NATIONAL UNIV of SCIENCE & TECHNOLOGY PRESENTATION BY FRANK S. FACULTY OF APPLIED SCIENCES Summer 2013

 Passive methods detect anomalies or changes in the Earth without introducing any energy.  Magnetic method is a passive method.  These include magnetometry, gravity and magnetotellurics.  Active methods introduce some sort of energy into the ground and then detect subsurface responses.

THE GOAL OF MAGNETIC METHODS IS TO MAP CHANGES IN THE MAGNETIZATION WHICH ARE IN TURN RELATED TO THE DISTRIBUTION OF MAGNETIC MINERALS. a) To investigate subsurface geology on the basis of the anomalies in the earth's magnetic field resulting from the magnetic properties of the underlying rocks. b) In general, the magnetic content of rocks is extremely variable depending on the type of rock and the environment it is in.  Common causes of magnetic anomalies include dykes, faults and lava flows.  In a geothermal environment, due to high temperatures, the susceptibility decreases.

1. Several minerals containing iron and nickel display the property of ferromagnetism. 2. Rocks or soils containing these minerals can have strong magnetization & produce significant local magnetic fields. 3. Sedimentary rocks have a very small magnetic susceptibility compared with igneous or metamorphic rocks. 4. Igneous rocks also show a wide range of magnetic properties. 5. Homogeneous granitic plutons are weakly magnetic 6. Metamorphic rocks- Make up the largest part of the earth's crust Rocks combine, to give complex patterns of magnetic anomalies over areas of exposed metamorphic terrain.  Most magnetic surveys are designed to map the geologic structure on or inside the basement rocks (the crystalline rocks that lie beneath the sedimentary layers) or to detect magnetic minerals directly.

 All materials, including minerals, possess the property of becoming magnetized, or polarized, in the presence of an applied magnetic field, thus altering the applied field.  where M is the magnetic dipole moment per unit volume, J is the magnetic polarization or magnetization, H is the magnetic field, and k is the magnetic susceptibility.  In weak fields, like the Earth's main magnetic field, the magnetization is approximately linearly proportional to the magnetizing field,  Iron, nickel, cobalt and some of the rare earths have high the magnetic susceptibility.  In these materials, the induced field is in the opposite direction of the inducing field, hence the total field is less than the applied field.  All materials possess a weak negative susceptibility known as diamagnetism.  In atoms with complete subshells (like He or Zn), therefore, no net magnetization in absence of field in a field, orbital moments precess, therefore are tilted so as to reduce field slightly  The fundamental property of all matter susceptibility is negative.

 Strongly magnetic materials are often referred to as ferromagnets they contain Iron.  Ferromagnetism refers to magnetic ordering where neighboring electron spins are aligned by the exchange interaction.exchange interaction  Paramagnetism is a weak positive response to a magnetic field due to rotation of electron spins.  Paramagnetism occurs iron-bearing minerals.  Magnetic minerals diamagnets and paramagnets contribute a weak magnetism and have no remanenceremanence  Diamagnetism occurs in all substances.  In response to an applied magnetic field, electrons precess and by Lenz LAW they shield the interior of a body from the external magnetic field.  Thus, the moment produced is in the opposite direction to the field and the susceptibility is negative.susceptibility  Strongly magnetic materials are often referred to as ferromagnets they contain Iron.  Ferromagnetism refers to magnetic ordering where neighboring electron spins are aligned by the exchange interaction.exchange interaction  Paramagnetism is a weak positive response to a magnetic field due to rotation of electron spins.  Paramagnetism occurs iron-bearing minerals.  Magnetic minerals diamagnets and paramagnets contribute a weak magnetism and have no remanenceremanence  Diamagnetism occurs in all substances.  In response to an applied magnetic field, electrons precess and by Lenz LAW they shield the interior of a body from the external magnetic field.  Thus, the moment produced is in the opposite direction to the field and the susceptibility is negative.susceptibility

 In general, the magnetic content of rocks is extremely variable depending on the type of rock & environment.  Common causes of magnetic anomalies include dykes, faults and lava flows.  Where the rocks have high magnetic susceptibility, the local magnetic field that is detected will be strong Where they have low magnetic susceptibility, it will be weaker.  In a geothermal area, due to high temperatures, the susceptibility decreases so it is useful in exploration of geothermal energy.  Ground magnetics provide more detailed information on sub- surface structures that could act as heat sources in comparison to aeromagnetic data.

The magnetization can be either a. Remnant (a permanent magnetization created by the earth's magnetic field during some process in the history of formation of the mineral) or b. Induced magnetization created by the presence of the earth's magnetic field. In most rocks both are present.  When the external field is shutoff, the induced magnetisation disappears at once, but some materials retain a permanent magnetisation

Ground Magnetics- Use of a Handheld Magnetometer to measure the local Magnetic Field.

 Ground magnetic measurements are usually made with portable instruments at regular intervals along parallel lines which cover the survey area.  The magnetometer is operated by a single person.  Two can be used so as to reduce errors  One magnetometer measures the local field whilst another measures the field due to the Earth.

 Measures the magnetic field strength & orientation are called magnetometers.  Measure small, localised variations in the Earth's magnetic field.  Magnetometers work on the principle of Electromagnetic Induction.  The result is actually a measure of the field as it is being effected by the earth’s magnetic field, as well as any other large bodies of magnetic rock which are near by.  Magnetometers are highly accurate  Magnetization of the rocks depends on the inducing field so that the resulting field from an object depends on how the induced field interacts with the inducing field to alter it.

