Volume 111, Issue 4, April 2002
Index of content:
- GENERAL LINEAR ACOUSTICS 
111(2002); http://dx.doi.org/10.1121/1.1461836View Description Hide Description
Fine-grained water saturated sediments like silty clays have the curious property that the speed of sound through its bulk medium is lower than that of the interstitial pore fluid. When a fine-grained sediment is at the water–sediment interface, classical theory predicts that there is an angle at which the reflection coefficient is zero, and there is total transmission of sound into the seabed. This angle is called the angle of intromission and has been directly observed at the seafloor only rarely. Data from a new measurement technique show this phenomenon with remarkable clarity. The presence of the angle of intromission creates an opportunity for a direct (i.e., without search) inversion for the sediment sound speed and density. Though acoustic techniques generally do not estimate sediment density very precisely, this technique is quite sensitive to density. Geoacoustic inversion results from reflection measurements in the Straits of Sicily compare very favorably with independent “ground truth” data indicating that the method is robust.
111(2002); http://dx.doi.org/10.1121/1.1459462View Description Hide Description
An acoustic backscatter technique proposed by Oelze et al. [J. Acoust. Soc. Am. 109, 1826–1832 (2001)] was used to characterize the roughness of porous soil surfaces. Roughness estimation errors are minimized when the effective flow resistivity of the porous soil is high, e.g., above 300 000 mks Rayls/m. Four soil plots were constructed by roughening soil with farming implements. Three plots were sealed using Saran powder dissolved in methyl ethyl ketone (MEK) and then covered to prevent further weathering. A fourth plot was left in the open and exposed to rainfall, which also acted to seal the surface and further change the roughness. In sealing the surface the effective flow resistivity of the surface was increased above 300 000 mks Rayls/m, which is typical for weathered agricultural surfaces. The roughness power spectra of the soil surfaces were measured by acoustic backscatter and alternatively by a laser profiler. Regression analysis was used to approximate each roughness power spectrum versus roughnesswave number with a best-fit line. The best-fit line was used to calculate the rms height and the correlation length of the rough surface by integrating the approximate roughness power spectrum over a range of roughnesswave number values. The range of roughnesswave number values defines the roughness length scales used in the statistical calculations. High-roughness wave numbers correspond to smaller length scales of roughness and low-roughness wave numbers correspond to larger length scales of roughness. Over certain ranges of roughnesswave number values the statistics from the acoustic backscatter and laser profiler measurements is in good agreement. However, as the low-cutoff roughnesswave number is decreased and the high-cutoff roughnesswave number is increased, agreement between the laser and acoustic techniques diminishes.