Steven G. Schock

Marine sediment classification using the chirp sonar

The chirp sonar is a calibrated wideband digital FM sonar that provides quantitative, high‐resolution, low‐noise subbottom data. In addition, it generates an acoustic pulse with special frequency domain weighting that provides nearly constant resolution with depth. The chirp sonar was developed with the objective of remote acoustic classification of seafloor sediments. In addition to producing high‐resolution images, the calibrated digitally recorded data are processed to estimate surficial reflection coefficients as well as a complete sediment acoustic impulse profile. In this paper, surficial sediments in Narragansett Bay, RI are used to provide ground truth for an acoustic model. Quantitative acoustic returns from the chirp sonar are used to estimate surficial acoustic impedance and to predict sediment properties. A robust acoustic sediment classification model that uses core samples to account for the local depositional environment has been developed. The model uses an estimate of acoustic impedance t...

Chirp subbottom profiler for quantitative sediment analysis

A wide‐band, frequency‐modulated, subbottom profiling system (the chirp sonar) can remotely determine the acoustic attenuation of ocean sediments and produce artifact‐free sediment profiles in real time. The chirp sonar is controlled by a minicomputer which performs analog‐to‐digital and digital‐to‐analog conversion, correlation processing, and attenuation estimation in real time. The minicomputer generates an FM pulse that is phase‐ and amplitude‐compensated to correct for the sonar system response. Such precise waveform control helps suppress correlation noise and source ringing. The chirp sonar, which has an effective bandwidth of 5 kHz, can generate chirp (Klauder) wavelets with a tuning thickness (Rayleigh’s criterion for resolution) of approximately 0.1 ms. After each return is correlated, a computationally fast algorithm estimates the attenuation of subbottom reflections by waveform matching with a theoretically attenuated waveform. This algorithm obtains an attenuation estimate by minimizing the m...

Buried object scanning sonar

A sonar, designed to scan for objects buried in the seafloor, generates images of pipe and cable sections and ordnance buried in sand. The sonar operates by illuminating a broad swath of the seabed using a line array of acoustic projectors while acoustic backscattering from the illuminated sediment volume is measured with a planar hydrophone array. The line transmitter performs along track beamsteering to improve the SNR of buried target images by illuminating major target surfaces at normal incidence and to reduce volume scattering by limiting the volume of sediments illuminated. The output of the planar-hydrophone array undergoes nearfield focusing which allows the sonar to operate near the seabed where target images have the highest SNR and resolution. The nearfield focusing reduces scattering noise by approximately 12 dB, an improvement measured by comparing the SNR of target echoes in single channel data with the SNR of buried targets in the focused imagery. Plan and side views of the seabed generated from a three-dimensional matrix of focused data provide the position and burial depth of targets covered by sand off Hawaii. An energy detector automatically locates targets in the focused image data.

A method for estimating the physical and acoustic properties of the sea bed using chirp sonar data

This paper proposes a method, based on the Biot model, for estimating the physical and acoustic properties of surficial ocean sediments from normal incidence reflection data acquired by a chirp sonar. The inversion method estimates sediment porosity from reflection coefficient measurements and, using the estimated porosity and the measured change in fast wave attenuation with frequency, estimates the permeability of the top sediment layer. The spectral ratio of echoes from the interface at the base of the upper sediment layer and from the sediment-water interface provides a measure of the change in attenuation with frequency. Given the porosity and permeability estimates, the Kozeny-Carman equation provides the mean grain size and the inversion method yields the acoustic properties of top sediment layer. The inversion technique is tested using chirp sonar data collected at the 1999 Sediment Acoustics Experiment (SAX-99) site. Remote estimates of porosity, grain size, and permeability agree with direct measurements of those properties.

Sonar attenuation modeling for classification of marine sediments

An attenuation‐based model for classification of marine sediments is developed for the chirpsonar operating in the frequency range of 2–10 kHz. A relaxation‐time model is proposed that combines the various dissipative energy loss mechanisms of sound in marine sediments into a single parameter. Historical data were analyzed by converting attenuation values reported in ‘‘dB/m@kHz’’ to a single relaxation time value. Analysis of these previous attenuation measurements supports the use of a relaxation‐time model. Based on this large collection of data, an empirical equation is developed that relates relaxation time to grain size (in phi units). Using this model, very little phase dispersion is observed for a correlated chirp pulse traveling through 40 m of sand, silt, or clay. Yet, this is not so for a pulse in the ultrasonic frequency range (0.2–1.0 MHz) traveling through only 10 cm of clay. Here, significant dispersion is noted. Because of the unique Gaussian‐like shape of the correlated chirp pulse power spectrum, pulse elongation due to attenuation is minimized. Using the center frequency shift in the pulse spectrum, a new ‘‘instantaneous frequency’’ method of attenuation estimation is proposed that overcomes the problems associated with interfering reflections. Based on the relaxation‐time model, the correlated chirp pulse was synthetically attenuated to establish a relation between the relaxation time and the center frequency shift. I n s i t u sediment‐type predictions from chirpsonar data using the instantaneous frequency method and analyses of core samples taken in the Narragansett Bay, Rhode Island are in good agreement.

