Fernando Simonet

PROPELLER NOISE FROM A LIGHT AIRCRAFT FOR LOW-FREQUENCY MEASUREMENTS OF THE SPEED OF SOUND IN A MARINE SEDIMENT

The sound from a light aircraft in flight is generated primarily by the propeller, which produces a sequence of harmonics in the frequency band between about 80 Hz and 1 kHz. Such an airborne sound source has potential in underwater acoustics applications, including inversion procedures for determining the wave properties of marine sediments. A series of experiments has recently been performed off the coast of La Jolla, California, in which a light aircraft was flown over a sensor station located in a shallow (approximately 15 m deep) ocean channel. The sound from the aircraft was monitored with a microphone above the sea surface, a vertical array of eight hydrophones in the water column, and two sensors, a hydrophone and a bender intended for detecting shear waves, buried 75 cm deep in the very-fine-sand sediment. The propeller harmonics were detected on all the sensors, although the s-wave was masked by the p-wave on the buried bender. Significant Doppler shifts of the order of 17%, were observed on the microphone as the aircraft approached and departed from the sensor station. Doppler shifting was also evident in the hydrophone data from the water column and the sediment, but to a lesser extent than in the atmosphere. The magnitude of the Doppler shift depends on the local speed of sound in the medium in which the sensor is located. A technique is described in which the Doppler difference frequency between aircraft approach and departure is used to determine the speed of sound at low-frequencies (80 Hz to 1 kHz) in each of the three environments, the atmosphere, the ocean and the sediment. Several experimental results are presented, including the speed of sound in the very fine sand sediment at a nominal frequency of 600 Hz, which was found from the Doppler difference frequency of the seventh propeller harmonic to be 1617 m/s.

An Autonomous Open-Ocean Stereoscopic PIV Profiler

Abstract Over the past decade, a novel free-fall imaging profiler has been under development at the Scripps Institution of Oceanography to observe and quantify biological and physical structure in the upper 100 m of the ocean. The profiler provided the first detailed view of microscale phytoplankton distributions using in situ planar laser-induced fluorescence. The present study examines a recent incarnation of the profiler that features microscale turbulent flow measurement capabilities using stereoscopic particle image velocimetry (PIV). As the profiler descends through the water column, a vertical sheet of laser light illuminates natural particles below the profiler. Two sensitive charge-coupled device (CCD) cameras image a 25 cm × 25 cm × 0.6 cm region at a nominal frame rate of 8 Hz. The stereoscopic camera configuration allows all three components of velocity to be measured in the vertical plane with an average spatial resolution of approximately 3 mm. The performance of the PIV system is evaluated ...

Deep Sound: A Free-Falling Sensor Platform for Depth-Profiling Ambient Noise in the Deep Ocean

Ambient noise in the deep ocean is traditionally monitored using bottommounted or surface-suspended hydrophone arrays. An alternative approachhas recently been developed in which an autonomous, untethered instrument platform freefallsundergravityfromthesurfacetoapreassigneddepth,whereadropweight isreleased,allowingthesystemtoreturntothesurfaceunderbuoyancy.Referredto as Deep Sound, the instrument records acoustic, environmental, and system data continuously during the descent and ascent. The central componentof Deep Sound is a Vitrovex glass sphere, formed of two hemispheres, which houses data acquisition and storage electronics, along with a microprocessor for system control. A suite of sensors on Deep Sound continuously monitor the ambient noise, temperature, salinity, pressure, and system orientation throughout the round trip from the surface to the bottom. In particular, several hydrophones return ambient noise time series, each with a bandwidth of 30 kHz, from which the noise spectral level, along

AUE: An Autonomous Float for Monitoring the Upper Water Column

A new autonomous underwater vehicle for exploring the upper water column has been developed. The Autonomous Underwater Explorer (AUE) is a 25 cm diameter sphere whose mission is to monitor currents in the upper 100 m of the ocean via either underwater acoustic tracking or by providing a (lat, Ion) fix via GPS after surfacing. Equipped with both temperature and depth sensors the float is capable of tracking both isotherms and isobars by adjusting its density via a piston that changes the volume of the vehicle. Equipped with an acoustic modem we envision fleets of such systems tracking water parcels in coastal regions while reporting their positions to each other and a surface ship in real time. Equipped with more advanced sensors such as fluorometers, nutrient sensors, or other miniature advanced devices, such small autonomous systems have the promise to provide a more synoptic view of the water column than heretofore possible.

Estimation of In Situ 3-D Particle Distributions From a Stereo Laser Imaging Profiler

In this paper, an image processing system for estimating 3-D particle distributions from stereo light scatter images is described. The system incorporates measured, three-component velocity data to mitigate particle blur associated with instrument motion. An iterative background estimation algorithm yields a local threshold operator that dramatically reduces bias in particle counts over the full image field. Algorithms are tested on simulated particle distributions and data from an open-ocean profile collected near the Santa Barbara Channel Islands, CA. They yield over a 50% reduction in root-mean-squared error in particle size estimates, and a 30% reduction in the magnitude of the motion blur point spread function. In situ particle distributions are estimated and compared to several models. It is demonstrated that quantitative, 3-D particle distributions can be accurately estimated from these data for particles with diameter larger than 4 pixels (0.8 mm).

