Why FTG?
On a moving platform, the vessel, whether a ship or airplane, continuously has changes in direction caused by the movement of the vehicle. The change of motion could be large sudden changes caused by turbulence or waves or subtle changes caused by the pilot steering the vessel. Each time the vessel changes its direction, forces are exerted onto the vessel. Since the force of gravity is directly related to the gravimeter measurement, the gravimeter also measures forces related to changes in the vessel motion. Any measured force not caused by the earth's gravity would be noise that decreases the ability of the instrument to discern minor variations of geologic structure. To reduce the effects caused by the vessel motion, correction factors from an independent measurement are applied to the data. To calculate the gradient from measured marine or airborne gravity data, however, adjacent values must be subtracted. Any errors in the measurement are then added, increasing uncertainties in the data.
Just like a gravimeter, each accelerometer in the gradiometer measures both the acceleration caused by gravity and the accelerations of the moving vessel. Unlike the gravimeter, however, each unit in the pair of accelerometers measures nearly the same motion of the vessel. When the signals from the accelerometers are subtracted, the first order motion of the vessel is removed leaving primarily the gradient of the earth in the signal.
By measuring all five independent gradient components, the full tensor data allows further noise reduction. Since the gradient components are derived from the same geologic sources, the response of each tensor component, as well as the gravity, can be predicted from the other components. By using the full tensor, up to six independent measurements (the five tensor components and the gravity) can be used to predict and remove noise in the signal.
The full tensor data provides horizontal measurements to better determine lineaments such as fault zones, dykes, buried river channels, etc. in addition to the vertical data for targeting specific features.