Full Tensor Gradient Gravimetry

Textbooks, MSc courses and proponents have long argued the case for potential field, i.e. gravity and magnetic, techniques as a reconnaissance oil & gas exploration tool, especially the air-borne versions. Unfortunately, I think it’s the experience of explorers that the promise is almost never fulfilled, the lack of resolution and the interpretation ambiguities not at all being compensated for by the cheapness (and what does magnetic susceptibility have to do with petroleum anyway!).

Until now!

Full tensor gravity gradiometry (FTG) brings vastly superior resolution and definition to the potential field arena; this is important as it offers a relatively inexpensive regional screening tool that can be used onshore, focusing thinking prior to committing to seismic data acquisition (which can be an order of magnitude more expensive onshore than offshore).
FTG technology derives from initial US military applications – as I recall, for submarine warfare, detection of underground 'bunkers' etc – in which highly sensitive gradient gravimeters are deployed in stable platforms, at sea or air-borne. While a conventional gravity survey records a single component of the 3-component gravitational force, FTG measures the derivatives of all 3 components in all 3 directions, yielding 9 tensors from which can be derived 5 independent components. Compared with conventional gravity, the extra independent components reduce the ambiguity inherent in potential field methods; from the maths it can be shown that FTG measurements are richer in shorter wavelength data and also the differential measurements deal far better with the effects of a moving platform (a plane or a ship*),yielding more accurate shorter wavelength data. The shorter wavelengths are key to subsurface description at the depths of interest for petroleum exploration.

A handful of companies have developed a profile in providing FTG services to the oil & gas (and also the mining) industry, two of these being Bell Geospace and ArkEx. Bearing in mind that they first needed to replicate what the military had achieved, this has taken them a while. However, there are now regular reports of FTG surveys being implemented; for example, back in April, Bell announced that they were going to undertake a FTG survey for NAM in the Dutch sector of the North Sea, the aim being to ‘gradient optimize’ 3D seismic data to image the tops and flanks of salt masses within the sector, interpretation of the ‘gradient optimized’ 3D seismic data being done jointly by NAM, Shell and Bell Geospace.
Now to be perfectly honest with you, what ‘gradient optimize’ actually means escapes me! Perhaps they mean that if you are working in a very mature area where you have tons of 3D seismic, it’s certainly best to integrate the FTG and the seismic to get the best out of both?

However, what I hope I’ve made clear is the exciting possibility of using FTG onshore, to focus where expensive seismic should be shot.

Or maybe not so expensive; see my next blog!