Biomedical subjects

I Kalikhman

Publications and source records attributed to I Kalikhman.

7 recordsLinked to original sources

PubMed2006

Patchy distribution fields: a spiral survey design and reconstruction adequacy.

A mathematical model was used to examine the effects of a spiral survey design on the adequacy of reconstructing patchy distribution fields. The model simulates fish or plankton patches (or gaps) of different shapes and spatial orientations, and acoustic surveys by a spiral of Archimedes; for comparison, surveys by parallel or zigzag transects are imitated. Adequacy of the reconstructed fields to those originally generated was evaluated by calculating their correlations (r). The mathematical experiments conducted showed that spiral surveys ensure, practically speaking, the same adequacy of field reconstruction (both in cases of immovable or movable patches) as do surveys by parallel or zigzag transects with greater sampling effort (overall path). In the case of a spiral survey, a patchy field can be reconstructed properly (r(2) = 0.70) if the overall survey path is not less than S/R(av) = 20-30, where R(av) is the autocorrelation radius averaged for various directions. Thus, the results obtained allow us to conclude that a spiral survey design is expedient in cases that the minimal duration of a survey is a decisive factor for its conduction and there is a priori information that no onshore-offshore gradients of fish density exist in a region under study.

Acoustics↗

PubMed2006

Patchy distribution fields: sampling distance unit of a spiral survey and reconstruction adequacy.

A mathematical model is used to examine the effects of choosing various units of sampling distance of a spiral acoustic survey on the adequacy of reconstructing patchy distribution fields. The model simulates fish or plankton patches (or gaps) of different shapes and spatial orientations, and an acoustic survey by a spiral of Archimedes along which a unit of sampling distance is set. For comparison, surveys are imitated by parallel transects along which the same unit of sampling distance is set. Adequacy of the reconstructed fields to those originally generated is evaluated by calculating their correlations (r). In the case of a spiral survey, the mathematical experiments conducted show that an immovable field can be reconstructed properly (r(2) > 0.70) if the ratio of the units of sampling distance to the autocorrelation radius for the field averaged in various directions d/R(av) < 1.0-1.5. Regarding immovable fields, the spiral surveys ensure, practically speaking, the same adequacy of the field reconstruction as do surveys by parallel transects with the same unit of sampling distance. In regard to movable fields, a comparison of the results of spiral surveys with those of surveys by parallel transects indicates that the former may ensure even higher adequacy of the field reconstruction than do the latter, provided that the units of sampling distances in these surveys are equal to each other.

Animals↗

PubMed2006

Patchy distribution fields: sampling distance unit of an interleaved survey and reconstruction adequacy.

A mathematical model was used to examine the effects of choosing various units of sampling distance of an interleaved (two-pass) acoustic survey on the adequacy of reconstructing patchy distribution fields. The model simulates fish or plankton patches (or gaps) of different shapes and spatial orientations, and an interleaved survey by parallel or zigzag transects, along which a unit of sampling distance is set. The efficiency of a survey is determined by the adequacy of a reconstructed field to that originally generated, which is evaluated by calculating their correlations (r). Regarding immovable fields, the experiments conducted indicate that a patchy field can be reconstructed properly (r2 > 0.70) if the distance between transects D < (1.5-2.0)R and the unit of sampling distance d < (1.5- 2.0)R(p). As, for regular surveys d < (1.0-1.5)R(p), it may be concluded that interleaved surveys are more efficient than regular ones because of the factor studied. In regard to movable fields, a comparison of the results of interleaved surveys with those of regular surveys directly indicates that the former may ensure a more adequate field reconstruction than the latter do. This fact confirms the previous conclusion that an interleaved survey is expedient in cases where there is no preference regarding the position of a vessel for further work.

Acoustics↗

PubMed2005

Patchy distribution fields: an interleaved survey design and reconstruction adequacy.

