Ocean Seismo-Acoustics: Low-Frequency Underwater Acoustics (Nato Conference Series, 16)

Seafloor investigation has long been a feature of not only seismology but also of acoustics. Indeed it was acoustics that produced depth sounders, giving us the first capability of producing both global and local maps of the seafloor. Subsequently, better instrumentation and techniques led to a clearer, more quantitative picture of the seabed itself, which stimulated new hypotheses such as seafloor spreading through the availability of more reliable data on sediment thickness over ocean basins and other bottom features. Geologists and geophysicists have used both acoustic and seismic methods to study the seabed by considering the propagation of signals arising from both natural seismic events and man-made impulsive sources. Although significant advances have been made in instrumentation, such as long towed geophysical arrays, ai r guns and ocean bot tom seismometers, the pic ture of the seafloor is still far from complete. Underwater acoustics concerns itself today with the phenomena of propagation and noise at frequencies and ranges that require an understanding of acoustic interaction at both of its boundaries, the sea surface and seafloor, over depths ranging from tens to thousands of meters. Much of the earlier higher frequency (>1 kHz) work included the characterization of the seafloor in regimes of reflection coefficients which were empirically derived from surveys. The results of these studies met with only limited success, confined as they were to those areas where survey data existed and lacking a physical understanding of the processes of reflection and scattering. Front Matter....Pages i-xv Sound Pulse Propagation in a Weakly Range Dependent Shallow Ocean....Pages 1-10 Intrinsic Modes in a Wedge-Shaped Ocean with Stratified Elastic Bottom....Pages 11-20 A Green’s Function Method for One-Way Wave Propagation in a Range-Dependent Ocean Environment....Pages 21-28 Factorization and Path Integration of the Helmholtz Equation: Numerical Algorithms....Pages 29-38 Range Dependent Propagation Codes Based on Wave Field Factorization and Invariant Imbedding....Pages 39-46 An Acoustic Reflectivity Method for Laterally Varying Layered Media....Pages 47-55 A Vector Parabolic Equation Model for Elastic Propagation....Pages 57-66 Influence of Bottom Refraction on the Propagation of Underwater Sound....Pages 67-73 Bottom Interaction Effects on Normal Modes: an Algebraic Approach....Pages 75-85 Gaussian Beams and 3-D Bottom Interacting Acoustic Systems....Pages 87-101 Development of a Parabolic Approximation for the Computation of Propagation Loss in a Range-Dependent Environment....Pages 103-114 Linear and Nonlinear Ocean Acoustic Propagation Models....Pages 115-128 Modeling of Pulse Response Functions of Bottom Interacting Sound Using the Parabolic Equation Method....Pages 129-137 A Study of Propagation Loss Dependence on Sediment Layer Thickness Using the Fast Field Program....Pages 139-148 Shear Wave Effects on Propagation to a Receiver in the Substrate....Pages 149-157 Description and Some Results of the Operational Propagation Loss Model ALMOST (= A coustic L oss M odel for O perational S tudies and T asks)....Pages 159-168 Experimental Confirmation of Horizontal Refraction of Sound Propagation in a Wedge-Like Ocean....Pages 169-178 Acoustic Propagation over Large-Scale Linear Ocean Slopes....Pages 179-189 Predicted Partitioning of VLF Acoustic Energy in a Range-Dependent Environment....Pages 191-197 Experiments on Sound Propagation over Sloped Bottoms....Pages 199-206 The Effect of Unconsolidated Sediment Rigidity on Low Frequency Acoustic Propagation....Pages 207-215 A Propagation Anomaly Observed in the Barents Sea....Pages 217-222 Long Range Low Frequency Propagation Measurements in Deep Water Using a Seismic Towed Array....Pages 223-231 Some Characteristics of Virtual Modes in Shallow Water with High Speed Bottom....Pages 233-242 The Deduction of Approximate Values of Sediment Depth from Propagation Loss Measurements....Pages 243-252 Range and Time Dependence of Acoustic Intensity Fluctuations....Pages 253-267 Shallow Water Tomography....Pages 269-279 Medium-Induced Low-Frequency Fluctuations in Acoustic Transmission Loss: Examples from Measurements in Selected Geographical Areas....Pages 281-292 Wave Propagation in Anisotropic Media Using Born Series Through 4th Order....Pages 293-303 Coupling of Acoustic Modes in the Ocean, A Comparison of Approximate Solutions....Pages 305-312 Long Range, Low Frequency Acoustic Backscattering: A Survey....Pages 313-326 Directional Measurements of Low-Frequency Acoustic Backscattering from the Seafloor....Pages 327-333 Seamount Height Estimation from Long-Range, Low