Page 6 - js expolaratıon

Interpreting a Ground Magnetic Survey
Back in the office, a geophysicist processes the data removing artefacts produced from surface items
such as steel drums and wire fences. Filters and corrections are used to enhance the data to reveal
subtle features and structures within the underlying rock.
Reduced-to-pole (RTP) is a very important correction made to the data, particularly for locations far
from the earth’s magnetic poles. The RTP filter corrects for the artificial effect of being of the distance to
the earth’s magnetic poles, allowing for accurate drill-hole location and targeting.
Geophysical interpretation is as much an art as a science. An experienced scientist will use a variety of
filters that highlight different aspects of the magnetic gradients in the data, and reveal subtle changes
which may point to concealed structures and associated mineral occurrences.
Other high level filters used by geophysicists include tilt derivative and horizontal derivative, each
providing a slightly different way of viewing the data.
A correction may be applied to standardize the data to a specific height above the ground. Upward
continuing helps to remove or minimize erratic surface magnetism, while enhancing longer
wavelengths associated with deep objects and features..
The geologist in close consultation with the geophysicist interprets the magnetic images. The use of
specialized GIS mapping software allows for other data such as stream sediment, soil and rock chip
geochemistry to be combined with the magnetic images. Any coincident anomalies become potential
drill targets.
Drill-Hole Planning
A successful soil sampling programme will result in a map of geochemical anomalies. The next step in
exploration is for the geologist to design a drilling programme.
The exploration geologist uses several lines of evidence, both geophysical and geochemical, when
developing a drilling proposal.

Rationale
The purpose of drilling is to confirm if anomalies in data are real and provide further evidence of a
concealed ore deposit. Anomalies can be geochemical and/or geophysical. Anomalies are often ranked
according to their strategic and geological rating. Those ranked highest are normally drilled first.

Hypothetical plan for a drilling
programme showing a
coincident geochemical and
geophysical anomaly. Base
image: Google Earth.

A geochemical anomaly is an
unusually high level for a
particular element. To be
anomalous the concentration of
the element needs to be at least
a magnitude or higher than the
element’s background level. When the anomaly involves several sample points and has a consistent
geometric shape, it’s more likely to exist, and a greater confidence is placed on the existence of
mineralization.
Geophysical anomalies are unusually high or low points in the data which define a region of peculiarity.
Geophysical techniques are indirect methods of mineral exploration. A mineral deposit may affect a
certain geophysical parameter, which results in an anomaly. Not every geophysical anomaly indicates
an ore body so geophysics is usually used together with geochemistry, not as a stand-alone technique.
The exploration geologist uses several lines of evidence, both geophysical and geochemical, when
developing a drilling proposal. The proposal needs to have robust evidence, preferably from several
different sources, before seeking a budget from directors and shareholders.

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