FS–239–95 OCTOBER 1997

Introduction to Potential Fields: Gravity

Introduction

Gravity and magnetic 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 (density and magnetization, respectively). Gravity and magnetic exploration can help locate faults, 

mineral or petroleum resources, and ground-water reservoirs. Potential-field surveys are relatively inexpensive and can quickly cover large areas of ground.

What is gravity?

Gravitation is the force of attraction between two 

bodies, such as the Earth and our body. The strength 

of this attraction depends on the mass of the two bodies and the distance between them.

	 A mass falls to the ground with increasing velocity, and the rate of increase is called gravitational 

acceleration, g, or gravity. The unit of gravity is the

Gal (in honor of Galileo). One Gal equals 1 cm/sec2.

	 Gravity is not the same everywhere on Earth, 

but changes with many known and measurable factors, such as tidal forces. Gravity surveys exploit the 

very small changes in gravity from place to place 

that are caused by changes in subsurface rock density. Higher gravity values are found over rocks that 

are more dense, and lower gravity values are found 

over rocks that are less dense.

How do scientists measure gravity?

	 Scientists measure the gravitational acceleration, 

g, using one of two kinds of gravity meters. An 

absolute gravimeter measures the actual value of g

by measuring the speed of a falling mass using a 

laser beam. Although this meter achieves precisions 

of 0.01 to 0.001 mGal (milliGals, or 1/1000 Gal), 

they are expensive, heavy, and bulky.

	 A second type of gravity meter measures relative changes in g between two locations. This instrument uses a mass on the end of a spring that 

stretches where g is stronger. This kind of meter can 

measure g with a precision of 0.01 mGal in about 5 

minutes. A relative gravity measurement is also 

made at the nearest absolute gravity station, one of a 

network of worldwide gravity base stations. The 

relative gravity measurements are thereby tied to the 

absolute gravity network.

What is a gravity anomaly?

	 Gravity meters measure all effects that make up 

the Earth’s gravity field. Many of these effects are 

caused by known sources, such as the Earth’s rotation, distance from the Earth’s center, topographic 

relief, and tidal variation. Gravity caused by these 

sources can be calculated using realistic Earth models and removed from the measured data, leaving 

gravity anomalies caused by unknown sources. To 

the geologist, the most important unknown source is 

the effect of the irregular underground distribution 

of rocks having different densities.

	 A sequence of gravity corrections are applied to 

the original gravity reading and result in various 

named gravity anomalies. The observed gravity

anomaly has been corrected for Earth rotation, latitude, tidal effects, and gravity meter fluctuations. 

A modern pioneer—


U.S. Geological


Survey geophysicist


takes a gravity read-


ing on the Oregon 


Trail.
The free air gravity anomaly has been corrected for 

the gravity effect caused by the elevation difference 

between the station and sea level (a correction for distance) and is a standard for oceanic gravity interpretation. The Bouguer (pronounced Boo-gay ́) gravity 

anomaly has been further corrected for the mass that 

may exist between sea level and the observer (a correction for mass) and is a standard used in geologic 

interpretation on land. A simple-Bouguer anomaly 

has undergone a simplified removal of topographic 

effects, which suffices in relatively flat areas. A complete-Bouguer anomaly contains a terrain correction 

that uses a more complete representation of the local 

topography, which is necessary for accurate gravity 

values in mountainous areas. The isostatic (pronounced iso-stat ́-ic) gravity anomaly is calculated by 

subtracting the gravitational effect of low-density 

mountain roots below areas of high topography. 

Although these roots have never been seen, their isostatic effect has been measured and models calculated 

using topography. Isostasy is typified by floating icebergs that have 90% of their mass of ice below water 

that supports a smaller mass of ice projecting above 

water.

What is a gravity map?

	 A gravity map is made using numerous gravity 

measurements across the area of interest. Gravity surveying by aircraft is still a new science, so most gravity measurements are made on the ground at discrete 

stations. Because access is often a problem, gravity 

stations may be randomly spaced, although detailed 

surveys are usually made at regular intervals.

	 Gravity measurements are often processed to a 

complete-Bouguer or isostatic gravity anomaly. These 

data are then gridded, so that the randomly spaced data 

are converted to a representation of the gravity field at 

equally spaced locations. The distance chosen between 

grid points depends on the average distance between 

gravity stations. Too large a grid interval would not 

use all the information from the original data set, 

whereas too small a grid interval fragments the continuity of anomalies across a region—either result is a 

poor representation of the true gravity field.

	 Gravity anomaly maps can be shown as color figures—with warm colors (reds and oranges) showing 

areas of higher gravity values and cool colors (blues 

and greens) showing lower values—or as contour line 

maps, where each contour line follows a constant 

gravity value.

What is rock density?

	 Density is a rock property described by the ratio 

of mass to volume. Rock densities commonly range 

between 2.0 and 4.0 grams per cubic centimeter 

(g/cm3). Pure water, by comparison, has a density of 

1 g/cm3. Each rock type can have a range of density 

values, and tables in the scientific literature show the 

general range of densities for various rock types. 

Often, the geoscientist will collect samples of exposed 

rocks in the study area and measure their densities to 

estimate the actual density of the rock unit where it 

is buried.

	 Various rock types within a study area often 

contrast enough in density to cause gravity anomalies. For example, sedimentary rocks that fill basins 

almost always have low densities and are characterized by gravity lows on anomaly maps. Mafic 

rocks, which contain high-density minerals, often 

are associated with gravity highs. The scientist can 

use these differences to map large regions where 

rocks are inaccessible or concealed, to look for 

faults that juxtapose rocks of different densities, or 

to infer structures such as basins, arches, and buried 

intrusions.

What is a derivative gravity map?


	 A gravity anomaly map contains information 

about rock density, and depth and distribution of 

anomaly source rocks. Maps can be derived from 

the original gravity anomaly grid by using 

mathematical tools to enhance parts of the gravity 

field. Derivative maps can show, for example, 

anomalies that have been mathematically filtered 

for size and that show deeper or shallower sources. 

Other derivative techniques can magnify gravity 

gradients, places where the gravity field changes 

from high to low—these places often mark edges of 

rock units or faults, or they can mimic a geologic 

map by converting (or “terracing”) the gravity 

anomalies into discrete, bounded units representing 

rock units. 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 gravity maps, and it is 

available on the web site listed below.

	 Information on gravity base stations and availability of gravity maps and data in specific areas 

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

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

web site: http://minerals.er.usgs.gov

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