Peter H. Dahl

Underwater Mach wave radiation from impact pile driving: theory and observation.

The underwater noise from impact pile driving is studied using a finite element model for the sound generation and parabolic equation model for propagation. Results are compared with measurements using a vertical line array deployed at a marine construction site in Puget Sound. It is shown that the dominant underwater noise from impact driving is from the Mach wave associated with the radial expansion of the pile that propagates down the pile after impact at supersonic speed. The predictions of vertical arrival angle associated with the Mach cone, peak pressure level as function of depth, and dominant features of the pressure time series compare well with corresponding field observations.

On bistatic sea surface scattering: Field measurements and modeling

The bistatic scattering cross section of the sea surface, σ, is studied, along with a model for σ and its comparison with field data. The data are horizontal spatial coherence and ensemble-averaged intensity, which represent integral measures of sea surface bistatic scattering, and the model for σ is used to generate these same properties for comparison with the field data. The data are from an experiment conducted in shallow waters off southern Florida, using a sound frequency of 30 kHz. Directional wave measurements were made with a wave buoy positioned within 100 m of the acoustic measurements, with the environment characterized by rms wave heights of O(10) cm and wind speeds of 1–4 m/s. In the model σ is divided into two components: σr associated with scattering from the rough, air/sea interface, and σb associated with scattering from near-surface bubbles. The second-order small slope approximation is used to compute σr, which is a much improved approach over the traditionally used composite roughness...

High-frequency forward scattering from the sea surface: the characteristic scales of time and angle spreading

Forward scattering from the sea surface is discussed in the contest of a forward bounce path, or channel, through which high-frequency sound energy is transmitted. Such a channel might be used in an underwater communication or imaging task. Both time and angle spreading are inherent to the process of forward scattering by a roughened sea surface. Spreading in each domain relates, via Fourier transform, to a conjugate or coherence separation variable, e.g., angle spreading and spatial coherence. The measurement and modeling of time and angle spreading are discussed, with the modeling incorporating the bistatic cross section of the sea surface. A characteristic scale for each spread variable is defined: L for the time spread and /spl sigma//sub /spl theta/h/ and /spl sigma//sub /spl theta/v/ for the horizontal and vertical angular spread, respectively. Simplified expressions for these characteristic scales as a function of array acquisition geometry and sea surface conditions are also obtained. Data from two field experiments are discussed, one conducted in shallow waters of 30-m depth, and one conducted in deep, pelagic waters of 4000-m depth. Both experiments utilized frequencies /spl ges/20 kHz. The role of bubbles in forward scattering is illustrated using measurements from the deep-water experiment. It was demonstrated that bubbles can attenuate the forward-scattered signal, but otherwise have little effect on L and /spl sigma//sub /spl theta/h,v/ until their concentrations approach those necessary to nearly extinguish the signal scattered from the air/sea interface.

Forward scattering from the sea surface and the van Cittert-Zernike theorem.

The van Cittert-Zernike theorem is used to generate models for the spatial coherence of a sound field that has been forward scattered from the sea surface. The theorem relates the spatial coherence of an observed wave field to the distant source intensity distribution associated with this field. In this case, the sea surface upon ensonification is taken to be the source, and the sea-surface bistatic cross section corrected for transmission loss is taken as a surrogate for the source intensity distribution. Improvements in methodology for generating an estimate of the 2D autocorrelation function for sea surface waveheight variation, necessary to compute the bistatic cross section, are documented in the Appendix. Upon invoking certain approximations, simple expressions for the characteristic length scales of vertical, horizontal, and horizontal-longitudinal coherence, are derived from the theorem. The three coherence length scales identify a coherence volume for the spatial coherence of a sound field arriving via the surface bounce channel. Models for spatial coherence derived from the van Cittert-Zernike theorem without these approximations compare reasonably well with measurements of complex vertical coherence made at 8 kHz and 20 kHz in the East China Sea as part of the 2001 ASIAEX field program. In terms of the ASIAEX field geometries and sea-surface conditions, at frequency of 20 kHz the coherence volume is a vertical layer 0.5 m thick by 3 m in each of the two horizontal dimensions; at 8 kHz these dimensions increase by a factor of 2.5, representing the ratio of the two frequencies.

