Bistatic Backscattering of Low-Frequency Underwater Sound from the Ocean Surface

Abstract

The bistatic scattering strength S(φi,φs) = 10 logs (φi,φs) of the ocean surface has been measured in 12-oct bands over the frequency range 62.5–4000 Hz for a wide range of grazing angles of incidence and scattering, φi and φs. Explosive charges at the bottom of a vertical string of hydrophones were used to ensonify the surface. Concurrent measurements of surface roughness were taken with an accelerometer float. S was obtained for rms waveheights Ĥ between 1.9 and 2.8 ft and rms omnidirectional slopes tanΘ̂ between 0.10 and 0.13. For frequencies between 62.5 and 1440 Hz, the bistatic scattering coefficient s may be represented in the form s=CeD[sinφi+sinφ2]2, where C and D are functions of the surface roughness parameters kĤ and tanΘ̂. The functions have one form when surface shadowing of the scattered radiation is weak (φi > Θ̂, φs > Θ̂), and another form when surface shadowing is strong (φi > Θ̂, φs ⪅ Θ̂).

Simulation of wind-generated surface waves and effects of bubbles on scattering, transmission, and attenuation of low frequency sound at the sea surface

“…Another approach in this area which is based on the classical physics methods was proposed by pioneers like Rayleigh, Helmholtz, Kirchhoff and Born [71]. To provide experimental data in this regard, Chapman and Harris [72], among others [73][74][75], conducted extensive measurements of surface scattering strength using omni-directional hydrophones and by applying explosive sources for low frequencies in spectral region from 400 to 6400 Hz. Also, the sea surface backscattering strength at low frequencies (70-1000 Hz) have been presented by Ogden and Erskine [76,77] through critical sea tests (CST), covering a variety of wind speeds from 3.0 to 17.5 m/s as well as mean grazing angles from 5° to 30°.…”