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Introduction to Potential Fields: Magnetics

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Introduction to Potential Fields: Magnetics FS–236–95 OCTOBER 1997 Introduction to Potential Fields: Magnetics Introduction magnetometers. Magnetic surveys usually are Magnetic and gravity exploration, also conducted from an aircraft. Ground surveys referred to as “potential fields” exploration, is also can be made and are especially useful for used to give geoscientists an indirect way to locating buried metallic objects such as waste “see” beneath the Earth’s surface by sensing dif- barrels. ferent physical properties of rocks (magnetization An aeromagnetic survey is flown using an and density, respectively). Gravity and magnetic aircraft (airplane or helicopter) to which a exploration can help locate faults, mineral and magnetometer is attached. The most common petroleum resources, and ground-water reser- aircraft magnetometers measure the total voirs. Potential-field surveys are relatively inex- intensity of the magnetic field, but not its pensive geophysical methods and can quickly direction, along continuous flight lines that are cover large areas of ground. a fixed distance apart. The aircraft can be flown at a constant barometric elevation (such What is magnetism? as 9,000 ft above sea level) or at a constant The force a magnet exerts on an iron filing or distance above the ground (such as 500 ft the force the Earth’s magnetic field exerts on the above terrain, also called a “draped” survey). needle of a compass are two common examples Magnetometers measure all effects of the of magnetism. A magnetic field has both intensity Earth’s magnetic field. Because the field and direction. The strength of the magnetic force changes slowly over time, models of this field, depends on the amount of magnetic material called the International Geomagnetic Refer- present and its distance and direction relative to ence Field (IGRF), are updated every 5 years. the detector. The Earth’s magnetic field probably The IGRF for the time and location of a mag- is caused by movement of partially molten iron in netic survey is calculated and removed. The the Earth’s outer core. The magnetic field magnetic field is also subject to complex short- strength increases from 25,000 nanoteslas (nT) at term variations such as magnetic storms. For the magnetic equator to 70,000 nT at the mag- purposes of correcting aeromagnetic survey netic poles. (One nanotesla equals 1 gamma, and data, a base magnetometer records magnetic 105 gammas equals 1 oersted.) The Earth’s mag- levels at a fixed location within the study area, netic field changes in intensity and direction and these variations are removed from the air- slowly over time. borne magnetic data. What remains is the mag- Like all dipole magnets, the Earth has a mag- netic field largely associated with magnetic netic field (also called the core or main field) that minerals in crustal rocks. has a North and South Pole. The angle between a compass needle and true north is called the mag- What is a magnetic anomaly? netic declination. The north-seeking end of a Although the force of the Earth’s magnetic compass needle that is free to orient itself in an field is not very strong, it is large enough to up-down direction will point down in the North- magnetize certain kinds of rocks that contain ern Hemisphere and up in the Southern Hemi- iron or other magnetite-bearing minerals. Mag- sphere. The angle between the needle and hori- netic anomalies, therefore, are the differences zontal is called the magnetic inclination. between measured magnetic values and the values predicted from the model of the Earth’s How do scientists measure the magnetic field? core field. They are caused by variations in Geoscientists measure the Earth’s magnetic magnetization of crustal rocks. Measurements field intensity to an accuracy of 0.1 nT using of many rock samples show that most sedi- mentary rocks are generally not magnetic, wher- and depth to the source of the anomaly. Maps eas igneous rocks rich in iron minerals often are can be derived from the original magnetic very magnetic. anomaly grid by using tools to enhance parts of Because of the dipolar nature of magnetism, a the magnetic field. single magnetic body can cause either a positive In general, the deeper the magnetic source, or negative magnetic anomaly or both (especially the broader and gentler the gradients of the if the Earth’s magnetic field at the time of rock resulting anomaly will be. Also, in general, the formation is reverse of the present-day field), or shallower the magnetic object, the sharper and it can cause a more complex magnetic pattern narrower the resulting anomaly. Derivative caused by tilting of the magnetic body itself. maps can show anomalies that have been fil- tered for size and shape to emphasize either What is a magnetic anomaly map? shallow or deep sources. Another type of A magnetic anomaly map is made from derivative map, called “reduced to the pole,” recorded flight-line measurements across the area can correct the anomalies for inclination and of interest from which the Earth’s field has been declination differences caused by location and removed. These data are then gridded so that the produce the magnetic field of the bodies as flight-line magnetic data are converted to a repre- though the area were moved to the North Pole. sentation of the magnetic field at equally spaced This simplifies complex anomaly shapes locations along and between the flight lines. caused by dipole effects of the Earth’s mag- Magnetic anomaly maps can be shown as color netic field and centers the anomaly over its images—with warm colors (reds and oranges) source. Another derivative method can mag- showing areas of higher magnetic values and cool nify magnetic gradients, places where the mag- colors (blues and greens) showing lower val- netic field changes from high to low—these ues—or as contour line maps, where each contour places often mark edges of rock units or faults. line follows a constant magnetic value. All of these maps can be used together to make a geologic interpretation. What is rock magnetism? Magnetic susceptibility is a rock property Additional information describing the amount of magnetizable material U.S. Geological Survey Open-File Report in a rock. It is a dimensionless unit, and 1 electro- 95–77 lists many USGS computer programs magnetic unit (emu) in the cgs (centimeter-gram- and databases used to create magnetic maps: second) system equals 4π SI (System Internation- http://minerals.er.usgs.gov al) units. Information on the availability of magnetic Rocks containing magnetic minerals may maps and data in specific areas and other gen- have two kinds of magnetization: induced and eral information on USGS airborne coverage remanent. Induced magnetization exists only in can be obtained from: the presence of an external magnetic field. Remanent magnetization, however, is frozen Pat Hill within the rock, and the rock remains magnetized U.S. Geological Survey in a field-free area. Sometimes the direction of Box 25046, MS 964, Denver Federal Center the Earth’s field at the time of rock formation or Denver, CO 80225 alteration is preserved. The study of rock paleo- (303) 236-1343 magnetism is based on this property and, in some [email protected] places, can be used to show rock movement (136.177.80.14) through time. Studies of remanently magnetized rock show that the magnetic North and South Viki Bankey (same address) Poles have reversed through geologic time. (303) 236-1348 Remanent magnetization, therefore, can also give [email protected] some indication of the age of magnetization. Both induced and remanent magnetization vanish Vicki Langenheim above the Curie temperature (about 580°C for U.S. Geological Survey magnetite). 345 Middlefield Road, MS 939 Menlo Park, CA 94025 What is a derivative magnetic map? (415) 329-5313 A magnetic map contains information about [email protected] both rock magnetization changes across an area (130.118.4.68).
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