Thesis (Elec.E)--M.I.T., Dept. of Electrical Engineering and Computer Science, 1979. / Supervised by Alan V. Oppenheim and George V. Frisk. Bibliography: leaves 132-133.

Thesis--Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution. / Includes bibliographical references (p. 181-186).

Efforts to extract information regarding the surficial composition of the ocean bottom have increased in the last decade as increases in the availability of computing power have corresponded with advances in signal processing techniques. The ability to extract information from acoustic echosounders is especially desirable due to the relatively low cost and ease of deployment of such systems. Products already exist for the acquisition and logging of echosounder returns. An acoustic return is comprised of the incoherent backscatter from individual scatterers within the annulus of insonification that occurs when a spherically-spreading transmit pulse intersects with the ocean floor. The return is a convolution of the source ping, and the impulse response modeled by the backscatter profile. Most echosounders, generate an envelope of the received signal. The bottom impulse response undergoes a dilation linear with depth due to simple geometry which can be corrected with time-scale normalization. Under certain circumstances it may be necessary to deconvolve the source ping from the envelope of the return prior to time-scale normalization. It is shown that this can be done by modelling the envelope generation function with a finite sum discrete convolution and the Hilbert transform of the source signal. A second-order Volterra kernel can be derived using a standard predictor network with constrained optimization. Other factors which contribute to the quality of the return include off-vertical transducer angles which in fact improve the classification by eliminating the nulls that occur in the bottom impulse response due to transducer beam pattern. Spatial averaging can have the effect of beam widening if the transducer angle varies. Simple feature extraction algorithms are shown to be moderately effective in providing separability. The computational cost of combining the resulting feature sets can be reduced if the individual feature sets are scaled appropriately, reduced and then combined, prior to a reduction to the final dimensionality. The resulting feature space axes contain contributions from both the principal axes of the individual feature sets, as well as cross-algorithmic terms. Blind clustering of the data is provided through a two-step modification of the K-means algorithm. The first step generalizes it to use arbitrary classification metrics, and the second embeds this generalized kernel within a second kernel which modifies the covariance. The resulting K-stats kernel is very robust when successively applied to a growing number of clusters. / Graduate

The bathymetry, magnetic anomalies, gravity anomalies and sediment layer is presented to support a young Western Mediterranean no older than 25 million years and an Eastern Mediterranean as old as 200 million years. A buried salt layer is detected in seismic reflection profiles lying below a sediment cover whose base is 4 to 5 million years in age. The seafloor in the eastern Mediterranean is being actively deformed by compressional folding and thrusting beneath the Mediterranean Ridge. The Western Mediterranean opened by rifting and is floored by oceanic crust.

No description available.

No description available.

A three-dimensional parabolic equation (PE) and perturbation approach is used to invert for the depth- and range-dependent geoacoustic characteristics of the seabed. The model assumes that the sound speed profile is the superposition of a known range-independent profile and an unknown depth- and range-dependent perturbation. Using a Green’s function approach, the total measured pressure field in the water column is decomposed into a background field, which is due to the range-independent profile, and a scattered field, which is due to the range-dependent perturbation. When the Born approximation is applied to the resulting integral equation, it can be solved for the range-dependent profile using linear inverse theory. Although the method is focused on inverting for the sound speed profile in the bottom, it can also invert for the sound speed profile in the water column. For simplicity, the sound speed profile in the water column was assumed to be known with a margin of error of ± 5 m/s. The range-dependent perturbation is added to the index of refraction squared n2(r), rather than the sound speed profile c(ro). The method is implemented in both Cartesian (x,y,z) and cylindrical (r,q,z) coordinates with the forward propagation of the field in x and r, respectively. Synthetic data are used to demonstrate the validity of the method [1]. Two inversion methods were combined, a Monte Carlo like algorithm, responsible for a starting approximation of the sound speed profile, and a steepest descent method, that fine-tuned the results. In simulations, the inversion algorithm is capable of inverting for the sound speed profile of a flat bottom. It was tested, for three different frequencies (50 Hz, 75 Hz, and 100 Hz), in a Pekeris waveguide, a range-independent layered medium, and a range-dependent medium, with errors in the inverted sound speed profile of less than 3%. Keywords: Three-dimensional parabolic equation method, geoacoustic inversion, range-dependent sound speed profile, linear inversion, Born approximation, Green’s functions. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2017. / FAU Electronic Theses and Dissertations Collection

The seafloor properties are of high importance for many applications such as marine biology, oil and gas exploration, laying cables, dredging operations and off-shore construction. Several approaches exist to classify the properties of the seabed. These include taking direct samples of the seabed (e.g., coring), however, these methods are costly and slow. Underwater acoustic remote sensing techniques are of interest because they are lower cost and faster. The information about the seabed properties can be extracted by studying the energy of single beam echo sounders (SBES). This can be done by either phenomenological or numerical methods [1], [2]. This research investigates a numerical, model-data fitting method using a high frequency backscattering model developed by Jackson et al [3]. In this "inversion modeling" method, the matching process between the model and average echo envelope provides information about the sediment parameters, namely the sediment mean grain size (Mz) as the indicator of the seabed type, spectral parameter (W2) as the indicator of seabed roughness and normalized sediment volume parameter σ2 as the indicator of the scattering due to sediment inhomogeneities.

Crustal and upper mantle structures in the West Philippine Basin, along 17-18°N, have been determined using explosions as sources and ocean bottom seismometers to measure refracted compressional waves. Seismic refraction profiles out to nearly 500 km were completed. Shallow structure was measured using small shots, 1-240 lbs., and the deeper structure was probed with large explosions, 0.9-1.8 tons. A velocity-depth inversion using short range data shows the upper crust to have strong velocity gradients which gradually decrease with depth. The lower crust is characterized by a nearly constant velocity gradient of 0.24 sec⁻¹. Standard delay-time functions and a modified function accounting for lateral velocity gradients were also used in travel time inversion. Results from the two methods are comparable and yield ~1.5 km transitional zone thicknesses in the basin. Although they vary slightly in magnitude between methods, West Philippine Basin oceanic layer thicknesses are abnormally thin, by about 2 km, when compared to average crust. Total crustal thicknesses are shown to be thinner in the eastern part of the basin, approaching only 3 km. Crustal thinning toward the east is consistent with the Palau-Kyushu Ridge being a remnant transform fault connecting Philippine and Kula-Pacific ridges in the past. Predicted water depths in the basin are about 300 meters shallower than observed depths when compensated to average mantle depths found for the Western North Pacific. The depth anomaly cannot be fully reconciled by thin crust, and requires a deeper-seated anomaly to be present in the West Philippine Basin. Temperature and pressure modeling using experimental measurements from proposed mantle constituents indicate high seismic gradients in the upper mantle and may suggest that a multi-component or graded mantle exists beneath the marginal sea. / Graduation date: 1983

Graduation date: 1964