Noise modeling, noise experiment, and noise inversion

Abstract

Recent developments in inverting ambient noise to obtain bottom reflection properties originated from an understanding of noise behavior and from noise modeling. Sea‐surface noise can be modeled equivalently as a ray, wave, or mode phenomenon, but often the ray or flux approach provides the most insight (explaining, e.g., the noise‐notch or low‐frequency noise enhancement). Theoretically, noise vertical directionality is surprisingly sensitive to the effective depth of the sheet of sound sources, and this is borne out in the spread of behaviors found experimentally. A number of noise inversion approaches have been proposed based on coherence or directionality of sea‐surface noise, individual ships or shipping, and airplane doppler. Using a drifting vertical array as a means to measure directionality, one can infer reflection coefficient versus angle and frequency. One can then use this directly for propagation calculation, or alternatively invert to geoacoustic parameters with the help of a fast noise model. A further development is to recover the reflection phase using spectral factorization, then Fourier transform to get the impulse response, which becomes a sub‐bottom profile for a drifting array. At 4‐kHz design frequency, the layer structure thus determined compares well with a boomer down to 15 m.