Juan I. Arvelo

Plumbing the depths of Ligeia: considerations for depth sounding in Titan's hydrocarbon seas.

Saturns moon Titan is the only satellite in this solar system with a dense atmosphere and hydrocarbon seas. The Titan Mare Explorer (TiME) mission would splashdown a capsule to float for 3 months on Ligeia Mare, a several-hundred-kilometer wide sea near Titans north pole. Among TiMEs scientific goals is the determination of the depth of Ligeia, to be achieved with an acoustic depth sounder. Since Titans surface temperature is known to vary around 92 K, all instruments must be ruggedized to operate at cryogenic temperatures. This papers contributions include an approach to infer key acoustic properties of this remote environment and the extraterrestrial environments influence on the development of a cryogenic depth sounder. Additionally, an approach is formulated to infer the transducers response, sensitivity, and performance when in situ calibration is impossible or when replicating key environmental conditions is too costly.

Adiabatic normal‐mode theory of sound propagation including shear waves in a range‐dependent ocean floor

A normal‐mode model, including absorptive effects and elastic waves in the ocean floor, is presented for a source in the water column and a receiver in the water or the elastic medium. The appropriate boundary conditions between all liquid and solid layers are taken into account, and an absorptive semi‐infinite basement is assumed, which causes the radiating wave‐number spectrum to be discrete. The adiabatic approach has been adopted for the case where the acoustic properties or the boundaries vary slowly with range. Results of these calculations display the importance of even a slight range dependence, and of bottom elasticity in shallow‐center sound propagation.

A wideband moving coil electrodynamic transducer system for autonomous underwater vehicle-based geoacoustic inversion

A small expendable wideband low-frequency sound source that will be deployed on the seafloor is being developed to be used for geoacoustic inversion surveys in conjunction with a terrain-hugging AUV. This low-cost deployable source contains a transducer that produces a relatively flat transmit response over the broad frequency band of 100 to 4000 Hz. In operation, a seafloor interface wave will be excited and exploited for geoacoustic inversion by the deployed sound source and a receiving array on the bottom-hugging AUV. A feasibility study is also being performed that includes physics-based sonar simulations to infer the performance of geoacoustic inversion in a number of AUV scenarios and environmental conditions. Based on this study, design trade-offs will be determined to finalize key factors of the transducer, such as its physical size, weight, and production cost. Battery technology is also being developed to optimize the source level, the duty cycle, and the operating life of the signals that will ...

Arctic marine mammal passive monitoring and tracking with a single acoustic sensor

The Arctic Ocean is a unique environment in the number of physical mechanisms that may be potentially exploited with much simpler acoustic systems than would be required in other oceans. The Arctic sound speed profile forms a surface duct with favorable cylindrical-spreading for near-continuous detection of marine mammal vocalizations. This ducted waveguide exhibit low seasonal variability, particularly under the ice cap, forcing under-ice sound to heavily interact with this rough elastic stratified boundary. The ice roughness introduces steeper slopes that enhance water-to-ice sound penetration [Arvelo, POMA 2012]. The ice elasticity is responsible for the excitation of a radially polarized longitudinal wave and a transverse-horizontal shear wave with group velocities around 2700-3000 m/s and 1550-1650 m/s, respectively. A third dispersive flexural vertical plate wave propagates at much slower speeds (<1200 m/s) at low frequencies [Stein, Euerle & Parinella, JGR 1998]. Vocalization distance may be estima...

Aggressive adaptive beamforming for ambient noise inversion with a limited‐aperture sonar on an autonomous platform.

Bottom loss inference from ambient noise inversion has been shown to yield accurate and robust predictions of undersea sound propagation and sonar detection performance using vertical line arrays and a conventional beamformer [Arvelo, J. Acoust. Soc. Am., 123, 679, 2008]. However, any array with a vertical aperture should be good candidates for ambient noise inversion. The main problem with vertical line arrays is that they are not well suited to rapidly survey a large area. Platform translation with a vertical array is expected to cause extreme array shape distortions that could seriously limit the accuracy of the results. To mitigate array shape distortion, a smaller array with vertical aperture could be mounted on the survey platform, which would allow it to survey the area at greater speeds. The platforms diameter certainly limits the vertical aperture of the array. A limited vertical aperture limits the frequency range, resolution, and accuracy of the estimated bottom loss. Since the array is rigidl...

