| Popis: |
The spreading‐loss portion of the total propagation loss of sound reflected or scattered from a rough ocean bottom is not firmly established in the literature. Many investigators of bottom backscattering assume, for non‐normal incidence, spherical spreading both from the source to the bottom and for the return path; this procedure yields a spreading‐loss correction term of −40 logR (where R is the one‐way range in yards). At normal incidence, the mirror‐image solution is generally assumed for bottoms which are not too rough relative to the wavelength, resulting in a spreading loss term of −20 log (2R). To clarify the actual range dependence of the returning echo, experimental measurements were made in a laboratory tank on the echo from randomly rough model surfaces having known statistical properties. Data are available for normal incidence at 100 kHz. The echo from a plane surface was checked and confirmed to result in mirror‐image behavior, with a spreading loss of 6 dB over the return path. The echo from a random rough surface having surface irregularities comparable to a wavelength fluctuated greatly with range and with small changes in the particular area of surface insonified, but an average of many range dependence curves shows an echo‐amplitude decay rate of 13.5 dB for a tenfold increase in range, beginning about 2 ft from the reflecting surface. |
