ONR Award Research Articles

Acoustic surveys in the Alboran Sea to map the presence and distribution of Cuvier’s beaked whales.

In 2008 and 2009, visual and acoustic surveys were organized by NURC and other institutions to estimate the diversity, abundance, and distribution of marine mammals in the Alboran Sea (Western Mediterranean Sea). The mapping of the presence of Cuvier’s beaked whales (Ziphius cavirostris) with wideband towed arrays of hydrophones was particularly successful and further demonstrated the efficacy of passive acoustic surveys conducted with a quiet dedicated platform, the NRV Alliance, equipped with high‐quality detection and monitoring instruments connected to a real‐time GIS system. Techniques and constraints for the detection and classification of echolocation clicks from beaked whales are discussed. [Research Cruises SIRENA08 and MED09. Work supported by ONR Award No. N00014‐09‐1‐0804.]

Sonar‐induced strains and stresses in an absorbing solid elastic sphere.

The interaction of harmonic plane and spherical pressure waves with an absorbing solid elastic sphere is modeled. The analytic, boundary‐element‐method (BEM) and finite‐element‐method (FEM) solutions for the internal displacement, strain, and stress fields are evaluated numerically for 50‐mm‐diameter solid spheres of poly(methyl methacrylate) (PMMA) and high‐density polyethylene (HDPE) in an immersion medium of mass density 1000 kg/m3 and sound speed 1500 m/s. The mass density and longitudinal and transverse sound speeds of the PMMA sphere are 1191 kg/m3, 2690 m/s, and 1340 m/s, respectively. The corresponding properties of the HDPE sphere are 957 kg/m3, 2430 m/s, and 950 m/s. The two materials are assumed to have a hysteresis‐type absorption, hence with constant product of absorption coefficient and wavelength. The respective values of this product for longitudinal and shear waves for PMMA are assumed to be 0.19 and 0.29 dB, and for HDPE, 0.40 and 1.20 dB. Each of two frequencies is considered, 10 and 100 kHz, for which the wavenumber‐radius product is π/3 and 10π/3, respectively. Results for the three solution methods are compared. [Work partly supported by NOPP through ONR Award No. N000140710992.]

Quantifying fish behavior with high‐frequency scientific echo sounder and parametric sonar.

Schools of Norwegian spring‐spawning herring (Clupea harengus) were observed in their wintering area off the northwest coast of Norway in November 2009 with the Simrad EK60 scientific echo sounder operating at 18, 38, 70, 120, and 200 kHz and Kongsberg TOPAS parametric sub‐bottom profiling sonar, with nominal difference‐frequency bandwidth 0.5–6 kHz. The beamwidths are 7 deg except at 18 kHz, where it is 11 deg. The TOPAS difference‐frequency beamwidth decreases with both range and frequency, but lies within the range 3–5 deg at the depths of the detected schools, typically 50–300 m. The registered depths of the upper surfaces of the detected schools have been compared, indicating that the apparent depth often, but not always, is inversely related to beamwidth. The phenomenon seems to be related to the pattern of backscattering with respect to frequency within the same school, which appears to have two distinct modes. The results are analyzed and discussed relative to the phenomenon of gas release and avoidance reaction. [Work partly supported by Norwegian Research Council Grant No. 184705 and ONR Award No. N000140910482.]

Target strength spectrum of Atlantic herring over a low‐kilohertz frequency band.

Atlantic herring (Clupea harengus) in the stock of Norwegian spring‐spawning herring were observed in their wintering area off the northwest coast of Norway in November 2009. The volume backscattering coefficient sV was measured with the Simrad EK60/38‐kHz scientific echo sounder, yielding numerical values of fish density ρ through the fundamental equation of echo integration sV = ρσ, where ρ is the mean backscattering cross section of the ensonified fish. Derived values of ρ were used to convert measurements of sV made with the Kongsberg TOPAS PS18 parametric sub‐bottom profiling sonar to infer σ, thence its logarithmic measure target strength, over the approximate difference‐frequency band 1.5–6 kHz. [Work supported by ONR Award No. N000140910482 and Norwegian Research Council Grant No. 184705.]

Pressure and displacement fields inside an absorbing fluid sphere ensonified by a point acoustic source: comparison of analytic and numerical solutions.

