Magnetic methods, Physics tutorial

Basic Concepts of Magnetic Methods:

Basically the sources in the core cause the magnetic field posses by the earth.  The shape of the field is roughly the same as would be caused due to associated sub parallel along geographic axis a dipole or bar magnet placed close to the Earth's center.  In standard S.I. units as nanoteslas (nT) is customarily the intensity of the Earth's field is follows or in an earlier unit, gamma (γ): 1 γ = 1 nT = 10-3 μT.  The intensity of the Earth's field about 25 to 80 μT over the conterminous United States except for local perturbations.

In the earth's field several minerals and rocks are weakly magnetic or are magnetized by induction and cause spatial perturbations or "anomalies" in the Earth's main region.  Iron or steel are highly magnetized Man-made objects which locally can cause large demerits up to several thousands of nT.  Magnetic methods are normally used to map the size of ferrous objects and address.  Engineering geophysicist done by the considerations of the capability of the magnetic method should be taken by an experience. Incorporation and modeling of auxiliary information may be compulsory to produce an adequate work plan.

Theory:

The Earth's magnetic field guided then earlier all magnetic measurements made or close to the surface of the Earth.  The Earth's total field intensity differs significantly by location over the surface of the Earth.  Almost all materials except permanent magnets,  due to the nature of the material if the material is in a strong field as the Earth's determines an induced magnetic field magnetic polarization  sometimes called induced magnetization refers to the material on the action of the feild whereas itself to continue as a magnet the ambient field is enhanced causing the material  The field created by such a material is directly proportional to the intensity of the ambient field and to the capacity of the material to boost the local field--a features known as magnetic susceptibility.  is the volume magnetic susceptibility is equal to the The induced magnetization and the inducing field of the Earth:

I = KF                                                                  

Where,

K = Volume magnetic susceptibility (unitless)

I = Induced magnetization per unit volume

F = Field intensity in tesla (T)

 K is smaller than 1 for most materials, in fact, the order for rock materials are usually of 10-6 is magnetite. Susceptibility is approximately 0.3 for the most imperative exclusion by a geologic point of stand; magnetite and its division consider the magnetic features of rocks. In mining prospecting, there are other significant magnetic minerals but the amount and state of magnetite inside a rock find the reaction of most rocks to an inducing field. The stainless steel has a less susceptibility that is the exception and other ferromagnetic alloys, Iron and steel posses searching ability to varies orders of magnitude greater than magnetite. The value of magnetite may not be overstated. rock materials have taken some test that a rock consist of 1% magnetite may have searching ability as more as 10-3, or 1,000 times more than most rock materials. Some typical values are provided for rock materials by the table given below. In the rock magnetite amount depended by values of range provided for each sample that usually mentioned.

Table: Approximate magnetic susceptibility of representative rock types

Rock Type                                    Susceptibility (k)

Altered ultra basics                           10-4 to 10-2

Basalt                                                   10-4

Gabbro                                             10-4 to 10-3

Granite                                              10-5 to 10-3

Andesite                                              10-4

Rhyolite                                            10-5 to 10-4

Metamorphic rocks                            10-4 to 10-6

Most sedimentary rocks                      10-6 to 10-5

Limestone and chert                                10-6

Shale                                                  10-5 to 10-4

Thus, it can be seen that in most engineering and environmental scale investigations, sufficient contrast will not shown by the sedimentary and alluvial sections such that magnetic measurements will be of use in mapping the geology. However, the presence of ferrous materials in common place civic garbage and in most industrial waste does permit the magnetometer to be effective in direct detection of landfills.  Other ferrous objects, which may be detected, underground storage tanks, involve pipelines, and some ordnance.          

Instrumentation:                             

 Today for different purposes there are various types of magnetometers is used. For environmental and engineering investigations the current standards are normally proton procession and cesium vapor. The magnetic field is measured magnetometer type reflects the physical process. Proton procession instruments posses a sensor occupied with a hydrogen rich fluid. A strong magnetic field in the fluid resulting in the alignment of protons is generated by an inductor. If the protons return to ambient magnetic conditions is recorded then the inducted current is suspended, the relaxation rate. This rate is directly proportional to the magnetic field. An over Hauser magnetometer, presents a variations on the proton procession magnetometer by using radio frequency magnetic fields to produce the polarizing signal this improves the results of a proton procession magnetometer, as the RF field does not interfere with the precession signal.

