Acoustic tomography in the autonomous oceanographic sampling network

Abstract

Ocean acoustics plays a central role in the new autonomous oceanographic sampling network (AOSN) concept. Thus, as in most other underwater systems, acoustics provides the only reliable means for wireless communication and navigation. Also, the sensor suite applied in the AOSN includes various sonars for object avoidance and imaging, and acoustic Doppler current profilers. By adding a tomographic component, the performance of the AOSN in highly dynamic littoral environments can be significantly improved. Thus, the tomography can provide a low resolution, but also a synoptic image of the oceanography which can eliminate the space-time aliasing inherent to the point measurements made by the AUVs. Using real-time processing the tomographic map is used to adaptively focus the AUV measurements in areas with high gradients or large uncertainty. In addition, the AUVs can enhance the tomographic coverage by equipping them with acoustic sources. The feasibility of this acoustically focused oceanographic sampling (AFOS) concept was investigated in the June 1996 Haro Strait PRIMER experiment. The experiment clearly demonstrated the feasibility of the tomographic components. Robust and efficient algorithms have been developed, capable of performing real-time tomographic inversion within a littoral AOSN. The results of the experiment are reviewed, including frontal mapping performed in more than 80 AUV missions. The experiment also clearly demonstrated the severity of the littoral acoustic environment in terms of ambient noise, reverberation, and variability, significantly affecting the robustness of the acoustic communication and navigation systems crucial to the AOSN. [Work supported by ONR.]