Echo sounding

Abstract: Part 1 Sound transmission fundamentals: sound speed the propagation equation, logarithmic power measurement, the "new" reference unit sound reflection and refraction pressure reflection and transmission coefficients behaviour at normal incidence reflection and transmission with varying incidence reflection and transmission - the "fast" bottom and the "slow" bottom. Part 2 The Solar Equations: source intensity calculation source directivity transmission loss target strength reflection intensity loss coefficient sea floor loss sea surface loss noise reverberation calculating the signal excess. Part 3 Characteristics and analysis of sonar waveforms: swept frequency spectrum analyzers filter bank spectrum analyzers fast Fourier transform analyzers prony analysis further model-building techniques for spectral estimation four dimensional space-time waveform analysis. Part 4 Ray trace modelling of sonar propagation: ray tracing sonar models ray trace calculations some examples of ray modelling modelling transmission in the shelf-seas the Lloyd mirror effect. Part 5 Normal mode modelling of sonar propagation: a Heuristic treatment of normal modes in an acoustic waveguide normal mode solution for long ranges normal modes as interfering plane waves the normal mode solution formalized normal mode solution for all ranges the horizontally stratified channel. Part 6 Noise and reverberation: deep sea ambient noise level the variability of ambient noise level with time and depth the angular distrubution of the ambient noise field ship-generated noise reverberation scattering. Part 7 Acoustic transduction: the basic principles of acoustic transduction Piezo-electric transduction the Langevin projector ring and tube transducer designs resonance behaviour of transducers multiple matching layer transducers polar response measurements on transducers admittance measurements on terminal response hydrophones. Part 8 Transducer arrays: the linear hydrophone array the Fourier transform approach to pattern synthesis array beamsteering directivity index the parametric sonar synthetic aperture sonar. Part 9 Sonar engineering and applications: the basic echo sounder sub-bottom profiling fishing sonars side-scan terrain-mapping sonars Seismic survey acoustic positioning and navigation Doppler measurements. Part 10 Acoustic communications: the gross attributes of the received signal the channel transfer function combatting multipath diversity reception equalization communication using parametric transmission.

Abstract: Experimental results are presented from a traverse over the ice sheet of north western Greenland in 1964, during which a continuously recorded profile of ice thickness was obtained for the first time. Interpretation of data from this traverse is consistent with results of subsequent work to December 1967. The parameters of the apparatus are presented briefly, while the details of electronic circuits are being published separately. Theoretical problems of radio wave propagation in an ice sheet and, in particular, the factors affecting accuracy are discussed. The uncertainty in depth, over a small area, is ±5 m ±1.5% and this is verified by comparison with the seismic results for a range of depths up to 1.5 km. It is found that the only real uncertainty arises in irregular terrain. The effectiveness of the radio echo technique is dependent on the absorption of radio waves in ice. Temperature, and to a lesser extent the impurity content of ice, appear to be the main variables affecting field performance. Earlier laboratory results on the variation of absorption with temperature for ice cores from northwest Greenland, together with theoretically predicted temperature distributions throughout the ice mass, have provided estimates of the total loss by absorption. These estimates are reasonably consistent with the observed echo strengths over most of the traverse. Consequently, it is predicted that echoes can be obtained over considerable areas of the ice sheets of Greenland and Antarctica, as has been verified by subsequent observations. The reflexion coefficient at the ice/rock interface is of the order of —15 dB. It could rise to 0 dB for an ice/water interface and one area was found in Greenland where it appeared to fall to — 30 dB. Results from this traverse have shown that local surface slopes on the ice sheet are largely controlled by variations of longitudinal stress along the line of flow. Regional slopes over several kilometres vary with the velocity of movement of the ice, but appear to be less dependent on basal ice temperatures than laboratory results would suggest. The velocity of ice movement increases in proportion to the square or cube of the basal shear stress, but the stress itself shows no obvious dependence on basal ice temperature. Partially reflecting layers discovered within the ice mass are discussed mainly in terms of small density variations between adjacent layers of ice. One particularly prominent layer is calculated to be about 1000 years old and its variation of depth with position provides evidence in favour of the steady state model of the ice sheet.

Abstract: [1] Analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment. Echoes from a general interface consist of both specularly reflected and diffusely scattered contributions. Specular reflection results from smooth uniform interfaces, whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media. This article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data. Reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of Whillans Ice Stream and Ice Stream C, West Antarctica. Additionally, reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over Ice Stream C. Analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material. Echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates. Comparison of low- and high-resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and/or scattering interface. Combining the results from these independent analyses provides classification of the subglacial environment. Classified regions include smooth seawater, smooth saturated sediments, accreted marine ice, rough bottom crevasses, mixed conditions with partial subglacial water, and dry frozen conditions.

Abstract: Preface. About the Author. Introduction. Sound. Arrays. Propagation of Sound in the Sea. Target Strength. Noise in Sonar Systems. Reverberation. The Sonar Equations. Passive Sonar. Active Sonar. Echo Sounding and Side Scan Sonars. Mine Hunting Sonars. Intercept and Communications Sonars. Active Sonar Design. Conclusion. Solutions to Problems. Index.