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8 results on '"Lindwall, D."'

1. Io - The near-infrared monitoring program, 1979-1981

Author

Lindwall, D, Cheigh, F, Tittemore, W. C, and Sinton, W. M

Subjects
Lunar And Planetary Exploration
Abstract

The results of a program to monitor the brightness of Io in the near infrared are reported. While Io's 2.2 micron flux can be explained very well by conventional albedo analysis, at 3.8 and 4.8 microns Io has large intrinsic variations presumably associated with volcanism. At the latter two wavelengths, Io appears brighter and more active on its trailing hemisphere. These wavelengths also have possibly significant terms at the Jupiter corotational periods of 13 and 6.5 hr. These are particularly significant for observations of the trailing hemisphere and have a marked peak when Io is in the active sector. The data also show that the variability occurs in outbursts. Eight of these were observed and four were studied, deriving color temperatures near 700 K which decayed to about 300 K after several hours. It is argued that the apparent peak and the significance of the corotational terms may be fortuitous.

Published
1983

2. Seismic Oceanography -- A New View of the Ocean.

Author

Wood, W. T., Lindwall, D. A., Book, J. W., Wesson, J., Carniel, S., and Hobbs, R. W.

Subjects
OCEANOGRAPHY, MERIDIONAL overturning circulation, WATER depth, SIGNAL processing, ACOUSTIC reflection, NAVAL research
Abstract

NRL scientists are collaborating with international colleagues in developing a new way of observing water masses in the ocean: seismic oceanography. Using low-frequency seismic systems designed to image sub-seafloor geologic structures, thermohaline contrasts between water masses can be mapped, characterized, and quantified. Seismic oceanography uses the relationship between oceanic temperature contrasts and acoustic reflectivity to generate a quantitative measure of vertical temperature gradient throughout the water column at vertical and lateral resolution of 5 to 10 meters, several orders of magnitude finer than traditional methods. Such resolution opens up a new window on small-scale processes and mixing of the coastal and deep ocean. In 2009, we executed an ambitious field effort in the Adriatic Sea (ADRIASEISMIC-09) to acquire seismic data together with state-of-the-art oceanographic data as a way to compare the datasets and evaluate the limits and capabilities of seismic methods to characterize ocean processes. This was only the third such combined field effort ever performed, and the first in shallow water, thus adding to the useful development of this new approach. Incorporating seismic oceanography into standard oceanographic practice will require integrating field acquisition practices and developing new signal processing algorithms, with the potential benefit of gaining a new view of ocean structure. [ABSTRACT FROM AUTHOR]

Published
2010

6. Imaging marine geophysical environments with vector acoustics.

Author

Lindwall D

Subjects
Animal Migration, Animals, Oceans and Seas, Acoustics instrumentation, Environment, Marine Biology instrumentation, Models, Theoretical
Abstract

Using vector acoustic sensors for marine geoacoustic surveys instead of the usual scalar hydrophones enables one to acquire three-dimensional (3D) survey data with instrumentation and logistics similar to current 2D surveys. Vector acoustic sensors measure the sound wave direction directly without the cumbersome arrays that hydrophones require. This concept was tested by a scaled experiment in an acoustic water tank that had a well-controlled environment with a few targets. Using vector acoustic data from a single line of sources, the three-dimensional tank environment was imaged by directly locating the source and all reflectors.

Published
2006
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7. Inversion of sonobuoy data from shallow-water sites with simulated annealing.

Author

Lindwall D and Brozena J

Abstract

An enhanced simulated annealing algorithm is used to invert sparsely sampled seismic data collected with sonobuoys to obtain seafloor geoacoustic properties at two littoral marine environments as well as for a synthetic data set. Inversion of field data from a 750-m water-depth site using a water-gun sound source found a good solution which included a pronounced subbottom reflector after 6483 iterations over seven variables. Field data from a 250-m water-depth site using an air-gun source required 35,421 iterations for a good inversion solution because 30 variables had to be solved for, including the shot-to-receiver offsets. The sonobuoy derived compressional wave velocity-depth (Vp-Z) models compare favorably with Vp-Z models derived from nearby, high-quality, multichannel seismic data. There are, however, substantial differences between seafloor reflection coefficients calculated from field models and seafloor reflection coefficients based on commonly used Vp regression curves (gradients). Reflection loss is higher at one field site and lower at the other than predicted from commonly used Vp gradients for terrigenous sediments. In addition, there are strong effects on reflection loss due to the subseafloor interfaces that are also not predicted by Vp gradients.

Published
2005
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