Grant C. Eastland

Enhanced backscattering in water by partially exposed cylinders at free surfaces associated with an acoustic Franz wave.

When a smooth curved object is lowered into water, the initial specular contribution to high frequency backscattering of sound is weak before the specular point on the object becomes illuminated by incident sound. The associated transition in reflection from a metallic cylinder viewed at grazing incidence was previously studied [Baik and Marston, IEEE J. Ocean. Eng. 33, 386-396 (2008)]. The present research involves analogous measurements of backscattering performed using short tone bursts facilitating improved temporal resolution of distinct mechanisms contributing to the backscattering. The measurements reveal the presence of a delayed contribution to the backscattering that evolves in time in a way consistent with a scattering contribution of an acoustic Franz wave. The wave appears to be partially reflected at the free surface after having been excited on the cylinder by the incident acoustic wave. For slightly exposed cylinders viewed at grazing incidence, the Franz wave mechanism dominated the observed backscattering.

Calibration of a broadband acoustic transducer with a standard spherical target in the near field

This paper investigates the applicability of calibrating a broadband acoustic system in the near field. The calibration was performed on a single transducer with a mono-static configuration using a single standard target, a 25-mm tungsten carbide sphere in the nearfield of both the transducer and the sphere. A theoretical model was developed to quantify the nearfield effect. Numerical simulations revealed that the frequency responses at different distances varied significantly, the null positions were essentially invariant-a unique characteristic for determination of the compressional and shear wave speeds in the calibration sphere. The calibration curves obtained in the near field could be applied to farfield once the nearfield effects were accounted for. Since the transducer was located in the near field, the signal-to-noise ratio was high, resulting in a much wider useable bandwidth than the nominal bandwidth. The resultant calibration uncertainty, i.e., root-mean-square uncertainty over the entire usable frequency band was 1.05 dB and reduces to 0.33 dB when the regions corresponding to nulls were excluded. The methods reported here could potentially be applied to the calibration of multibeam and broadband echosounder/sonar systems since it is difficult to meet the farfield condition for outermost beams when shipboard calibrations are needed.

Calibration of a broadband acoustic system in near-field

This paper investigates the applicability of calibrating a broadband acoustic system in Near-field. The calibration was performed on a single transducer with a monostatic or backscattering configuration using a standard target, a 25-mm tungsten carbide sphere, in the near-field of both the transducer and the sphere. Theoretical model to quantify the near-field effect was developed in the paper. Theoretical simulations revealed that although the shape of the frequency responses of the received echoes at different distances varied significantly, the null positions were essentially invariant, a unique characteristic that was used to determine the compressional and shear wave speeds in the calibration sphere. The calibration curves obtained at different distances in the near-field by taking into account the near-field effect were consistent with each other. Since the transducer was located in the near-field, the signal-to-noise ratio was high, resulting in a much wider usable bandwidth, between 300 and 800 kH...

Acoustic scattering enhancements for partially exposed cylinders in sand and at a free surface caused by Franz waves and other processes

Creeping waves on solid cylinders having slightly subsonic phase velocities and large radiation damping are described as Franz waves because of association with complex poles investigated by Franz. For free-field high frequency broadside backscattering in water, the associated echoes are weak due to radiation damping. It was recently demonstrated, however, that for partially exposed solid metal cylinders at a free surface viewed at grazing incidence, the Franz wave echo can be large relative to the specular echo when the grazing angle is sufficiently small [G. C. Eastland and P. L. Marston, J. Acoust. Soc. Am. 135, 2489–2492 (2014)]. The Fresnel zone associated with the specular echo is occluded making it weak while the Franz wave is partially reflected at the interface behind the cylinder. This hypothesis is also supported by calculating the exact backscattering by half-exposed infinitely long rigid cylinders viewed over a range of grazing angles. Additional experiments concern the high frequency backsca...

Computation of backscattering enhancements by a half-exposed rigid cylinder at a free surface caused by Franz waves

Recent observations of the backscattering by partially-exposed solid aluminum cylinders in water viewed at grazing incidence at a free surface [G. C. Eastland, Ph.D. thesis, Wash. State Univ., 2012] indicate that the generation, propagation, and reflection of Franz-type creeping waves can be important. The present investigation gives additional support for this hypothesis by calculating the exact backscattering by a half-exposed infinitely long rigid cylinder viewed over a range of grazing angles. The calculation begins with the known frequency domain expression for the complex amplitude given in an Appendix of [K. Baik and P. L. Marston, IEEE J. Ocean. Eng. 33, 386 (2008)]. Numerical Fourier transforms were used to construct the time-domain response for various excitations and the evolution of that response was investigated as a function of the grazing angle. This procedure reveals from the timing of the computed features there is a significant delayed echo having the expected timing of a Franz wave part...