Depth profiling ambient noise in the deep ocean

Deep Sound is an untethered instrument platform designed to free‐fall from the sea surface to a preassigned depth, at which point a burn wire releases a weight, allowing the system to return to the surface under buoyancy. The descent and ascent rate is 0.6 m/s. A Vitrovex glass sphere houses lithium‐ion batteries and a suite of microprocessor‐controlled electronics for data acquisition, data storage, power management, and system control. Outside the sphere, several hydrophones are arranged in vertical and horizontal configurations, a CTD returns environmental data, and motion sensors monitor pitch, roll, and yaw. Data may be downloaded, and the batteries may be recharged, via throughputs in the sphere. The hydrophones, with a bandwidth of 30 kHz, are rated to a depth exceeding 11 km, and the sphere itself has a depth‐rating of 9 km. The system made three descents in the Philippine Sea in May 2009, to depths of 5100, 5500 and 6000 m; and in November 2009, two descents were made in the Mariana Trench to a d...

Vertical profiling of ambient noise with Deep Sound.

Deep Sound is a free‐falling high‐bandwidth acoustic recording system designed to profile ambient noise from the surface to depths of 9 km. The recording platform is autonomous and descends under gravity to its preprogramed maximum depth, where a burn‐wire releases weight, permitting the system to return to the surface under its own buoyancy. Two hydrophones are mounted at half meter vertical spacing allowing the noise spectrum and vertical coherence (directionality) to be obtained over four decades of frequency (10 Hz–100 kHz). The acoustic recordings are made continuously as the instrument descends and ascends along with measurements of sound speed and depth. The systems low power and large data storage capabilities allow round trips to the deepest trenches of the ocean. Alternative modes of operation include (1) synthetic aperture signal detection and (2) residence on the seabed with return to the surface at a later time. Deep Sounds design and acoustic characteristics will be described and data from...

Acoustic reflectivity measurments of sirenia (Florida manatees) at high frequencies

The Florida manatee (Trichechus manatus latirostris) is an endangered sirenian inhabiting shallow coastal waters of Florida (USA). Its population (∼3300) is dwindling, with 25%–30% of mortalities occurring due to collisions with boats (95 losses in 2002). An active sonar system that detects the presence of the animals, and hence alerts the boater, could help reduce the collisions. Experiments were performed on six captive manatees with several calibrated transducers in a 33×10×3 m3 pool to learn how much sound is reflected by these animals. At a frequency of 171 kHz we transmitted a brief sinusoid. An underwater video camera was aimed along the axis of the range direction of the sound transmission, permitting the co‐registration of animal and acoustics. The camera and sonar were calibrated together by translating a 38‐mm tungsten carbide sphere (TS=−39 dB@ 171 kHz) in a separate test tank facility. Results indicate that the reflectivity of the animals (not strictly target strength) is somewhat low, in the...

Estimation of the acoustic reflectivity of a Florida manatee from physical measurements of animal tissue

In order to better understand the cause of low acoustic reflectivity of the Florida manatee (Trichechus manatus latirostris), a series of measurements were taken from a stranded animal post‐mortem. The samples came from an adult male that was struck and killed by a boat in April 2004. These measurements were used to calculate the speed of sound, density and attenuation within each sample. To achieve a good statistical representation, many subsamples (connective tissue, blubber and muscle) were extracted from ventral, dorsal and lateral samples collected from approximately the midpoint of the body (at the umbilicus). Results for the connective tissue samples indicate an average sound speed, density and attenuation of 1725 m/s, 1089 kg/m3 and 5.21 dB/mm, respectively. These results establish a reflection coefficient of 0.12, or, in other words, only 12% of the incident acoustic wave is reflected in the water–skin interface.

Sound from a light aircraft for underwater acoustic inversions

Experiments are being conducted in shallow (30 m) water off La Jolla, CA, to investigate the potential usefulness of the sound from a single‐engine, propeller‐driven, light aircraft for performing underwater acoustic inversions. The sound signature of the aircraft contains harmonics between 50 Hz and 1 kHz, which return the low‐frequency geoacoustic properties of the seabed. A microphone approximately 1 m above the surface monitors the sound in air, a seven‐element vertical array detects the acoustic arrivals underwater and a single, buried hydrophone receives the signals in the sediment. Aircraft overflights have been made at altitudes between 33 m and 330 m, yielding the altitude‐dependence of the peak levels received underwater. Using the vertical array, the reflection coefficient of the seabed is being measured as a function of grazing angle. From the reflection coefficient, the critical angle of the sea floor and hence the sound speed in the sediment are inferred. The sound speed in the sediment should also be available directly from the Doppler shift on the buried hydrophone. These techniques and the available data sets will be discussed in the presentation. [Work supported by ONR.]