Magnetic Geophysical Method - Essay Example

of Learning: Magnetic Geophysical Method Introduction The magnetic geophysical method is a scientific approach that is used to measure variations or anomalies that are localized and small in the magnetic field of the earth. Materials found on the surface of the earth such as the basic igneous rocks, and the ore magnetic bodies are helpful when it comes to the identification of the magnetic variations because they have the magnetic properties. Steel objects buried in the earth also help in the production of the strong anomalies or magnetic fields. The magnetic field of the earth can be approximated in the sense that the magnetic dipole is situated slightly off the center of the earth and that the dipole is incline close to fifteen degrees to the axis of the rotation (Sherriff97). The magnetic field of the earth is believed to be developed by a dynamo that is driven by convection currents in the molten core of the earth. Instrumentation Magnetometers are the instruments which are used in this method to take measurements of the magnetic field of the earth. The field of strength or the intensity of the magnetic field of the earth is actually what is measured by the magnetic geophysical method, and it is measured in terms of Tesla (T) (Kaufman23). Since the main objective of the method is to measure the variations, the magnetic variations are measured in terms of NanoTesla (nT) or gammas (?) (1 nT = 10?9 T = 1 ?). There are various types of magnetometers used in this method. The magnetometer used usually reflects the physical process whereby the magnetic field of the earth is measured. These types of magnetometers used are highly accurate and reliable in the field. They can be able to measure the local magnetic field of the earth to a high precision degree. These kinds of magnetometers normally function close to similar principles by utilization of the proton rich fluids (Kaufman65). A fluxgate magnetometer tends to use the concepts of electromagnetic induction. For it to work, the instrument is composed of two coils that are permeable, and are wounded in the opposite directions. The coils are normally driven by the AC signal. The major function of the fluxgate magnetometer is to detect the changes in the magnetic field of the earth. If there is no external magnetic field on the earth, there will be a cancellation of the primary coils. If there is the presence of the external magnetic field, one of the primary coils in the instrument will tend to saturate before the other primary coil. This then leads to the creation of an imbalance in the magnetic field that is to be detected through the induction of EM in the secondary coil (Mishra40). The proton precession instrument that is used by environmentalists and engineers is characterized by a sensor that has a fluid similar to kerosene that is hydrogen rich. For the instrument to be successful in the magnetic geophysical method, a strong magnetic field is created by an inductor to the fluid which leads to the alignment of the protons. After the suspension of the inducted current, the rate of relaxation is recorded as the protons tend to return to the ambient magnetic conditions. The magnetic field of the earth is directly proportional to the relaxation rate (Mishra45). A variation on the proton precession magnetometer can be presented by an overhauser magnetometer by the use of the magnetic fields from the radio frequency to produce the polarizing signal. By this, the results given by the proton precession magnetometer is improved because the procession signal is not interfered with by the radio frequency used. The gradiometer magnetometer tends to measure the gradient of the magnetic field of the earth rather than measuring the total strength of the field. Though the magnetic gradient anomalies are not highly useful to geological operations, they are helpful in the provision of a better definition of shallow features that are buried for example drums and tanks. The gradiometer magnetometers are useful in areas with clay soils, saline groundwater and high rates of contamination. This follows the fact that high electrical ground conductivities do not affect the penetration depth of the magnetic surveys (Sherriff89). On the other hand, a cesium vapor magnetometer is composed of an absorption chamber, a photon emitter, a photon detector, and a buffer gas. Application of photons gives room for the displacement of electrons, and this is a well known property of the cesium atom. Cesium can only be affected by the photons at the third and the ninth energy levels although it can exist at any level of the available nine energy levels. For this reason, photons are able to pass through the cesium vapor with no obstructions and electrons will be no longer transferred (Lutgens and Tarbuck28). The application of external AC magnetic field brings about the establishment of variations in the energy levels of electrons by the ambient magnetic field. The photons are then allowed to the electrons by the new field established, and this is measured by the quantity of light that reaches the photon detector. This leads to an increased performance of the magnetometer (Mishra78). Application of the method The property of ferromagnetism can be indicated by various minerals that contain nickel and iron. When such minerals are present in soils and rocks, there can be an occurrence of a strong magnetization in the rocks and soils. As a result of this, there can be a production of significant local magnetic fields. The magnetization can either be induced magnetization developed by the presence of the magnetic field of the earth or remanent magnetization. The formula M = KH, x 10-S defines the induced magnetization. In the equation, H, is the applied field intensity in gammas while K denotes the susceptibility in cgs units. Remanent magnetization is a permanent form of magnetization that is developed by the presence of the magnetic field of the earth during the historical process of mineral formation (Mishra26). There are various combinations of remanent and induced magnetization that are contained in the magnetic rocks of the earth which tend to perturb the primary field of the earth. Both the remanent and induced magnetization magnitudes rely majorly