How do we Measure Gravity?
As you can imagine, it is difficult to construct instruments capable of measuring gravity anomalies as small as 1 part in 40 million. There are, however, a variety of ways it can be done, including:
- Falling body measurements. These are the type of measurements we have described up to this point. One drops an object and directly computes the acceleration the body undergoes by carefully measuring distance and time as the body falls.
- Pendulum measurements. In this type of measurement, the gravitational acceleration is estimated by measuring the period oscillation of a pendulum.
- Mass on spring measurements. By suspending a mass on a spring and observing how much the spring deforms under the force of gravity, an estimate of the gravitational acceleration can be determined.
As will be described later, in exploration gravity surveys, the field observations usually do not yield measurements of the absolute value of gravitational acceleration. Rather, we can only derive estimates of variations of gravitational acceleration. The primary reason for this is that it can be difficult to characterize the recording instrument well enough to measure absolute values of gravity down to 1 part in 50 million. This, however, is not a limitation for exploration surveys since it is only the relative change in gravity that is used to define the variation in geologic structure.