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At least 685 records · Page 38

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NASA NTRS1973

Particle track record in Apollo 15 deep core from 54 to 80 cm depths.

Particle track measurements have been made in nearly 500 individual grains from 13 levels in the 54-80 cm depth range of the Apollo 15 deep core. They reveal a wide range of track densities at all depths and some systematic variations within layers, indicating that both predepositional mixing and subsequent layering are present and that separate sublayers exist within larger regions where no sublayers are visible. Minimum track densities are inferred to be useful measures of maximum residence times for undisturbed layers. Using the observed minimum track densities, we conclude that the average deposition rate in this section of soil column was greater than 0.4 cm/million years.

Fleischer, R. L.↗

NASA NTRS1973

Depth profile of Mn-53 in the lunar surface

Measurements of cosmic-ray produced Mn-53 are reported for a series of lunar surface samples down to a depth of 416 g/sq cm. These results clearly illustrate the decrease in activity with depth as the incident galactic cosmic rays are absorbed. Below 60 g/sq cm the production rate decreases exponentially with a mean length of about 220 g/sq cm. These results indicate that, at the Apollo 15 site, the lunar regolith has been unmixed, on a meter scale, for the last 5 m.y. The neutron activation technique for Mn-53, which allowed samples smaller than 200 mg to be used for these measurements, is described.

Imamura, M.↗

NASA NTRS1975

Estimating snow depth using VHRR data from NOAA environmental satellites

The NOAA environmental satellites provide daily coverage of the earth in the visible and thermal spectral bands. The ground resolution of the very high resolution radiometer is 1 km at nadir. This improved resolution in the visible permits more detailed observations of snow features than was possible with previous operational satellites. A densitometer examination of a visible-band image from Feb. 11, 1973, which shows heavy snow cover in considerable detail over areas extending from Alabama to North Carolina, indicates that, in general, there is direct correlation between increasing brightness and increasing snow depths. A power regression analysis of greatest satellite brightness versus greatest snow depth for 201 data pairs produced a correlation coefficient of 0.86. Similar analysis of five late winter and early spring cases resulted in much lower correlations.

Mcginnis, D. F., Jr.↗

NASA NTRS1976

An estimate of the influence of sediment concentration and type on remote sensing penetration depth for various coastal waters

Under the assumptions of collimated light, a homogenous water column, zero molecular scattering, and constant ratio of volume scattering function to scattering coefficient, estimates of the remote sensing depth parameter, Z90, are made for various coastal waters at 540 nm. Calculations indicate that sediment concentration and type have a strong influence on remote sensing depth when concentrations are below 5 mg/theta. Above 5 mg/theta, the absorption coefficient of the sediments becomes large in comparison to that of water, causing Z90 values to be less than 2 m with only small differences between various sediment types.

Whitlock, C. H.↗

NASA NTRS1975

Design parameters for a stereoptic television system based on direct vision depth perception cues

Remotely controlled systems which use television for visual feedback require that depth cues be available to the operator. A number of techniques have been developed to provide stereoptic video. An analysis of parameters of such systems as related to the depth cues of convergence and retinal disparity was carried out. Parameter requirements were determined for the provision of natural and exaggerated stereoptic cues and expressions were developed for range resolution limits based on the retinal disparity threshold. An empirical study was conducted using a stereoptic video system to determine threshold values

Shields, N. L., Jr.↗

NASA NTRS1976

Depth-diameter relation for large Martian craters determined from Mariner 9 UVS altimetry

The depth/diameter ratio for 87 craters on Mars is determined using Mariner 9 ultraviolet-spectrometer (UVS) altimetry. The sample includes craters 12 to 100 km in diameter and 0.4 to 3.3 km in depth. The largest depth/diameter ratios on Mars are comparable to those of fresh craters on Mercury. However, more than half of the present sample consists of degraded craters whose depths are significantly shallower than those of fresh craters of similar diameter on Mercury, confirming the interpretations of earlier photoanalysts.

