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Seismic surveys are carried out to support the investigation of geological formations. Marine seismic surveys enable the mapping of detailed geological features below the seabed. Shell’s proposed work program in Bulgaria includes acquisition of seismic data and detailed geological studies.
Short of drilling a well, seismic surveying produces the best data available about the oil and gas potential of an area. That makes it a key part of oil and gas exploration and development. A seismic survey produces images of underground rock formations that specialists can analyse to determine whether they are likely to contain oil or gas. The information can also show where best to drill a well.
Seismic technology enables us to see through solid matter in the same way ultrasound helps us see an unborn baby. The process involves sending sound waves into the earth. We measure how long it takes for the rock layers underneath to reflect the waves back to the surface. This measurement can indicate the characteristics of the rocks, just like a ball bounced on a hard floor will rebound faster than from a carpet. Seismic waves reflecting off dense rock layers will behave differently to those that rebound from porous rock.
Understanding the density of rocks is crucial in the search for oil and gas. Dense rocks on top of softer rocks can form traps in which oil and gas might be found. These traps are required for further exploration; without them, any oil or gas within the rocks would simply have seeped towards the surface and dissipated over the millions of years since it formed. The computerised maps produced by a seismic survey are analysed by geologists and geophysicists to look for these structural traps that could contain oil and gas.
Seismic on land and sea
The method for conducting seismic surveys is based on the same principle whether on land or at sea, although the tools we use are different. On land a seismic crew – which could number from a few hundred to a few thousand people depending on the area being surveyed – will set out a line or grid of acoustic listening devices called geophones. Typically we use GPS technology to precisely arrange thousands of geophones per square kilometre.
The sounds that travel underground are usually generated by either small explosive charges set off individually at intervals; or through vibrations made by a truck-mounted vibrating plate, which pounds the surface of the ground to create the sound waves. As these sound waves hit successive layers of rock under the ground, part of their energy is reflected back to the surface and is picked up by the geophones, which convert the waves into electrical signals.
At sea we conduct the surveys firing an air-gun towed behind a ship, sending pulses of sound toward the bottom of the ocean. To record the echoes that bounce back, we use two methods: either we trail a string of sensitive sound recording device known as hydrophones farther astern; or we lay out a grid of hydrophones on the seabed. Our project in Bulgaria uses the former method – trailing a string of hydrophones behind a specifically built for that purpose ship.
The seismic data is computer processed after being recorded. Analyses of the processed seismic data allow for interpretation of subsea geological formations.
During surveying, vessels travel at a speed of four to six knots. The sound source is situated some 300 m behind the vessel at a depth of approximately 7 meters below the surface. A seismic vessel would typically be active for several weeks, depending on size of the seismic survey.
Visualising and Interpreting
Processing the recorded signals is a highly specialised job involving complex mathematics, physics and computer analysis. Fortunately, big advances in computing power in recent years have allowed us to carry out much more detailed seismic processing. We can use three-dimensional (3-D) seismic images to create virtual reality animations revealing the thicknesses and densities of the rock in a way that can offer clues as to where oil and gas might be trapped.
The process takes time and work. For instance, the seismic processing in a 3-D survey can take between four to six months. But a computer alone cannot determine the presence of hydrocarbons. Like doctors studying X-rays in a hospital, it requires specialists who can interpret the images precisely to decide the prospects of finding hydrocarbons in the surveyed area. As we go deeper and explore in more complex subsurface structures, obtaining a clear image of the subsurface is essential in order for us to decide where to drill our exploration wells.