Demonstration of a Technique for Rapid Inversion of Geo-Acoustic Properties of Littoral Marine Locations
NAVAL RESEARCH LAB STENNIS SPACE CENTER MS GEOLOGY/GEOPHYSICS SECTION
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This report describes how to invert seismic data collected with sonobuoys in littoral marine environments to obtain sea floor geo-acoustic properties. The inversion method is an adaptation of simulated annealing SA which can rapidly and automatically find the best Earth model for each data set. The established SA algorithm was enhanced with envelope fitting and layer stripping. The first example is a synthetic data set to demonstrate the method for a known Earth model. The second example is at a 750 m water depth site using a watergun source that has a very clean and sharp pulse. The offsets were accurately determined prior to the inversion calculation for this case and a clear subbottom reflector was modeled in only 5323 iterations. The third example was at a site with 250 m water depth using an airgun source. The direct airgun signal saturated the sonobuoys at small offsets causing distortions in the waveforms and amplitudes. Data offsets could not be accurately measured and had to be included in the inversion problem which tripled the number of variables. This more difficult third example was solved with 35,421 iterations. This demonstration is with highly data. The receivers are sparsely and irregularly spaced and have too limited a dynamic range for the source strengths and sea floor reflections encountered. However, by adapting the inversion technique to the challenges presented by the field data, the geoacoustic parameters of the sea floor were measured using rapidly deployed inexpensive and expendable equipment. The geoacoustic properties that I modeled include the sea floor depth, the water sound velocity and the compressional wave velocity of the uppermost 150 m of sediment. In the last case I also inverted for the sonobuoy to airgun offsets.
- Physical and Dynamic Oceanography
- Geology, Geochemistry and Mineralogy