Lester R. LeBlanc

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...

An underwater acoustic sound velocity data model

A computer model for generating world ocean sound velocity profile (SVP) information is presented. It employs a ’’least‐squares’’ predictor to combine National Oceanographic Data Center (NODC) archival SVP data with any amount of available new sound velocity measurements that might be available. A technique is presented in the paper for the analysis of NODC World Ocean SVP data which is highly efficient. The technique is called empirical orthonormal function (EOF) analysis and it is capable of a very large compaction of the data set. This method provides a very compact presentation of the total statistical nature of the SVP data bank. The end result is a computer model which permits the optimum utilization of all archival data and any new data at a given place and time in world oceans to produce a new complete SVP. Of even greater significance is the fact that the form of the predicted SVP profile is such that it is easily employed in any propagation loss prediction model that is currently in use.

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.

High‐resolution sonar volume scattering measurements in marine sediments

This paper develops a simple volume scattering model for use with high‐resolution sonar measurements. A model of volume scattering for sediments is used to analyze data from the Canadian Hy‐Gro experiment. Measurements made at four sites are used in a comparison of acoustic volume scattering coefficient with core data. The volume scattering coefficient is a relative measure of the scattering strength of a cubic meter of sediment. Scattering measurements in a sediment depth interval of 0–2 m revealed two separate scattering probability density functions. Volume scattering is characterized by an exponential density function and was found to originate from incoherent scatterers in a 0.5‐ to 2‐m depth interval. In the 0‐ to 0.5‐m depth interval, signals were reflected from the seafloor boundary and the density function corresponded to a Rayleigh density function. In comparing the estimated sediment scattering coefficient to sediment properties, in general, sediments with large grain components produced larger...

High‐resolution wave‐number‐frequency methods for towed arrays

In passive narrow‐band or broadband target detection sonars, line arrays are used to enhance performance. In large towed arrays, wave‐number‐frequency (k‐ω) data analysis methods have been used on an experimental basis to detect and identify the presence of noise generators. The noise sources are composed of acoustically radiating sources located in the ocean, vibrational energy within the array, and pressure disturbances along the array. In this paper, a hybrid autoregressive (AR) technique is presented that will generate k‐ω solutions having better spatial resolution than is currently possible with methods that make use of a two‐dimensional fast Fourier transform (FFT). The hybrid AR method is implemented by applying a one‐dimensional FFT on the sampled time series data and using this result to obtain an averaged array cross‐spectral density matrix. The AR method is now applied to the spatial cross spectral density matrix to obtain a wave‐number‐frequency spectrum. For the hybrid AR method, equations ar...

Adaptive beamforming for underwater acoustic communication in shallow water

An adaptive beamformer was developed for use with existing underwater acoustic modems and for measurement of the underwater acoustic communication channel. The output of the beamformer is made up of many independent channels, each derived from the application of orthogonal vectors to the array elements. Each channel represents a vertical beam that is focused in the direction of correlated energy arriving at the array of vertically spaced transducers. Within each beam, nulls are created in the direction of interference. Experiments were conducted in shallow water to characterize the acoustic communication channel at many frequencies and ranges so as to evaluate modem performance using various encoding methods and waveforms. It was found that high‐reverberation levels exist at our test site in the shallow water near the shore environment. In this environment, adaptive processing reduced reverberation and transmission errors significantly. Wideband PSK was used to evaluate channel characteristics. In the pro...

Frequency−domain seismic deconvolution filtering

Reverberation in shallow water seismic profiles is a frequent problem that sometimes makes the geophysical interpretation of the records difficult. This can be further complicated if the acoustic source emits a bubble pulse. This paper describes a frequency−domain approach to deconvolution of the seismic signals to remove these artifacts. The resulting computation speed was found to exceed that of a time−domain Wiener filter by a factor of 10, for the same length dereverberation operator. Guidelines are established for using the FFT approach to deconvolution of seismic signals. The guidelines established are for the statistical stability and deconvolution ’’reach’’ of the filter, and prevention of FFT wrap−around effects. A digital filtering technique for obtaining a replica of the source signal from the received data record is presented and utilized in processing continental shelf air−gun data.

Correlation between estuarine sea floor acoustic reflection signatures and engineering sediment measurements

A recently developed technique is investigated for the analysis of sea floor acoustic reflection data. The technique uses Empirical Orthonormal Functions (EOF’s) to analyze the entire acoustic echo from the sea floor. In previous work [Milligan et al., J. Acoust. Soc. Am. 64, 795–807(1978)], the technique was used to ’’cluster’’ areas of similar acoustic behavior. This paper explores the relationship between the acoustic behavior of a sea floor sediment and classical engineering measurements. To achieve the objective, an experiment was configured so as to obtain acoustic reflection data and sediment measurements at the same spot on the sea floor. The results of this analysis showed that significant correlation exists between the acoustic waveform and certain measured sediment properties, and that an acoustic sediment classification scheme can be implemented.