Asturias mentioned airborne gravity which has only become technically viable in the last 15-20 years with the advent of sophisticated airborne equipment and critically, highly precise GPS readings.
I was involved with an airborne survey in Chad over ten years ago. The accuracy of the survey was astounding. We had reliable gravity readings of 1 milligal, which is about one-millionth of a "g". These data were extracted from raw data that included "noise" of plus or minus 1 to 2 "g".
However, even with that accuracy, you can't use the data to choose prospective structures. We were using it to identify the edges of a Cretaceous rift basin that was buried beneath 500 m of flat-lying Tertiary sediments, after which we were able to properly position our seismic survey over the rift basin. A $500,000 survey saved us millions.
As Asturias said, this survey over Kent and Sussex will almost certainly be for some mundane purpose, like updating maps. Anti-frackers can relax!
So they know for certain what's down there and how much it will produce.
WB627, based on a whole career of looking for oil and gas and occasionally finding some, I would like to modify your statement.
So they think they know what's down there and they have guessed at how much it will produce.