Big improvements in gravity survey technology
Feature Articles, Feb  27  2009 (Digital Energy Journal)

- Big improvements in gravity survey technology means that it is being used to determine the best prospects to drill, not just to get a quick overview of the potential of a region

Gravity surveys are an essential part of explorationist’s tool kit. Being able to measure the gravitation signal from the earth helps determine the rock’s density and thereby creating a picture of the subsurface geology. But conventional airborne gravity surveys have their limitations, says industry specialist ARKeX. With low signal bandwidth and a low signal to noise ratio, conventional gravity surveys are good at mapping geology on a regional or basin scale, but not down to prospect (potential drilling target) level.

ARKeX is utilizing a new technology called Gravity Gradiometry to obtain ultra high resolution data with a high bandwidth and a high signal to noise ratio. The resulting information is then used to map the geology down to prospect level and show features that would be invisible to conventional gravity surveys.

One of the problems with conventional airborne gravity surveys is that it is very difficult to correct for the acceleration of the aeroplane in the gravity reading. A conventional gravity surveying device (explained simply) is a weight hanging on a spring – the greater the gravitational pull, the more the string stretches. The sensor needs to be very sensitive to detect the precise changes in gravity, which indicates the density of the rock beneath.

Unfortunately, acceleration of the plane in different directions will also impact how much the spring is stretching. In order to correct for that, you need to know how much, and in which direction, the plane is accelerating. This is done using GPS (global positioning satellite) but it is not a precise correction so the final data contains a lot of ‘noise’ and a lot of the detail is lost.

Gravity Gradiometry, by contrast, doesn’t measure gravity, but the gravity gradient. That is the rate of change of gravity over a unit distance. Again, explained simply, it uses two separate weights on two separate springs, one above the other. They move in time with the aeroplane (or ship) so any acceleration experienced by the weights is common to both. If however, there is a change in gravity (due to a rock structure beneath), the distance between the weights will change and this is what is measured. This is the gravity gradient. Because gravity gradiometry can record minute gravitational changes, it can map structural rock density in such high resolution, features that conventional gravity shows as noise can be seen as distinct features.

Gravity gradiometry technology will shortly be improved even further, with a new device called the EGG (Exploration Gravity Gradiometer), which uses superconductivity. This will be even more sensitive and be able to map an even wider range of geologies.

“It’s phenomenally sensitive,” says Dr. Mark Davies, Chief Scientist with ARKeX, one of the leading companies in the field. “The EGG will be able to measure rock structures with small density contrasts, which would be impossible with today’s technology.”

Gravity Gradiometry has already been used extensively in North America, Africa and the Middle East. In these areas it has proved to be extremely useful across many exploration settings. In a mountainous Thrust Belt region of Muskwa Kechikia, British Columbia (notoriously difficult and expensive to survey with seismic technology), gravity gradiometry successful showed why a Major oil company drilled a dry well in the region and where the main structure could have been found. It has also been used to map salt bodies in West Africa, again extremely difficult using seismic.

“The technology is starting to become accepted in the industry. Most [of our customers] are on their 2nd or 3rd survey,” says Dr. Davies.

Gravity Gradiometry looks to have a bright future in oil & gas exploration. High resolution data that can be acquired at low cost, and when mounted in a plane, can fly over difficult terrain that would hinder other exploration, means Gravity Gradiometry is becoming an essential tool for independents and majors alike.

Mark Davies was speaking at the OilVoice geophysics forum in London on January 21st.

Arkex

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