Units Associated with Gravitational Acceleration
As described on the previous page, acceleration is defined as the time rate of change of the speed of a body. Speed, sometimes incorrectly referred to as velocity, is the distance an object travels divided by the time it took to travel that distance (i.e., meters per second (m/s)). Thus, we can measure the speed of an object by observing the time it takes to travel a known distance.
If the speed of the object changes as it travels, then this change in speed with respect to time is referred to as acceleration. Positive acceleration means the object is moving faster with time, and negative acceleration means the object is slowing down with time. Acceleration can be measured by determining the speed of an object at two different times and dividing the speed by the time difference between the two observations. Therefore, the units associated with acceleration is speed (distance per time) divided by time; or distance per time per time, or distance per time squared.
If an object such as a ball is dropped, it falls under the influence of gravity in such a way that its speed increases constantly with time. That is, the object accelerates as it falls with constant acceleration. At sea level, the rate of acceleration is about 9.8 meters per second squared. In gravity surveying, we will measure variations in the acceleration due to the earth's gravity. As will be described next, variations in this acceleration can be caused by variations in subsurface geology. Acceleration variations due to geology, however, tend to be much smaller than 9.8 meters per second squared. Thus, a meter per second squared is an inconvenient system of units to use when discussing gravity surveys.
The units typically used in describing the graviational acceleration variations observed in exploration gravity surveys are specified in milliGals. A Gal is defined as a centimeter per second squared. Thus, the Earth's gravitational acceleration is approximately 980 Gals. The Gal is named after Galileo Galilei . The milliGal (mgal) is one thousandth of a Gal. In milliGals, the Earth's gravitational acceleration is approximately 980,000.