Mid-Frequency Noise Notch in Deep Water. W.S. Hodgkiss / W.A. Kuperman. June 1, 2012 May 31, 2013

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1 Mid-Frequency Noise Notch in Deep Water W.S. Hodgkiss and W.A. Kuperman June 1, 2012 May 31, 2013 A Proposal to ONR Code 322 Attn: Dr. Robert Headrick, Office of Naval Research BAA UCSD ABSTRACT Technical Contact: Administrative Contact: W.S. Hodgkiss / W.A. Kuperman Nancy Wilson whodgkiss@ucsd.edu / wkuperman@ucsd.edu nwilson@ucsd.edu / Marine Physical Laboratory Scripps Institution of Oceanography (SIO) / University of California, San Diego 9500 Gilman Drive, La Jolla CA We propose a combined experimental and modeling program to study the mid-frequency noise notch in deep water. A very sensitive 100 element vertical array will be used for noise measurements in the 1-10 khz band. Experimental results will be interpreted using in situ environmental data, theory and modeling. The overall results will also be interpreted in the more general context of arbitrary array configurations. Since no sources will be deployed, there will be no need for a source-related EIS. 1

2 Mid-Frequency Noise Notch in Deep Water W.S. Hodgkiss and W.A. Kuperman June 1, 2012 May 31, 2013 A Proposal to ONR Code 322 Attn: Dr. Robert Headrick, Office of Naval Research BAA UCSD VOLUME I: TECHNICAL PROPOSAL Technical Contact: Administrative Contact: W.S. Hodgkiss / W.A. Kuperman Nancy Wilson whodgkiss@ucsd.edu / wkuperman@ucsd.edu nwilson@ucsd.edu / Marine Physical Laboratory Scripps Institution of Oceanography (SIO) / University of California, San Diego 9500 Gilman Drive, La Jolla CA Table of Contents Objective p. 2 Background p. 2 Approach p. 2 Experimental Program p. 3 Modeling Program p. 3 Array and Signal Processing Analysis p. 4 Deliverables p. 4 Transition Potential p. 4 References p. 4 1

3 Objective: To measure and model the deep-water noise notch at mid frequencies. Background: Signals need to be extracted from noise, the latter having been studied in various details over the years (Carey, Jensen et al.). Understanding the physics of the deep-water mid frequency noise notch is key to utilizing the noise notch for an assortment of convergence zone scale ASW applications. In particular, the actual minimum level of the noise notch under various array-processing configurations is indicative of the potential for utilizing this ocean acoustic phenomenon. The noise notch is a well-known phenomenon resulting from noise sources near the surface. It has been particularly studied in the lower frequency, ship generated noise regime (see for example Wagstaff). There, the noise notch results from a combination of downward refraction and the dipole structure associated with distant sources near the pressure release ocean surfaces. At these lower frequencies, the common mechanisms for filling in the deep water noise notch primarily originating from shipping lane sources are shoaling of the sound channel between polar to temperate regions and continental slope, heavy shipping traffic environments in which down slope propagation converts steeper rays to more horizontal paths. In shallow water, the lower frequency noise notch case is discussed in papers by Rouseff and Ferat et al. and Yang et al. (also see references contained therein) where, for example, Rouseff takes internal wave activity as the notch filling mechanism. At mid-frequencies, however, the contributions from surface noise sources (see for example, Kennedy) will typically come from within a few convergence zones. Rather than the above effects, the notch filling mechanisms in deep water areas are likely to include ocean variability such as that originating from internal waves. In any event, there is in an important need for calibrated measurements of mid-frequency, beamformed noise to quantitatively ascertain the lowest levels of the noise notch (see for example Holden and references therein but in which their array did not have resolution to measure the notch) and for a model to relate these results to the oceanographic environment. While this total problem is rich in basic and applied research issues in oceanography, acoustic propagation through a fluctuating medium and signal/array processing, it has an important, potentially near-term impact on understanding and improving a class of ASW operations. Approach: The approach will involve at-sea measurements/data analysis and modeling. There are no plans to include an acoustic source in these measurements so there will be no need for an associated EIS. 2

