An airborne magnetic
survey is the predominant method of remotely collecting geophysical data using a
specially modified aircraft outfitted with magnetometers to fly a grid pattern
over a targeted area. Airborne magnetic surveys are an effective geological
exploration technique to acquire gridded data describing the magnetic anomaly
caused by variations in earth materials and structure. Magnetic databases are compiled and interpreted
to identify the presence of magnetic anomalies to determine the geological
framework of the region and potentially the direct targeting of mineral and
hydrocarbon commodities.
Primary types of sensors used in airborne geophysical surveys include:
-
Magnetic (Including Total magnetic intensity, horizontal, and vertical gradiometry)
-
Radiometric
-
Electromagnetic (Either helicopter borne frequency domain electromagnetic, or passive VLF-EM)
-
Gravity
The type of aeromagnetic survey
specifications, instrumentation and interpretation procedures will largely
depend on the objective of the survey. Airborne magnetics are highly effective
in identifying the geological framework for; Diamonds, Lode Gold, VMS Deposits ,
MVT Lead-Zinc Deposits, SEDEX Deposits, Porphyry Copper Deposits, Uranium
Deposits, Olympic Dam – Type Deposits, Magmatic Ni-Cu-PGDs Deposits.
Airborne magnetics are highly effective
in direct targeting deposits of; Diamonds, VMS Deposits, SEDEX Deposits, Porphyry
Copper Deposits, Olympic Dam – Type Deposits
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. Typically a magnetometer is housed in either a stinger or in wingtip pods at the end of the aircraft wing providing 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
Benefits of Airborne Magnetic Surveys
Airborne surveys
greatly improve;
· the
rate at which magnetic data can be acquired
· the
area which can be surveyed
· acquisition
of data in challenging and remote terrain
· economical,
efficient data acquisition
Important
Considerations for Aeromagnetic Survey Planning
· Flight
line spacing – determined in conjunction with survey altitude and budget,
typically ranging from 50m for detailed surveying to 500m for regional surveys
· Survey
Altitude – low as is safely permitted given the MTC, ~70 m using a pre-planned
drape surface model
· Line
Direction – typically oriented normal to a predominate geological strike
direction
· Control
Line Spacing – 90 degrees to the flight lines and typically 10x the spacing for
correcting the data during the data processing stage
· Sampling
Interval - typically 10 times per second, equivalent to a ground spacing of 7
metres per second.
· Noise
Envelope – a specified noise envelope sets the target that must be met in
eliminating aircraft effects and temporal effects in the execution of a survey
Source; Colin
Reeves (Aeromagnetics Surveys – Principles, Practices and Interpretation)