A bit of a read for some further research to help the discussion.
Dipolar effect is what you need to look at... here's a hint read from start to finish and you may be enlightened...
FS¡V236¡V95 OCTOBER 1997 Introduction to Potential Fields: Magnetics Introduction Magnetic and gravity exploration, also referred to as ¡§potential fields¡¨ exploration, is used to give geoscientists an indirect way to ¡§see¡¨ beneath the Earth¡¦s surface by sensing different physical properties of rocks (magnetization and density, respectively). Gravity and magnetic exploration can help locate faults, mineral and petroleum resources, and ground-water reservoirs. Potential-field surveys are relatively inexpensive geophysical methods and can quickly cover large areas of ground. What is magnetism? The force a magnet exerts on an iron filing or the force the Earth¡¦s magnetic field exerts on the needle of a compass are two common examples of magnetism. A magnetic field has both intensity and direction. The strength of the magnetic force depends on the amount of magnetic material present and its distance and direction relative to the detector. The Earth¡¦s magnetic field probably is caused by movement of partially molten iron in the Earth¡¦s outer core. The magnetic field strength increases from 25,000 nanoteslas (nT) at the magnetic equator to 70,000 nT at the magnetic poles. (One nanotesla equals 1 gamma, and 105 gammas equals 1 oersted.) The Earth¡¦s magnetic field changes in intensity and direction slowly over time. Like all dipole magnets, the Earth has a magnetic field (also called the core or main field) that has a North and South Pole. The angle between a compass needle and true north is called the magnetic declination. The north-seeking end of a compass needle that is free to orient itself in an up-down direction will point down in the Northern Hemisphere and up in the Southern Hemisphere. The angle between the needle and horizontal is called the magnetic inclination. How do scientists measure the magnetic field? Geoscientists measure the Earth¡¦s magnetic field intensity to an accuracy of 0.1 nT using magnetometers. Magnetic surveys usually are conducted from an aircraft. Ground surveys also can be made and are especially useful for locating buried metallic objects such as waste barrels. An aeromagnetic survey is flown using an aircraft (airplane or helicopter) to which a magnetometer is attached. The most common aircraft magnetometers measure the total intensity of the magnetic field, but not its direction, along continuous flight lines that are a fixed distance apart. The aircraft can be flown at a constant barometric elevation (such as 9,000 ft above sea level) or at a constant distance above the ground (such as 500 ft above terrain, also called a ¡§draped¡¨ survey). Magnetometers measure all effects of the Earth¡¦s magnetic field. Because the field changes slowly over time, models of this field, called the International Geomagnetic Reference Field (IGRF), are updated every 5 years. The IGRF for the time and location of a magnetic survey is calculated and removed. The magnetic field is also subject to complex shortterm variations such as magnetic storms. For purposes of correcting aeromagnetic survey data, a base magnetometer records magnetic levels at a fixed location within the study area, and these variations are removed from the airborne magnetic data. What remains is the magnetic field largely associated with magnetic minerals in crustal rocks. What is a magnetic anomaly? Although the force of the Earth¡¦s magnetic field is not very strong, it is large enough to magnetize certain kinds of rocks that contain iron or other magnetite-bearing minerals. Magnetic anomalies, therefore, are the differences between measured magnetic values and the values predicted from the model of the Earth¡¦s core field. They are caused by variations in magnetization of crustal rocks. Measurements of many rock samples show that most sediƒn mentary rocks are generally not magnetic, whereas igneous rocks rich in iron minerals often are very magnetic. Because of the dipolar nature of magnetism, a single magnetic body can cause either a positive or negative magnetic anomaly or both (especially if the Earth¡¦s magnetic field at the time of rock formation is reverse of the present-day field), or it can cause a more complex magnetic pattern caused by tilting of the magnetic body itself. What is a magnetic anomaly map? A magnetic anomaly map is made from recorded flight-line measurements across the area of interest from which the Earth¡¦s field has been removed. These data are then gridded so that the flight-line magnetic data are converted to a representation of the magnetic field at equally spaced locations along and between the flight lines. Magnetic anomaly maps can be shown as color images¡Xwith warm colors (reds and oranges) showing areas of higher magnetic values and cool colors (blues and greens) showing lower values¡X or as contour line maps, where each contour line follows a constant magnetic value. What is rock magnetism? Magnetic susceptibility is a rock property describing the amount of magnetizable material in a rock. It is a dimensionless unit, and 1 electromagnetic unit (emu) in the cgs (centimeter-gramsecond) system equals 4„jƒnSI (System International) units. Rocks containing magnetic minerals may have two kinds of magnetization: induced and remanent. Induced magnetization exists only in the presence of an external magnetic field. Remanent magnetization, however, is frozen within the rock, and the rock remains magnetized in a field-free area. Sometimes the direction of the Earth¡¦s field at the time of rock formation or alteration is preserved. The study of rock paleomagnetism is based on this property and, in some places, can be used to show rock movement through time. Studies of remanently magnetized rock show that the magnetic North and South Poles have reversed through geologic time. Remanent magnetization, therefore, can also give some indication of the age of magnetization. Both induced and remanent magnetization vanish above the Curie temperature (about 580„aC for magnetite). What is a derivative magnetic map? A magnetic map contains information about both rock magnetization changes across an area and depth to the source of the anomaly. Maps can be derived from the original magnetic anomaly grid by using tools to enhance parts of the magnetic field. In general, the deeper the magnetic source, the broader and gentler the gradients of the resulting anomaly will be. Also, in general, the shallower the magnetic object, the sharper and narrower the resulting anomaly. Derivative maps can show anomalies that have been filtered for size and shape to emphasize either shallow or deep sources. Another type of derivative map, called ¡§reduced to the pole,¡¨ can correct the anomalies for inclination and declination differences caused by location and produce the magnetic field of the bodies as though the area were moved to the North Pole. This simplifies complex anomaly shapes caused by dipole effects of the Earth¡¦s magnetic field and centers the anomaly over its source. Another derivative method can magnify magnetic gradients, places where the magnetic field changes from high to low¡Xthese places often mark edges of rock units or faults. All of these maps can be used together to make a geologic interpretation. Additional information U.S. Geological Survey Open-File Report 95¡V77 lists many USGS computer programs and databases used to create magnetic maps: http://minerals.er.usgs.gov Information on the availability of magnetic maps and data in specific areas and other general information on USGS airborne coverage can be obtained from: Pat Hill U.S. Geological Survey Box 25046, MS 964, Denver Federal Center Denver, CO 80225 (303) 236-1343 [email protected] (136.177.80.14) Viki Bankey (same address) (303) 236-1348 [email protected] Vicki Langenheim U.S. Geological Survey 345 Middlefield Road, MS 939 Menlo Park, CA 94025 (415) 329-5313 [email protected] (130.118.4.68)
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