Sediment classification based on impedance and attenuation estimation

This paper presents a remote marine sediment classification model that can be implemented in real time while underway in a survey. The model is based on the estimation of impedance and attenuation of subbottom sediments from normal incident reflection seismograms. A robust impedance inversion model utilizing layer detection was developed which is implemented with significantly less computation when compared to full inverse methods, and hence runs in real time. A new ‘‘weighted least‐squares fitting’’ procedure is proposed for evaluating the impulse response of the sediment column. The acoustic attenuation in the sediment is determined by measuring the frequency shift of the pulse spectrum using an instantaneous frequency method. The impedance inversion model requires an input of the estimated attenuation to account for the loss in signal energy due to absorption. A recently developed model relating sediment acoustic properties to sediment physical properties for a given depositional environment is employed. The constants appearing in the classification model are evaluated using measurements from a few core samples. Impedance and attenuation estimates are used to predict sediment properties such as porosity, density, mean grain size, and sound speed. The reflection data for the present study were acquired by a linear wideband (full spectrum) sonar. It is used because of its linear system components, high resolution, and wide bandwidth. Analysis of acoustic data acquired by the full spectrum sonar demonstrates the feasibility of remote acoustic seafloor sediment classification.

Remote estimates of physical and acoustic sediment properties in the South China Sea using chirp sonar data and the biot model

A chirp sonar acquires sea-bed reflection data in the South China Sea, generates imagery of the sediment layering, and estimates sediment properties along the acoustic propagation paths selected for the Asian Sea International Acoustics Experiment (ASIAEX). Normal incidence reflection data, collected by the chirp sonar, are processed with an inversion technique based on the Biot model. The inputs for the inversion are reflection coefficient and attenuation rolloff measurements extracted from chirp sonar reflection data. The inversion provides the acoustic and physical properties of the sediments for the top layer of the sea bed. Porosity, predicted by the inversion, agrees with directly measured porosity. Estimates of porosity, mean grain size, permeability, fast wave velocity, and attenuation (in decibels/meter) are given for the uppermost sediment layer along each of the acoustic propagation lines.

Spatial and temporal pulse design considerations for a marine sediment classification sonar

A linear FM sonar system was developed to support the objective of remote acoustic classification of seafloor sediments. It is a calibrated, wideband, digital, frequency modulated sonar that provides quantitative, high-resolution, low-noise sub-bottom data. Since the linear sonar system can precisely transmit a specified waveform, the calibrated digitally recorded reflection data can be processed to estimate the acoustic impulse response of the seabed and sediment attenuation. An acoustic pulse with special frequency domain weighting characteristics is designed to provide low temporal sidelobe levels and a nearly constant resolution with depth even after passing through a sediment with high losses such as sand. After correlation processing, the wideband acoustic pulse yields an effective beam pattern with high spatial resolution and insignificant sidelobe levels. Data sets generated with the FM profiler indicate that the required temporal and spatial characteristics of the sonar are realized in practice. >

Synthetic aperture processing of buried object scanning sonar data

Synthetic aperture processing improves the resolution and signal to scattering noise performance of buried object scanning sonar (BOSS) imagery. Time delay focusing coherently sums acoustic data measured by a line hydrophone array located in the wings of the BOSS vehicle. Synthetic aperture processing improves the along track spatial resolution by coherently summing the data over a sequence of transmissions. SAS (synthetic aperture sonar) motion compensation is implemented by calculating the changes in projector and hydrophone positions between transmissions using IMU (inertial measurement unit) and DVL (Doppler velocity log) data. The coherent summation is performed at each location in a 3D volume of focal points including the upper meter of sediments and the sediment-water interface. Sonar images are projections of the 3D data set onto orthogonal planes. A set of three cylinders with diameters of 5,7.5 and 10 cm, buried in sand, are used to measure the relationship between SAS aperture length and the SNR and spatial resolution of BOSS imagery. The BOSS data sets containing the cylindrical targets were collected in September 2004 as part of SAX-04 (Sediment Acoustics Experiment - 2004), located off Fort Walton Beach, Florida.

Buried object scanning sonar for AUVs

A 252 channel FM sonar is developed to generate images of objects buried in sediments using reflection tomography. An omnidirectional source, transmitting FM pulses over the band of 2 to 12 kHz, illuminates buried targets. The backscattered signals are measured with 252 hydrophones and processed with a digital matched filter. Coherent nearfield focusing generates a 3D map of acoustic intensity for each transmission event. As the sonar approaches and passes buried targets, the processor generates 3D matrices of acoustic intensity, overlapping in space and referenced to a coordinate system fixed to the seabed. The co-located pixels are added incoherently to generate a multi-aspect image of the target. The change in vehicle position between transmissions is measured with a DVL (Doppler Velocity Log) and IMU (Inertial Measurement Unit). The resulting reflection tomographic images provide target shape information useful for target classification. The sonar can be mounted on small AUVs by replacing the 252 channel, 1.5 m diameter array with one meter long line arrays mounted as wings. The receiver aperture is generated synthetically using the near field focusing processor which performs time delay focusing based on the positions of the hydrophones in the line array and vehicle motion data. A comparison between synthetic swath and tomographic images of a rigid spherical target in water shows the improvement provided by reflection tomography.