Over the past few years, the location and recovery of artifacts buried in marine sediments has been enhanced by the utilization of several technological innovations. Acoustical technology appears to offer a potential application in the high-resolution delineation of archeological sites. Good acoustic penetration of marine sediments, however, usually requires a low-frequency sonar, and this requirement is not compatible with the realization of good angular resolution, at least for systems of reasonable size. Parametric arrays with their low-frequency, high-resolution capability, appear to be well suited to these conflicting requirements. The results of several measurements made with one such array are presented in support of this contention. These results include acoustic propagation and beam pattern data obtained in sea water on a 20-kHz difference frequency radiation generated by the nonlinear interaction of 190- and 210-kHz carriers that were transmitted from a 17-in.-diam line-in-cone source. It is also shown that a 5-in. diam aluminum sphere, buried to a depth of 6 in. in a fine sand sediment, was detected with the difference-frequency radiation but not with the carriers. [This work sponsored by the Office of Naval Research.]
The use of high-frequency acoustic techniques for detection and study of submerged objects and geological features in rivers can be limited by both attenuation and reverberation. These parameters were measured in a pilot study of the lower reaches of the Brazos River near Houston, Tex. The attenuation was due to the suspended particulate matter carried by this muddy river. At a frequency of 200 kHz and for a particulate concentration of 0.025% by weight, the attenuation was found to be 1.5 dB/100 yd. Reverberation from the bottom sediment was measured at a frequency of 85 kHz as a function of transducer grazing angle. The reverberation data are interpreted as a scattering strength per unit area of bottom insonification and fall in the range of scattering strengths reported by others for fine mud. [This work was supported by the Office of Naval Research.]
The nonlinear interaction of two high-frequency high-intensity waves to produce a highly directive difference frequency wave is known to be of value for systems requiring good angular resolution at operating frequencies not limited by absorption. The technique has recently been successfully applied to problems in marine bathymetry [J. Acoust. Soc. Amer. 50, 1085–1087 (1971)] and archeology [Paper F6, 83rd Meeting, Acoust. Soc. Amer.]. An analysis of the potential utilization of this technique in ultrasonic diagnostics is presented in the present paper. This analysis includes a comparison of parameters attainable by both nonlinear and conventional methods, as well as a design description of a prototype parametric echoscanner. This device utilizes a 0.25-in. piston to transmit interacting waves at 7.25 and 8.75 MHz. The results of measurements on the amplitude and beampattern of the 1.5-MHz difference-frequency wave are presented. Echolocation data obtained with several simple targets are also presented to demonstrate the resolution capability of the instrument.
Investigation into the self-noise of a SPOCAL (solion polarized cathode acoustic linear) transducer has identified the mechanisms responsible for this self-noise. Measurements on this transducer have shown it to have a noise threshold of 10−5 μbar, or less than 10−8 mm Hg. Therefore, the linear dynamic range of this type of transducer is 100 dB in the pressure range from 10−5 to 10+3 μbar, with a frequency range from 0.0002 to 3 Hz. The noise thresholds of conventional liquid-pressure measuring devices are several orders of magnitude higher than those of SPOCAL tranducers. The SPOCAL transducer has been applied in a seismometer where it senses the motion of a mercury column as the inertial mass. This SPOCAL seismometer obtains valid data from seismic activity at levels far below the capability of the instruments currently in use. Subject Classification: [43]85.24, [43]85.40; [43]40.50.
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