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It is not intended to describe the physical details or operation of the considerable number of gravity field instruments which have been developed over the years. Some twenty or more different land gravity meters have had very extensive use in the field.

Unit of Gravity Measurement

A body on the surface of the earth has a “weight” which results from the gravitational attraction of the entire earth. If the body is allowed to fall, it is accelerated by this weight. The unit of acceleration is the gal, named after Galileo, and one gal is simply one cm/sec2. The average acceleration on the earth's surface is about 980 cm/sec2 = 980 gals (but increases by about 1/2 percent from equator to pole).

In geophysical prospecting we are interested in “anomalies” in the gravitational field produced by variation in density as defined in the previous chapter. These are only very small fractions of the earth's total field, and a smaller unit is needed. The commonly used unit is the miilligal or mgal which is .001 gal. Anomalies from local geologic structures are commonly of the order of one to ten mgals.

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