Kaustubha Raghukumar

Spatial diversity in passive time reversal communications

A time reversal mirror exploits spatial diversity to achieve spatial and temporal focusing, a useful property for communications in an environment with significant multipath. This paper presents the impact of spatial diversity on passive time reversal communications between a single probe source and a vertical receive array using at‐sea experimental data, while the probe source is either fixed or moving at about 4 knots. The performances of two different approaches are compared: (1) time reversal alone and (2) time reversal combined with adaptive channel equalization. In the presence of source motion, the motion‐induced Doppler shift is coarsely estimated using a decision‐feedback phase‐locked loop with a training sequence and then the received time series is resampled prior to the demodulation process. The time‐varying channel responses due to source motion require an adaptive channel equalizer such that time reversal combined with the equalizer outperforms time reversal alone by up to 13 dB as compared to 5 dB for a fixed source case. The experimental results around 3 kHz with a 1‐kHz bandwidth illustrate that even two or three receivers (i.e., 2‐ or 4‐m aperture) can provide resonable performance at 4.2‐ and 10‐km ranges in a 118‐m deep water.

The Kauai Experiment

The Kauai Experiment was conducted from June 24 to July 9, 2003 to provide a comprehensive study of acoustic propagation in the 8–50 kHz band for diverse applications. Particular sub‐projects were incorporated in the overall experiment 1) to study the basic propagation physics of forward‐scattered high‐frequency (HF) signals including time/angle variability, 2) to relate environmental conditions to underwater acoustic modem performance including a variety of modulation schemes such as MFSK, DSSS, QAM, passive‐phase conjugation, 3) to demonstrate HF acoustic tomography using Pacific Missile Range Facility assets and show the value of assimilating tomographic data in an ocean circulation model, and 4) to examine the possibility of improving multibeam accuracy using tomographic data. To achieve these goals, extensive environmental and acoustic measurements were made yielding over 2 terabytes of data showing both the short scale (seconds) and long scale (diurnal) variations. Interestingly, the area turned out...

High frequency normal mode statistics in a shallow water waveguide: The effect of random linear internal waves

Using transport theory and Monte Carlo numerical simulation, the statistical properties of mode propagation at a frequency of 1 kHz are studied in a shallow water environment with random sound-speed perturbations from linear internal waves. The environment is typical of summer conditions in the mid-Atlantic bight during the Shallow Water 2006 experiment. Observables of interest include the second and fourth moments of the mode amplitudes, which are relevant to full-field mean intensity and scintillation index. It is found that mode phase randomization has a strong adiabatic component while at the same time mode coupling rates are significant. As a consequence, a computationally efficient transport theory is presented, which models cross-mode correlation adiabatically, but accounts for mode coupling using the mode energy equations of Creamer [(1996). J. Acoust. Soc. Am. 99, 2825-2838]. The theory also has closed-form expressions for the internal wave scattering matrix and a correction for an edge effect. The hybrid transport theory is shown to accurately reproduce many statistical quantities from the Monte Carlo simulations.

Pressure sensitivity kernels applied to time-reversal acoustics

Sensitivity kernels for receptions of broadband sound transmissions are used to study the effect of the transmitted signal on the sensitivity of the reception to environmental perturbations. A first-order Born approximation is used to obtain the pressure sensitivity of the received signal to small changes in medium sound speed. The pressure perturbation to the received signal caused by medium sound speed changes is expressed as a linear combination of single-frequency sensitivity kernels weighted by the signal in the frequency domain. This formulation can be used to predict the response of a source transmission to sound speed perturbations. The stability of time-reversal is studied and compared to that of a one-way transmission using sensitivity kernels. In the absence of multipath, a reduction in pressure sensitivity using time reversal is only obtained with multiple sources. This can be attributed both to the presence of independent paths and to cancellations that occur due to the overlap of sensitivity kernels for different source-receiver paths. The sensitivity kernel is then optimized to give a new source transmission scheme that takes into account knowledge of the medium statistics and is related to the regularized inverse filter.

High-frequency normal-mode statistics in shallow water: The combined effect of random surface and internal waves

In an earlier article, the statistical properties of mode propagation were studied at a frequency of 1 kHz in a shallow water environment with random sound-speed perturbations from linear internal waves, using a hybrid transport theory and Monte Carlo numerical simulations. Here, the analysis is extended to include the effects of random linear surface waves, in isolation and in combination with internal waves. Mode coupling rates for both surface and internal waves are found to be significant, but strongly dependent on mode number. Mode phase randomization by surface waves is found to be dominated by coupling effects, and therefore a full transport theory treatment of the range evolution of the cross mode coherence matrix is needed. The second-moment of mode amplitudes is calculated using transport theory, thereby providing the mean intensity while the fourth-moment is calculated using Monte Carlo simulations, which provides the scintillation index. The transport theory results for second-moment statistics are shown to closely reproduce Monte Carlo simulations. Both surface waves and internal waves strongly influence the acoustic field fluctuations.

