Gravitational field of the Earth

According to Newton's Law, the gravitational force between two masses is proportional to each mass and inversely proportional to the square of their distance.

The Earth's gravity field is the result of the mass distribution inside the Earth. The Earth's gravitational field is dominated by a decrease in gravitational attraction from the poles towards the Equator. Superimposed on this are subtle variations that are largely the result of density variations near the Earth's surface. The main characteristics of the Earth's gravity field are:

Since the aim of gravity surveys is to investigate the gravity field caused by geological structures, called the gravity anomaly field, the large variations with latitude need to be removed from the raw survey data.

Rock density

Density is defined as mass divided by volume. Different rock types have different densities due to variations in composition and due to physical/chemical processes that affect density. Sedimentary rocks are generally low in density, while deep seated rocks have densities of 3 g/cm³or 3000 kg/m³ or higher.

Geology and gravity

The gravitational acceleration at the surface of the Earth, once corrected for geographic position and altitude, shows subtle variations that depend on the density distribution in the underlying rocks. Below, some examples are listed:

Note that even within a particular rock type, the range of densities is quite substantial - this is caused by many different factors that have influenced the rock throughout its formation and history. For example, metamorphism has an effect on density, and so has fracturing. Generally, sedimentary rocks are less dense than metamorphic and igneous ones, but their density may very a lot depending on fluids present, and depth of burial. It should be noted that the maximum variation in rock density in the Earth's crust is about a factor of 2. Thus, density variations are much smaller than variations in magnetic susceptibility, which spans a range of 10⁵.

Survey design

Gravity surveys are generally carried out on the ground. In remote areas, helicopters are used to reach the sites, but the actual measurement is done by placing a gravimeter on the Earth's surface. Recently, gravity data have been acquired from aircraft, but the resolution is generally only suitable for reconnaissance surveys. Another interesting development is the use of radar to map the surface of the oceans. The average sea surface height reflects the local gravity, and thus the marine gravity field can be determined from accurate maps of the sea surface.

Survey mode

There are different types of gravity surveys present in the National Gravity Data Base:

Gravity data acquisition system

A typical gravity survey data acquisition system consists of a number of instruments:

Most survey instruments are relative gravity meters. This means that they accurately measure the difference in gravitational acceleration between stations. In order to tie these observations to a national datum, the gravimeter needs to be calibrated at the beginning and at the end of each day by taking a measurement at a point where the gravity is accurately known. In Canada, over 5000 points have been established that form the Gravity Standardization Network. These points are used to tie relative survey observations to the national datum. Information about specific control points can be obtained from the Geodetic Survey Division, Geomatics Canada, Department of Natural Resources Canada.

Data processing

Once the survey data is collected, it is further processed to make corrections for each measurement location, both horizontally and vertically:

After these steps, the survey data is ready for displaying as maps or images.

Data presentation

Gravity data are usually displayed in one of the following ways:

Presently, most data is presented as colour maps or images where the colour denotes the amplitude of the anomalous gravity field (Bouguer or Free-Air).

Enhancements

To help interpretation of the gravity data, many types of enhancements (or transformations) can be applied to the maps or images to emphasize features of interest. The gravity anomaly field not only reflects near surface variations in density, but also large scale variations in crustal thickness and deeper structures. Since density variations near the ground surface produce gravity anomalies that have smaller areal extent than anomalies produced by deep sources, suppressing or removing the broader anomalies will leave a map with only the shallow source anomalies highlighted. Shading or artificially illuminating the gravity images can be used to emphasize gravity anomaly trends in a certain direction.

Interpretation

Interpretation of gravity data is performed on either profile or map data. Profiles are selected to run roughly perpendicular to geological structures, and a vertical cross-section is constructed that has a density distribution in accordance with the gravity anomaly observations. The shape of a particular gravity anomaly is usually indicative of the dip and depth extent of a geologic unit if there is a significant density contrast with the surrounding rock units. For example, over granite plutons, the extent of the gravity low (related to the lower density granites) can be use to accurately determine the shape of the pluton at depth and the dip angle of the contact with the surrounding rocks.

Map data are most often interpreted by the use of inversion techniques. Under certain assumptions (e.g. a constant thickness of the crust, or a constant density), a geological model that produces a gravity field that matches the observations can be calculated by inversion of the data. These types of models are often used to study large scale variations in crustal thickness or composition, for example along continental margins. On the largest scale, gravity anomaly data can tell us something about the state of equilibrium in the crust and mantle.