Discrete Reflectors Research Articles

Spectral reflectance of the human ocular fundus

Reflectance spectra from discrete sites in the human ocular fundus were measured with an experimental reflectometer in the visible and near-infrared parts of the spectrum. The principal study population consisted of ten subjects 22 to 38 years of age with a wide range of degree of fundus melanin pigmentation. Reflectance spectra were obtained from the nasal fundus, the fovea, and an area 2.5 degrees from the fovea. Spectra were also recorded from several older subjects and from one aphakic patient with a coloboma. The reflectance spectra were found to be influenced by the degree of individual and local melanin pigmentation of the fundus, the amount of blood in the choroid, the transmission properties of the ocular media, and the discrete reflections in the stratified fundus layers. Mathematical models of the optical properties of the stratified layers are proposed and are fitted to the experimental fundus reflectance spectra. The models account for the absorption by blood, melanin, macular pigment, and ocular media, and incorporate tissue scattering and discrete reflectors corresponding to anatomical layers.

A method to analyze and verify deep crustal reflections offshore Costa Rica

A multichannel seismic reflection profile across the oceanic crust seaward of the Middle America Trench off the Nicoya Peninsula, Costa Rica, shows discontinuous, low‐frequency events at 6.5 to 7.0 s. These events might first be interpreted as reflections from the Moho. However, careful analysis of the seismic data suggests that these events represent three‐dimensional (3-D) scattered energy from the rough basaltic basement. Velocity analysis indicates that root‐mean‐square (rms) velocities for these deep “reflection events” are too low to emanate from the Moho. Also, the ghost separation caused by the streamer depth decreases for increasing record time, suggesting that incident angle for these “reflections” increases with time. Furthermore, these events are approximately 13 dB stronger than would be expected for a Moho reflection. Common‐depth‐point (CDP) stacking and velocity filtering were used to attenuate the scattered noise and sideswipe from the basalt. The results show a 21 dB total reduction of scattered energy. However, Moho reflections still cannot be discerned. The results suggest (1) ambient noise after processing is 20 dB below the expected Moho level and is not a factor in detection of the Moho; (2) Moho reflectivity may be smaller than 0.1 (reflectivity is calculated from assumed velocities and densities) and could be as small as 0.05 (the detection threshold); (3) the Moho may not be a discrete reflector and may therefore represent a transition zone; and (4) Moho events may be disorganized by transmission through rough basalt so the CDP stacking process is not effective.

Distribution of volcanic ash reflectors in the eastern equatorial Pacific Ocean

Volcanic ash erupted from vents in Central America, southern Mexico and northern South America has accumulated as discrete layers in sediment of the surrounding ocean basins. Many of these ash layers form acoustic reflectors which are observed on 3.5 kHz seismic records from the eastern equatorial Pacific. Twelve discrete reflectors within 40 m of the seafloor may be traced in the region. Shallow (<20 m) seismic reflectors can be correlated to ash layers in piston cores where some ashes were correlated by Bowles et al. (1973) and Drexler et al. (1980) using geochemical methods. In addition, deeper seismic reflectors (>20 m) can be correlated with ash layers in two DSDP cores and used to indirectly establish that all seismic reflectors in this study are ash layers. Based on areal distribution patterns, most seismic reflectors indicate the direction of possible volcanic sources. Moreover, using an assumed constant sedimentation rate of 3 cm × 10 3 yr −1, the average estimated age of each reflector can be calculated. All ash reflectors are Quaternary in age. Reflectors 1 to 3 are correlated to marine ash layers “D”, “I”, and “L”, and the assigned ages correlate well with the previously established ages of 83,000, 133,000 and 230,000 yrs B.P., respectively, for these layers.

Developments in Seismic Processing for Geologic Interpretation: ABSTRACT

Since the introduction in the early 1960s of digital recordings and processing of seismic data, the effectiveness of the seismic method as a petroleum exploration tool has improved substantially by providing better solutions to many problems such as water reverberations and resolution of discrete reflectors. Thus far, the primary emphasis has been on the development and application of new signal-processing techniques through the utilization of communication-theory concepts. With the development of large rapid-access bulk-storage devices and high-resolution on-line display capabilities, the scope of computer processing can be enlarged to include interpretation and the integration of geophysical and geological data. Continuous interval-velocity information, with an estimated associated error, can now be extracted from seismic data on a routine basis. Examples from field tests show that, in favorable cases, lateral variations in interval velocity of the order of 1% may be detected. In addition to obtaining a major increase in the accuracy of structural information, continuous velocity data provide a means for detecting lithologic and stratigraphic variations. This capability coupled with a means for extracting and displaying quantitative information about reflector amplitude and waveform provides new possibilities for stratigraphic-trap exploration. Examples from the Gulf of Mexico show how the computer may be utilized to obtain an interpretation of a grid of seismic data with the assistance of an interpreter to make difficult interpretation decisions and to correct the inevitable errors which can occur in the processing sequence. The capability of the computer to deal with and migrate all the relevant data in three dimensions will lead to a more accurate and complete three-dimensional model of the subsurface from seismic data. As this capability is realized, it will become useful to record seismic data on a tighter grid than is done with current practices. End_of_Article - Last_Page 742------------

Stability of Active Transmission Lines with Arbitrary Imperfections

Two sufficient conditions for the stability of one-dimensional active transmission lines with arbitrary imperfections (i.e., discrete or continuous reflections) are derived. The first stability condition guarantees stability for any arbitrary distribution of reflection. The second stability condition is restricted to a special case of interest that includes discrete reflectors with nominally equal magnitude and spacing; the stability condition for this restricted class is greatly improved over the general stability condition described above. These results, aside from their own interest, provide rigorous justification for previous calculations for the gain statistics of such a device with random discrete reflectors. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> They may also be used to find an upper bound on the probability of instability of such a device with random reflectors. Certain types of optical maser amplifiers and traveling-wave tubes provide examples of practical devices with distributed gain to which these results, or similar ones, might be applied.