Seismologist John Collins and geologist Susan Humphris with an Ocean Bottom Seismometer (OBS). OBSs are placed on the seafloor in precise lines or grid patterns and are usually left in place for several months to over a year. The orange plastic "hard hats" house instruments that record ground movements. Many OBSs are also fitted with a hydrophone, or underwater microphone, that records pressure (sound) waves, which may be made by ground movements, ships, or whales. (Tom Kleindinst, Woods Hole Oceanographic Institution)[back]
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