Experimental studies were made of the propagation of longitudinal waves in several mixtures of water with quartz sands fractionated as to particle size. The most probable particle size in the several samples ranges from 0.01 to 0.07 cm. The experimental frequencies were approximately 400 to 1000 kc/sec. Additional measurements in the same frequency range cover the reflection and scattering of underwater sound from essentially plane surfaces of these aggregates. The velocity and attenuation data are related satisfactorily to a well-known analysis for a porous acoustical material having a pliable skeleton. The reflected signal, as a function of angle of incidence, behaves approximately as expected from the conditions for continuity at the plane surface, when the appropriate complex index of refraction is used. The scattering data are compared with the analysis based on a random distribution of scattering amplitude per unit volume, with autocorrelation distance proportional to the particle size. The analysis predicts the limiting behavior of scattering coefficient with respect to particle size and frequency, but predicts a more rapid fall of scattering amplitude beyond the critical angle than was observed. The magnitude and autocorrelation properties of the fluctuations in scattered signal as the apparatus was translated to scan the sand surface were observed, and found to correspond approximately to a model based on a Gaussian distribution of local scattering amplitude.
Experiments were performed to determine the noise characteristics of a hydrophone streamer that had incorporated a number of noise reduction features. In the original system, the channels to which the depth‐controller birds were attached were 3 to 4 times noisier than nonbird channels. Fortunately, the bird noise is near‐field and is eliminated simply by increasing bird/hydrophone separation to 9 ft. On this cable, no other discrete noise sources are evident. The boat, propulsion system, lead‐in cable, tail buoy, and ambient sea conditions (moderate seas) do not generate significant noise at towing speeds above 5 knots. The noise on individual hydrophones not near birds is mainly random with only a small coherent component traveling horizontally through the water from the direction of the boat. However, since the 145-ft hydrophone arrays of 20 detectors are much more effective in reducing random noise than coherent noise, the array output consists of approximately equal portions of each. A twofold decrease in the total noise‐to‐signal ratio would result from doubling the array length (to 290 ft) while maintaining the same hydrophone density. This would result in a four to fivefold decrease in the coherent noise‐to‐signal ratio and a 30 percent decrease in the random noise‐to‐signal ratio. Additional noise reduction would result from increasing the hydrophone density and decreasing the motion sensitivity of the hydrophones. (The streamer hydrophones are not the motion canceling type.) At a towing speed of 5.3 knots, the noise level recorded on an array (not near a bird) is equivalent to pressures of 1 μbar. In normal operations with an 8-gun sleeve exploder source, a stacked section signal‐to‐towing noise ratio of 3 was obtained at 3.0 sec. However, the towing noise increases as the cube of the boat speed, and the S/N ratio would decrease by a factor of 11 if the boat speed were doubled. Conversely, decreasing the boat speed by 18 percent would double the signal‐to‐towing noise ratio.
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Source‐generated seismic noise has been studied in several areas of Oklahoma and Texas having distinctly different near‐surface layering and velocities. Surface‐wave velocities, particularly those of the propagating modes, are closely related to near‐surface layer thickness and velocity. Modal structure is strongly influenced by the shooting parameters, of which charge depth is the most important. The Rayleigh or first propagating mode and the third propagating mode are readily identifiable on the noise records and agree satisfactorily with theory, if frequency‐dependent attenuation and modal overlapping are taken into account. Leaky modes are identifiable on the basis of their high phase and group velocities but to date have defied quantitative comparison with theory.
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