Daniel D. Sternlicht

Remote sensing of sediment characteristics by optimized echo-envelope matching.

A sediment geoacoustic parameter estimation technique is described which compares bottom returns, measured by a calibrated monostatic sonar oriented within 15 degrees of vertical and having a 10 degree-21 degree beamwidth, with an echo envelope model based on high-frequency (10-100 kHz) incoherent backscatter theory and sediment properties such as: mean grain size, strength, and exponent of the power law characterizing the interface roughness energy density spectrum, and volume scattering coefficient. An average echo envelope matching procedure iterates on the reflection coefficient to match the peak echo amplitude and separate coarse from fine-grain sediments, followed by a global optimization using a combination of simulated annealing and downhill simplex searches over mean grain size, interface roughness spectral strength, and sediment volume scattering coefficient. Error analyses using Monte Carlo simulations validate this optimization procedure. Moderate frequencies (33 kHz) and orientations normal with the interface are best suited for this application. Distinction between sands and fine-grain sediments is demonstrated based on acoustic estimation of mean grain size alone. The creation of feature vectors from estimates of mean grain size and interface roughness spectral strength shows promise for intraclass separation of silt and clay. The correlation between estimated parameters is consistent with what is observed in situ.

Time-dependent seafloor acoustic backscatter (10–100 kHz)

A time-dependent model of the acoustic intensity backscattered by the seafloor is described and compared with data from a calibrated, vertically oriented, echo-sounder operating at 33 and 93 kHz. The model incorporates the characteristics of the echo-sounder and transmitted pulse, and the water column spreading and absorption losses. Scattering from the water-sediment interface is predicted using Helmholtz-Kirchhoff theory, parametrized by the mean grain size, the coherent reflection coefficient, and the strength and exponent of a power-law roughness spectrum. The composite roughness approach of Jackson et al. [J. Acoust. Soc. Am. 79, 1410-1422 (1986)], modified for the finite duration of the transmitted signal, is used to predict backscatter from subbottom inhomogeneities. It depends on the sediments volume scattering and attenuation coefficients, as well as the interface characteristics governing sound transmission into the sediment. Estimation of model parameters (mean grain size, roughness spectrum strength and exponent, volume scattering coefficient) reveals ambiguous ranges for the two spectral components. Analyses of model outputs and of physical measurements reported in the literature yield practical constraints on roughness spectrum parameter settings appropriate for echo-envelope-based sediment classification procedures.

Active rapid geoacoustic characterization using a seismic survey source

A survey of received acoustic energy levels from a seismic profiler were performed in Long Beach Harbor, CA, for compliance with the Marine Mammal Protection Act (MMPA). In addition to direct acoustic measurements, a rapid geoacoustic inversion algorithm was applied to the data to estimate the sediment properties acoustically. This inversion algorithm has matching criteria based on time spread, range-frequency interference patterns, and the range dependence of transmission loss. Self-consistency was checked by comparing acoustic measurements with predictions based on the inversion. With an estimated geoacoustic profile, predictions of received levels as a function of position in the range-dependent environment of Long Beach Harbor were then performed.

Stacking and averaging techniques for bottom echo characterization

Bottom‐looking sonars operating at frequencies greater than 30 kHz transmit acoustic signals which penetrate at most a few meters into seafloor sediments, making them well suited to characterize the water/sediment interface. Acoustic wavelengths at these frequencies are small compared to the rms relief of the interface, resulting in bottom echoes dominated by incoherent energy and varying significantly in amplitude and shape as the sonar translates longitudinally above the interface. Because of this variability, echoes must be treated stochastically and determining the average shape of the bottom echo involves careful selection of stacking and averaging algorithms. Comparisons of a geoacoustic temporal model with 33‐ and 93‐kHz backscatter echoes from sand and silt substrates reveal that a representative echo shape is obtained by averaging echoes along‐track over a distance roughly equivalent to the 6‐dB footprint diameter of the sonar beam pattern. To be meaningful, such averaging must be performed on ec...

Guest Editorial Special Issue on Synthetic Aperture Sonar

The eight papers in this special issue focus on synthetic aperture sonar. The focus is on signal processing and performance characterization for synthetic aperture imaging sonars, with emphasis on systems that operate in stripmap mode--a monostatic approach utilizing broadside beams, and which represents the majority of designs currently seeing practical application.