Burt, J.↗

NASA NTRS1978

Non-destructive ultrasonic measurements of case depth

Two ultrasonic methods for nondestructive measurements of the depth of a case-hardened layer in steel are described. One method involves analysis of ultrasonic waves diffused back from the bulk of the workpiece. The other method involves finding the speed of propagation of ultrasonic waves launched on the surface of the work. Procedures followed in the two methods for measuring case depth are described.

Flambard, C.↗

NASA NTRS1978

On the apparent source depth of planetary magnetic fields

Two simple assumptions regarding the ratios of the strengths of the field contributions of the multipole moments of the terrestrial magnetic field at its effective source depth are used to examine the consistency between the apparent source depths for the magnetic fields of Mercury and Jupiter and the present understanding of their interior structure. Both fields are consistent with the present understanding. However, the comparison would be facilitated by further measurements of the magnetic fields at both planets, especially at Mercury.

Elphic, R. C.↗

NASA NTRS1975

The simulated depth history of dust grains in the lunar regolith

Trajectories giving the individual depth variations of lunar dust grains with time are randomly generated by a Monte Carlo code, where the variables are the mass and speed distribution of meteorites at the lunar surface and the geometrical shape of impact craters. A statistical analysis of a great number of such trajectories is then used to define the 'average' depth history of lunar dust grains for two grain radii: 1 and 50 microns. This yields: (1) estimates for time constants involved during the dynamic evolution of the regolith, (2) a model for the layering of the regolith, and (3) a better understanding of the basic dust-grain mechanisms responsible for the formation of the most mature lunar soil samples. The validity of various soil models proposed for the dynamic evolution of the regolith is discussed in terms of experimental constraints on the models.

Duraud, J. P.↗

NASA NTRS1978

Aerosol size distributions obtained by inversion of spectral optical depth measurements

Columnar aerosol size distributions have been inferred by numerically inverting particulate optical depth measurements as a function of wavelength. An inversion formula which explicitly includes the magnitude of the measurement variances is derived and applied to optical depth measurements obtained in Tucson with a solar radiometer. It is found that the individual size distributions of the aerosol particles (assumed spherical), at least for radii greater than or approximately equal to 0.1 micron, fall into one of three distinctly different categories. Approximately 50% of all distributions examined thus far can best be represented as a composite of a Junge distribution plus a distribution of relatively monodispersed larger particles centered at a radius of about 0.5 micron. Scarcely 20% of the distributions yielded Junge size distributions, while 30% yielded relatively monodispersed distributions of the log-normal or gamma distribution types. A representative selection of each of these types will be presented and discussed. The sensitivity of spectral attenuation measurements to the radii limits and refractive index assumed in the numerical inversion will also be addressed.

King, M. D.↗

NASA NTRS1979

Estimation of the depth of sunlight penetration in natural waters for the remote sensing of chlorophyll a via in vivo fluorescence

In attempting to measure remotely the constituents of the ocean through spectral analysis of diffusely reflected sunlight, it is important to know the depth over which constituent concentrations can be estimated. Recently, considerable interest has been generated in the use of sunlight-excited fluorescence of chlorophyll a contained in photoplankton (in vivo) to determine remotely the chlorophyll a concentration in surface waters. In the present paper an estimate is provided for the depth to which chlorophyll a concentration can be determined from observations of the fluorescence.

Gordon, H. R.↗

NASA NTRS1980

Active notch filter network with variable notch depth, width and frequency

An active notch filter having independently adjustable notch frequency, width, and depth is provided by three equal capacitors connected in series with an operational amplifier (connected in a voltage follower configuration), a potentiometer across the series connected capacitors for notch depth adjustment, and a potentiometer (for notch frequency connected across the center capacitor); with its tap connected to receive a voltage feedback signal from a variable voltage divider comprised of another potentiometer for notch width. Adjusting the voltage dividing potentiometer will independently set the notch width, and adjusting the tap on the potentiometer across the center capacitor will independently adjust the notch frequency of the filter. A second operational amplifier connected in a voltage follower configuration may be used to connect the voltage divider output to the adjustable tap of the potentiometer across the center capacitor.

