Laboratory measurements of shear wave attenuation as a function of frequency were made using recently developed ceramic bimorph bender transducers to excite transverse particle motion in a medium grain water-saturated sand. The measurements were made at 13 frequencies from 450 to 7000 Hz. Multicycle sine-wave pulses were used to insure steady-state vibration at the measurement frequency. Attenuation was determined from the slope of a linear least-squares fit to the maximum received level versus transducer separation data. This not only affords an estimate of the attenuation but allows confidence intervals to be placed around that estimate. The attenuation values’ which increased with frequency from 5 dB/m at 450 Hz to 120 dB/m at 7000 Hz’ did not exhibit a simple first-power frequency dependence. The results were compared with predictions based on a two-component model developed by Stoll and were consistent in both amplitude and frequency dependence.
A promising approach in the prediction of acoustic wave speeds and attenuations in marine sediments is the use of the Biot theory as implemented by Stoll. The Biot–Stoll physical sediment model is examined here in a field application. Both the geophysical inputs and the geoacoustic outputs were measured at three shallow-water sites in the Mediterranean Sea. This permitted the investigation of the accuracy of the model for a variety of natural marine sediments including silty clay, sand, and gravel. A difficulty in the use of the Biot–Stoll model is the problem of accurately determining the 13 geophysical inputs since many of the inputs cannot be directly measured. Ten of the input properties are derived by empirical means and the other three properties are determined by measurement. Comparisons are made between the Biot–Stoll model predictions of compressional velocity, compressional attenuation, and shear velocity with in situ and laboratory measurements. The model predictions, on the whole, show excellent agreement with the measured data.
Signal transmission loss and spatial coherence data for source-receiver separations between 100 and 250 km were acquired in the Gulf of Mexico with a calibrated seismic-streamer measurement system at 400-m depth, a towed projector at 100-m depth which emitted 67-and 173-Hz tones, and a moored Webb sound source at 988-m depth driven at 175 Hz. Rangedependent bathymetry and sound velocity profiles and other environmental data were measured. The 67-Hz data showed a persistent sound transmission with a mean of measured range-averaged loss values (corrected for cylindrical spreading) of 41 dB ranging between 37 and 45 dB; the 173-Hz data showed several pronounced transmission loss minima with a mean measured range-averaged loss value of 51 dB ranging between 41 and 60 dB as well as a rapid increase in loss over the slope at ranges greater than 225 km and water depths less than 1.2 km. Slope enhancements were found to be on the order 2-4 dB at 67 Hz and 6 dB at 173 Hz when compared to flat bottom calculations. Pairwise coherence data showed the effect of signal-tonoise ratio variations due to multipath interference. Estimates of signal coherence length from the coherent summation of streamer hydrophones yielded coherence lengths ranging between 70m (8A) and 300m (35A) with an average of 181 m (20A) at a frequency of 173 Hz (A --8.67 m). Fast asymptotic coherent and normal mode transmission loss calculations produced results in qualitative agreement with measured data for the deep flat portion of the measurement track when measured geoacoustic profiles or the derived bottom loss curves were used. The results of implicit finite difference parabolic equation calculations were consistent with range-averaged data for the flat portion of the track as well as on the slope. These results show that if proper descriptions of the subbottom velocity profiles are used, then computations employing either parabolic equation or normal mode techniques provide qualitative agreement with experimental results.
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