Resolving meso-scale seabed variability using reflection measurements from an autonomous underwater vehicle

Abstract

Seabed geoacoustic variability is driven by geological processes that occur over a wide spectrum of space-time scales. While the acoustics community has some understanding of horizontal fine-scale geoacoustic variability, less than O(10(0)) m, and large-scale variability, greater than O(10(3)) m, there is a paucity of data resolving the geoacoustic meso-scale O(10(0)-10(3)) m. Measurements of the meso-scale along an ostensibly "benign" portion of the outer shelf reveal three classes of variability. The first class was expected and is due to horizontal variability of layer thicknesses: this was the only class that could be directly tied to seismic reflection data. The second class is due to rapid changes in layer properties and/or boundaries, occurring over scales of meters to hundreds of meters. The third class was observed as rapid variations of the angle/frequency dependent reflection coefficient within a single observation and is suggestive of variability at scales of meter or less. Though generally assumed to be negligible in acoustic modeling, the second and third classes are indicative of strong horizontal geoacoustic variability within a given layer. The observations give early insight into possible effects of horizontal geoacoustic variability on long-range acoustic propagation and reverberation.

Seabed Characterization Using Acoustic Communication Signals on an Autonomous Underwater Vehicle With a Thin-Line Towed Array

“…However, the level of the stacked peak is not expected to be a good measure of the individual correlation peaks, because it is known from experience that bottom reflections contain a significant component of random spatial variability. At lower frequencies below approximately 10 kHz, mesoscale spatial variations O(1 to 10 ) m as mentioned in [6] will have a significant effect. At higher frequencies, the causes of variability include smaller scale variations O(10 to 1) m, such as interface roughness and volume inhomogeneities [12], variations in the bulk sediment properties, and bottom dwelling biological organisms [13].…”

“…Sound is projected at the seabed and the reflected signals are received by the line array and processed to measure the reflected signal, which is then used to invert for the properties and layering within the seabed. Recently, the source and receiving array have been miniaturized and deployed on an autonomous underwater vehicle (AUV) [4]- [6]. Instead of a dedicated sound source, sources of opportunity, such as the self-noise of the tow ship, have been used in conjunction with a matched-field technique to achieve a model-based inversion of the sediment strata [7].…”

Seafloor sound-speed profile and interface dip angle characterization by the image source method

“…Some geoacoustic inversion methods employ long-range (km to tens of km) propagation from a single receiver 1,2 or from a vertical array. 3,4 Other techniques [5][6][7] use shorter ranges to isolate a single interaction with the seabed. These techniques generally use inverse methods to estimate the geoacoustic properties.…”

Layer filtering for seafloor scatterers imaging

“…2,4 The array is 30 m long with 26 receivers spaced every 1.2 m. The first hydrophone is 10 m behind the source. Source and receivers are 20 m above the seafloor (Fig.…”