What is Seismic Migration?

Seismic migration is a data-processing technique that creates an image of earth structure from the data recorded by a seismic reflection survey.

The oil and gas industry developed seismic reflection surveying methods between about 1970 and 1990. 40-ton shaker trucks send seismic waves into the ground, which echo from earth structures miles below. Miles-long arrays of thousands of sensors set in the ground, each like a 2-inch phonograph needle, pick up the echoing vibrations and send them to a recording truck along cables.

Industry has used simple reconstructions of these echoes, called stacked sections, to locate buried oil reservoirs and pockets of natural gas. Stacked sections are most useful in areas where the rocks are relatively uniform and uncomplicated, such as the Gulf Coast. Where the rocks are more complex, such as in the Great Basin, seismic survey techniques have not been as successful in locating deposits of oil, gas, or geothermal hot water. More complex rocks scatter the echoing seismic waves in unexpected directions, and stacked sections show subsurface features in the wrong location, if they image them at all.

The term migration comes about because, compared to stacked sections, the echoes ``migrate'' to their true subsurface position. Migration processing is only needed in geologically complex areas. This image shows features to a few miles depth below a producing geothermal power field in Dixie Valley, central Nevada. Advanced seismic migration processing of the data from previously recorded reflection surveys shows a possible reservoir of geothermal hot water caught within a complex of earthquake faults, about 21/2 miles down. Although seismic migration was the only way to make a coherent subsurface image from the data in Dixie Valley, it does introduce some artifacts. Any interpretation of migrated images has to ignore these upward-curving ``false arcs''.

Above is a very similar type of echo-shounding, a medical ultrasound image taken of a baby in the womb. The data-gathering and processing techniques are almost exactly the same. In ultrasound, of course, all the echo sensors are in a probe just an inch wide, instead of miles long; and the image represents human structures just a few inches instead of miles deep. Note that the ultrasound image has the same kind of upward-curving ``false arcs'' as the migrated earth section.

Click on any of the illustrations above for a high-resolution Adobe Acrobat PDF version, between 1/8 and 1/4 megabyte in size.


J. Louie, 2/2/97
URL of this page: http://crack.seismo.unr.edu/cemat/migration.html