Turkmenistan is in the news this week after confirming that its South Yolotan-Osman natural-gas field is among the five biggest in the world. That assertion is based on a survey done by an independent British firm.

Such estimates are of vital importance to energy-rich countries, as they cannot plan their national strategies without some idea of what resources they have. So how do surveyors determine how much wealth lies under their feet? And just how accurate are these estimations anyway?

Knowing you have crude oil or natural gas on your territory is one thing, calculating how big your reserves are is quite another.

Suppose that your geologists have used seismic imaging to pinpoint the likelihood of oil. That's a technique in which sound waves are fired into the ground, and the reflected waves allow the geologists to build a picture of the rock types underground. Oil will usually be found trapped in porous rock formations covered by impenetrable rocks.

So your seismic imaging has indicated promising geological conditions, and you drill an exploratory well. That produces oil, and you think you might have a real bonanza.

Manochehr Takin, an analyst with the Center for Global Energy Studies in London, cautions that this is where things start to get complicated.

"It is important to know what is the size of the oil and gas deposit you have discovered, because when you drill a well into the subsurface and find oil, it's as though you have this huge mass of [material, like a haystack,] and you have one needle, one long needle which goes down and gives you some information, but how do you know the size of this haystack?" Takin says.