In June 2003 a series of acoustic propagation experiments were conducted off the coast of Panama City, Florida. The experiments were designed to measure and provide an understand of signal phase and amplitude fluctuations, and signal spatial and temporal coherence over several large horizontal and vertical arrays. The propagation measurements were conducted in a water depth of 8.8m and at ranges of 70 m and 150 m. The acoustic measurements cover frequencies from 1 to 140 kHz. The propagation measurements were supported by data obtained by wave rider buoys, CTD's, thermister chains and current meters. Bottom penetration data was also obtained using a buried hydrophone array. The experiments will be outlined and the data sets described.
The effects of variations in water sound-speed parameters on the performance of four matched-field processing algorithms (Bartlett, maximum likelihood, sector focusing, and multiple constraint) have been investigated. The SNAP propagation model was used to generate the replica acoustic pressure field for a shallow-water channel with a depth variable sound-speed profile typical of a midlatitude summer environment. It was also used to simulate a ‘‘detected’’ field due to an acoustic source. These were then correlated using the four algorithms for selected degrees of mismatch of the water sound-speed profile. The maximum likelihood and multiple constraint estimators achieved good peak resolution and high accuracy for small degrees of mismatch. The maximum likelihood estimator deteriorated quickly as the mismatch increased. The multiple constraint performed significantly better, but both failed to give correct location estimates at the higher mismatch values. In contrast, the sector focusing estimator continued to give accurate location estimates over the whole range of sound-speed mismatch, with slightly less peak resolution at small mismatch values. It also performed much better overall than the Bartlett estimator, which was found to give accurate location estimates over the whole range of mismatch values used, but with poor peak resolution against a background containing many high sidelobes. [This work was supported by the Naval Res. Lab.]
The effects of variations in water sound-speed parameters on the performance of four matched-field processing algorithms (Bartlett, maximum-likelihood, sector-focusing, and multiple constraint) have been investigated. The SNAP propagation model was used to generate the replica acoustic pressure field for a shallow water channel with a depth variable sound-speed profile typical of a midlatitude summer environment. It was also used to simulate a ‘‘detected’’ field due to an acoustic source. These were then correlated using the four algorithms for selected degrees of mismatch of the water sound-speed profile. The maximum likelihood and multiple constraint estimators achieved good peak resolution and high accuracy for small degrees of mismatch. The maximum likelihood estimator deteriorated quickly as the mismatch increased. The multiple constraint estimator performed significantly better, but both failed to give correct location estimates at the higher mismatch values. In contrast, the sector focusing estimator continued to give accurate location estimates over the whole range of sound-speed mismatch, with slightly less peak resolution at small mismatch values. It also performed significantly better overall than the Bartlett estimator, which was found to give accurate location estimates over the whole range of mismatch values used, but with poor peak resolution against a background containing many high sidelobes.
During the month of June, 2003, the Naval Research Laboratory conducted a series of coherence experiments in shallow water (approximately 9 meters) off Panama City Beach, Florida. Examined here are preliminary mid frequency (1-10 kHz) results of analyzed temporal coherence data. For this experiment, a G34 omnidirectional source, mounted approximately 2.7 meters from the bottom, ensonified a vertical and horizontal array of hydrophones mounted on a submerged tower 70 and 150 meters down range in the along shore direction. Results will be shown for both macro (5 minutes), and micro events (<20 seconds).
A fine-scale acoustic bottom scattering experiment is planned for the summer of 1993 on the Mid-Atlantic Ridge flank just north of the Kane Fracture Zone as a part of the Acoustic Reverberation Special Research Program (ARSRP). The site (or sites) of the detailed experiment will be about 5 × 5 km. The experimental design includes the use of four vertical line arrays moored near the bottom within the site. Several factors complicate the geometry of the experiment, one of which is that the scattering patch will be in the near field of the arrays. This will require the use of focused beamforming for viewing a scattering patch within the experimental site. Added difficulties arise in the fact that, within the experimental specifications, the arrays may not be straight and vertical and the bottom may not be horizontal. An algorithm has been formulated for the phase shifts required at elements along a distorted vertical line for focusing on points on a sloped flat plane for use in processing the data to arrive at a valid value for scattering strength. Presented are calculations that show effective “beam patterns” in the experimental geometry and other geometric considerations affecting the experiment and the calculation of scattering strength. [Work supported by ONR.]
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