There are a variety
of geophysical survey methods that are used both in mineral prospecting or
regional mapping. Using either ground or airborne techniques, geophysical
companies employ the use of magnetic, radiometric and electromagnetic equipment
to detect concentrations of minerals in the Earth's crust.
Magnetic Surveys:
The most commonly
used geophysical survey method is the aeromagnetic survey, in which a
magnetometer is either attached to an aircraft on a stinger or towed behind on
a long line to measure the intensity of the earth's magnetic field, thereby
permitting the detection of ambient magnetic fields caused by the minerals that
are present in the ground. The
separation of the magnetic sensor from the aircraft is critical to the quality
of the data, hence the need for specially modified aircraft for geophysical exploration.
The resolution of the
data is dependent upon, among other things, the distance between the traverse
line spacing and, as such a survey can be categorized as either regional or
detailed.
A regional 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. Typically a regional survey is an area of at
least 5000 Lkm with a traverse line spacing of at least 250 meters or more.
A detailed
aeromagnetic survey, as the name suggests, offers data at a higher resolution
and can be used as a means of direct 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 (30-50m AGL). A detailed survey will offer improved data on
the presence of economic minerals and for mapping geology and structure.
There are two basic
approaches to the application and interpretation of magnetic data: direct and
indirect exploration. Many economic
minerals are either themselves magnetic creating a characteristic magnetic
field which can be detected by a survey or they may be intimately associated
with or hosted by minerals or rock types that possess a magnetic signature. In these cases these economic minerals can be
identified directly by magnetic surveys; this allows the mining companies to
progress rapidly to ground based, detailed exploration and delineation of the
resource.
In the case of
indirect exploration, the magnetic data are used to map the geology and
structure in far greater detail than is available by the ground mapping of
outcrops which are commonly separated by large areas of overburden. Once the detail of the stratigraphy and
structure has been improved, the geologist is then able to other apply types
data (eg geochemistry, drilling, seismic, etc.) to enhance the predictability
of geologic modeling. This procedure
models all observed data with the known characteristics of different types of
economic mineral deposits in order to detect any similarities and thereby
identify any new potential mineralization.
For example, hydrocarbons themselves are not magnetic, but information
about the surrounding geology may be modeled to identify a typical depositional
environment for the potential accumulation and entrapment of hydrocarbons.
Radiometric Surveys:
Terraquest primarily uses airborne digital, 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)
Potassium (K), 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 product in many 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)
Airborne radiometric methods provide detailed, systematic coverage of
large areas which are invaluable to improving the mapping, especially when used
in conjunction with other survey products such as magnetics.
Electromagnetic
Surveys:
Electromagnetic
surveys (EM) are designed to measure the conductivity in the surface of the
earth in either a "target mode” for identifying conductive minerals such as
graphite or massive sulphides (base metal minerals) or in a "mapping mode” to
identify long conductive structures. The successfulness of electromagnetic
surveys is impaired in geological environments where the country rock is highly
conductive or where the overburden is both thick and conductive.
There are two types
of EM surveys, active and passive.
Active EM requires the utilization of a powerful source to transmit a
local electromagnetic field (referred to as the primary field) from the aircraft. This field penetrates the ground
and ideally energizes conductors within the bedrock, which in turn generate
weaker secondary fields. The airborne survey measures both primary and
secondary fields but through processing, the secondary field can be enhanced.
There are two modes
of active EM surveys, frequency domain and time domain. The frequency domain
system utilizes different frequencies to identify different EM characteristics
of the ground conductor. The time domain
system uses a pulse transmitter and the EM characteristics are identified
according to the decay of the signal strength.
The second type,
passive EM utilizes natural (eg AFMAG) and/or more commonly, existing man made
sources (VLF) to energize the conductors.
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.
Terraquest uses a
recently developed proprietary method of measuring Very Low Frequency (VLF) EM,
called XDS VLF-EM to map structure. The
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 include all three North American VLF stations), to measure separately
the X, Y and Z components of the VLF field.
Continuous monitoring of all three, combined VLF frequencies reduces the
reliance on any one transmitter during periodic maintenance downtime.
Individual three-axis component data provides more detailed information about
the nature of the earth's conductivity than simple total field measurements
could, primarily because the coils respond differently to different
conductors. The horizontal components
tend to be strongest where currents are present (over conductive zones) while
the z component tends to peak over contacts.
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.
Summary:
The combination of
magnetic data with gamma-ray spectrometry
and electromagnetic data, in a modern digital system, yields powerful
mapping and exploration tools for both direct and indirect exploration
techniques for economic minerals for mining companies.