on the size, quantity, and composition of the magnetic mineral grains (Christiansen and Hamblin106). In hydrological studies, the magnetic geophysical method can be applied technically for purposes of locating the occurrence of buried metallic material in the earth’s crust that may lead to ground water contamination. The magnetic measurements have been used by geologists for a longer period to carry out exploration of minerals and mapping the regional geologic structures. In hydrological studies, suppose metallic drums and tanks have been buried in shallow trenches, magnetometers can be used in the location of the trench boundaries (Lutgens and Tarbuck28). The magnetic geophysical method is normally used in various fields among them archaeology. The method was first used in the field of archaeology in the 1950’s, and up to date the method is the backbone of the archeological field. In archaeology, the method is used in the application to the exploration of minerals, and oil and also to the regional geology studies. The method is applicable in the archaeology field because once the soils are disturbed by a burial or compacted by the occupation of humankind; there is normally an indication of a variation from magnetic susceptibility background values (Campana 43). In most occasions, archaeologists have found out that burials tend to lead to localized oxidation which brings about a void in the content of magnetite. Archaeologists tend to use the susceptibility meters for purposes of mapping the soils that have been modified by the activities of man. Still in the field of archaeology, the use of microcomputer-controlled systems used for automatic data gathering and the use of magnetic gradiometers is rampant in today’s world by archaeologists. These systems are effective in the high productivity surveys and are low in terms of noise production. To speed up the process of data gathering in the field of archaeology, some of the archaeologists tend to use the fluxgate magnetometers (Oswin58). Airplanes can be used in the acquisition of data in the magnetic geophysical method in the sense that the magnetic field of the earth is continuously recorded when an individual is flying over a path that is predetermined. During the acquisition of magnetic data when using an airplane, it should be clear that readings on the magnetometer in use are corrected for causes of magnetic intensity variations and diurnal variations. During the process, there is normally an indication of magnetic anomalies in closed contours which are caused by the anomalous depths to the basement or by the local ferromagnetic ore bodies (Sherriff76). The magnetic variations or anomalies that are measured by the magnetic geophysical method tend to occur as a result of lateral contrast in rock structure and in rock composition. Magnetization of the earth will be flat in cases where there is the lack of lateral contrast. The magnetic anomalies are usually created by a combination of what is called remanent and induced magnetization (Christiansen and Hamblin67). In most occasions, the remanent magnetization tends to be larger than the induced magnetization. When the rocks of the earth tend to cool bellow the Curie point temperature of the constituent magnetic minerals, remanent magnetization is obtained. Remanent magnetization may have an opposite and a different direction to the present magnetic field of the earth. This may rely on the direction of the field of the earth at the time when the magnetization was acquired (Garland97). It is quite difficult for the magnetic geophysical method to obtain magnetic anomalies from the sedimentary rocks because the rocks lack enough proportions of the magnetic minerals to generate the variations. Even with the presence of little amounts of magnetic minerals in the sedimentary rocks, there will be no magnetic variations produced unless the sedimentary materials with magnetic properties are brought to the surface of the earth through processes such as erosion, faulting or folding (Lutgens and Tarbuck103). There are various conditions under which aeromagnetic anomalies produced by the sedimentary rocks can be detected. The conditions include lack of young volcanic units that overlie the sedimentary rocks, the absence of the magnetic basement rocks in the upper few kilometers of the sedimentary rocks, faulting or folding of the sedimentary rock section leading to outcropping contacts, and favorable magnetization contrast within the sedimentary rock section (Garland102). Advantages and disadvantages of the method There are various advantages which are related to the use of the magnetic geophysical method. The method is advantageous in the sense that it is used to measure the variations in the magnetic field of the earth. It is of help because the variations in the magnetic field of the earth occur due to various reasons such the influence by the geology that is underlying. As compared to other geophysical methods, the magnetic method is advantageous simply because it makes the deployment in the regions whereby vegetation and rough terrain may be an obstruction to the process of data gathering easy. This is because the magnetic method does not use large instruments in the process of data gathering like the electromagnetic coils (Christiansen and Hamblin89). The method is beneficial because the analysis of the magnetic data obtained can be used to approximate the regional extent of the ferrous materials that are buried in the surface of the earth. By use of the method, the depth of the buried ferrous materials can be identified from the magnetic data gathered. Normally, the high priority areas can be indicated by the certainty of the ferrous materials. Through the application of the magnetic geophysical method can make it possible for an interpreter who is highly experienced in the field to identify the type of the rock by inspecting the magnetic anomalies. The method also makes it possible for the identification of different magnetic properties between units on the magnetic maps. This is possible because significant magnetic rock units are capable of producing magnetic anomalies that characterize the presence or absence of the rock (Christiansen and Hamblin59). It is also advantageous to use some of the magnetometers in