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To measure the thickness of oceanic crust near the East Pacific Rise, WHOI scientists measured the velocity of airgun-generated seismic waves that travel through the crust, refract off rock layers, and are recorded by ocean bottom seismometers. A single seisimic wave from a single airgun shot consists of several phases, or rays, depending on the number of layers it penetrates. Pg rays (blue) are refracted within the crust and provide information on the speed of waves traveling through it. PmP rays (orange) reflect off the Moho (the mantle-crust transition boundary) and provide data on crustal thickness. Pn rays (green) are refracted in the upper mantle and offer data on the speed of waves traveling through the mantle. Only one of every 10 rays is plotted.
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