Airborne Geophysical Surveys – Aeromagnetic Surveys are the Most Commonly Employed Method of Airborne Geophysical Data Acquisition

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Airborne Geophysical SurveysAirborne geophysical surveys are a cost-effective and optimal way of securing geophysical data to measure the ambient magnetic fields of a widespread area in the search for a broad spectrum of commodities. Aerial geophysical surveys can provide a broad overview of the geology of a particular area, as well as measure the fluctuating magnetic signatures found in the Earth's crust that may be indicative of mineral concentrations. Once geophysical data has been acquired, processed and interpreted, the data is evaluated to determine if further exploration is warranted.
 
There are four key types sensors used in airborne geophysical surveys:
  • Magnetic (Including Total magnetic intensity, horizontal, and vertical gradiometry)
  • Radiometric
  • Electromagnetic (Either helicopter borne frequency domain electromagnetic, or passive VLF-EM)
  • Gravity
Each type of survey sensor is unique in the type of information it gathers and also in how each survey must be performed. Magnetic data may be acquired at a higher altitude and aircraft speed than radiometrics which requires a low and slow flying aircraft for superior data. Gravity also requires a typical acquisition rate of 60 meters / second.
 
Out of these four, aeromagnetic surveys are the most commonly employed method of airborne geophysical surveys as magnetics are commonly used in conjunction with each of the other technologies. In this type of survey a magnetometer is attached to an aircraft in either a stinger or in wingtip pods which provide distance from the aircraft itself or it may be towed behind an aircraft on a long line cable. The magnetometers test the magnetic fields in their current state, as magnetic fields are in constant flux as they are dependent upon spatial variations and solar winds.
 
There are three main types of temporal magnetic variations:
  • Diurnal
  • Magnetic Storms
  • Micropulsations
Aeromagnetic surveys must take the above into consideration when acquiring data.
 
There are two classifications applied to airborne geophysical surveys; regional and detailed.
 
Detailed surveys are done to identify targets. The mining industry relies on aeromagnetic surveys as a means of uncovering high concentrations of minable commodities. Detailled surveys are used to determine the structure, geometry and depth of a body of interest, map out weak magnetic fields, and discriminate between non-metallic and metallic conductors. The line spacing of detailed surveys is less than 250 meters.
 
The line spacing of regional surveys, on the other hand, are wide at more than 250 meters and will cover up to 10,000 square kilometers. Regional surveys are conducted in order to attain precise data to compile both geological maps and basement depth mapping.
 
Regardless of whether it is a regional or detailed survey, it is important that each expedition is readily equipped with advanced technological equipment in order to render the most precise and consistent data available.
 
While there are many data acquisition systems to select from, a data acquisition system is a mandatory tool for each of the four main geophysical sensors. All sensor data gathered is transmitted to the data acquisition system. The following acts as a guide of characteristics to ensure a modern data acquisition system is capable of:
  • Data is recorded in the exact way it is acquired
  • Data is recorded during the aircraft turns
  • Multi-channel graphical display
  • Real time graphical display
  • Multiple analog inputs
Typically the more comprehensive an airborne survey is, using as many geophysical exploration tools as possible, the more precise the end outcome of an exploration program. A typical survey will utilize two or even three types of different sensors to gather as much data as feasible for the interpreting geophysicists and geologists to work with.
 
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