Microwave and acoustic scattering from parasitic capillary waves

We report simultaneous microwave and acoustic Doppler backscattering measurements made in a wind-wave tank. The microwave system operated at 35 GHz (0.857 cm), while the acoustic system transmitted at 190 kHz (0.777 cm). The two systems were mounted to view the surface at the same incidence angle, which was varied. The measurements showed that when both systems looked upwind, horizontal transmit and receive polarization (HH) microwave backscatter from the rough water surface was 2 to 12 dB stronger than acoustic backscatter, depending on incidence angle and wind speed. When the acoustic system looked downwind, however, its backscattering level was consistently about 1 dB lower than that of the upward-looking microwave system. We interpret these results to indicate that both the acoustic and microwave systems were scattering from parasitic capillary waves in addition to freely propagating, wind-generated waves. The tilt of the parasitic capillary waves accounts for the observed differences in microwave and acoustic backscatter. We show that Bragg scattering theory predicts both the intensity and the Doppler shift of the microwave and acoustic signals very well using known properties of parasitic capillary waves. Spectral densities of the parasitic capillary waves derived from this Bragg scattering model are in good agreement with those predicted recently by Fedorov and Melville [1998] and observed by Fedorov et al. [1998].

Geoacoustic inversion results from the ASIAEX East China Sea experiment

This work presents the results of geoacoustic inversions carried out using data from the Asian Seas International Acoustics Experiment East China Sea. Broadband data from small explosive sources were used for the inversions. Compressional wave speeds in the sediment and basement layers were estimated using a nonlinear, long-range, tomographic inversion technique based on group speed dispersion. This tomographic technique is a hybrid approach that combines a genetic algorithm for global parameter search with a Levenberg-Marquardt method for fine-scale parameter tuning. The results were compared with data from gravity and piston cores and a geophysical survey conducted at the experimental location using a watergun and towed hydrophone array.

On bubble clouds produced by breaking waves: An event analysis of ocean acoustic measurements

A field experiment was carried out to measure the time-varying properties of bubble clouds as they evolve from ocean breaking waves. The experiment was conducted from the floating instrument platform Flip and consisted of simultaneous, colocated acoustic measurements of subsurface bubbles and video recordings of the sea surface. Details of two breaking wave events are presented. The results of the acoustic measurements of bubble density and their entrainment depth are reported for the time t/Tc ≳ 3, where t is the time from breaking and Tc is a characteristic timescale based on the speed of the breaking wave crest as estimated from the video recordings. Order of magnitude values for the air-void fraction β, vertical eddy diffusivity kν, and kinetic energy dissipation e associated with breaking waves are presented. The wind speed dependence of average acoustic backscattering properties over the course of the experiment is also given.

Simultaneous acoustic and microwave backscattering from the sea surface

Simultaneous and coincident measurements of acoustic and microwave backscatter from the air/sea interface were obtained during Phase II of the SAXON-FPN experiment in December 1992 and again in March 1993. The acoustic and microwave grazing angles were both set to 17°, and the wavelengths were matched, being set to 2.14, 3.00, and 5.66 cm, corresponding to, respectively, acoustic frequencies of 26.5, 50, and 70 kHz and microwave frequencies of 5.3, 10, and 14 GHz. Backscattering cross sections normalized by ensonified area for the acoustic (σ0a) and microwave (σ0m) returns were determined, and their dependence on wind speed was investigated. The acoustic scattering strength is defined as 10 log10(σ0a) and the microwave scattering strength is defined as 10 log10(σ0m)−10 log10(4π). The results of these experiments show that the two scattering strengths are comparable at wind speeds below about 3 m/s but that the acoustic scattering strength increases much faster than the microwave scattering strength with i...

Sea-bed acoustic parameters from dispersion analysis and transmission loss in the East China Sea

As a part of the Asian Seas International Acoustic Experiment (ASIAEX) in the East China Sea, sound propagation signals from wideband explosive sources were measured using a 32-element vertical line array. Measurements were made as a function of range in two perpendicular tracks. Sea-bed geoacoustic parameters based on a fluid half-space geoacoustic model (sound speed, density, and attenuation) are inverted from the sound propagation in the frequency range 100-500 Hz. The sea-bed sound speed and density were first derived from mode arrival time differences which were obtained using a spatial mode filtering technique. Sea-bed acoustic attenuation was subsequently estimated by comparing measured transmission loss with model results.

Acoustic tomography for monitoring the Sea of Japan: a pilot experiment

A pilot experiment was conducted in the Sea of Japan (also called the East Sea) in September-October 1999, to assess the possibility of using acoustic tomographic techniques for monitoring water mass structure and dynamics. Acoustic m-sequence signals at various frequencies between 250 and 634 Hz were transmitted from bottom-mounted acoustic sources in shallow water off the coast of Vladivostok to vertical-array receiving systems deployed off the north coast of Ulleung-Do island (S. Korea), 558 km to the south. The data are analyzed for temporal correlation, time spread, and transmission loss and are interpreted in terms of a tomographic system for monitoring the East Sea.