The common ones used for commercial applications are the proton precession, fluxgate, caesium vapour and gradiometer magnetometer systems. (a). Proton precession magnetometer The proton free-precession magnetometer exploits the tendency of a free proton (H+) to align its magnetic moment with an ambient magnetic field and precess about that field direction when disturbed. A quantity of a liquid rich in protons - such as water or alcohol - in a sensor bottle is subjected to a strong applied magnetic field by way of passing a current through a coil wound round the bottle. Turning off that current causes the protons to search for the direction of the only remaining magnetic field - that of the earth - and to precess around it. (b). F lux-gate magnetometer The sensor element of the flux-gate magnetometer consists of two identical rods of a material with high magnetic permeability. If one such rod is wound with a coil through which an alternating current (about 1000 T is the magnetic total field and the magnetic permeability of free space. Variable transformer Optical absorption magnetometer The physical principle on which the optical absorption magnetometer is based is known as Larmor precession.

a) Measurements are usually made along parallel lines to cover a sample area. b) The magnetometer is used to sweep across an area near the ground. i. In rocky magnetite the sensor has to be up to 4 m high above the ground to remove errors from buried material. ii. Also because of errors from the collections of soil magnetite near the ground. a) A compass should be more than 3 m away from the magnetometer when measuring the field. b) Often a rapid reading magnetometer is used c) Intense fields from Steel and other ferrous metals in the vicinity of a magnetometer are removed.

The magnetic method involves the measurement of the earth's magnetic field intensity. Typically the total magnetic field and/or vertical magnetic gradient is measured. The shape and amplitude of an induced magnetic anomaly gives a. the orientation, geometry and magnetic susceptibility of the body b. the intensity and inclination of the earth's magnetic field in the survey area.

 Data are usually displayed in the form of a contour map of the magnetic field,  Interpretation is often made on profiles.  When the lines are close together they represent a steep gradient or change in values. When lines are widely spaced they represent shallow gradient or slow change in value. A modern technique is to plot the magnetic data as a colour image  red=high, blue=low  From these maps we can locate magnetic bodies.  interpret the nature of geological boundaries at depth, find faults etc.  Magnetite can also be weathered or leached from rocks and re-deposited in other locations, such as faults.  In a geothermal environment, this is a very useful feature as it may indicate the presence of faults, target for drilling.

 It certainly seems plausible that it depends upon the rotation of the fluid metallic iron which makes up a large portion of the interior, and the  Convection drives the outer-core fluid and it circulates relative to the earth. Convection  This means the electrically conducting material moves relative to the earth's magnetic field.earth's magnetic field  rotating conductor model leads to the term "dynamo effect" or "geodynamo", evoking the image of an electric generator.electric generator  If it can obtain a charge by some interaction like friction between layers, an effective current loop could be produced. The magnetic field of a current loop could sustain the magnetic dipole type magnetic field of the earth. Large-scale computer models are approaching a realistic simulation of such a geodynamo. current loop

 The primary contributions to the geomagnetic field are 1. The main field from the Earth's core 2. Core flow inferred from the secular variation of the magnetic field 3. The lithospheric field caused by magnetized crustal rocks 4. Motional induction in the oceans 5. Tsunami-generated magnetic fields 6. Ionospheric fields 7. Magnetospheric fields

 Because different rock types differ in their content of magnetic minerals, the magnetic map allows a visualization of the geological structure of the upper crust in the subsurface,geological structure  A geophysicist can use mathematical modeling to infer the shape, depth and properties of the rock bodies responsible for the anomalies.geophysicist  Particularly the spatial geometry of bodies of rock and the presence of faults and folds.  This is particularly useful where bedrock is obscured by surface sand, soil or water.

 Magnetometer surveys indicate that there are many unexpected variations in this model, called “magnetic anomalies”.  Definition: an anomaly is where the measured field strength is higher than the value predicted by the global model, and a magnetic low follows the same.  Induced magnetic anomalies are the result of secondary magnetization induced in a ferrous body by the earth’s magnetic field.  Possible causes for magnetic highs include the presence of magnetically charged rocks in the subsurface.  Magnetic prospecting looks for variations in the magnetic field of the earth that are caused by changes in the subsurface geologic structure or by differences in the magnetic properties of near-surface rocks.

1. Surveying:- Magnetic Imaging 2. Mining:- iron sulfide deposits 3. Shallow (Engineering and Environmental contaminants, toxic waste, pipes, cables and metals 4. Geotectonics:- major player in discovery of, and current analysis of tectonic processes& to map basement faults and basic igneous intrusives 5. For the exploration of geothermal energy, both ground and aero-magnetic data have been used to investigate the presence of a geothermal resource. 6. Oil and groundwater exploration: depth to magnetic basement in basins, detection of faults which are easy areas to drill boreholes. 7. Military:- location of UXO’s of Unexploded ordnanceUnexploded ordnance 8. Archeology:- Investigate archaeological sites 9. e.g. buried walls, old fire pits

Useful in exploration of minerals Often useful to delineate geologic features related to hydrogeology, e.g. bedrock lineations, intrusives, geologic contacts, heat sources. Geothermal energy findings. Very effective for identifying infrastructure hazardous to drilling or excavation. Effective technique for screening an area for steel, buried drums, water mains. It is quick and relatively uncomplicated It gives immediate indications of defects It shows surface and near surface defects, and these are the most serious ones as they concentrate stresses Large or small objects can be examined

 It is restricted to ferromagnetic materials - usually iron and steel, and cannot be used on austenitic stainless steel  Depth resolution is poor except for small, shallow targets.  Local traffic and other magnetic disturbances  Often requires gradient (two sensor) techniques.  Lower cost proton precession units tend to be less effective than more expensive sensors.  Most methods need a supply of electricity

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