering and
erosion, these elements become further distributed in the soil, groundwater,
stream sediments or plants and this pattern is called a secondary chemical halo.
Secondary halos aid in the search for deposits as they normally cover a greater
area and therefore the chance of a chemical survey selecting a sample from these
areas is greater than from a primary halo area.
elements have different "mobility" in the environment which is based
their readiness to dissolve in water
their ability to form compounds with other elements and
the acidity (pH) of the environment.
the secondary halo may not contain the "metal" for which a geochemical
survey is searching but other "marker" elements.
is used to obtain very detailed information about rock types, mineral content,
rock fabric and the relationships between rock layers close to the surface and
at depth. Drilling is only used in
areas that have been selected as "targets" from geological,
geophysical and/or geochemical methods. Four
drilling methods are widely used, each depending on the type of information
required and/or the rock types being drilled.
this is a quick and economical method of producing a sample.
Compressed air is forced down the drill pipe and, as it returns to the
surface, it carries with it rock chips made by the rotating bit.
This method is used in soft rock materials to about 25m deep, or to drill
through the top layer of decomposed rock and soil to get a fresh rock sample
from the bedrock below.
this method is used for penetrating hard rock types to a depth of around 300m.
Rather than a rotating bit, a hammer bit is forced through the rock by
compressed air. As in air-rotary drilling, rock chips are brought to the surface
by the returning air.
this method is used for drilling through soft rocks, sand and clay layers
especially in the search for coal, oil or gas. A special mix of clay and water
is forced down the drill hole turning a rotary bit, with rock chips returning in
the "mud" slurry. This method is used for holes up to 3km deep and can
be done from ships or offshore
platforms especially in the search for oil and/or gas. A person who collects the
chips and identifies the rock type and other information is known as a
this method involves a pipe encrusted in industrial diamonds being used to drill
through rock layers, with a "core" of rock being left in the centre of
the pipe. This core is recovered and gives information not only about the rock
types, but also about the relationships between the rock layers in detail.
This website is hosted by
Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)