Airborne geophysical survey methods for mineral exploration

Airborne geophysical data can be used indirectly to map some geological features in detail, including contacts, faults, shear zones, folds, alteration zones and other structures.

 

To obtain this data, survey areas are systematically traversed by fixed wing or rotary 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 geophysical survey methods that are used in mineral prospecting. Through either ground or airborne surveys, geophysical companies employ the use of magnetic, radiometric, electromagnetic and gravity surveys to detect anomalous responses which may indicate the presence of mineral deposits.

 

 

The most commonly used first step in geophysical exploration is the aeromagnetic survey.  A magnetometer or array of magnetometers are installed in a stinger, in wingtip pods, or towed beneath the aircraft.  These magnetometers measure variations in 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 magnetometers from the aircraft is critical to the quality of the data, hence the need for specially modified aircraft and equipment for aeromagnetic surveys. The magnetometer data is processed to remove the magnetic effects of the aircraft, and non-geological sources such as diurnal solar activity.  Geophysicists can then analyze the processed airborne magnetic data and make recommendations on the next step in mineral exploration.  Using this method, mining companies are able to better understand geological structures and have a better idea where significant concentrations of ores may be located. 

 

Aeromagnetic surveys can also aid in the detection and mapping of hydrocarbons, uranium, titanium, base and precious metals. The resolution of the aeromagnetic data is dependent upon;

• the spacing between the traverse lines of the survey
• the height of the aircraft above the ground
• the aircraft speed during data acquisition
• the magnetic signature of the aircraft itself
• variations in the diurnal activity 

The size of an aeromagnetic survey is measured in line kilometres, which is the linear 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 or ground survey follow up.

 

A detailed aeromagnetic survey acquires data at a higher resolution and can be used as a means of prospecting by mining companies.  Typically performed at 50 meters line spacing and as low and slow to the ground as is possible within the safety parameters of the aircraft (typically 30-70m AGL). The data from detailed surveys can be used for mapping structures and may show the presence of magnetic ores. Magnetic maps are often used in conjunction with other geophysical survey methods such as radiometrics, gravity and electromagnetics 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 map 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 element 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 mineralized system, corresponding radioelement anomalies provide direct exploration guidance. (Source; www.nrcan.gr.ca)

 

Airborne radiometric surveys provide valuable, systematic coverage of large areas which are invaluable when used in conjunction with other survey types such as magnetics.

 

 

EM Surveys are useful in most geological environments except for where the country rock is highly conductive or where overburden is both thick and conductive.

 

Airborne electromagnetic surveys measure the strongest EM responses from massive sulfides and use transmitted primary electromagnetic fields to measure the electromagnetic properties of rocks.

 

Terraquest uses a proprietary method of measuring Very Low Frequency (VLF) EM, called Matrix 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, Matrix VLF-EM can be useful as a mapping tool, particularly when combined with magnetics and radiometrics.

 

VLF signals in the range of 15 to 40 kHz range are transmitted around the world, primarily for communication purposes with submarines. In North America there are four transmitters; a very powerful one in Cutler, Maine (24.0 kHz), another of medium power at LaMoure, North Dakota (25.2 kHz), one at Jim Creek, Washington (24.8), and another at Puerto Rico (40.8 kHz).  Around the world, additional VLF stations exist in Iceland, UK, Germany, Norway, Italy, Turkey, India, South Korea, Japan, Australia and Hawaii.  Signals from these transmitters 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 which can be measured by the Matrix VLF-EM system.

 

The Terraquest Ltd Matrix VLF-EM system by Magenta Inc. is a newly developed digital frequency specific VLF system as well as broadband capability.  It utilizes three orthogonal coils mounted in the aircraft stinger, coupled with a receiver-console.  The primary signals originate from up to four VLF frequencies, and full parametrization of these VLF-EM signals are recorded.  Terraquest has recently added optional inversion modelling to create resistivity products which further enhance the Matrix data to focus on project specific geological features.   

 

The gravity method is a passive technique to measure naturally occurring variations in the Earth's gravitational field.  A gravity survey is an indirect means of calculating densities of subsurface materials.  The higher the gravity values, the denser the rocks beneath.  This technique can be used to identify certain features such as intrusions, ores, basin fills, faults and other geological features. 

A gravimeter is an instrument used to measure the local gravitational field of the Earth.  It is a type of accelerometer, specialized for measuring the constant downward acceleration of gravity, which varies by about 0.5% over the surface of the Earth.  Gravimeters are designed to be highly sensitive in order to measure these minute changes caused by nearby geological structures or variations in rock types.  Gravimeters record data in units of gals, milligals or microgals.  For airborne gravity surveys, Terraquest utilizes the GT-2A gravimeter.

 

 

It is important to emphasize that properly calibrated airborne geophysical systems acquire 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 and government agencies.