Acoustic remote sensing for source localization and atmospheric tomography: Applications of the cross-correlation Green's function retrieval method.

In this work, the application of the cross-correlation Green's function retrieval method for source localization and atmospheric acoustic tomography is presented. Open field experimental measurements of an acoustic source, with an impulsive waveform, are conducted for the evaluation of an array system. Of particular interest are the source localization and sound speed estimation capabilities of the array system. The cross-correlation delay-and-sum beamformer is used to estimate source directivity and sound speed. This beamformer inherently employs the cross-correlation Green's function retrieval method between a pair of receivers. The beamforming results adequately identify the various source directions as well as the scatterers along the propagation path. Reasonable sound speed estimates are obtained at the peak frequency of the retrieved Green's functions. In the case of atmospheric acoustic tomography, the estimated sound speed from the array system can serve as an average background sound speed in a tomographic inversion algorithm. Utilizing a tomographic inversion algorithm with radial basis functions and the estimated sound speed, the reconstruction of temperature and wind velocity profiles are demonstrated.

Green's function extraction by crosscorrelation and multidimensional deconvolution for outdoor sound propagation.

In this work, the Green's function is estimated from outdoor measurements of controlled-sources. Crosscorrelation and multidimensional deconvolution have successfully been employed for Green's function retrieval. Crosscorrelation assumes a lossless medium and equipartitioned wavefield; when these assumptions are not satisfied it may result in a Green's function smeared with the source point-spread function. Multidimensional deconvolution removes the point-spread function from the retrieved Green's function. Both methods are employed to estimate the Green's function between two array stations for a single and multiple controlled-sources. The results demonstrate that the source-to-center radius has a negligible effect on the retrieved Green's function, if the source-to-center radius is larger than the distance between the two array stations.

Direct on-strip analysis of size- and time-resolved aerosol impactor samples using laser induced fluorescence spectra excited at 263 and 351 nm.

We report a novel atmospheric aerosol characterization technique, in which dual wavelength UV laser induced fluorescence (LIF) spectrometry marries an eight-stage rotating drum impactor (RDI), namely UV-LIF-RDI, to achieve size- and time-resolved analysis of aerosol particles on-strip. The UV-LIF-RDI technique measured LIF spectra via direct laser beam illumination onto the particles that were impacted on a RDI strip with a spatial resolution of 1.2mm, equivalent to an averaged time resolution in the aerosol sampling of 3.6 h. Excited by a 263 nm or 351 nm laser, more than 2000 LIF spectra within a 3-week aerosol collection time period were obtained from the eight individual RDI strips that collected particles in eight different sizes ranging from 0.09 to 10 µm in Djibouti. Based on the known fluorescence database from atmospheric aerosols in the US, the LIF spectra obtained from the Djibouti aerosol samples were found to be dominated by fluorescence clusters 2, 5, and 8 (peaked at 330, 370, and 475 nm) when excited at 263 nm and by fluorescence clusters 1, 2, 5, and 6 (peaked at 390 and 460 nm) when excited at 351 nm. Size- and time-dependent variations of the fluorescence spectra revealed some size and time evolution behavior of organic and biological aerosols from the atmosphere in Djibouti. Moreover, this analytical technique could locate the possible sources and chemical compositions contributing to these fluorescence clusters. Advantages, limitations, and future developments of this new aerosol analysis technique are also discussed.