The concept of gravity is as old as science itself. Galileo Galilei realized that bodies fall with the same acceleration when air resistance is accounted for. Issac Newton quantified the gravity attraction between two masses: F = G m1m2/r2, where r is the distance between them. This is the (simple) basis for gravity work.
Earth’s acceleration due to gravity (near the surface) is about 9.8 m/s2. The unit “Gal” (in honor of Galileo, 1 cm/s2) is commonly used. Gravity is mapped on Earth’s surface to an accuracy of about 1 mGal (about 1 part per million).
But the earth is not a static, smooth sphere of uniform density, so there are complications to any generalities that need to be taken into account when measuring gravity:
- There is an increase in gravity of about 0.5% from equator to the poles.
- There is a decrease in gravity of about 3 microGal per cm of height increase.
- Gravity earth tides have amplitudes of up to 300 microGal, and can be modeled with an accuracy of about 1 microGal.
- Ocean tides influence seafloor gravity with peak amplitudes of 50 – 100 microGal, can be modeled to an accuracy of about 10 microGal and measured to an accuracy of 1 microGal or better.
- Effects of the variable water load on the crust (ocean loading) peaks at about 20 microGal. Models of this has largest uncertainties far from shore.
From an environmental impact perspective, gravity measurements are passive measuring techniques that are friendly, unlike more active techniques that emit strong acoustic or electromagnetic signals and have lead to sea mammal disorientation and even death.