Frequency Acoustic Backscatter....Pages 335-344 An Acoustic Model for Bathymetric Scattering with Low-Frequency Applications....Pages 345-353 Evaluation of Low-Frequency Bottom Backscattering Strength vs Grazing Angle by Means of Multiple Beamforming....Pages 355-364 Near Grazing Propagation over a Low Roughness Hard Ocean Bottom....Pages 365-372 Estimation of Ice Surface Scattering and Acoustic Attenuation in Arctic Sediments from Long-Range Propagation Data....Pages 373-386 Low Frequency Attenuation in the Arctic Ocean....Pages 387-395 Effect of Sub-Bottom Inhomogenieties on Shallow Water Spatial Coherence....Pages 397-405 Mode Coupling from Subbottom Roughness....Pages 407-415 Acoustic Waves in Marine Sediments....Pages 417-434 Pore Shape and the Biot-Stoll Model for Saturated Sediments....Pages 435-444 Low-Frequency Anomalies in the Reflection Behaviour of Marine Sediments....Pages 445-453 The Interaction of Low-Frequency Acoustic Waves with a One-Dimensional Random Sediment Model....Pages 455-461 Propagator Matrix for Acoustic Wave Propagation through Anisotropic Porous Media....Pages 463-472 Acoustic-to-Seismic Coupling at Porous Ground Surfaces....Pages 473-482 Geotechnical Characteristics of the Sea Bed Related to Seismo-Acoustics....Pages 483-500 Dynamic Properties of Marine Sediments....Pages 501-510 Relationship between the Acoustical Characteristics of Deep Sea Sediments and their Physical Environment....Pages 511-517 Low Frequency Measurements of the Acoustic Properties of Marine Sediments....Pages 519-525 Spatial Variability of Surficial Shallow Water Sediment Geoacoustic Properties....Pages 527-536 A Bottom Shear Modulus Profiler: Inverse Analysis of the Field Data on Wave-Induced Bottom Motion....Pages 537-545 Estimation of Subsurface Parameters in Lossy Media....Pages 547-552 Analysis of Ocean-Subbottom Seismograph (OSS) Data....Pages 553-563 The Relative Amplitudes of Primary and Multiple Signals Refracted in the Ocean Crust....Pages 565-578 The Propagation of P n ....Pages 579-587 Effects and Consequences of Transverse Isotropy in the Seafloor....Pages 589-597 Seismic Anisotropy in the Upper Crust at DSDP Site 504B....Pages 599-607 A Broad-Band Study of Attenuation in Ocean Bottom Sediments....Pages 609-621 On the Role of Bottom Interface Waves in Ocean Seismo-Acoustics: A Review....Pages 623-641 Observations of Interface Waves and Low-Frequency Acoustic Propagation Over a Rough Granite Seabed....Pages 643-652 A Comparison of Seismic and Hydroacoustic Measurements at Very Low Frequencies in Different Shallow Water Areas....Pages 653-662 Interface Wave Studies on the Ligurian Shelf Using an OBS Array: Experimental Results and Propagation Models....Pages 663-672 A Study of Sea Floor Structure Using Ocean Bottom Shots and Receivers....Pages 673-681 Shear Properties of Ocean Sediments Determined from Numerical Modelling of Scholte Wave Data....Pages 683-692 The Determination of Geoacoustic Models in Shallow Water....Pages 693-702 The Estimation of the Density, P-Wave, and S-Wave Speeds of the Top-Most Layer of Sediments, from Water Bottom Reflection Arrivals....Pages 703-710 Effects of Upper Crustal Geoacoustic Parameters on Low Frequency Sound....Pages 711-720 Evaluation of Experimental Techniques for Determining the Plane Wave Reflection Coefficient at the Sea Floor....Pages 721-730 A Study of Ocean and Seismic Noise at Infrasonic Frequencies....Pages 731-741 Geo-Acoustic Noise Levels in a Deep Ocean Borehole....Pages 743-751 Low Frequency Noise Fields and Signal Characteristics....Pages 753-766 Time Dependence of Infrasonic Ambient Seafloor Noise on a Continental Shelf....Pages 767-778 Infrasonic Attenuation and Ambient Noise....Pages 779-784 Parametric Sources — Design Considerations in the Generation of Low-Frequency Signals....Pages 785-800 The Postcritical Penetration of Low Frequency Parametric Beams through a Water-Sediment Interface....Pages 801-810 The Use of Low and High Frequency Intensity Fluctuations in Sea-Bed Surface Classification....Pages 811-820 Ocean Bottom Seismology: History and Current Status....Pages 821-840 The Use of Love Waves to Determine the Geoacoustic Properties of Marine Sediments....Pages 841-852 Development of a Deep-Towed Seismic System A New Capability for Deep-Ocean Acoustic Measurements....Pages 853-862 Sea-Floor Seismology : Mobile Ocean Bottom Vertical Seismic Array (OBVSA) for Controlled Source Experiments....Pages 863-870 A 12-Channel Marine Eel for Shallow Refraction Surveying of the Seabottom in Coastal Waters....Pages 871-880 Characterization of Average Geoacoustic Bottom Properties from Expected Propagation Behavior at Very Low Frequencies (VLF) Using a Towed Array Simulation....Pages 881-889 Back Matter....Pages 891-915