Eric M. Giddens

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.

Theory of sound propagation from a moving source in a three-layer Pekeris waveguide

A theory is developed for the acoustic field in a three-layer waveguide, representing the atmosphere, shallow ocean and sediment. The unaccelerated source is moving horizontally in the atmosphere. Two solutions are presented. The first, for a line source normal to the direction of travel, is a single wavenumber integral yielding the two-dimensional (2-D) field in each layer; and the second, for a point source, is a double wavenumber integral for the 3-D field in each layer. In both cases, the moving-source dispersion relationship for the three-layer environment is derived. From the 2-D dispersion relation, asymmetries fore and aft of the source, due to source motion, are shown to exist in the field in all three layers. In the water column, complex Doppler effects modify asymmetrically the effective depth of the channel and hence also the mode shapes. Evidence of the fore-aft modal asymmetry appears in the high-resolution spectrum of the field in the channel, which exhibits several sharp peaks on either side of the unshifted frequency, each associated with an up- or downshifted mode. A numerical evaluation of the 3-D solution provides a graphic illustration of the asymmetrical character of the field in all three layers.

Inversion of the propeller harmonics from a light aircraft for the geoacoustic properties of marine sediments

In a recent series of experiments, the sound from a light aircraft flying over a shallow ocean channel was detected on acoustic sensors in the atmosphere, throughout the water column and buried in the sediment. The predominant feature of the sound signature is a series of propeller harmonics extending over the frequency range from about 100 to 800 Hz. As the aircraft flies over the sensor station, a significant Doppler-downshift in the frequency of a given harmonic occurs. The difference in the Doppler-shifted frequency on approach and departure provides the basis of a technique for estimating the speed of sound in the sea bed. Once the sound speed is known, the remaining geoacoustic parameters of the sediment may be determined from the correlations that are known to exist between them.

A LIGHT AIRCRAFT AS A LOW-FREQUENCY SOUND SOURCE FOR ACOUSTICAL OCEANOGRAPHY

The sound from a propeller-driven aircraft is largely generated by the propeller itself. Its acoustic signature consists of a series of harmonics, the lowest of which, the fundamental, is typically in the region of 100 Hz, with the higher harmonics appearing at multiples of the fundamental. Experiments have recently been conducted off the coast of southern California, using several different types of single-engine, light aircraft, which show that the first 10 or so propeller harmonics are detectable not in the atmosphere, in the water column and also on sensors buried to a depth of about 1 m in the seabed. The Doppler-shifted comb of frequencies produced by an aircraft propeller is the basis of a recently developed technique for making point measurements of the low-frequency sound speed and attenuation in marine sediments

Geoacoustic inversions in shallow water using Doppler‐shifted modes from a moving source

Underwater acoustic measurements have been made of aircraft overflights of a shallow‐water (15 m deep) environment off the coast of La Jolla, CA. The narrow‐band harmonics generated by the engine and propeller penetrates the air–sea interface and excites the normal modes of the system with each mode having a characteristic Doppler shift. From a high‐resolution FFT, the modal eigenvalues are determined from the modal frequencies. These eigenvalues are dependent on the acoustic properties of the sediment. For a Pekeris waveguide, the eigenvalues can be found from the transcendental dispersion relationship, providing a direct method for inverting for the density and sound speed in the sediment without the use of a complicated numerical model. The method is relatively straightforward and only requires a single sensor in the water column. The sensitivity of the inversion technique will be discussed in the context of simulated experiments as well as real data collected in the experiments performed north of Scri...

Inversion of underwater sound from a high‐Doppler airborne source (a light aircraft) for the geoacoustic properties of the sea bed

An inversion technique has been developed that utilizes the sound from a light aircraft to obtain estimates of the low‐frequency (100–500 Hz) sound speed and attenuation in shallow‐water sediments. As the aircraft flies over the channel, each engine and propeller harmonic excites a set of normal modes in the water column. The modes are Doppler‐upshifted ahead of the aircraft and downshifted behind. In addition to the mode number, these frequency shifts depend on the sound speed in the sediment. A high‐resolution FFT of the subsurface sound from the aircraft returns the shifted modal frequencies, allowing the sound speed in the sediment to be determined. The sediment attenuation is obtained from the spatial gradient of the modal amplitudes along the aircraft track. Experiments on the inversion technique, using a variety of light aircraft, have been conducted in the Pacific Ocean about 1 km from shore between La Jolla and Del Mar, southern California, and as part of the ONR‐sponsored SAX04 in the northern G...

A method for obtaining the geoacoustic properties of marine sediments using sound generated by aircraft

A low‐frequency geoacoustic inversion method has been developed for an isovelocity waveguide using light‐aircraft as a source of sound. The high Doppler shift of the acoustic field facilitates the inversion, allowing directional information to be gathered by a single, omnidirectional receiver. The inversion method has been applied to simulations and to field experiments conducted in shallow water (14.4 m depth) off the coast of La Jolla. The inversion results will be discussed along with an error analysis using the Cramer‐Rao Lower Bounds. [Work supported by ONR and the ARCS Foundation.]

Numerical modeling of air‐to‐sea transmission of light aircraft noise

Recent experiments at SIO have shown that the acoustic signature of a light aircraft can be detected by sensors in the water column as well as buried in the underlying sediment and a method for extracting the sound speed and attenuation from this Doppler shifted signal has been proposed. To test the accuracy of this geoacoustic inversion technique, a numerical model of the air‐water‐sediment acoustic propagation, including the effects of a high‐speed airborne source, has been developed based on the spectral method. Simulated acoustic data have been generated representing an aircraft flying over a microphone in the atmosphere, a vertical line array in the ocean, and a hydrophone buried 1‐m deep in the sediment. The results of the geoacoustic inversion for sound speed and attenuation are compared to the known input parameter values of the model, giving a sense of the relative errors that may be expected when applying the technique to experimental data. [Work supported by ONR.]

Estimating the low‐frequency (0.1–1 kHz) sound speed in marine sediments using the harmonics from the propeller of a light aircraft

During ONR’s Sediment Acoustics Experiment 1999 (SAX99) in the northeastern Gulf of Mexico, several research groups made high‐precision, in situ measurements of dispersion in the medium‐sand sediment at frequencies greater than 20 kHz. Comparable precision at lower frequencies is difficult to achieve with in situ time‐of‐flight techniques because of wavelength issues which, inter alia, dictate an inconveniently large and costly acoustic source. Yet low‐frequency (1 kHz) sound speed measurements are sorely needed to distinguish between competing theoretical predictions. An alternative to the traditional travel‐time approach employs a single hydrophone buried in the sediment and, instead of an in situ sound source, the low‐frequency harmonics from the propeller of a light aircraft. Essentially, the airborne‐source technique relies on the difference between the Doppler‐shifted frequencies on aircraft approach and departure, as detected on the buried hydrophone, to yield a direct measure of the local sound sp...

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.]