Robb, Gary Benjamin O'Neill
The in situ compressional wave properties of marine sediments
University of Southampton, Institute of Sound and Vibration Research,
The inversion of compressional wave properties is presently emerging as a technique for determining the geotechnical properties of marine sediments. However, the relationships required to perform such an inversion are still under debate, with further research required to resolve the dependence of compressional wave properties on both frequency and geotechnical properties. Though the use of in situ probes provides the most promising manner of examining these relationships, previous work in this field has encountered a number of experimental difficulties.
This work presents a series of well-constrained in situ transmission experiments. These were undertaken on inter-tidal sediments using a purpose built in situ device, the Sediment Probing Acoustic Detection Equipment (SPADE). Compressional wave properties were measured from 16 to 100 kHz in a range of sediment types (medium to fine sands and medium to fine silts), with several closely spaced locations examined at each general site to assess the local variability in compressional wave properties. Spreading losses, which were adjusted for sediment type, were incorporated into the data processing. Also included were a thorough error analysis and an examination of the repeatability of both the acoustic wave emitted by the source and the coupling between the probes and the sediment.
The results indicate that sands possess greater group velocities, greater effective attenuation coefficients and lower quality factors than silts, while the low velocities measured in silts imply that the bulk moduli of the silt sites examined are lower than expected owing to a considerable fraction of organic matter. Significant variations were observed in compressional wave properties, which were more reliably related to variations in geotechnical properties in sands than in silts. Group velocities were observed to be independent of frequency in sands within 95 % confidence limits, with no reliable frequency-dependence being determined in silts owing to variability in the measured values. Effective attenuation coefficients were proportional to frequency within 95 % confidence limits for the majority of the sand and silt locations examined. Results indicate that compressional wave properties can be used to determine porosity, bulk density and sand fraction, while the reliable determination of mean grain diameter from compressional wave properties in inhibited by the scatter in the data.
The results from this study were also used to assess the effectiveness of Biot Theory to predict the compressional wave properties of these sediment types. In sands, the Biot phase velocities agreed with measured group velocities, while Biot absorption coefficients were less than measured effective attenuation coefficients, owing to scattering or squirt flow not accounted for in the Biot Theory. In silts, Biot phase velocities are greater than measured group velocities, while Biot absorption coefficients generally agree with or are greater than measured effective attenuation coefficients. In silts, predicted velocities are greater than those measured, while absorption coefficients generally agree with or are greater than measured attenuation coefficients. The discrepancy between the measured attenuation coefficients and predicted absorption coefficients can be explained through the over-estimation of in situ porosities by the geotechnical measurement techniques adopted.
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