Scheer Edward K.

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Scheer
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Edward K.
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  • Technical Report
    Technical report on a portable gravity meter platform and first test results
    (Woods Hole Oceanographic Institution, 1983-01) Goldsborough, Robert G. ; Scheer, Edward K. ; Bowin, Carl O.
    This report describes the new portable platform and gravity meter system which has been assembled at the Woods Hole Oceanographic Institution. It consists of three functionally distinct parts. The first of these is a recently developed gyro-stabilized two-axis platform. This platform has been designed to carry the vibrating string accelerometer (VSA) and its associated oven assembly as the gravity sensor. The new platform represents a major reduction in both size and weight over other platforms suitable for gravity measurement. The second major part of this system is a new gravity readout which interfaces with the VSA, processes the VSA output, and prepares the resulting filtered acceleration data for output to the acquisition system. The readout has been designed to allow flexible use of the gravity system on a variety of vehicles, including ships, submarines and aircraft. The third part of this new meter is the data acquisition system. It consists of a microprocessor interfaced to a Kennedy 9-track tape drive. Both the platform and the readout are connected to the microprocessor. Results are presented from Endeavor cruise 88 that demonstrate the ability of the platform to stabilize the gravity meter and for the gravity system to produce raw data with a resolution of 48 milligals at a sampling rate of 10 Hz. Digital signal processing techniques which were used to filter the data and extract the gravity signal with a resolution of 0.48 milligals are also discussed.
  • Article
    Deep water towed array measurements at close range
    (Acoustical Society of America, 2013-10) Heaney, Kevin D. ; Campbell, Richard L. ; Murray, James J. ; Baggeroer, Arthur B. ; Scheer, Edward K. ; Stephen, Ralph A. ; D'Spain, Gerald L. ; Mercer, James A.
    During the North Pacific Acoustic Laboratory Philippine Sea 2009 experiment, towed array receptions were made from a towed source as the two ships transited from a separation of several Convergence Zones through a Closest Point of Approach at 3 km. A combination of narrowband tones and broadband pulses were transmitted covering the frequency band 79–535 Hz. The received energy arrives from two general paths—direct path and bottom bounce. Bearing-time records of the narrowband arrivals at times show a 35° spread in the angle of arrival of the bottom bounce energy. Doppler processing of the tones shows significant frequency spread of the bottom bounce energy. Two-dimensional modeling using measured bathymetry, a geoacoustic parameterization based upon the geological record, and measured sound-speed field was performed. Inclusion of the effects of seafloor roughness and surface waves shows that in-plane scattering from rough interfaces can explain much of the observed spread in the arrivals. Evidence of out-of-plane scattering does exist, however, at short ranges. The amount of out-of-plane scattering is best observed in the broadband impulse-beam response analysis, which in-plane surface roughness modeling cannot explain.
  • Thesis
    Estimates of crustal transmission losses using MLM array processing
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1982-05) Scheer, Edward K.
    Seismic refraction experiments have been used extensively in the past thirty five years in investigations of the structure of the oceanic crust. The longer range of the refraction or wide angle reflection technique, on the order of tens of kilometers, permits a deeper and wider area of examination, although with less resolution, than the spatially limited seismic reflection experiment. Observations of arrivals from the Mohorovicic discontinuity, at an average depth of seven kilometers below the sea floor, are routinely made. The major focus in interpreting refraction data has been the analysis of travel time/range data and the "inversion" of this data for the purpose of determining a velocity versus depth profile of the crust. The most frequent application of this procedure is the geophysicist's use of velocities for postulating geologic structures and rock types below the sediment (Christensen & Salisbury, 1975). Another area using refraction data, less widely seen, falls into the ocean acoustician's domain. In studying the behaviour of sound in the ocean, the sea floor is often modelled as a boundary with a half space below, and with some form of reflection characteristic and/or loss mechanism. If acoustic energy, upon encountering the bottom, was either reflected or transmitted directly, this would be appropriate, and the determination of reflection and transmission coefficients for the sea-sediment interface would probably be sufficient. However, sound energy does penetrate beneath the sea floor and is both reflected and refracted back to the water. In an active acoustical experiment, especially at longer ranges, a significant amount of the received energy may come from waves that have interacted with the earth's crust and have been reinjected into the water. Since these arrivals can be detected in the ocean, their study is of concern for the acoustician. The role of bottom interaction, especially at low frequencies, is now an area of intense research activity in modelling acoustic propagation. In particular, in the language of the sonar engineer, the TL, or transmission loss, of this energy is of major importance for i) predicting the character of the sound field at a receiver in future experiments, ii) for comparing crustal loss with the better known TL of paths remaining primarily in the water layer, and iii) expanding the role of arrival amplitudes in inversion theory. Just as there may be a number of possib1e paths in the sea between a source and receiver, each with a different loss characteristic, trajectories in the crust are variegated and exhibit different TL behaviors. It is important to be able to differentiate the energy partitioned among the different paths, and to determine which paths are most important. Resolving the locus of a particular acoustic path is intimately tied to the problem of determining the velocity structure of a medium. To the limits of the geometrical optics approximation of acoustic behaviour, sometimes sorely pressed at low frequencies, a completely detailed knowledge of sound speed variations, both laterally and with depth, plus known source characteristics and attenuation losses in the medium, enables one in principle to predict signals observed at a receiver. For an ocean acoustician, the requirement of environmental knowledge of the sound speed profiles, both in water and crust, needed to predict the amplitude and timing of data, is clearly very burdensome. In the past twenty five years, however, models of the oceanic crust have been formulated which are statistically consistent over much of the oceans. These models divide the crust into three or more horizontal layers with certain average thicknesses and velocities (Raitt, 1963). At least within the confines of these models, if a typical transmission loss were known for each of these layers, an acoustician can make predictions of the expected strength and timing of crustal arrivals at other stations. Most of this environmental information has been obtained from refraction and/or wide angle reflection data, usually via travel time analysis. Little has been done in developing models accounting for amplitude dependence.