BOX 2.1 How do geophysicists measure gravity?


1. Earth's gravity field is 980 Gals (roughly 1000 Gals); a gravity meter is capable of measuring 0.01 mGal.
How many 0.01 mGals in 1000 Gals? 1000x1000x100 equals 100 million. A gravity meter is capable of measuring one-hundred-millionth of the Earth's field. Or one-millionth of one percent!

2. A spring becomes less rigid as it is heated. It is easier to elongate the spring when it is warm.

3. An absolute gravimeter measures the real value of g, but is very heavy and takes a long time to measure g. The other type of meter discussed in box2.1 doesn't measure the real value of g (it has to be referenced to a spot where g is known), but it is easily transported and measures g quickly and, therefore, is most often used by geophysicists to cover large areas.

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