Geophysical data can be used indirectly to map some geological features in detail,
including faults, shear zones, folds, alteration zones and other structures.
To obtain this data,
survey areas are systematically traversed by fixed wing aircraft carrying
geophysical equipment along parallel flight lines. The traverse lines are oriented to intersect the
geology and structure so as to maximize the signal response when acquiring geophysical data.
There are a variety
of methods of geophysical surveys that are used in mineral prospecting. Through either ground or airborne methods,
geophysical companies employ the use of magnetic, radiometric and
electromagnetic surveys to detect anomalous responses which may be indicative of concentrations of minerals.
The most commonly
used first step in geophysical exploration process is the aeromagnetic survey. A magnetometer or often a series of magnetometers are attached to an aircraft in a stinger, or wingtip pods to measure the intensity of the earth's magnetic field,
thereby permitting the detection of magnetic anomalies caused by the
minerals that are present in the ground.
The physical separation of the magnetic sensors from the aircraft is critical to
the quality of the data, hence the need for specially modified aircraft and equipment for
geophysical exploration. The geophysical data is processed to remove the magnetic signature of the aircraft, and non-geological sources such as diurnal solar activity. Geophysicists can then analyze the airborne data and make recommendations on the next step
in a mineral exploration. Using this method, mining companies are able
to understand where significant concentrations of ores may occur in the Earth's crust.
can also aid in the detection of hydrocarbon, uranium, titanium, petroleum,
coal, base and precious metals. The resolution of the data is dependent upon,
among other things;
- the distance between the traverse line spacing
- the distance between the aircraft and the ground
- the magnetic signature of the aircraft itself
- variations in the diurnal activity
An aeromagnetic survey is
measured in Line kilometers, which is the distance that the aircraft must
travel to cover the entire survey area flying in a grid pattern. Regional aeromagnetic surveys are flown at a wider line spacing (often 500 m or greater), with the intention of acquiring a generalized understanding of magnetic features, useful to identify areas for further detailed aeromagnetic follow up.
aeromagnetic survey offers data at a higher resolution
and can be used as a means of prospecting by mining companies. Typically
performed at 50m meters line spacing and as low and slow to the ground as is
possible within the safety parameters of the aircraft (typically 50-70m AGL). A detailed survey will offer data on the
presence of ores and for mapping structure. Magnetic maps are often used in conjunction
with other geophysical survey methods such as radiometrics, gravity and VLF-EM to create
a more comprehensive geophysical and geological understanding of the survey area.
Terraquest primarily uses digital airborne gamma-ray spectrometers which are designed for
the detection and measurement of low-level radiation from both naturally
occurring and man-made sources, associated with the radioactive elements;
thorium, potassium, and uranium. Gamma
Ray Spectrometry provides a direct measurement of the surface of the earth,
with no significant penetration, but permits reliable measurement of the radioactive
element contacts to the mapped bedrock and surficial geology. (Source; www.nrcan.gr.ca
uranium (U) and thorium (Th) are the three most abundant, naturally occurring
radioactive elements. K is a major constituent of most rocks and is the
predominant alteration element in most mineral deposits. Uranium and thorium
are present in trace amounts, as mobile and immobile elements, respectively. As
the concentration of these different radioactive elements varies between
different rock types, we can use the information provided by a gamma-ray
spectrometer to map the rocks. Where the 'normal' radioelement signature of the
rocks is disrupted by a mineralizing system, corresponding radioelement
anomalies provide direct exploration guidance. (Source; www.nrcan.gr.ca
provide valuable, systematic coverage of large areas which are invaluable when
used in conjunction with other survey products such as magnetics.
EM Surveys are used
in most geological environments except where the country rock is highly
conductive or where overburden is both thick and conductive.
electromagnetic surveys generate the strongest EM responses from massive
sulfides and can use man-made primary electromagnetic fields to measure the
electromagnetic properties of rocks.
Terraquest uses a
proprietary method of measuring Very Low Frequency (VLF) EM, called XDS VLF-EM
to map structure. The system typically responds to variations
in overburden conductivity, to large faults or shear zones, and to graphitic
formational conductors. Because of these
characteristics, XDS VLF-EM can be useful as a mapping tool, particularly when
combined with magnetics.
The VLF signal is transmitted
around the world by governments, primarily for communication purposes. In North America there are three
transmitters, a very powerful one in Cutler, Maine (24.0 KHz) another of medium
power at LaMoure, North Dakota (25.2 KHz) and another at Jim Creek, Washington
(24.8). Signals from these transmitters
cover most of the continent and act as primary
fields that are capable of energizing conductive bodies (such as graphite,
metallic minerals and structures) in the ground. Once energized, the current within these
bodies emits a secondary field
forming the basis for a geophysical exploration.
The Terraquest Ltd proprietary XDS VLF-EM system
uses three orthogonal coils mounted in the aircraft stinger, coupled with a
broadband receiver to record all frequencies between 22.0-27.0 KHz, to measure
the X, Y and Z components of the VLF field.
Three component data provides more detailed information about the nature
of the earth's conductivity than simple total field measurements could. The horizontal components tend to be
strongest where currents are present (over conductive zones) while the z
component tends to peak over contacts.
It is important to
emphasize that properly calibrated airborne and gamma-ray spectrometry
surveys produce quantitative geophysical data. The combination of gamma-ray
spectrometry with magnetic and electromagnetic sensors, acquired by a modern digital
system, yields powerful mapping and exploration tools for exploration professionals, from
individual prospectors to mining and exploration companies.