| Popis: |
At previous meetings it was shown that when a water wave breaks onshore a seismic pulse, or “surfseism,” is communicated to the sandy bottom. The spectral character of pulses measured on the beach at Fire Island, New York, were accounted for in terms of statistical parameters characterizing the surf “noise generator” and geophysical parameters characterizing the intervening “earth filter.” Using experimental values derived from these beach measurements, an estimate has been made of the absolute output levels of the surf noise generator under certain physical assumptions which will be discussed. Expected pressure spectrum levels from pounding surf were computed at several ranges offshore using a simple two‐layered liquid half space as an approximation to coastal conditions off Fire Island. Between 22.5 and 65 cps, where only first mode propagation need be considered, the spectrum levels produced by breakers several feet in height at a range of 1000 ft from shore are less than the extrapolated values obtained from Knudsen's spectrum level curve for the ocean at zero sea state. For coastal areas of the type considered here, surfseism, contributions to the ambient spectrum a few thousand feet offshore appear to be completely negligible. The effect of different geophysical conditions on these contributions will be discussed in conjunction with the specific calculations. (This work was supported by the Office of Naval Research.) |
