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Stressed-out holograms.

Stressed-out holograms

Stress isn't what makes the world go 'round, but it does make the world shake, at least during earthquakes. To study quake-producing stresses, which are simply the forces within the crust, geophysicists use a range of techniques that can determine the direction of these forces. Historically, however, it has been much more difficult to measure the size of the stress.

Now scientists are adding holograms to their bag of stress-measuring tricks--a promising technique that can resolve both the direction and size of crustal stress, says Catherine L. Smither, who is working with Thomas J. Ahrens at the California Institute of Technology in Pasadena and with Douglas R. Schmitt of Stanford University.

In a trial of this technique, Smither and her colleagues inserted a holographic camera into a borehole in the wall of a mine and took a reference picture of the area. Then, after drilling a small hole in the side of the wall, they used the same photographic plate to take a second picture of the area. Since the hole reduced the stress on the borehole wall, the wall moved a minute distance between the two pictures, creating an interference pattern on the hologram. By analyzing this pattern and repeating the technique on different sides of the borehole, Smither's group could measure the size of the stress in all three dimensions. They hope to reduce the size of the camera so that they can soon test the technique in other locations.
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Title Annotation:holograms used to measure stress in earth's crust
Publication:Science News
Date:Dec 19, 1987
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