Closed, Illegal and Abandoned (CIA) Disposal Sites

Gravitational Field Technologies

Gravity is defined as the force of attraction between two masses. The most commonly understood gravitational force is between the sun and the earth. However, lateral density changes in the earth’s subsurface cause a change in the force of gravity at the surface of the earth. A subsurface body of a different density from its surroundings will attract a mass on the surface to a greater or lesser extent than the surrounding earth. By analyzing the change in gravitational attraction along the surface of the earth these subsurface anomalies can be detected.

Geophysicists can use gravity measurements to help them understand the internal structure of the earth while an environmental scientist can use gravity measurements to locate underground gravitational anomalies. A gravimeter is an instrument designed to measure spatial variations in gravitational acceleration. There are various types of gravimeters in existence today. The most common gravimeter used in surveys is based on a simple mass-spring system.

Gravity measurements alone are very difficult to analyze, there are many solutions and interpretations to the observed measurements. Gravity surveys and measurements are commonly used in conjunction with other studies to confirm theories drawn by geologists. Mathematical corrections to measured anomalies are often necessary and should be applied with the discretion of a geologist or geophysicist.

More information on gravity and gravitational surveys:

• Geovision Geophysical Services
• Colorado School of Mines
• Northeast Geophysical Services

Defining Landfill Geometry Using a Gravimetric Survey

Roberts et al.* performed a gravity survey of a municipal solid waste landfill in order to investigate the effectiveness of the method in establishing the lateral boundaries and the vertical extent of the landfill. The survey consisted of approximately 200 gravity stations spaced at 5-meter to 10-meter intervals. Bouguer and terrain corrections were made to the raw data. The information gathered from the gravity survey was compared to data from boreholes and pre-landfill and post-landfill topographic maps. The results of the survey closely correlated with the topographic maps and borehole data.

To illustrate the importance of obtaining independent geologic information about the site prior to performing a gravity survey, Roberts stated “the gravity method depends on precise gravity and surface elevation measurements, careful computations, and constraining information obtained from collateral geologic and geophysical studies for the interpretations to be valid.” (Roberts et al p. 259).

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*  Roberts, R.L., Hinze, W.J., and Leap, D.I., 1990, Application of the gravity methods to investigation of a landfill in the glaciated mid-continent, USA in Ward, Stanley., ed., Geotechnical and environmental geophysics; Volume 2: Environmental and groundwater: Society of Exploration Geophysicists Investigations in Geophysics No. 5, p. 253-266.

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Last updated: April 18, 2008