Tarun K. Chandrayadula

Bio: Tarun K. Chandrayadula is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Signal processing & Time of arrival. The author has an hindex of 4, co-authored 17 publications receiving 61 citations. Previous affiliations of Tarun K. Chandrayadula include George Mason University & Naval Postgraduate School.

Coupled mode transport theory for sound transmission through an ocean with random sound speed perturbations: Coherence in deep water environments

Abstract: Second moments of mode amplitudes at fixed frequency as a function of separations in mode number, time, and horizontal distance are investigated using mode-based transport equations and Monte Carlo simulation. These second moments are used to study full-field acoustic coherence, including depth separations. Calculations for low-order modes between 50 and 250 Hz are presented using a deep-water Philippine Sea environment. Comparisons between Monte Carlo simulations and transport theory for time and depth coherence at frequencies of 75 and 250 Hz and for ranges up to 500 km show good agreement. The theory is used to examine the accuracy of the adiabatic and quadratic lag approximations, and the range and frequency scaling of coherence. It is found that while temporal coherence has a dominant adiabatic component, horizontal and vertical coherence have more equal contributions from coupling and adiabatic effects. In addition, the quadratic lag approximation is shown to be most accurate at higher frequencies and longer ranges. Last the range and frequency scalings are found to be sensitive to the functional form of the exponential decay of coherence with lag, but temporal and horizontal coherence show scalings that fall quite close to the well-known inverse frequency and inverse square root range laws.

Observations and transport theory analysis of low frequency, acoustic mode propagation in the Eastern North Pacific Ocean.

Abstract: Second order mode statistics as a function of range and source depth are presented from the Long Range Ocean Acoustic Propagation EXperiment (LOAPEX). During LOAPEX, low frequency broadband signals were transmitted from a ship-suspended source to a mode-resolving vertical line array. Over a one-month period, the ship occupied seven stations from 50 km to 3200 km distance from the receiver. At each station broadband transmissions were performed at a near-axial depth of 800 m and an off-axial depth of 350 m. Center frequencies at these two depths were 75 Hz and 68 Hz, respectively. Estimates of observed mean mode energy, cross mode coherence, and temporal coherence are compared with predictions from modal transport theory, utilizing the Garrett–Munk internal wave spectrum. In estimating the acoustic observables, there were challenges including low signal to noise ratio, corrections for source motion, and small sample sizes. The experimental observations agree with theoretical predictions within experimental uncertainty.

Observations of phase and intensity fluctuations for low-frequency, long-range transmissions in the Philippine Sea and comparisons to path-integral theory

Abstract: In the Philippine Sea, from April 2010 to March 2011, a 330-km radius pentagonal acoustic transceiver array with a sixth transceiver in the center transmitted broadband signals with center frequencies between 172 and 275 Hz and 100 Hz bandwidth eight times a day every other day. The signals were recorded on a large-aperture vertical-line array located near the center of the pentagon at ranges of 129, 210, 224, 379, 396, and 450 km. The acoustic arrival structures are interpretable in terms of ray paths. Depth and time variability of the acoustic observations are analyzed for six ray paths (one from each transceiver) with similar vertical sampling properties in the main thermocline. Acoustic-field statistics treated include: (1) variances of phase and intensity, (2) vertical coherence and intensity covariance, (3) glinting and fadeout rates, and (4) intensity probability density functions. Several observed statistics are compared to predictions using Feynman path-integral theory assuming the Garrett-Munk internal-wave spectrum. In situ oceanographic observations support this assumption and are used to estimate spectral parameters. Data and theory differ at most by a factor of two and reveal the wave propagation regimes of unsaturated, partially saturated, and fully saturated. Improvements to the evaluation of path-integral quantities are discussed.

Interpolation methods for vertical linear array element localization

Abstract: Array element localization is crucial for applications such as ocean acoustic tomography. Loss of navigation data makes it difficult to compensate for array motion when implementing operations such as mode filtering or beamforming. This paper presents a method for estimating missing array navigation data using an empirical orthogonal function (EOF) model. The method can be applied to estimate the location of some vertical array elements based on the location of the other elements. It assumes that second order statistics can be estimated from a set of navigation measurements for the full array. The paper applies the EOF-based method to estimate missing navigation data for the long range ocean acoustic propagation experiment (LOAPEX). The results are evaluated by examining how the errors in mooring motion estimation affect mode processing. In particular the paper analyzes the degradation in array gain and the errors in time of arrival for the low order modes. The error statistics indicate that use of the EOF method has a negligible effect on mode processing.

Monterey Bay ambient noise profiles using underwater gliders

Abstract: In 2012, during two separate week-long deployments, underwater gliders outfitted with external hydrophones profiled the upper 100-200 m of the Monterey Bay. The environment contained various noises made by marine mammals, ships, winds, and earthquakes. Unlike hydrophone receivers moored to a fixed location, moving gliders measure noise variability across a wide terrain. However, underwater mobile systems have limitations such as instrument and flow noise, that are undesired. In order to estimate the system noise level, the hydrophones on the gliders had different gain settings on each deployment. The first deployment used a 0 dB gain during which the ambient noise recordings were dominated by the glider. The second used two hydrophones, one with a 0 dB gain and the other with 20 dB. Apart from system sounds, the higher-gain hydrophone also recorded far-away sources such as whales and ships. The noise recordings are used to estimate the spectrograms across depth and record time. The spectrograms are integrated with the glider engineering data to estimate histograms of noise power as a function of depth and glider velocity. The statistics from the two different deployments are compared to discuss the value of gliders with external hydrophones in ambient noise studies.

