Unit-III surveys mineral exploration-airborne versus ...
Transcript of Unit-III surveys mineral exploration-airborne versus ...
Geophysical methods of mineral exploration-airborne versus ground
surveys
Unit-III
Geophysics
Ø Geophysics is the study of earth by making use of established principles of physics.
Ø Geophysical investigations involve methods of study made on the surface with the aim to determine the subsurface details.
Ø this is done by measuring certain physical properties and interpreting them mainly in terms of subsurface geology.i.e. Rock type, ore deposits, structure, and ground water conditions.
Exploration geophysicsDefinition
Exploration geophysics Is an applied branch of geophysics, which uses physical methods at the surface of the Earth to measure the physical properties of the subsurface, along with the anomalies in those properties.
These anomalies areDensityMagnetismElectrical resistivity Elasticity
Airborne Geophysical Survey§ Airborne geophysical survey is carried out using low flying
helicopters or light aircraft which fly in a grid pattern.
§ The airborne survey is quicker, cost effective and may be carried out over the inaccessible area in terms of terrain, forestry and of other reasons.
§ The survey may comprise of magnetic, radiometric, gravity or electromagnetic methods.
§ These surveys provide general geological information for an area and are often used in the initial stages of exploration.
§ In this survey, instruments may be either mounted on the aircraft or towed underneath.
§ The aircraft may fly between 25 and 60 m above the ground and flight lines spaced between 25 and 200 m apart.
The most important parameters measured by using the surveys are;
Conductivity Magnetic susceptibility Rock density Radioactive element concentration
Airborne Geophysical Survey
Important types of Airborne Geophysical survey
There are two chief airborne geophysical procedures utilizing;
1. Airborne radiometric methods/ Aero-radiometric method 2. Airborne electromagnetic/Aero-electromagnetic method
The nuclei of certain elements are unstable and change spontaneously in to the nuclei of other elements. This change is accompanied by emission of radiations. This natural phenomena is called radioactivity. It is used in rock because it can be detected.
Energy is released in the form of radiation;
• Alpha Particle (or helium nuclei) - Least Energy- Travels few cm of air. • Beta Particle (or electrons)- Higher Energy-Travels upto a meter in air• Gamma Rays- Highest Energy-Travels upto 300 meters in air.
q This survey employs a gamma ray detector.
1. Aero-radiometric Survey (Gamma ray detector)
• Gamma ray sensors detect natural radioactive emanations, called gamma rays, from rocks and soils.
• The radiometric, or gamma-ray spectrometric method is a geophysical process used to estimate concentrations of the radio-elements potassium, uranium and thorium by measuring the gamma-rays which the radioactive isotopes of these elements emit during radioactive decay.
• Airborne gamma-ray spectrometric surveys estimate the concentrations of the radioelements at the Earth's surface by measuring the gamma radiation above the ground from low-flying aircraft or helicopters
• This method is capable of detecting only the presence of U, Th, and K at and near the surface of the ground.
Instruments for detecting radioactivity
There are two principal instruments1. Geiger Muller counter2. Scintillometer
Geiger Muller counter
§ The Geiger-Muller counter is a simple and cheap device that responsible primarily to Beta radiation.
§ It must held close to the outcrop to detect the Beta rays, and thus tool for limited applications, seldom used in modern prospecting.
Scintillometer§ The scintillometer works by counting scintillations produced in
a detector by gama radiation.
§ A logical extension of the scintillometer is a spectrometer that distinguishes characteristic gamma rays from K, U, and Th.
§ Therefore gamma radiation from different sources has different energy levels.
The data generated is plotted by computer and prepared a map of the surveyed area. The map shows individual value of uranium, potassium, or total gamma ray activities or ratio of U/Th, U/K, and Th/K.
Applications of Airborne radiometric method
I. As a reconnaissance geologic mapping tool in most areas.
II. As changes in the concentration of the three radioelements U, Th, and K accompany most major changes in lithology.
III. Identification of primary geological processes such as the action of mineralizing solutions or metamorphic processes and secondary geological processes like supergene alteration and leaching that may be indicated by variations in radioelement concentrations.
IV. Radiometric surveys, both airborne and ground, have been primary tools in the exploration for uranium.
§ The (AEM) survey is widely used in airborne geophysical survey.
§ Electromagnetic method provide a means to measure subsurface electrical conductivity and to identify subsurface metal objects.
§ The general objective of AEM is to search for metallic conductors, e.g. massive sulphides , located in bedrock and often under a cover of overburden.
§ This method can be applied in most geological environments except where the country rock is highly conductive or where overburden is both thick and conductive.
2. Airborne electromagnetic Survey (AEM)
§ Electrical conductivity is a function of soil and rock type, porosity and permeability, as well as the composition of fluids that fill the pore spaces.
§ electrical conductivity values are given in units of milliSiemens/meter (mS/m).
§ Higher the conductivity, the more current will flow in the earth for a given electrical field strength.
§ The higher the resistivity, less current will flow for a given electrical field strength.
§ Conductivity and resistivity are inversely related.
Instruments used
Airborne electromagnetic surveys are carried out with
§ fluxgate magnetometer, and § proton precession magnetometer.
§ Helicopter AEM systems carry transmitter and receiver coils mounted in a frame slung beneath the helicopter.
§ The helicopter flies at a height of approximately 60m and tows the circular frame approximately 30m above the ground surface.
§ An electrical current is pulsed through the transmitter coil which produces a primary magnetic field. This field induces eddy currents in the ground, which then create their own secondary magnetic fields.
§ The strength of the secondary field fluctuates with varying ground conductivity which may be influenced by changes in soil salinity or changes in the quality of groundwater.
§ The decay of the secondary field is measured by the receiver coil located beneath the helicopter and the response can be used to tell us about ground conductivity as it varies with depth.
How do airborne AEM systems work?
§ Airborne Geophysics is a powerful means available to the earth scientist for investigating very large areas rapidly.
§ Airborne methods are quick and cost-effective method.
§ Compared with ground-based methods, airborne techniques offer the advantages of rapid acquisition of data at scales that are suitable for many geophysical problems.
§ Airborne surveys provide the capability of traversing regions that are otherwise difficult or impossible to cover.
§ Airborne methods are advantageous for surveys over areas that are physically accessible but that have social, economic, or political barriers or environmentally hazardous.
Advantages of Airborne geophysical survey
§ It can provide detailed information near to the surface as well as to depths greater than 100 m.
§ It is now routinely employed as an investigative technology for mapping groundwater quality and the characteristics of aquifer systems, and for soil salinity mapping.
Limitation and disadvantages of air borne survey
1. It is not final but anomalies obtained require confirmation by other methods of exploration.
2. Interpretation can vary due to the number of variables which have to be taken into account.
3. Support by geological ground work is essential.
4. The resolution and accuracy vary with the speed and height of aircraft.
5. Locations are also subject to lack of high precission but this can be greatly circumvented by the use of helicopters.