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Why do gravity values decrease as elevation above sea level increases?

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Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

Gravity values decrease as elevation increases primarily because of the increased distance from the center of the Earth. According to Newton's law of universal gravitation, the gravitational force between two bodies diminishes as the square of the distance between them increases.123

In the context of geophysics and geodesy, this phenomenon is accounted for by the free-air correction factor, which is approximately −0.3086 milligal per meter of elevation. This calculation assumes that the space between the point of observation and sea level is occupied by material of zero density.1

When conducting gravity surveys, geologists must also apply the Bouguer correction to account for the actual mass of rock material located between the observation point and sea level, as this mass exerts an additional gravitational attraction that partially offsets the decrease caused by elevation.12