Shake it. Seismic vibrator trucks provide a source of low-frequency sound waves for imaging of oil and gas deposits.
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Focus: A Better View Underground

Published February 22, 2013 | Physics 6, 21 (2013) | DOI: 10.1103/Physics.6.21

A new technique allows improved acoustic imaging of oil deposits and other underground structures and may also work for medical imaging.

A new algorithm will allow a cleaner view of subterranean structures using sound waves. A team that previously proposed an imaging technique for an idealized arrangement of geological layers now presents a generalized theory that would be practical for almost any situation. Their technique should allow improved imaging of underground water reservoirs, plumes in the earth’s mantle, or even non-geological systems, such as the innards of airplane wings or human bodies.

To image an underground structure such as an oil or gas deposit, geophysicists can use acoustic waves produced by one or more seismic trucks, vehicles that generate strong vibrations that propagate kilometers into the ground. Instruments underground or at the surface detect the reflections from these waves, allowing researchers to construct an image—essentially a subterranean sonogram.

But the ideal acoustic source would be much closer to the underground structure, just as a flashlight works better up close to an object than from far away. So researchers have developed so-called virtual sources, a technique for reorganizing the reflection data to make it seem as though there is a single, strong acoustic source kilometers below ground, near the structure under study. And the virtual source can be easily moved to different locations—in effect, shining a flashlight all around the layers of earth, getting a close look at the nooks and crannies. The mathematical description of virtual sources includes the concept of time-reversal, where the waves that travel down to a location in the ground can be seen as being emitted from that spot instead.

Read more: Physics – A Better View Underground.

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