Black, J. M.↗

NASA NTRS1980

Effective depth of spectral line formation in planetary atmospheres

The effective level of line formation for spectroscopic absorption lines has long been regarded as a useful parameter for determining average atmospheric values of the quantities involved in line formation. The identity of this parameter was recently disputed. The dependence of this parameter on the average depth where photons are absorbed in a semi-infinite atmosphere is established. It is shown that the mean depths derived by others are similar in nature and behavior.

Lestrade, J. P.↗

NASA NTRS1979

Electron, electron-bremsstrahlung and proton depth-dose data for space-shielding applications

A data set has been developed, consisting of depth-dose distributions for omni-directional electron and proton fluxes incident on aluminum shields. The principal new feature of this work is the accurate treatment, based on detailed Monte Carlo calculations, of the electron-produced bremsstrahlung component. Results covering the energy region of interest in space-shielding calculations have been obtained for the absorbed dose (a) as a function of depth in a semi-infinite medium, (b) at the edge of slab shields, and (c) at the center of a solid sphere. The dose to a thin tissue-equivalent detector was obtained as well as that in aluminum. Various results and comparisons with other work are given.

Seltzer, S. M.↗

NASA NTRS1980

Water depth measurement using an airborne pulsed neon laser system

The paper presents the water depth measurement using an airborne pulsed neon laser system. The results of initial base-line field test results of NASA airborne oceanographic lidar in the bathymetry mode are given, with water-truth measurements of depth and beam attenuation coefficients by boat taken at the same time as overflights to aid in determining the system's operational performance. The nadir-angle tests and field-of-view data are presented; this laser bathymetry system is an improvement over prior models in that (1) the surface-to-bottom pulse waveform is digitally recorded on magnetic tape, and (2) wide-swath mapping data may be routinely acquired using a 30 deg full-angle conical scanner.

Hoge, F. E.↗

NASA NTRS1980

Depth-dependence of turbulence in stellar atmospheres

Recent observations concerning the depth dependence of motions in stellar atmospheres are reviewed. With regard to turbulence in stellar photospheres, attention is given to evidence from the sun and Arcturus for an increase in the total micro- plus macro-velocities with height, starting at mid-photospheric levels. Concerning stellar chromospheric velocity fields, it is noted that Mg II data point to changes in the hydrodynamic structure of the upper atmosphere. Ca II and Mg II asymmetry observations and other studies are used to describe the upper atmospheric structure of cool evolved stars, particularly coronae and circumstellar envelopes, revealing a depth dependent effect of velocity fields among a wide range of such stars.

Stencel, R. E.↗

NASA NTRS1980

Chemical depth profiles of the GaAs/native oxide interface

The final-state oxidation products and their distribution in thin native oxides (30-40 A) on GaAs have been studied using X-ray photoelectron spectroscopy in conjunction with chemical depth profiling. Extended room-temperature-oxidation conditions have been chosen to allow the native oxide to attain its equilibrium composition and structure. The work emphasizes the use of chemical depth-profiling methods which make it possible to examine the variation in chemical reactivity of the oxide structure. A minimum of two distinct regions of Ga2O3 with differing chemical reactivity is observed. Chemical shift data indicate the presence of As2O3 in the oxide together with an elemental As overlayer at the interface. A change in relative charge transfer between oxygen and both arsenic and gallium-oxide species is observed in the region of the interface.

Grunthaner, P. J.↗

NASA NTRS1980

Mechanisms of wave transformation in finite-depth water

Mechanisms of wave transformation in finite-depth water are investigated. The linear mechanisms examined are percolation, bottom motion, shoaling, and refraction. The nonlinear mechanisms examined are wave-wave interaction and bottom friction. New exact computations of the nonlinear transfer for finite-depth waves are presented for some directional wave spectra. These mechanisms are found to explain satisfactorily wave decay observations obtained at several sites with different bottom sediment properties. The decay rates at these sites are found to be dominated by different mechanisms which are determined by the bottom conditions. As an example, detailed calculations are presented for data obtained at the Jonswap site.

Shemdin, O. H.↗