Airborne Surveys – Airborne Magnetic Surveys Depend Heavily Upon High Precision Geophysical Systems for Success

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Airborne SurveysEven though airborne surveys are a relatively low cost and efficient method of exploration, each survey must be as economically viable as possible in the grassroots stage of exploration.  Therefore it is extremely important that your airborne survey is thoroughly planned and executed according to the predetermined survey plan.  Integral to the planning process are considerations such as;
  • flight line direction / orientation in relation to the predominant strike of the known geology
  • traverse and tie line spacing
  • survey height in relation to the terrain and the equipment utilized
  • distance to the nearest airport (permitting more time over the grid)
Airborne Surveys are normally flown along a series of equally spaced parallel flight lines.  Fore general reconnaissance mapping purpose the flight line direction is usually oriented north-south or east-west depending on the predominant strike of the known geology.  For more specific surveys, such as the definition of mineral exploration targets, the flight-line direction will be oriented across the strike to maximize the magnetic signature. (Source; ASO Journal of Australian Geology and Geophysics.)
    
The cost of an airborne magnetic survey varies drastically depending on the type of geophysical sensors utilized; however the rate is usually measured by a cost per line kilometer.  In the interest of remaining competitive, geographical survey companies utilize state of the art data acquisition systems and strive to develop new and improved  survey techniques for more cost effective acquisition.

When considering an airborne geophysical survey, consider the type of sensor that is most applicable to the your targeted resource.
 
There are four key types of sensors used in airborne geophysical surveys:
  • Magnetometers; (Including Total magnetic intensity, horizontal, and vertical gradients
  • Cesium spectrometers for radiometric acquisition
  • Electromagnetic (Either passively acquired XDS VLF-EM or helicopter borne frequency domain electromagnetic)
  • Gravimeter
Each type of survey sensor is unique in the type of information it gathers and yet the equipment also dictates how each survey must be performed. Gravity and Magnetic data may be acquired at a higher altitude and aircraft speed, with a wider line spacing between the flight lines.  Radiometric surveying which requires extremely low and slow flying due to the the attenuation of the gamma-ray signal in the air column, and the flight lines are typically no more than 200m apart.
 
Airborne Survey Technology;
 
While the purposes of conducting airborne magnetic surveys vary, the accuracy of the geophysical system is of utmost importance. A typical system includes: the data acquisition system, a variety of different geophysical sensors, and a specially modified aircraft equipped with stingers and pods to isolate the sensors from the noise of the aircraft.
 
Reliable data acquisition systems are the cornerstone of any airborne magnetic survey. The basic function of the data acquisitions system (DAS) for airborne geophysical surveying is to digitally record all geophysical, navigation, altitude, temperature and pressure data.  Two commercial DAS are readily available, from Picodas and RMS Instruments DAARC C500.

Geophysical survey companies should be able to offer their clients a selection of sensors to acquire that data that will best suit the needs of any expedition. All data is then handled via the data acquisition systems.  Consider the following when selecting a superior geophysical acquisition system:
  • Graphical Display – Is the data viewable in real time?
  • Magnetometers – How many are there?
  • Is the system capable of real time correction, or is post flight correction necessary?
  • Is the navigation interface available to all GPS instrumentation?
  • Is detailed flight path planning available?
  • How many channels of analog recording are there?
The above are just some of the questions that should be asked when sourcing a geophysical survey company along with which data acquisition system will best suit the needs of the survey.
 
Survey Size and Scope;
 
The quality of data acquisition system required will vary depending on whether the airborne magnetic survey is regional or detailed. Regional surveys provide data that is typically of a lower resolution, as the traverse line spacing is usually more than 250 meters and the area covered is at least 5000 square kilometers.
 
Detailed surveys have a lower traverse line spacing of less than 250 meters. The data resolution is higher. Detailed surveys are used as a means of mineral prospecting for magnetic ores such as kimberlites, asbestos-bearing ultramafic rocks, and magnetic iron ores. Detailed surveys are also capable of discriminating between non-metallic and metallic conductors as well as locating specific basement targets.
 
Because of the specific reasoning behind conducting a detailed survey (direct and indirect mineral prospecting), it is of utmost importance that the data acquired is high resolution.
 
Aside from ensuring that a geophysical survey company is well-equipped with state of the art data acquisition systems, it is vital to consider the capabilities of the field equipment used and that the aircraft is "magnetically quiet”.
 
In short, the success of an airborne magnetic survey is dependent upon the equipment selected, the flight plan chosen and the resolution of the data acquired. The results may be acted upon for further exploration in the pursuit of valuable mine-able commodities.
 
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