Timothy M. Marston

Nonlinear propagation modeling: Guidelines for supporting measurements

In nonlinear propagation, evolution of waveforms is strongly dependent on the characteristics of the waveform. The commonly observed waveform steepening is a coherent process that produces a specific phase relationship between spectral components. High‐amplitude military jet noise may show peak pressures five to ten times the root‐mean‐square pressure and may have local rise times less than several microseconds. Measurement of these waveforms containing fast rise times requires large bandwidth and high fidelity in phase. If the bandwidth and phase are not preserved then the waveforms can lose precisely those features that distinguish them from linear propagation. Whether such waveforms are used as starting fields for nonlinear codes or as code validation, the impact of limitations in the measurement system must be understood—the simple frequency‐response magnitude is insufficient. By generating freely propagating weak‐shock waves with rise times comparable to those observed in full‐scale jet measurements,...

Volumetric Acoustic Imaging via Circular Multipass Aperture Synthesis

In this paper, volumetric imaging via multipass circular synthetic aperture sonar (CSAS) is demonstrated using an autonomous underwater vehicle (AUV). A multidimensional aperture is synthesized by performing a series of circular scans at varying grazing angles around targets and coherently combining the backscattering information from the set of scans to form high-resolution volumetric images. A data-driven technique for precision alignment of the individual scans comprising the multipass set enables synthesis of a multidimensional array. To beamform in the vertical dimension using the irregular and undersampled multipass aperture, a compressive-sensing-based approach is adopted which is similar to methods used in analogous synthetic aperture radar tomography applications but modified to accommodate for the wider fractional bandwidth of the synthetic aperture sonar (SAS) system. The modification exploits a joint sparsity assumption in the vertical scattering profile at different subbands and adapts a standard joint sparse solving algorithm to the relevant case in which the sparsity profile is common between solution vectors but the sensing matrices are different. Results are shown for a variety of targets, including proud and obliquely buried unexploded ordnance, a 2-1 solid aluminum cylinder, and a steel oil drum.

Image-Based Automated Change Detection for Synthetic Aperture Sonar by Multistage Coregistration and Canonical Correlation Analysis

In this paper, an automated change detection technique is presented that compares new and historical seafloor images created with sidescan synthetic aperture sonar (SAS) for changes occurring over time. The method consists of a four-stage process: a coarse navigational alignment that relates and approximates pixel locations of reference and repeat-pass data sets; fine-scale coregistration using the scale-invariant feature transform (SIFT) algorithm to match features between overlapping data sets; local coregistration that improves phase coherence; and finally, change detection utilizing a canonical correlation analysis (CCA) algorithm to detect changes. The method was tested using data collected with a high-frequency SAS in a sandy shallow-water environment. Successful results of this multistage change detection method are presented here, and the robustness of the techniques that exploit phase and amplitude levels of the backscattered signals is discussed. It is shown that the coherent nature of the SAS data can be exploited and utilized in this environment over time scales ranging from hours through several days. Robustness of the coregistration methods and analysis of scene coherence over time is characterized by analysis of repeat pass as well as synthetically modified data sets.

Automated Change Detection for Synthetic Aperture Sonar

In this paper, an automated change detection technique is presented that compares new and historical seafloor images created with sidescan synthetic aperture sonar (SAS) for changes occurring over time. The method consists of a four stage process: a coarse navigational alignment; fine-scale co-registration using the scale invariant feature transform (SIFT) algorithm to match features between overlapping images; sub-pixel co-registration to improves phase coherence; and finally, change detection utilizing canonical correlation analysis (CCA). The method was tested using data collected with a high-frequency SAS in a sandy shallow-water environment. By using precise co-registration tools and change detection algorithms, it is shown that the coherent nature of the SAS data can be exploited and utilized in this environment over time scales ranging from hours through several days.

Modeling real porous tube infrasonic arrays

Measured values of acoustic resistance in porous hoses have been implemented into a previously developed porous tube model for infrasonic arrays [J. Acoust. Soc. Am. 122, 2960 (2007)]. Garden soaker hoses are commonly used in porous infrasonic arrays, and a simple method for the measurement the acoustic resistance of a segment of soaker hose will be presented. This experimentally determined resistance value can be implemented into the porous tube model for an improved array response prediction. It is anticipated that this analysis will give rise to improved phase and magnitude predictions for this type of array. Applications include design optimization of porous tube arrays, and infrasonic propagation through porous ducts. [Research funded by the Applied Research Laboratory Educational and Foundational Fund.]

