Abstract

This article describes an approach for using broadband acoustic data to determine the depth dependence of the compressional (P) velocity in the upper several hundred meters of the seafloor in deep water areas. The technique used here does not rely upon ‘‘picks’’ of arrival time from acoustic data from many ranges or upon deconvolution of the received signal. Rather, the waveform of the first bottom bounce arrival at a single range is simulated and R (the ratio of the P-wave velocity of the sediment to the water sound speed) and g (the gradient of P-wave velocity) are adjusted to maximize the similarity between the measured and simulated signals. A low-frequency band is used to reduce complications caused by small scale layering in the seafloor and a long-range source is chosen to avoid acoustic interaction with the substrate. For an objective measure of similarity, Cmax, the maximum magnitude of the normalized cross-correlation function of measured and simulated time series is used. In the implementation discussed in this article, the simulated time series were produced by a ray-based computational model that includes eigenrays that travel through the seafloor. It was found that the dependence of Cmax on R and g is strong enough to allow precise estimates (within 10%) of their values. The procedure begins with waveforms simulated for the reported experimental geometry, measured sound velocity profile, measured or calculated source waveform, and initial estimates of geoacoustic parameters. First, the geometrical parameters are refined by maximizing Cmax with respect to the range and depth of source and receiver, and then the P-wave parameters are obtained by maximizing with respect to R and g. The method is demonstrated by obtaining R and g from explosive source data collected in an abyssal hills environment in the Pacific Ocean. Maximizing Cmax in the 25- to 250-Hz band yielded estimates of R and g that agree with reported values.

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call

Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.