the magnetic geophysical method. The use of fluxgate magnetometer is advantageous because it can be used to record magnetic data on a continuous mode, and it can be able to make directional measurements. The use of the proton precession magnetometer is useful due to its lightweight characteristics. The cesium vapor instrument makes the process fast though it is not continuous in detecting the magnetic anomalies in the earth, and that it does not have to align the bottle with the field. The magnetic gradiometer on the other hand is advantageous when used in the sense that it provides a high resolution of the near surface feature and that it does not have to be corrected for purposes of diurnal variation (Oswin60). Application of the magnetic geophysical method is beneficial because of its high cost to benefit ratio characteristic. The method is highly effective in the application to the field of geophysical exploration. The method is also effective since the magnetic data are usually collected by an aircraft hence covering larger grounds leading to a large amount of magnetic data acquired for interpretation. When the method is used for the initial assessment of the hazardous waste regions, the costs of carrying out the process are usually low. The method is usually cost effective especially in areas where tanks and drums are suspected to be buried (Garland65). The application of the magnetic geophysical method is fast and cheap per unit area. This happens so provided an airplane is used in the magnetic data acquisition. The airplanes are less expensive when it comes to the coverage of a larger area at once and that the magnetic data that are needed will be acquired in a faster rate unlike when the ground facilities are used (Christiansen and Hamblin98). The data handling capability of the magnetic geophysical method is normally high as compared to other similar methods. This is because of the incorporation of the non volatile and computers in the magnetometers used. The instruments then tend to keep track of the prompt for inputs of data storage for a whole day of work (Oswin89). The instruments used in the method have got a number of limitations hence making the method quite unreliable. The magnetic gradiometer is disadvantageous because it fails to measure the large scale features and that the ratio it gives for a signal to noise is low. The fluxgate magnetometer on the other hand only tends to take measurements of the field in the direction of the coils, and that it is sensitive to temperature, hence in areas where the temperatures are too low or too high, the instrument may not be effective. The only disadvantage of using the cesium vapor is the fact that it is unable to take measurements of the vector field. Finally, the proton precession is also unable to measure vector field just like the cesium vapor, and that it is unable to make continuous records of the magnetic data (Garland165). The magnetic geophysical method is disadvantageous in the sense that the magnetic data collected by the magnetometers can be easily distorted by steel and other ferrous metals present in the field. When measuring the field, a compass should be located at least three meters away from the magnetometer in order to avoid the distortion of the magnetic data gathered (Sherriff69). The magnetic geophysical method makes it difficult for the interpretation methods in distinguishing between various steel objects. It is actually quite impossible to determine a magnetic variation or anomaly which is due to a group of old washing machines or of steel drums. The method also does not give room for the interpreter to determine the contents that are available in the buried tank or drum (Kaufman108). The final limitation of the magnetic geophysical method is the fact that the manmade structures which have been constructed by use of ferrous material like steel tend to have a detrimental effect on the quality of magnetic data obtained by the method. During the process of data acquisition, some of the manmade structures should be avoided. If the structures are not avoided, then there locations should be clearly noted on the field notebook as well as on the site map (Garland135). Conclusion In conclusion, it is well known that the magnetic field of the earth can well be described by the magnetic lines that are not visible. These magnetic invisible lines tend to flow out of the south magnetic pole of the earth into the north magnetic pole. The primary applications of the magnetic geophysical method are included in the location of the ferromagnetic ore deposits in the earth and the mapping of the earth’s basement. The magnetic geophysical method has been accepted in the field of geophysics in the evaluation and exploration of metallic ore deposits on the surface of the earth. The magnetic anomalies that are identified by the magnetic geophysical method are caused by the differences in the magnetic characteristics of the metallic rocks beneath. As discussed in the previous paragraphs, the method is advantageous because it can be used in fields of various circumstances such as on the land, in the air or in the water. It should also be noted that rocks that have minerals with a remanent magnetization tend to maintain an independent magnetic field while the rocks that contain minerals with ferromagnetic susceptibility tend to be magnetized in the earth’s field (Kaufman90). Works Cited Campana, Stefano. Seeing the Unseen. Geophysics and Landscape Archaeology. London: Taylor & Francis, 2008. Print Christiansen, Eric and William Kenneth Hamblin. Earth's Dynamic Systems. New Jersey: Prentice Hall, 2004. Print Garland, George David. Introduction to geophysics: mantle, core, and crust. California: Saunders, 1979. Print Kaufman, Alexander. Geophysical Field Theory and Method: Gravitational, electric, and magnetic fields, Part 1. Salt Lake City: Academic Press, 1992. Print Lutgens, Fredrick and Edward Tarbuck. The Earth: An Introduction To Physical Geology, Volume. Indiana: Merrill Pub. Co., 1987. Mishra, Dinesh. Gravity and Magnetic Methods for Geological Studies. New York: CRC Press, 2011.Print Oswin, John. A Field Guide to Geophysics in Archaeology. New York: Springer, 2009. Sherriff, Robert. Geophysical Methods. New Jersey: Prentice Hall, 1989. Read More