Reversible bistatic and monostatic synthetic aperture sonar filtering.

Recent investigations into understanding scattering features of proud and partially exposed cylinders have revealed many interesting properties of scattering mechanisms. In the present study, the effect of cylinder location was explored by lowering a horizontal solid aluminum cylinder through a flat free surface into a water tank while performing a vertical transducer line scan to monitor the evolution of the scattering. Interactions with the air‐water interface partially simulate various aspects of interactions with flat sediment. The presence of the interface contributes many more paths for scattering to occur, even in the case of broadside illumination considered. Reversible SAS filtering of monostatic and bistatic data is able to identify different specular mechanisms involving the target and free surface, in addition to their corresponding elastic responses. Studying how these contribute to SAS images enables a clearer understanding of target classification and identification. It is also helpful to view the evolution of signal time‐domain loci as a function of transducer location and target location. This investigation emphasizes broadside and near‐broadside illumination. A delayed multiple scattering feature is visible involving one interfacial reflection and double reflected from the target. [Work supported by ONR.]

Evolution of acoustic feature timing and imaging for different cylinder exposures and applications of reversible SAS filtering

When looking at acoustic scattering from a target near a flat surface, scattering mechanisms exist that may be attributed to interactions with the surface. These different mechanisms can be identified using the timing slope dt/dh, where dh signifies the change in the cylinder exposure. The slopes for the different mechanisms can be determined using ray theory. A technique involving Synthetic Aperture Sonar imaging can be used to isolate acoustic paths scattering directly off the target from those paths which have interacted with the surface after scattering off the target. This method is useful for quickly determining what type of mechanism is involved with a particular path, and in this paper is utilized to verify the identification of slopes. The case studied in this paper is a cylinder near a free surface.

Exploration of acoustic spiral waves in an underwater environment

The work to be presented is regarding a research project to investigate the viability and possible application of spiral or helical acoustic waves for use in underwater environments. The investigation encompasses studies into the properties and generation of spiral or helical acoustic waves. Spiral waves can be generated by precisely controlling the phase of the wave, which could have different physical properties due to what is known as “topological charge.” This topological charge could be adjusted in some advantageous way, depending upon the desired application. In addition, reception of spiral or helical waves was investigated. The goal of the research is a proof of concept and presentation of the results of generation and reception of spiral acoustic waves. Spiral waves have been investigated for acoustic navigation and may lend itself well for use in other applications.

Reverberation characterization inside an anechoic test chamber at the Weapon Sonar Test Facility at NUWC Division Keyport

The Weapon Sonar Test Facility (WSTF) at NUWC Keyport, WA is a 34,500 gallon pressure tank currently used for test and evaluation of torpedo sonar arrays. The tank has many more possible uses and complete characterization of the testing environment needs to be performed. One of the methods of characterization being used is the determination of the reverberation time of the anechoic chamber. Applying Sabine-Franklin-Jaeger theory of reverberant rooms, an experimentally determined reverberation time T60 for a “live room” can be used to provide an upper bound for the reverberation time in the chamber. Utilizing the Eyring theory of “dead” rooms will provide a better determination of T60 for the anechoic chamber; hence provide a lower bound characterization. The experimental method involves determining the spatially averaged acoustic energy of an initial tone and the corresponding reflection from the chamber wall seen in a recorded signal. These values determined from the root mean square signal voltage deter...

Scattering enhancements for partially exposed cylinders at a free surface caused by Franz waves: Measurements and geometric model

Creeping waves on solid cylinders having slightly subsonic phase velocities and large radiation damping are usually described as Franz waves because of their association with complex poles investigated by Franz. For free-field high frequency broadside backscattering in water, the associated echoes are weak due to the radiation damping. It is demonstrated here, however, that for partially exposed solid metal cylinders at a free surface viewed at grazing incidence, the Franz wave echo can be large relative to the specular echo even for ka above 20 when the grazing angle is sufficiently small. The reason is that at small grazing angles and small exposures, the Fresnel zone associated with the specular echo is occluded so that the specular echo is weak [K. Baik and P. L. Marston, IEEE J. Ocean. Eng. 33, 386 (2008)] while the Franz wave is partially reflected at the interface. This has been confirmed from the evolution of echo timing with cylinder exposure and by SAS imaging. In the experiment a solid cylinder...