One goal of investigative signal processing techniques is to discriminate between types of materials composing an object. This paper explores a method for material discrimination and characterization using bispectral signatures acquired from an object actively probed with acoustic pulses. The mathematical foundations of the bispectrum are presented, and the proposed technique is tested with an ultrasonic apparatus. Results indicate that at ultrasonic frequencies this technique provides signatures with the potential of discriminating between classes of materials such as plastic, aluminum, and rock.
Methylvinylboranes and trivinyiborane were characterized as the result of an improved preparative method (4) through observation of their infrared, ultraviolet, nuclear magnetic resonance (NMR), and mass spectra vapor pressures, and by vapor density. The ultraviolet and NMR spectra are published elsewhere (4). The gas-liquid chromatographic (GLC) properties were determined, and with suitable calibration they can be used henceforth for identification. METHODS AND RESULTS
A method is presented for material discrimination and characterization using bispectral signatures acquired from an object actively probed with acoustic pulses. Although bispectral techniques have proven useful in a diverse array of fields including passive acoustic ranging, bispectral processing in active acoustic applications has not been widely explored. The mechanisms responsible for the bispectral signatures revealed using active acoustics have not been well studied and little is known about the relative contributions to the bispectrum originating in the physical properties of the target material itself rather than from target structural acoustics and the propagation media. In a pilot experiment, we determine bispectral signatures for three targets of differing composition but similar dimensions using a submerged ultrasonic apparatus. The experiment is designed to isolate effects due to target properties from those attributable to propagation path, source, or receiver. The source wavelet is a broad-spectrum linear frequency modulated pulse. The normalized bispectrum is calculated using conventional nonparametic methods, and is averaged across many frames. Results indicate that at ultrasonic frequencies this technique provides signatures with the potential of discriminating between classes of materials such as plastic, metal, and rock. [Work supported by Applied Research Laboratories.]
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