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.