Acoustic energy attenuation of 3DVSP airgun arrays operating in Long Beach Harbor

Increased awareness of the disruptive impact high-power sonars and airguns potentially have on marine mammal populations has resulted in the adoption of flexible survey protocols, which account for movements of these animals through the region. To effectively modulate source transmissions and maintain adequate standoff distances, accurate estimates of range-dependent sound pressure levels (SPLs) are required. To determine safe ranges of airgun operation during a three dimensional vertical seismic profile survey (3DVSP) in Long Beach Harbor, stationary hydrophones were deployed to measure SPLs received from 160 in/sup 3/, 320 in/sup 3/ and 560 in/sup 3/ airgun arrays fired along a 5 km track. Recordings from bow and stern source boat aspects are analyzed for pulse energy, pulse duration, peak instantaneous pressure, and rms pressure over the pulse duration. For each airgun array and aspect combination, range-limited linear regression curve fits are carried out to characterize transmission loss. Distances at which the SPL drops below 190 dB, 180 dB, and 160 dB rms re: 1 /spl mu/Pa are determined, where operation of airguns at distances from marine mammals greater than the 160 dB range is mandated by the National Marine Fisheries Service (NMFS). For the 160 in/sup 3/ array, the bow aspect 160 dB level is estimated at 1279 m. For the 320 in/sup 3/ array, the bow and stern aspect 160 dB levels are estimated at 1712 m and 1916 m respectively. For the 560 in/sup 3/ array, the bow and stern aspect 160 dB levels are estimated at 2057 m and 2672 m respectively. The data collection and analysis techniques described in this paper rapidly and inexpensively determine NMFS compliant airgun operating procedures.

Near bottom sediment characterization offshore SW San Clemente Island

Normal incidence, 23.5 kHz seafloor acoustic backscatter data and bottom video were measured with the Deep Tow instrument package of the Scripps Institution of Oceanography in 100 meter water depth south of San Clemente Island, CA. The collected data were processed using an echo envelope sediment characterization method, to derive geoacoustic parameters such as particle mean grain size and the strength of the power law characterizing the roughness energy density spectrum of the sediment-water interface. Two regions, sand and silt, were selected based on available ground truth, perceived along-track sediment homogeneity, data quality and tow fish stability. Distinction between sand and fine grain sediments can be accomplished by creation of feature vectors comprised of mean grain size (M/sub /spl Phi//) and interface roughness spectral strength (w/sub 2/). Estimates for mean grain size and roughness spectral strength (M/sub /spl Phi//, w/sub 2/) were (1.5, 0.0095) for sand, and (6.7, 0.0033) for silt, where M/sub /spl Phi// is expressed in PHI units, and w/sub 2/ has units cm/sup 4/. These results are consistent with local ground truth measurements and illustrate the potential of this sediment characterization method in survey mode.

Historical development of side scan sonar

The seabed mapping side scan sonar, conceived and developed in the 1950s at the U.S. Navy Mine Defense Laboratory (USNMDL) [Commander and Sternlicht, J. Acoust. Soc. Am. 137, 2307 (2015)], was inspired by the Royal Navy ASDIC Type-162, a late-WWII hull mounted side-looking sonar capable of recording the silhouettes of bottomed U-boats. The commissioned U.S. Navy C-MK-1 SHADOWGRAPH, which operated in the Megahertz band and employed curved apertures for cross-track focusing, represents the first in a long history of military and commercial innovations. Commercial side scan sonars, typically operating in the tens or hundreds of Kilohertz, reached the marketplace in the 1960s. The SHADOWGRAPH series led to the Navy’s AN/AQS-14 and French DUBM-series mine detection sonars, and the active tow vehicles of the C-MK-1 served as test platforms for the first synthetic aperture sonars (SAS) fielded at USNMDL (now the Naval Surface Warfare Center Panama City Division) during the 1970s. Through the present day these sy...

Pioneers in side scan sonar: Julius Hageman and the shadowgraph

The concept of the side scan sonar was developed during the early 1950s at the U.S. Navy Mine Defense Laboratory, Panama City Florida—now known as the Naval Surface Warfare Center Panama City Division. In technical reports and laboratory notebooks, Dr. Julius Hageman, a German scientist who relocated to the Laboratory after World War II and worked there until his death in 1964, outlined the proposed “short-range high-definition mine location sonar” that would eventually become the C-MK-1 mine classification sonar system, more commonly known as “Shadowgraph.” Hageman’s patent for the concept (US Patent 4,197,591) was first disclosed in 1958, but remained classified until finally issued in 1980. The Shadowgraph was contracted by the U.S. Navy in 1957 and towed primarily from Oceangoing Mine Sweepers. It was operated as an undersea search and survey tool for more than 25 years before decommissioning in 1991. The Shadowgraph was a 1.5 MHz dual-sided side scan sonar, with range of 100 feet, and imaging resolution of approximately 3 in. square at a range of 75 feet. During its service life, it located numerous lost objects and aircraft on the sea floor and to this day has influenced the development of commercial and military sonars.

Downslope measurements from a bottom mounted tomography source

Long‐range tomography experiments seek to measure the temporal acoustic fluctuations due to thermal changes in the ocean structure. To map these changes to specific depths in the ocean, accurate travel times, and the ability to predict ray paths are required. Both of these become difficult in an environment that is bottom interacting (even though the water may be deep). To examine near source effects of interaction with the bottom for a tomographic source set on the bottom (at a depth of 800 m) measurements were taken off of Kauai. A short VLA was deployed from a small vessel and broadband recordings were taken at 6 ranges, from directly overhead to 55 km away. The issues to be addressed are the accuracy of the source timing and the relative strength of the bottom bounce and direct path. Evidence exists that interaction with the sea‐floor can lead to up to a 0.5 s delay in the measured travel time, from that predicted.