Seismic mapping is a means of seeing the marine underland. A specialist ship carrying a low-frequency, high-volume air gun fires sound pulses into the water. These pulses are powerful enough to penetrate some distance into the sea floor, before reflecting back upwards to where they are recorded by seismic sensors dragged on long cables behind the ship. The blasts can occur at intervals of under a minute, for weeks or months at a time. Barely audible above the surface, they fathom the seabed. But the sound blasts also travel for hundreds of miles laterally below water, sending thunderclaps
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