Gravity Gradiometry

Gravity Gradiometry Imaging (GGI) is not a new technique; however it was not until 1994 that the previously classified U.S. Defence Dept. 3D full tensor gradiometer was trialled in the Gulf of Mexico for commercial use. Since these initial marine trials several systems have been deployed. Now built by Lockheed Martin, ARKeX operates three such systems for airborne and marine use.

Combining high bandwidth and high resolution imagery with an airborne or marine platform, makes GGI an obvious choice for the challenges of today’s exploration.

Why Gravity Gradiometry?

So what are the advantages of this once classified military technology over conventional gravity?

While a conventional gravity survey records a single component of the three-component gravitational force, usually in the vertical plane, Full Tensor Gravity Gradiometry (FTG) measures the derivative of all three components in all three directions. The method measures the variation of the vertical component of the gravitational force in the vertical direction and in two horizontal directions and, similarly, it measures the variation of the horizontal components of gravity in all three directions. Figure 1 shows the nine tensors of gravity gradiometry.

Figure 1: Conventional Gravity measures ONE component of the gravity field Gz (LHS) Gravity Gradiometry measures ALL components of the gravity field (RHS).

Although there appear to be nine components of the gravity gradient tensor, there are only five independent components. Firstly the tensor is symmetric as the order of differentiation of a scalar quantity does not matter (implying, for example that G_xy= G_yx).

The ability to measure five spatially independent gravity components has some significant advantages over conventional gravity measurements, which only recovers the vertical component (G_z); the multi component measurement helps to constrain the non-uniqueness of conventional potential fields.