| Popis: |
A preliminary attempt has been made to determine to what extent ambient noise levels in a relatively large isolated body of water are accounted for by seismic disturbances communicated to the water through the land‐water boundary. Using the Naval Electronics Laboratory barge facility at Lake Pend Orielle, measurements of ambient noise pressure levels in the frequency range from 11 to 250 cps have been made under various surface conditions, in water of 750‐ft depth. Of particular interest is the lowness of the levels recorded during a rare period when wind and surface waves were absent: e.g., at a depth of 147 ft, using a half‐octave filter with a center frequency of 63 cps, the mean of four individual 1‐min averages of the pressure level taken over an 11‐min interval was 2.8×10−3 dyne/cm2/cycle band width, about a factor of ten smaller than corresponding typical levels in the ocean for low sea states. The results suggest the possibility that for remote inland water bodies under average weather conditions, contributions to the ambient noise levels from the earth together with those conceivably associated with the medium itself are relatively minor, in comparison with those contributions arising from wind‐wave interaction at the surface. (This work has been supported by the Office of Naval Research.) |
