FS–236–95 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 sedi-
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—with warm colors (reds and oranges) 

showing areas of higher magnetic values and cool 

colors (blues and greens) showing lower values—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 4π SI (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°C 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—these 

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 

95–77 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

pathill@musette.cr.usgs.gov

(136.177.80.14)

Viki Bankey (same address)

(303) 236-1348

viki@musette.cr.usgs.gov

Vicki Langenheim

U.S. Geological Survey

345 Middlefield Road, MS 939

Menlo Park, CA 94025

(415) 329-5313

zulanger@mojave.wr.usgs.gov

(130.118.4.68)