The orientation and strength of a magnetic field both are measured by a more complex device called a magnetometer. When the, the result is actually a measure if the magnetic field of a rock sample is measure of the field as it is being affected by the magnetic field of the earth which are nearby  as well as any other large bodies of magnetic rock. Small, localized variations in the magnetic field of the earth measured by surveys of magnetometer. Allowing the local magnetic field to be measured to accuracies of 0.002% magnetometers are highly accurate instruments. On the market there are several types of instruments. For commercial applications common ones is used are the cesium vapour, fluxgate, and gradiometer magnetometer systems and proton precession. The systems work on mostly same principles applying proton rich fluids bounded by an electric coil. A momentary current is utilized by the coil that generates a corresponding magnetic field which temporarily polarizes the protons. When the current is detached the protons realign or process into the point of reference of the Earth's magnetic field. The precession produces a small electrical current in the wrapping coil, at a frequency directly proportional to the local magnetic field intensity. Rather than total field strength, Gradiometers measure magnetic field gradient which permit the removal of background noise. Gradiometers measure the magnetic field gradient except total field strength that permits the elimination of background noise.

Data Acquisition:

At regular interval ground magnetic calculated are normally made by mobility devices  along more or less straight and parallel lines that cover the survey area. Often the gap between calculated locations (stations) towards the lines is less than the gapping between lines.

The magnetometer is measured by a single person. Though, surveying, grid layout, or the buddy system may want the use of another technician. If two magnetometers are present production is usually doubled as the ordinary operation of the instrument itself is straightforward. Extreme fields from man-made electromagnetic sources can be a difficulty in magnetic surveys. Steel and other ferrous metals in the vicinity of a magnetometer can distort the data. When operating the unit, large belt buckles, etc., must be eliminated. when measuring the field a compass should be greater  than 3 m far from the magnetometer take readings and immobilize the magnetometer while the operator travel towards the sensor  to a final test. If the readings do not exchanged by more than 1 or2 nT, the operator outcome must be seized 1 nT on very precise survey. Most magnetometers are designed to work in fairly intense 60-Hz and radio frequency fields. However caused by devices the switching of large alternating currents or using direct current can be a problem causes extremely low frequency fields. The sensor should be work well above the ground to acquire a representative reading. This procedure is done due to the possibility of collections of soil magnetite dividing the reading close to ground.  The rocks have some amount of magnetite in rocky terrain, sensor heights of equal to 4 m have been used to eliminate near-surface effects. The quantity of noise present and Data recording methods will differ with the reason of the survey and. Techniques include:  averaging the results and taking three readings, note down the three readings inside a meter of the station and recording the average or the each of the recordings. Some magnetometers can approve the techniques and even internally do the averaging.

  • In the field book for magnetic items to be recorded
  • Station location, with locations of lines with respect to permanent landmarks or surveyed points.
  • Potential interference nearby sources
  • For modern magnetometers the actual magnetic field readings, the time of the reading are stored internally by the meter.

Data Processing:                                         

Temporal changes in the field of the earth during the period of the investigation must be taken to make correct magnetic anomaly maps. Normal changes in a day, occasionally called diurnal drift, is a few tens of nT but modifications of thousands or hundreds of nT may takes place over a few hours in  magnetic storms. At the time of severe magnetic storms, which occur infrequently not be made magnetic surveys. The amendment for diurnal drift can be made by repeat calculation of a base station at regular intervals. between readings of the repeat base station at fixed base sites to monitor the temporal changes continuously The measurements at field stations are then accurate for temporal difference to assume a linear change of the field also use the recording magnetometers If time is accurately recorded at both field location by subtraction of the variations at the base site and base site, the field data can be corrected.

Magnetic survey data are usually displayed after all corrections have been made, as individual profiles  or as contour maps. Recognition of anomalies caused by cultural properties, such

As pipelines, bridges and railroads, is generally made using field observations and maps such properties are shown

Data Interpretation:                                            

In shape and amplitude Total magnetic disturbances or defects are extremely varies; they are, sometimes appear complex even from simple sources, almost always asymmetrical and usually portray the combined effects of several sources. An immeasurable number of possible sources can generate a given anomaly; bring growth to the term ambiguity.

Individual magnetic defects - magnetic signatures diverse from the background- consist of a high and a low (dipole) compared to the average field. In the Southern Hemisphere and the low to the south and the high is located to the north of the magnetic body. The size and position of the anomaly depend on the position and size of the magnetic body. The positioning of anomalies over the magnetic body is effected by the change in latitude will also affect. This permit the geoscientist to interpret the position of the body which has creates the anomalous reading. Often however the reading is complex due to the position of the body in relation to other rocks, its size, and what happens to the body at depth.

Data are usually shown in the form of a contour map of the magnetic field, but analysis is often made on profiles. From these maps and profiles geoscientists can address magnetic body, interpret the nature of geological boundaries at depth, find faults etc. Like all contoured maps, when the lines are close together they present a steep gradient or modifications in values. When lines are extensively gap they represent shallow gradient or slow change in value. A modern method is to plot the magnetic data as a color image. This gives an image which is easily can .be read

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