A filtering system is provided for acquiring and processing signals using a sampled filter for signal separation and signal enhancement; such as for determing locations, distances, and times in a geophysical exploration system. Signature signals permit mixing and seperation with sampled filters, such as for sharing common circuitry and for increasing the amount of acquired information. A sampled filter, such as a digital correlator, is provided for generating high resolution output data in response to low resolution input data processed with low resolution computation circuits. A real-time time-domain correlator is provided with single-bit resolution computational elements to provide improved correclation filtering. A high speed real-time correlator is provided to enhance signals with copositing-after-correlation and with correlation using a plurality of correlation operators.

Signature filter system

This is a review paper that surveys past work in, and the recent status of, active synthetic aperture sonar (SAS). It covers the early historical development of SAS with its provenance in synthetic aperture radar (SAR) and flows through into what work has been published in the open literature up to early 2007. The list of references is sufficiently complete to include most past and recent SAS publications in the open refereed literature.

Synthetic Aperture Sonar: A Review of Current Status

During mid-1967 two cruises of the Woods Hole Oceanographic Institution's R/V Chain provided nearly continuous seismic, geomagnetic, and gravity measurements along 8,000 km of ship track. These measurements supplement earlier ones from various sources to provide a comprehensive picture of the composition and geologic history of the continental margin off eastern North America, an area that is much larger than all of the United States east of the Mississippi River. The geomagnetic profiles portray a systematic pattern of positive and negative anomalies that are in accord with the concept of sea-floor spreading, whereby North America separated from Europe and Africa at the beginning of the Permian Period, and drifted westward from the site of rifting (the Mid-Atlantic Ridge) at average rates of 0.8-1.4 cm/year. During all this time the continent has been coupled firmly with the adjacent sea floor, as though both continent and sea floor were on the same conveyor belt. Gravity information suggests that a relict structure of the original rift is preserved in the same general area as the geomagnetic slope anomaly, beneath the seaward part of the continental shelf, the continental slope, or the upper continental rise. It has the form of a complex linear ridge of crystalline rocks that rises above the zone of sharpest landward slope of the Mohorovicic discontinuity. Seismic refraction measurements support the presence of such a ridge, bordered on both sides by linear trenches. The continuous seismic reflection profiles measured during the cruises reveal shallow acoustic basement in the form of a ridge complex that is shallower, but in the same general area, and probably is related to the deep ridge. The ridge and associated trenches served as dams and b sin sinks to trap land-derived sediments during the Mesozoic Era, so that only pelagic silts and clays could reach and be deposited on the irregular oceanic basement seaward of the barrier. During Late Cretaceous to middle Eocene time one or more thick deposits of probably chemical origin formed blankets of deep-sea chert throughout broad abyssal plains, which produce the acoustic reflector known as Horizon A. About middle Eocene time the land-derived sediments filled the trap west of the ridge and prograded eastward over the ridge top and built the present continental rise atop the Mesozoic abyssal plain. Continuous seismic reflection profiles show that the rise is a huge prism of generally seaward-dipping, interbedded pelagic sediments and turbidites that contain many masses of sediment displaced from higher on the continental rise and from the continental slope. Such slides continue to occur, a large one having occurred in 1929. The volume of the Cenozoic continental rise in the study region is nearly 3 million km3, about half the volume of all sediments deposited on basement during Mesozoic time. The interbedding of sandy turbidites with organic-rich silts and clays displaced from the continental slope may constitute a thick sequence of oil reservoir and oil source beds, but no exploratory drilling into them has been done.

Continental Rise off Eastern North America

Golf club heads are releasably engaged with shafts so that the club heads and shafts can be readily interchanged and/or so that the shaft position with respect to the club head can be readily changed. Assemblies for connecting the club head and shaft may include: (a) a shaft engaging member including a rotation-inhibiting structure; (b) a club head engaging member including a shaft-receiving chamber and a retaining structure for engaging the rotation-inhibiting structure; and (c) a securing system for releasably securing the shaft engaging member with respect to the club head engaging member. The club head and shaft may be changed by releasing the securing system and exchanging the original parts with different parts. Furthermore, the shaft may be bent or otherwise extend at an angle from the shaft engaging member so as to allow adjustment of the shaft position with respect to the club head.