Experimental demonstration of the utility of pressure sensitivity kernels in time-reversal

Pressure sensitivity kernels were recently applied to time-reversal acoustics in an attempt to explain the enhanced stability of the time-reversal focal spot [Raghukumar et al., J. Acoust. Soc. Am. 124, 98-112 (2008)]. The theoretical framework developed was also used to derive optimized source functions, closely related to the inverse filter. The use of these optimized source functions results in an inverse filter-like focal spot which is more robust to medium sound speed fluctuations than both time-reversal and the inverse filter. In this paper the theory is applied to experimental data gathered during the Focused Acoustic Fields experiment, conducted in 2005, north of Elba Island in Italy. Sensitivity kernels are calculated using a range-independent sound-speed profile, for a geometry identical to that used in the experiment, and path sensitivities are identified with observed arrivals. The validity of the kernels in tracking time-evolving Greens functions is studied, along with limitations that result from a linearized analysis. An internal wave model is used to generate an ensemble of sound speed profiles, which are then used along with the calculated sensitivity kernels to derive optimized source functions. Focal spots obtained using the observed Greens functions with these optimized source functions are then compared to those obtained using time-reversal and the inverse-filter. It is shown that these functions are able to provide a focal spot superior to time-reversal while being more robust to sound speed fluctuations than the inverse filter or time-reversal.

Model and data comparison for a three-dimensional, finite-difference, time-domain solver in Sequim Bay

Paracousti is a three-dimensional finite-difference, time-domain solution to the governing velocity-pressure equations. This program is directed at modeling sound propagation generated by marine hydrokinetic (MHK) sources in an ocean environment. It is capable of modeling complex, multi-frequency sources propagating through water and soil that have spatially varying sound speeds, bathymetry, and bed composition. Experimental sound data collected at Sequim Bay in Washington, USA, during the winter of 2017 is compared against several simulations modeled within Paracousti for a range of frequencies and receiver locations. This measurement campaign recorded ambient noise data and the sound from a source producing three-second long, sinusoidal pulses between 20 and 5,000 Hz at a depth of 3 m. Additionally, bathymetric, sanity, and temperature data for the bay were collected in order to calculate the sound speed. Data were recorded at six locations ranging in distance between 10 and 1,000 m from the source by stationary buoys. Each simulation was created to model the collected source profiles and has a total depth of 80 m, with the average soil depth occurring at 23 m, and compared via transmission losses. Paracousti is a three-dimensional finite-difference, time-domain solution to the governing velocity-pressure equations. This program is directed at modeling sound propagation generated by marine hydrokinetic (MHK) sources in an ocean environment. It is capable of modeling complex, multi-frequency sources propagating through water and soil that have spatially varying sound speeds, bathymetry, and bed composition. Experimental sound data collected at Sequim Bay in Washington, USA, during the winter of 2017 is compared against several simulations modeled within Paracousti for a range of frequencies and receiver locations. This measurement campaign recorded ambient noise data and the sound from a source producing three-second long, sinusoidal pulses between 20 and 5,000 Hz at a depth of 3 m. Additionally, bathymetric, sanity, and temperature data for the bay were collected in order to calculate the sound speed. Data were recorded at six locations ranging in distance between 10 and 1,000 m from the source by s...

The integrated ocean dynamics and acoustics project

The goal of timely and accurate acoustics modeling in the ocean depends on accurate environmental input information. Acoustic propagation modeling has improved to the point of possibly being ahead of ocean dynamical modeling from the standpoint that some significant ocean features having strong acoustic effects are not faithfully reproduced in many models, particularly data-driven ocean models. This in part stems from the fact that ocean models have developed with other goals in mind, but computational limitations also play a role. The Integrated Ocean Dynamics and Acoustics (IODA) MURI project has as its goals improving ocean models, and also making continued improvements to acoustic models, for the purpose of advancing ocean acoustic modeling and prediction capabilities. Two major focuses are improved internal tide forecasting and improved nonlinear internal wave forecasting, which require pushing the state of the art in data-constrained mesoscale feature modeling as well as developing specialized high-...

Analysis of sound speed fluctuations in the Fram Strait near Greenland during summer 2013

We analyze sound speed fluctuations in roughly 600 m deep polar waters from a recent experiment. The Thin-ice Arctic Acoustics Window (THAAW) experiment was conducted in the waters of Fram Strait, east of Greenland, during the summer of 2013. A drifting acoustic mooring that incorporated environmental sensors measured temperature and salinity over a period of four months, along a 500 km north-south transect. We examine the relative contributions of salinity-driven polar internal wave activity, and temperature/salinity variability along isopycnal surfaces (spice) on sound speed perturbations in the Arctic. Both internal-wave and spice effects are compared against the more general deep water PhilSea09 measurements. Additionally, internal wave spectra, energies, and modal bandwidth are compared against the well-known Garrett-Munk spectrum. Given the resurgence of interest in polar acoustics, we believe that this analysis will help parameterize sound speed fluctuations in future acoustic propagation models.

Comparison of shallow water mode transport theory with acoustic transmissions during Shallow Water Experiment 2006

Coupled-mode transport theory appears to have put on solid theoretical ground acoustical scattering by internal waves in both deep and shallow water, over a range of low, medium, and high frequencies [Raghukumar and Colosi (2014) and Colosi and Morozov (2009)]. Here, full-field theoretical calculations of the acoustic field moments are compared against experimental data gathered during the Shallow Water Experiment 2006. Transport theory at low frequency is validated using data gathered by the WHOI Shark array at 175 Hz with a source towed by R/V Sharp at several different speeds over distances of 1.5–5.5 km. At high frequencies, comparisons are made at 1.2 kHz using data received by a WHOI bottom-mounted single hydrophone unit with a source towed by CFAV Quest over distances of 0–20 km. Acoustic observables include the mean and variance of intensity. The effect of a range dependent stochastic internal wave field is examined in the context of data-model comparison, along with the effect of random surface w...