Detection, Estimation, and Modulation Theory

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Sound Propagation through the Stochastic Ocean

Abstract: Introduction and Historical Background In the undersea world the sound wave rules supreme, as is evidenced by the highly evolved acoustic physiology and sophisticated auditory processing capability of fish and marine mammals. Humans are relative latecomers to the world of undersea sound. Leonardo da Vinci (1483) developed a device to listen to approaching ships, writing If you let your ship stop, and dip the end of a long blowpipe in the water and hold the other end to your ear, then you can hear ships which are very far distant from you. There are also reports that for centuries Inuit whalers have been using acoustic methods to localize their prey. By placing the butt of a dipped oar against one'sjawbone, the underwater vibrations of the vocalizing whales could be sensed. Curiously the giants of classical physics (e.g., Newton, Euler, Lagrange, Laplace, Helmholtz) seem to have paid little attention to the subject, instead devoting their energies in acoustics toward musical problems. Even the great acoustical authority, Lord Rayleigh, makes scarce mention of underwater sound in his masterpiece publications The Theory of Sound , Parts 1 and 2, in 1887 and 1896. Indeed, the speed of sound in fresh water was not measured until 1826 (Colladon and Sturm, 1827), and the first crude measurements in sea water came roughly a century later (Wood, 1930). Developments in ocean acoustics seem to be inseparably tied to matters of military importance, the first of which was the problem of knowing the seafloor adequately so as to avoid vessel grounding. This was the genesis of the acoustic fathometer (echo sounder) invented shortly after the turn of the twentieth century, which was also in great favor with the European royals whose fleets suffered many losses due to grounding. The first measurements of the speed of sound in sea water were largely motivated by echo sounding and sound ranging. The addition of the submarine to the naval arsenal provided a particularly strong catalyst for advancement in the twentieth century. Though there were some significant developments during and after World War I (Wood, 1930), the independent discovery of the ocean sound channel by both US and Soviet scientists toward the end of World War II brought the field to an entirely new level (Ewing and Worzel, 1948; see also the discussion of the Soviet discovery in Munk et al.

Using Petrel II Glider to Analyze Underwater Noise Spectrogram in the South China Sea

Abstract: In this paper, a Petrel II autonomous underwater glider (AUG) was used to analyze underwater noise spectrogram in the South China Sea. For ocean observations, AUG has an excellent performance such as being efficient in all weather conditions, lasting for long time periods, being able to be remote controlled, which creates favorable conditions for analyzing underwater noise characteristics. In order to measure the underwater noise, a newly developed and customized underwater passive acoustic monitoring system was firstly installed on the Petrel II glider and tested in the South China Sea trial in August 2016. The observation data at a depth of 1000 meters range were taken as the research object and were used to analyze the noise levels over a long period through the method of the short-time Fourier transform. At the same time, the noise data have been compared in time to the actions of the glider during the sea trial. However, AUGs also have limitations such as undesired instrument and flow noise. By adopting the step-by-step method, these self-noise data of Petrel II glider were acquired and analyzed quantitatively, and the basic characteristics of self-noise under different working conditions are depicted. Removing the corresponding self-noise, the noise data at different depths measured by the Petrel II glider could be used to estimate underwater ambient noise levels across depth.

Modal analysis of the range evolution of broadband wavefields in the North Pacific Ocean: low mode numbers.

Abstract: The results of mode-processing measurements of broadband acoustic wavefields made in the fall of 2004 as part of the Long-Range Ocean Acoustic Propagation Experiment (LOAPEX) in the eastern North Pacific Ocean are reported here. Transient wavefields in the 50–90 Hz band that were recorded on a 1400 -m long 40 element vertical array centered near the sound channel axis are analyzed. This array was designed to resolve low-order modes. The wavefields were excited by a ship-suspended source at seven ranges, between approximately 50 and 3200 km, from the receiving array. The range evolution of broadband modal arrival patterns corresponding to fixed mode numbers (“modal group arrivals”) is analyzed with an emphasis on the second (variance) and third (skewness) moments. A theory of modal group time spreads is described, emphasizing complexities associated with energy scattering among low-order modes. The temporal structure of measured modal group arrivals is compared to theoretical predictions and numerical simulations. Theory, simulations, and observations generally agree. In cases where disagreement is observed, the reasons for the disagreement are discussed in terms of the underlying physical processes and data limitations.

Vertical line array measurements of ambient noise in the North Pacific

Abstract: Shipping noise and wind are the dominant sources of ocean noise in the frequency band between 20 and 500 Hz. This paper analyzes noise in that band using data from the SPICEX experiment, which took place in the North Pacific in 2004–2005, and compares the results with other North Pacific experiments. SPICEX included vertical arrays with sensors above and below the surface conjugate depth, facilitating an analysis of the depth dependence of ambient noise. The paper includes several key results. First, the 2004–05 noise levels at 50 Hz measured in SPICEX had not increased relative to levels measured by Morris [(1978). J. Acoust. Soc. Am. 64, 581–590] at a nearby North Pacific site three decades earlier, but rather were comparable to those levels. Second, at 50 Hz the noise below the conjugate depth decreases at a rate of −9.9 dB/km, which is similar to the rate measured by Morris and much less than the rate measured by Gaul, Knobles, Shooter, and Wittenborn [(2007). IEEE J. Ocean. Eng. 32, 497–512] for the ...