Releasable and Interchangeable Connections for Golf Club Heads and Shafts

This thesis contains the complete end-to-end simulation, development, implementation, and calibration of the wide bandwidth, low-Q, Kiwi-SAS synthetic aperture sonar (SAS). Through the use of a very stable towfish, a new novel wide bandwidth transducer design, and autofocus procedures, high-resolution diffraction limited imagery is produced. As a complete system calibration was performed, this diffraction limited imagery is not only geometrically calibrated, it is also calibrated for target cross-section or target strength estimation. Is is important to note that the diffraction limited images are formed without access to any form of inertial measurement information. Previous investigations applying the synthetic aperture technique to sonar have developed processors based on exact, but inefficient, spatial-temporal domain time-delay and sum beamforming algorithms, or they have performed equivalent operations in the frequency domain using fast-correlation techniques (via the fast Fourier transform (FFT)). In this thesis, the algorithms used in the generation of synthetic aperture radar (SAR) images are derived in their wide bandwidth forms and it is shown that these more efficient algorithms can be used to form diffraction limited SAS images. Several new algorithms are developed; accelerated chirp scaling algorithm represents an efficient method for processing synthetic aperture data, while modified phase gradient autofocus and a low-Q autofocus routine based on prominent point processing are used to focus both simulated and real target data that has been corrupted by known and unknown motion or medium propagation errors.

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Synthetic aperture imaging algorithms : with application to wide bandwidth sonar

Acoustic mapping apparatus

A sonar system particularly adapted for navigating a ship in shallow waters employing a forwardly directed narrow sonar beam emanating from a projector assembly carried by the ship. The projector assembly is suitably positioned between the upper surface and lower surface of a body of water so that there is substantially no reverberation from waves at the upper surface and irregularities along the bottom for sufficient distances forward of the ship to present a clear view within the body of water. In one embodiment, the narrow beam is provided by a nonlinear finite amplitude interaction of two beams of sonic energy of relatively high frequency to provide a low frequency highly directive beam. In a second embodiment, a second sonar which measures depth and doppler quantities is coupled together with the forward looking sonar to provide a space stabilized display of data obtained with the forward looking sonar.

Anti-collision sonar system

An apparatus for activating a remote underwater device comprising detecting means responsive to incident acoustic waves provides signal indication of the presence or absence of predetermined frequencies thereof. A logic arrangement decodes only those signal indications indicating a joint presence of some of the pre-selected frequencies and the absence of others of the pre-selected frequencies. This discriminates signals against broad-band acoustic noise. The logic arrangement further includes means for activating the remote underwater device only if the decoded successive signal indications match a predetermined pattern. This further discriminates between a present received acoustic wave and a prior transmitted acoustic wave present because of multipath or reverberation. Lastly, a timing and gating means responsive to a first decoded signal indication couples the detecting means to the logic arrangement only at periodic intervals thereafter. The time gating also reduces the chances of obtaining a prior multipath and reverberation signal because the pulse repetition period of the successive acoustic waves is predetermined.

Remote underwater device activating system

Sea bottom slope is measured by projecting an acoustic beam upon the sea bottom with reference to a local vertical and measuring the arrival angle between incident echos and the local vertical. Split receiving beams having separated phase centers are utilized to measure the arrival angle. In order to reduce the effects of noise, phase angle magnitude and sign are determined by signal processing means which utilize a greater proportion of the available energy in large time bandwidth product signals.

Sea bottom slope measuring apparatus

A waveform regenerator for use with a digital correlator having a first circulating memory and a second precessing circulating memory for storing and processing digital signal sequences. The apparatus comprises circuits responsive to each successive digital signal sample in the second memory for extracting the digital signal and reproducing each sample for a time interval equal to one circulating memory cycle. Reverse Order Logic is included in the second circulating memory for changing the order of succession of the digital signal samples. Means responsive to the contents of the first memory and the second memory replace the digital signal samples contained in the second memory by the digital signal samples stored in the first memory in the proper sequence to be extracted from the second precessing circulating memory.

George M Walsh

Papers

Acoustic mapping apparatus

Anti-collision sonar system

Remote underwater device activating system

Sea bottom slope measuring apparatus

Waveform regenerator for use with a digital correlator