Initial assessment of combustive sound source arrays as airgun alternatives for Arctic under-ice seismic exploration

Combustive sound source arrays consisting of submersible combustion chambers filled with a hydrogen/oxygen mixture are employed to assess their effectiveness for seismic exploration applications. The combustive mixture is ignited via spark and radiates acoustic pulses capable of undersea deep sub-bottom sediment penetration. Since electrolytic cells may be employed to generate the hydrogen/oxygen mixture from surrounding seawater, this source is an attractive alternative to airgun arrays for Arctic seismic exploration from under-ice platforms. Combustive sound source array configurations were tested in a central Virginia basin with hydrophones deployed in a line at another nearby basin. Seismic reflections are compared against nearby geologic cross sections of the central Virginia seismic zone. Funding provided via UAF sub-award under NOAA Grant NA09NOS4000262 and ONR.

Acoustic demonstrations for the iPod generation.

Today’s “iPod generation” can gain a unique and interactive exposure to physics by leveraging their musical interests, and their iPod, to explore the science of sound. Inside certain models reside a mini‐computer and a suite of acoustic and vibration sensors, including loudspeakers, microphones, and accelerometers. Combined with free or inexpensive software applications or “apps,” these highly portable devices can offer a tangible means to help students understand many physics, audio, and acoustics concepts that they can explore both inside and outside the classroom. We will demonstrate several apps that are currently available, for example, a seismometer, signal generator, noise level meter, spectrum analyzer, oscilloscope, and various musical instruments. Demonstrations utilizing an iPod Touch are guaranteed to excite students of all ages.

Validation of a physical model for surface clutter.

There is a strong need to more accurately represent active sonar system false contacts in various environments and conditions for the purpose of active acoustic simulation and synthetic training. A computationally viable approach for the generation of physics‐based false contacts in a raw beamformed time series that can be injected into a sonar processor was previously developed for bottom clutter. That model is now extended for clutter due to rough ocean surface scattering, and is compared with ASIAEX01 data. Considerations in extending the existing model include: characterizing appropriate statistics of the rough surface, adding surface scattering clutter to the sonar model, evaluating the realism of the simulation, and evaluating the computational burden (for real‐time trainers). The output of the simulation was analyzed for those considerations and then compared to the data. The use of a vertical line array allowed physical mechanisms in the data to be isolated, and their clutter data statistics separ...

Ambient noise inverted sonar performance sensitivity due to ocean variability

Undersea noise was collected on a vertical array affected by ocean currents and internal waves. Such fluctuations are known to negatively affect the accuracy of the inferred bottom loss over grazing angles [Arvelo and Prosperetti, J. Acoust. Soc. Am. 120, 3257 (2006)]. However, bottom loss is just an intermediate parameter to sonar performance predictions. The bottom loss at the dominant incident angles on the ocean floor is of most importance to the prediction of sound transmission. Therefore, the focus of attention should be on the effect of waveguide dynamics on the predicted acoustic propagation using the estimated bottom loss. Simultaneous transmission loss measurements are compared against predictions from several snapshots of collected element‐level wind‐driven noise to determine the bias and statistical moments associated with the in situ sonar performance estimations across frequency and range. In addition, dominant mode rejection will be applied on a subset of the data that is influenced by near...

Is higher resolution bathymetry always better for undersea sound propagation modeling

Bathymetry is one of the most important environmental parameters affecting undersea sound transmission in shallow waters. There has always been a preference towards bathymetry with the highest resolution and accuracy when conducting sound propagation modeling. However, extreme care must be exercised in the selection of the correct bathymetric resolution. It will be demonstrated that the correct resolution depends on the source of the geoacoustic model used. Higher resolution bathymetrics, such as multibeam surveys, introduce steeper slopes that yield larger bottom losses. If a lower resolution bathymetry was implemented to conduct geoacoustic inversion, then the same bathymetric resolution must be used to conduct subsequent sound transmission modeling. In addition, when geoacoustic inversion results are reported, the bathymetric resolution used should also be reported.