The airborne geophysics surveys, which they are performed by aircraft or helicopter,consist the variations measurement of the several earth physical parameters.
The most important of the measurable parameters such as conductivity which it equal to specific resistance reverse, the magnetic susceptibility, density and radioactive elements aggregation including potassium, and thorium and uranium, any change near the earth surface which it causes the measurable changes in these parameters show the practical applications of the air borne geophysics. The above parameters measurement devices are electromagnetic (FM), magnetic, gravity, gamma ray spectrometry (AGS), The EM surveys map the three- dimensional variations in conductivity which they have been caused due to change in lithology, alteration intensity and underground water volume or it’s salinity. The magnetic surveys are used for preparing the magnetic susceptibility alteration map which it often relates to rocks magnetite percent variation, and the gamma ray spectrometry surveys measure radiations made from one or more natural or man-made radio rays. The above methods are used for multiple aims. In some cases, one method put the direct sign from the desired are presence. For example, the situations which the magnetic method is used for finding Nickel and iron ores. In other cases, one method may provide only sign from being suitable of situation for the desired ore presence, for example, the magnetic method in oil exploration often is a means of detection in determining igneous beds rock depth, so that it is ensured the oil exploration with deposits thickness determination.
We can perform the geophysical surreys exploration in Land, sea and air. In areas which they are very large (more than hundred thousands km2) the airborne methods are often used. Because these methods are performed with more precision and very more fast.
Air borne magnetic method
The magnetic method is an oldest geophysical method which it is used for determining buried ores place and also for determining structures relating to oil and gas deposits. This method is a one which it’s origin is a natural and it is caused from earth magnetic field effects over rocks the earth magnetic field is an equivalent with one constant magnetic which it is in directional of north-south near land rotation axis. 99% land magnetic field has an internal origin which it accounts as main field and it changes a relatively slow, and the remaining 1% has external origin and it changes relatively fast.
The main field variations usually is very smaller than a main field, and it occurs from caused local magnetic anomalies near earth crust surface which it is a variations result in rocks magnetic ore content.
The review of these alterations constitute an exploratory geophysics aims, because we can interpret the local magnetic field anomalies relating to local earth structure. Of course u one of very important information sources in this field is rocks which it is possible that they have been magnetized at it’s formation constantly. Using rock magnetization measurement. We can deduce past history of the magnetic field.
In magnetic method, the obtained anomalies from field surveys are interpreted based on magnetic susceptibility alterations and/or constant magnetization. There are each two above figures at temperature below curie point in rocks, So the magnetic anomalies are only limited op to 30-40 km depths.
Air borne magnetic method applications
to search ore or magnetic host rock:
Consisting iron reserves, ultramafic rocks having chrome, some massive sulfate (usually Nickel), kimberlitic having diamond and tin- tungsten or rare earths having granite.
Geology maps preparation:
Consisting litho logy and structural maps in the weathered or covered area and the sediment basin geometry and also zone studies for the tectonic aims (For example crystal studies) and the suitable mineralization environments exploration.
Depth determination (basement):
It is for the hydrocarbons and non-metal ores exploration and also ores exploration having buried basement, (Such as gold).
4- If this method use from electromagnetic and induced polarization (IP) methods, it can be used in determining a metal and non-metal conductive, and interpreting the polarized or conductive masses geometry.
Engineering studies, underground water, archeology and hazard zonation:
Consisting faults map and berried joints preparation, curie point isotherm depth determination and metal objects search.
Airborne radiometry method:
Some elements isotopes are unstable and they are converted to more stable core with an active ion radiation diffusion. These isotopes are called radioactive isotopes or radioisotope. The main three type of ray is diffused from degrading the radioactive materials such as alpha, Beta and gamma rays, which alpha ray consists two protons and two neutron, as they have a charge and mass, they are attracted by few air combat particles transport a single negative charge and they can move up to 1 m in air. The annihilation of one alpha or beta usually remains a new core in excited state, and extra energy radiates as gamma rays.
These rays have not charge and mass , hence they we very penetrative, as they can penetrate up to 30 cm in rock and several hundred meter in air.
Each photon of gamma ray has a separate energy and this energy is a radiation source isotope characteristic. This method, forms a gamma ray spectrometry base (minty, 1997). The natural souces of gamma rays, which they are in air reviews, are potassion.Uranium and thorium. Therefore, radiometry method is based on the diffused gamma ray energy and current measurement during the decay of potassium, uranium and thorium radioactive isotopes and the relative approximation of these materials in surface rocks. The raw radiometry information, which they are recorded in 256 channel spectrum,
Consists 5 energy windows as follows
“Window” “Energy Range” “Channel Range”
Total count 410-2810 Key 35-240
Potassium 1370-1570 key 117 -134
Uranium 1660-1860 key 142-159
Thorium 2410-2810 key 206-240
Cosmic 3000-00 key 256
Because of the existing differences between radiations which are caused from potassium, thorium and uranium decay, Total radiations measurement in certain energy range is performed as a known channel. As with total gamma ray energy measurement, we will have total radiation parameter (Total count). The isometric method is used at cone exploration, but it depends on being appear the rocks order consideration, as the zone will be attracted by bed rocks radiation so lime covered deposits. The relative and absolute focus of the radioactive elements change con miserably with litho logy and the radiometry can reflect the adsorptive litho logy variations inside rather united rock units. On the other hand, kinds of very difference rocks can have a similar radiometry signs.
Falsie rocks, which they have outcropped, particularly granites and frolic volcanism have a clear radiometry.
A- Expansion of surface soils and other surface processes:
Bed rock radiation may decline by materials which they put between rock and detector, because the radiometry reviews can record information up to 35cm depth. The soil moisture rise also declines radiations.
B- The change in survey parameter, such as aircraft height which it transport a sensor above earth surface. (Terrain clearance), as in 150 m flight height, one 15m variation causes 10% change in the measurement amount .
- Beaker structure features such as old worked mines and/or road and rail which it entails fresh rocks application, and as linear features is seen which it can intercept geology boarders.
Potassium is a one of big earth crest compounds (%2,35) that it is alkali element, and it’s big hosts are pathetic feldspars (specially orthoclase and microcline with almost 13% potassium) and mica (biotitic, and muscovite with 8% potassium) . Potassium is rather high in falsie rocks (granite) and low in mafia basalts and very low in donates and peritonitis. During weathering the potassium site is destroyed in biotitic, pathetic feldspars and muscovite. The free potassium in weathering can be attracted by Ares having potassium such as iolite or other clay ores like Monte marinate in a suitable situation.
In gamma ray review, the potassium appears with mew 1/46 gamma ray measurement which it was caused from the decay k40. This isotope forms %2 from natural potassium and it is a correct measurement from existing potassium in earth. (Dickson&Scott, 1997).
The alteration patterns can be determined with me variations in radiometry signs often with increasing in potassium. Some of alkali and calico alkali porphyry (mot) gold and copper ore deposits accompany with prosaic hydrothermal alteration.