Post-stack velocity analysis in the dip-angle domain using diffractions

Interval velocity analysis in complex geological areas is often considered as an unresolved problem. A novel approach to improve the velocity analysis process is to perform the analysis in a non-conventional domain and to use seismic events that are usually ignored during standard data processing and imaging. In this study, a method to analyse diffraction data for migration velocity analysis in the time- or depth-domain is presented. The method is based on the clear distinction between diffractions and reflections in the post-migration dip-angle domain. The attractive possibility to perform the analysis, using only stacked data as an input, is demonstrated on synthetic and real data examples.     

Dynamic programming and the principle of optimality: A systematic approach

The dynamic programming recursive procedure has provided an efficient method for solving a variety of sequential decision problems related to water resources systems. In many investigations Bellman's principle of optimality is used as a proof for the optimality of the dynamic programming solutions. In this paper the dynamic programming procedure is systematically studied so as to clarify the relationship between Bellman's principle of optimality and the optimality of the dynamic programming solutions.Our main result is that although the principle is valid, in order to use it as a proof for the optimality of the dynamic programming solution certain modeling requirements should be met.The mathematical model presented in this paper provides a convenient framework for the modeling and analysis of dynamic programming problems encountered by in water resources management studies.The results derived here resolve few of the fundamental questions raised in the literature regarding the validity of Bellman's principle of oplimality and the optimality of the dynamic programming solutions.     

Spectral Line and Continuum Radiation Propagation in a Clumpy Medium

We discuss the propagation of spectral line and continuum radiation in a clumpy medium and give general expressions for the observed absorption or emission from a cloud population. We show that the affect of the medium clumpiness can usually be characterised by a single number multiplying the mean column opacity. Our result provides a simpler proof and generalization of the result of Martin et al. (1984). The formalism provides a simple way to understand the effects of clumping on molecular line profiles and ratios; for example, how clumping effects the interpretation of ¹³CO(1–0) to ¹²CO(1–0) line ratios. It also can be used as a propagation operator in physical models of clumpy media where the incident radiation effects the spectral line emissivity. We are working to extend the formalism to the propagation of masers in a clumpy medium, but in this case, there are special difficulties because formal expectation values are not characteristic of observations because they are biased by rare events.     

Stellar structure and mass loss on the upper asymptotic giant branch

We examine the envelope properties of asymptotic giant branch (AGB) stars as they evolve on the upper AGB and during the early post-AGB phase. Because of the high mass-loss rate, the envelope mass decreases by more than an order of magnitude. This makes the density profile below the photosphere much shallower, and the entropy profile much steeper. We discuss the possible role of these changes in the profiles in the onset of the high mass-loss rate (superwind) and the large deviation from spherical mass loss at the termination of the AGB. We concentrate on the idea that the shallower density profile and steeper entropy profile allow the formation of cool magnetic spots, above which dust forms much more easily.     

Short-term changes in the magnitude, frequency and temporal distribution of floods in the Eastern Mediterranean region during the last 45 years — Nahal Oren, Mt. Carmel, Israel

Short-term changes in Eastern Mediterranean precipitation affecting flow regime were documented in Nahal Oren, a 35 km² ephemeral stream in Mt. Carmel, a 500 m high mountain ridge located at the NW coast of Israel. The rainy winter of the Mediterranean type climate (Csa) in Mt. Carmel is characterized by average annual rainfall of 550 mm at the coastal plain to 750 mm at the highest elevation while the summer is hot and dry. Stream flow generates after accumulated rainfall of 120–150 mm while “large floods”, defined as flows with peak discharge of > 5 m³ s^− 1 and/or > 150,000 m³ in volume, are generated in response to rainfall of over 100 mm. Hence, large floods in Nahal Oren stream occur not earlier than December. Precipitation and flow data were divided into two sub-periods: 1957–1969 and 1991–2003 and compared to each other. The results indicate a clear increase in the frequency of large floods, their magnitudes and volumes during the second period with no parallel change in the annual precipitation. Similarly, an increase in storm rainfall–runoff ratio from < 5% to > 15% and a decrease in the threshold rainfall for channel flow by 16–25% were documented. These short-term variations in flooding behavior are explained by the clear decrease in the length of the rainy season and by the resulting significant shortening in the duration of the dry-spells. The increase in the number of large rainfall events and the large floods in each hydrological year together with the increasing number of years with no floods indicate strengthening of their uncertainty of behavior.     

Redefining high school catchment areas with varying effects of achievement equality

Among the several measures being adopted to upgrade the level of education in Israel is the policy of maintaining an equal achievement level in all public high schools. Short-run allocation of elementary school graduates to schools is therefore needed. This paper presents a spatial allocation procedure which aims at improving achievement equality simultaneously with upgrading accessibility to schools. The procedure allows the mutual effect of these two factors to be varied, as well as the measurement of the real-world meaning of such variations. The advantages of the proposed procedure in comparison with the common linear programming model used in spatial planning of school districts are discussed. Allocation patterns generated for the city of Beer Sheva illustrate its use.     

A new exact method for line radiative transfer

We present a new method, the coupled escape probability (CEP), for exact calculation of line emission from multi-level systems, solving only algebraic equations for the level populations. The CEP formulation of the classical two-level problem is a set of linear equations , and we uncover an exact analytic expression for the emission from two-level optically thick sources that holds as long as they are in the 'effectively thin' regime. In a comparative study of a number of standard problems, the CEP method outperformed the leading line transfer methods by substantial margins.
The algebraic equations employed by our new method are already incorporated in numerous codes based on the escape probability approximation. All that is required for an exact solution with these existing codes is to augment the expression for the escape probability with simple zone-coupling terms. As an application, we find that standard escape probability calculations generally produce the correct cooling emission by the C  ii 158-μm line but not by the ³P lines of O  i .