Variation in Gravitational Acceleration Due to Changes in Elevation

Imagine two gravity readings taken at the same location and at the same time with two perfect (no instrument drift and the readings contain no errors) gravimeters; one placed on the ground, the other place on top of a step ladder. Would the two instruments record the same gravitational acceleration?

No, the instrument placed on top of the step ladder would record a smaller gravitational acceleration than the one placed on the ground. Why? Remember that the size of the gravitational acceleration changes as the gravimeter changes distance from the center of the earth. In particular, the size of the gravitational acceleration varies as one over the distance squared between the gravimeter and the center of the earth. Therefore, the gravimeter located on top of the step ladder will record a smaller gravitational acceleration, because it is positioned farther from the earth's center than the gravimeter resting on the ground.

Therefore, when interpreting data from our gravity survey, we need to make sure that we don't interpret spatial variations in gravitational acceleration that are related to elevation differences in our observation points as being due to subsurface geology. Clearly, to be able to separate these two effects, we are going to need to know the elevations at which our gravity observations are taken.

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