The interaction of an external point acoustic source in a lossy immersion medium with an absorbing fluid sphere is modeled. The analytic boundary-element-method and finite-element-method solutions for the internal pressure and displacement fields are evaluated numerically for a 50-mm-diameter fluid sphere in an immersion medium of mass density of 1000 kg/m3 and sound speed of 1500 m/s. The mass density and sound speed of the fluid sphere are treated as parameters varying from 0.5 to 2 times the value of the corresponding medium property. Absorption is neglected in the immersion medium but is assumed to be 0 or 10 dB per wavelength in the fluid sphere. Each of two frequencies is considered, 10 and 100 kHz, for which the approximate wavenumber-radius products are 1 and 10, respectively. Results for the three solution methods are compared. [Work partly supported by NOPP through ONR Award No. N000140710992.]

Pressure and displacement fields inside an absorbing fluid sphere ensonified by a plane harmonic wave.

Analytic expressions have been developed for the pressure and displacement fields and dilatation in a lossy fluid sphere due to ensonification by a plane harmonic wave in a lossy immersion medium. These quantities have been computed for a 50-mm-diameter fluid sphere in water with a density of 1000 kg/m3 and a sound speed of 1500 m/s for each of two frequencies, 10 and 100 kHz, hence with wavenumber-radius products of order 1 and 10, respectively. The density and sound speed values in the fluid sphere have been varied to achieve contrast factors in the range of 0.5–2 relative to the respective medium property. Absorption in the fluid sphere has been varied over the range of 0–10 dB/m. Results are presented for each field quantity along the axis of the sphere, as defined by the direction of propagation of the incident wave, transverse to this axis from the center, and along the surface from the forward to reverse directions. [Work partly supported by NOPP through ONR Award No. N000140710992.]

An acoustic finite‐element model to study sonar interactions with marine mammals.

A computer model based on the finite‐element method (FEM) is being developed to study the interaction of sonar signals with marine mammals. This model solves the Helmholtz equation in a computational box that includes the animal and the surrounding medium, water. The FEM code has been validated with analytical solutions for the scattering of a plane wave by a fluid sphere over a range of parameters and frequencies of interest. Then it has been applied to a specimen, a 142‐cm‐long common dolphin (Delphinus delphis), and internal pressure and displacement fields were computed. The animal is represented in the computer model by a set of tissue groups whose acoustic properties, density, and sound speed are taken from the literature. The geometry of each tissue group was constructed from segmented computerized tomography images. Results are presented for harmonic signals in the 1–10 kHz frequency range. [Work supported by NOPP through ONR award No. N000140710992.]

An acoustic finite-element model to study sonar interactions with marine mammals

A computer model based on the finite-element method (FEM) is being developed to study the interaction of sonar signals with marine mammals. This model solves the Helmholtz equation in a computational box that includes the animal and the surrounding medium, water. The FEM code has been validated with analytical solutions for scattering of a plane wave by a fluid sphere over a range of parameters and frequencies of interest. The same FEM code has been applied to a 142-cm-long specimen of the common dolphin (Delphinus delphis); internal pressure and displacement fields have been computed. The animal is represented in the computer model by a set of tissue groups whose acoustic properties, density and sound speed, are taken from the literature. The geometry of each tissue group was constructed from segmented computerized tomography images. Results are presented for harmonic signals in the 1-10 kHz frequency range. [Work supported by NOPP through ONR award N000140710992.]

Internal field in an immersed, absorbing fluid sphere excited by a plane acoustic wave

Modeling of sonar interactions with complicated bodies whose characteristic dimensions are of the order of or greater than the acoustic wavelength generally requires use of numerical methods. For validation of associated computer code, a particularly useful example is the internal field excited in an immersed, homogeneous, absorbing sphere by an external plane acoustic wave, since this can be expressed analytically. Here, numerical results are given for a fluid sphere with wave number-radius products spanning the range 1–10 and absorption coefficients spanning the range 0–10 dB per wavelength. The density and sound speed contrasts of the fluid sphere relative to the immersion medium are both varied over the range 0.5–2. [Work supported by NOPP through ONR award no. N000140710992.]