Volume 123, Issue 4, April 2008
Index of content:
- UNDERWATER SOUND 
123(2008); http://dx.doi.org/10.1121/1.2875420View Description Hide Description
Measurements of the phase velocity and attenuation of sound in concentrated samples of bubblygels are presented. Hair gel was used as a matrix material to obtain well characterized distributions of bubbles.Ultrasonicmeasurements were conducted over a large range of frequencies, including the resonance frequencies of the bubbles. Surprisingly good agreement with Foldy’s prediction was found, even for monodisperse samples at resonance frequencies, up to volume fraction of 1%. Beyond this concentration, the effects of high-order multiple scattering were observed. These results support the feasability of ultrasonic techniques to investigate the size distribution of bubbles in a weak gel or liquid.
Acoustics of marine sediment under compaction: Binary grain-size model and viscoelastic extension of Biot’s theory123(2008); http://dx.doi.org/10.1121/1.2871839View Description Hide Description
This paper presents a model of acoustic wave propagation in unconsolidated marine sediment, including compaction, using a concept of a simplified sediment structure, modeled as a binary grain-size sphere pack. Compressional- and shear-wave velocities and attenuation follow from a combination of Biot’s model, used as the general framework, and two viscoelastic extensions resulting in complex grain and frame moduli, respectively. An effective-grain model accounts for the viscoelasticity arising from local fluid flow in expandable clay minerals in clay-bearing sediments. A viscoelastic-contactmodel describes local fluid flow at the grain contacts. Porosity, density, and the structural Biot parameters (permeability, pore size, structure factor) as a function of pressure follow from the binary model, so that the remaining input parameters to the acoustic model consist solely of the mass fractions and the known mechanical properties of each constituent (e.g., carbonates, sand, clay, and expandable clay) of the sediment, effective pressure, or depth, and the environmental parameters (water depth, salinity, temperature). Velocity and attenuation as a function of pressure from the model are in good agreement with data on coarse- and fine-grained unconsolidated marine sediments.
Spatial averaging of oceanic rainfall variability using underwater sound: Ionian sea rainfall experiment 2004123(2008); http://dx.doi.org/10.1121/1.2871485View Description Hide Description
An experiment to evaluate the inherent spatial averaging of the underwater acoustic signal from rainfall was conducted in the winter of 2004 in the Ionian Sea southwest of Greece. A mooring with four passive aquatic listeners (PALs) at 60, 200, 1000, and was deployed at 36.85°N, 21.52°E, west of a dual-polarization X-band coastal radar at Methoni, Greece. The acoustic signal is classified into wind, rain, shipping, and whale categories. It is similar at all depths and rainfall is detected at all depths. A signal that is consistent with the clicking of deep-diving beaked whales is present 2% of the time, although there was no visual confirmation of whale presence. Co-detection of rainfall with the radar verifies that the acoustic detection of rainfall is excellent. Once detection is made, the correlation between acoustic and radar rainfall rates is high. Spatial averaging of the radar rainfall rates in concentric circles over the mooring verifies the larger inherent spatial averaging of the rainfall signal with recording depth. For the PAL at , the maximum correlation was at , suggesting a listening area for the acoustic rainfall measurement of roughly .