Gravity surveys have been used for many years to search for...

Gravity surveys have been used for many years to search for potential oil bearing structures .
Airborne gravity surveys are more recent .
However it is a relatively cheap first pass method that is almost always followed up with seismic .

3D seismic gives much deeper insight into the target structures and after expensive manipulation of the data using high speed computers can even give some direct indications of hydrocarbons in the structure .
However 3D seismic is not completely definitive and only drilling can really prove what is there .
Plenty of structures have given very good indicators on 3D seismic but for the well to end up being non commercial .

Residual oil can give a false hydrocarbon indicator as can other non hydrocarbon geological deposits including volcanic ash and some clay particles taking up pore space and lowering permeability .
That said 3D has proved to be a great aid in assessing prospects .

Xanadu did have some sparse 2D seismic as well as the gravity data .
Given no 3D data it was a very Ballsy drill .

Let's hope the 3D says we have something special at Xanadu .