Automated Broadband Acoustic Sound Velocity Profiler

摘要

Scientific Fishery Systems, Inc. (SciFish) has investigated and validated the feasibility of making consistent acoustic measurements of absorption to obtain a temperature. Historically, absorption measurements were made via long baseline, open ocean experiments where absorption was not the primary measured quantity, or via exacting laboratory conditions where absorption was examined for specific chemical, temperature or pressure dependencies. Our goal has been to make a short baseline measurement of absorption in open waters. In prior work, we have found that a narrowband approach with a high resolution ADC was technically possible but economically infeasible for the scope of the existing effort. As such, an alternative method was devised using analog electronics that could perform the essential dynamic range expansion using the differences between successive range cells. The approach eliminated some of the most serious difficulties in making this measurement General attributes of the modified system are as follows: (1) It is ambient noise limited. Thermal noise constitutes the dominant generation mechanism at these frequencies, added source level should lead directly to added range. Favorable SNRs were achieved up to 30m in range; (2) It significantly reduces the effect of target strength variation, provided sufficient volume is averaged over the range cell to include a population with the full range of specular returns. (3) It obtains a salinity profile at the points of temperature reversal that occur in all of the theoretical absorption vs. frequency curves proposed by the research community. (4) It can provide a temperature profile with < 1.5% uncertainty (3-13m range, up to 3% at ranges up to 30m) based on measurements made in the harsh (low density, small reflectors) freshwater environment. An error analysis indicates that the best previous data for absorption had an error of +/- 1.5% that results in a temperature error of +/- 0.08 deg C.