Views from space help oil prospectors see deep underground

Views from space help oil prospectors see deep underground



It takes seismic force to make the ground give up its secrets. Through the years, those searching for oil and gas have used varied methods to send sound energy into the ground and to record the waves reflected by the geological features beneath the surface.

Modern methods include large vibrator trucks and many thousands of surface sensors called geophones, all precisely located to obtain the most useful information with which to explore for hydrocarbons. Today, seismic surveys planned with satellites are yielding clearer, deeper subterranean views at reduced cost. Often carried out in the remotest parts of the planet, these surveys are almost military in scale and expense; a seismic crew exploring a 500-square-kilometre area can require 400 people with up to 50 small and 15 large vehicles working with up to 600,000 geophones, and carrying out 600 seismic ’shots’ daily.

Seismic surveyor WesternGeco, has been working with ESA for the last three years to integrate satellite data into its working practices. What Earth Observation can provide is a detailed preview of a region’s topography and geology, valuable for assessing areas that will produce the best and worst seismic quality – meaning the sending and receiving of vibration signals – far in advance of commencing the survey. "Working on the surface, we deliver imaging and structural characterisation of the subsurface, down to 6000 metres or deeper," says Andreas Laake of WesternGeco. "Technology has moved on since the days of heavy explosives, but the principle remains the same."

Elastic waves are excited at the surface and propagate through the subsurface, partly transmitting, partly reflecting, and partly scattering. The reflected waves are then detected on the surface by a pre-planned array of geophones. Sophisticated processing of these sensor data creates a three-dimensional picture of the underlying geology of the survey area. "The modern vibroseis technique has spatial resolution sufficient not just to identify oil and gas reservoirs, but also to show internal details such as their fracture geometry," Laake adds. "This is vital, because our customers do not make money for the amount of hydrocarbons theoretically in the ground, but what they actually recover.” "The vibroseis method uses trucks with heavy masses and baseplates that vibrate the ground to provide a far more controlled source," Laake explains. "Depending on the target, the trucks can be tuned to work across a pre-defined frequency spectrum, providing ’multicoloured’ views in terms of elastic waves." ...cont'd


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