Simulation‐based response analysis of porous and non‐porous infrasonic arrays

A numerical simulation for porous and non‐porous infrasonic arrays has been developed to aid in the analysis of the response of these systems. For design flexibility, the simulation makes use of differential T‐elements, which may be used as building blocks to assemble pipes and junctions of various shapes and sizes. The simulation adds to the capabilities of a previously developed model [Alcoverro and Le Pichon, J. Acoust. Soc. Am. 117, 1717 (2005)], by including variable tube porosity, a calculation of wall‐losses based on a thermo‐viscous function, a calculation of the change in response as the azimuth and elevation of the incoming wave are varied, and a calculation of the wave shape as it propagates through the array in the time domain. [Research funded by the Applied Research Laboratory Educational and Foundational Fund.]

Utilization of aspect angle information in synthetic aperture images

Synthetic aperture sonar (SAS) involves the creation of high resolution images of a scene via scattered signals recorded at different locations. Each pixel of the reconstructed image includes information obtained from multiple aspects due to the changing position of the sources/receivers. The aspect dependent scattering at each pixel may be exploited to provide additional information about the scene; we will present a framework for converting and utilizing multi-aspect data, as well as several examples. This aspect dependency may be leveraged to separate objects of interest from the background, to understand the local bathymetry, or for visualizing acoustic shadowing in circular aperture sonar (CSAS) images. Additionally, the aspect dependence of low-frequency elastic scattering from objects may be used to understand underlying scattering physics; a laboratory example is presented.Synthetic aperture sonar (SAS) involves the creation of high resolution images of a scene via scattered signals recorded at different locations. Each pixel of the reconstructed image includes information obtained from multiple aspects due to the changing position of the sources/receivers. The aspect dependent scattering at each pixel may be exploited to provide additional information about the scene; we will present a framework for converting and utilizing multi-aspect data, as well as several examples. This aspect dependency may be leveraged to separate objects of interest from the background, to understand the local bathymetry, or for visualizing acoustic shadowing in circular aperture sonar (CSAS) images. Additionally, the aspect dependence of low-frequency elastic scattering from objects may be used to understand underlying scattering physics; a laboratory example is presented.

Extraction of infrasonic waveforms from conventional condenser microphones

There are several commercial measurement‐grade condenser microphones that are suitable for measurements down to about 0.1 Hz; however, these one‐half‐inch microphones reach those low frequencies with an in‐line attenuator that compromises the overall dynamic range. In a previous paper [T. M. Marston and T. B. Gabrielson, J. Acoust. Soc. Am. 119, 3378 (2006)], a process was described for digital reconstruction of the lowest frequencies that avoids the attenuator and the sacrifice in dynamic range. This work has been extended to evaluate a reconstruction process for a one‐quarter‐inch condenser microphone, which has significantly higher low‐frequency roll‐off than the one‐half‐inch infrasonic microphones. One reason for using the one‐quarter‐inch microphone is its higher peak pressure limit. By modeling the preamplifier input‐impedance bootstrapping and the pressure‐equalization leak, extension to a few tenths of a hertz can be performed. However, the pressure‐equalization leak characteristic frequency vari...

Comparison of measurement and theory for the acoustic pressure field in an infrasonic calibrator through the isentropic‐to‐isothermal transition

In a previous paper [T. M. Marston and T. B. Gabrielson, J. Acoust. Soc. Am. 119, 3378 (2006)], a simple total‐immersion calibrator for infrasonic microphones was described. This calibrator has been improved and so has the correspondence with theory. The Rott equations for thermoviscous acoustics have been incorporated in a stepwise solution for the acoustic field. These equations account for the transition between boundary‐layer flow in which the main field is almost isentropic, and the very‐low‐frequency approach to isothermal pressure oscillations. The entire microphone body is placed in the calibrator so that the pressure‐equalization leak functions normally. The calibrator operates from 0.02 Hz to 20 Hz and includes a DC‐coupled piezoresistive differential pressure sensor for comparison calibration; however, the model, which incorporates the geometry of the calibrator pipe, the temperature, and the piston stroke, permits response estimation without dependence on the reference sensor. [Research funded...

Microphone bandwidth extension by digital correction of low‐frequency roll‐off

When the low‐frequency roll‐off of a condenser microphone needs to be extended, the standard method of achieving extra bandwidth is to place a shunt capacitor in parallel with the input capacitance of the preamplifier. The result increases the bandwidth in the desired region, but sacrifices dynamic range across the entire bandwidth. It is possible to digitally extend the useful bandwidth of a microphone and only compromise dynamic range in the extended region. A technique for doing this has been developed and applied to microphones used to record sonic boom data. Microphones that have an insufficiently low‐frequency response to capture the characteristic N‐wave shape of sonic booms are corrected using the developed methods, and the results are compared to data captured by microphones with adequate low‐frequency response. During the development of the filtering technique, it was found that phase response is critical in the correction process and its importance is demonstrated. [Research funded by the Appli...