4 A. Experimental Program: We propose an experimental program that is comprised of two deep- water experiments: 1) a combination engineering trial and noise measurement experiment in which the vertical array is suspended from the ship in deep water; and 2) a deep-water deployment of the array suspended from a remote buoy. The experiments will use a one hundred element array emphasizing the 1-10 khz regime. The measurements will use a DURIPfunded array of sensitive hydrophones (the HTI-92-WB that has a preamp self noise of 27 db re 1 upa/sqrt(hz) at 1 khz and that drops to 15 db re 1 upa/sqrt(hz) at 10 khz). There will be two types of deployments (which also serves the purpose of risk mitigation). The first will be the array hung from the ship itself. The second will utilize a buoy based remote system with connectivity. As mentioned, during the engineering sea test, the array will be deployed over the side of the ship. We anticipate the array being deployed to a depth of ~300 m with the data stream being recorded directly on the ship. While not the eventual autonomous deployment mode, this approach has the advantage of testing the entire array electronics and data acquisition system while preserving the ability to quickly recover the hardware if any issues develop. The data from the engineering sea test also will provide a valuable initial look at the mid-frequency noise notch in deep water. During the primary data collection experiment, an off board buoy will be used to tether the array and data acquisition system. Here we also anticipate the array being deployed to a depth of ~300 m with the data stream being recorded on the buoy. This autonomous mode has the advantage of enabling the ship to stand off to mitigate any concerns regarding its own self-noise contaminating the ambient noise measurements. The buoy will have GPS navigation and wireless connectivity to the ship. This will provide us the opportunity to monitor the health of the system as well as to copy over modest amounts of data for analysis prior to recovery of the buoy. Both the engineering sea test (~4 days) and data collection experiment (~10 days) are planned for deep water west of San Diego off the Patton Escarpment. B. Modeling Program: We will modify our suite of existing noise codes to predict mid-frequency noise in a fluctuating ocean. Emphasis will be on a fast, RAM-based Parabolic Equation (PE) that includes a module to simulate internal wave dynamics for the propagation medium. For the PE, surface noise sources will be coupled into the water channel during each step of the marching algorithm. Other inputs into the model will be (measured) sound speed profiles, surface roughness and oceanographic information from which an estimate of internal dynamics can be made. The cross-spectral density function of the acoustic field will be computed and an ability to simulate data realizations will be developed. 3

5 C. Array and Signal Processing Analysis: We will interpret the experimental/modeling results in terms of predicting the performance relevant system configurations. Conventional and adaptive array processing methods will be used. Deliverables: Deliverables include research papers and focused reports. Transition Potential: The output of this program is relevant to some of the efforts of N875. References W. M Carey and R.B. Evans, Ocean Ambient Noise, Springer, New York (2011) F.B. Jensen, W. A. Kuperman, M.B. Porter and H. Schmidt, Computational Ocean Acoustics, 2 nd Edition, Springer, New York (2011) R. A. Wagstaff, Low-frequency ambient noise in the deep ocean sound channel-the missing component, J. Acoust. Soc. Am. 69, 1009 (1981). D. Rouseff and D. J. Tang, Internal wave effects on the ambient noise notch in the East China Sea;Madel/data comparison, J. Acoust. Soc. Am. 120, 1284 (2006). P. Ferat and J. Arvelo, Mid- to High-Frequency Ambient Noise Anisotropy and Notch- Filling Mechanisms, p.497, in High Frequency Ocean Acoustics, edited by M.B. Porter, M. Siderius, and W. A. Kuperman, AIP CP /04 (2004). T. C. Yang and K. Yoo, Modeling the environmental influence on the vertical directionality of ambient noise in shallow water, J. Acoust. Soc. Am., 101, 2541 (1997). R.M. Kennedy and T. K. Szlyk, Modeling high-frequency vertical directional spectra, J. Acoust. Soc. Am. 102, 673 (1991). A. Holden, Measurements and Predictions of High Frequency Ambient Noise, p. 508, in High Frequency Ocean Acoustics, edited by M.B. Porter, M. Siderius, and W. A. Kuperman, AIP CP /04 (2004). 4