A mathematical model was used to compare the effects of a regular (one-pass) or interleaved (two-pass) acoustic survey on the adequacy of reconstructing patchy distribution fields. The model simulates fish or plankton patches of different shapes and spatial orientations, and a set of parallel or zigzag transects forming a regular or interleaved acoustic survey. The efficiency of a survey is determined by the adequacy of a reconstructed field to that originally generated, which is evaluated by calculating their correlations. Regarding the immovable fields, the efficiency of a regular or interleaved acoustic survey was tested with the following two alternative assumptions: (1) the entire survey was completed; (2) the survey was interrupted for some reason at the moment when one transect remained non-accomplished. In the former case, the efficiencies of both acoustic surveys were nearly the same; in the latter case, the efficiency of an interleaved survey was superior to that of a regular one. With respect to movable fields, the efficiency of the completed interleaved surveys was even higher than that of the regular ones. Thus, the results obtained allow us to conclude that an interleaved survey is expedient in cases where there is no preference regarding the position of a vessel for further work.

Acoustics↗

PubMed2002

Patchy distribution fields: a zigzag survey design and reconstruction adequacy.

A mathematical model was used to examine the effects of a zigzag survey design on the adequacy of reconstructing patchy distribution fields. The model simulates fish or plankton patches (or gaps) of different shapes and spatial orientations, and an acoustic survey by a set of transects forming a zigzag. Adequacy of the reconstructed fields to those originally generated was evaluated by calculating their correlations (r). A priori information on the autocorrelation radius for the field in the direction of a survey (R) allows optimization of survey design and the algorithm of data analysis. A patchy field can be reconstructed properly (r > 0.80) if the distance between transects D < (1.0-1.5)R. If a priori information on the field is not available, the autocorrelation radius should be determined when reconstructing the field, i.e. a posteriori. In cases of field movement, the criterion for choosing a survey direction is based on the relationship between the dimension of moving patches in the direction of movement and that of the surveyed area. The results obtained indicate that, for a fixed transect spacing, zigzag pattern allow less adequate reconstruction of an original distribution field (in cases of both immovable and movable fields) than corresponding parallel pattern.

Acoustics↗

PubMed2002

Patchy distribution fields: sampling distance unit of a zigzag survey and reconstruction adequacy.

A mathematical model was used to examine the effects of choosing various units of sampling distance of a zigzag survey on the adequacy of reconstructing patchy distribution fields. The model simulates fish or plankton patches (or gaps) of different shapes and spatial orientations, and an acoustic survey by zigzag or parallel transects along which a unit of sampling distance is set. Adequacy of the reconstructed fields to those originally generated is evaluated by calculating their correlations (r). A priori information on the autocorrelation radii for the field in the directions of the survey (Rs) and perpendicular direction (Rp) allows optimisation of the survey design and the algorithm of data analysis. A field can be reconstructed properly (r2 > 0.70) if the distance between transects D < (1.0-1.5)Rs and the unit of sampling distance d < (1.0-1.5)Rp. A posteriori determination of patch orientation allows reconstruction of the best field attainable on the basis of the survey data. In cases of field movement, if the dimension of patches in the direction of movement exceeds that of a surveyed area, a survey in the opposite direction gives best results; in contrast, if the dimension of moving patches is smaller than that of a surveyed area, it is reasonable to carry out a survey in the same direction. The criterion remains valid when a survey is carried out by zigzag transects and a unit of sampling distance is set along them. The results obtained indicate that, for a fixed transect spacing and a given number of sampling points on each full transect, zigzag pattern allows less adequate reconstruction of an original distribution field (in cases of both immovable and movable fields) than corresponding parallel pattern.

Animals↗

PubMed2001

Tautomeric equilibrium between penta- and hexacoordinate silicon chelates. A chloride bridge between two pentacoordinate silicons.

The reaction of O-trimethylsilyl-1,1-dimethyl-2-trifluoroacetylhydrazine (1a) with chloromethyl(methyl)dichlorosilane affords an unexpected equilibrium mixture, 10a right arrow over left arrow 11a, between a neutral hexacoordinate silicon chelate with a covalent chloro ligand (10a) and an ionic pentacoordinate silicon complex (11a). The equilibrium reaction consists formally of a migration of the covalent chloro ligand from silicon to an adjacent ammonium nitrogen, as a chloride anion, and thus constitutes a novel type of tautomeric reaction. Crystallographic and NMR data provide evidence for the reaction. Temperature, solvent, substituent, and counterion effects on the tautomeric equilibrium are discussed: when the temperature of the mixture is raised, the equilibrium ratio 10a/11a increases. Formation of the mixture in toluene, a nonionizing solvent, shifts the equilibrium completely toward the neutral 10a. When the initial hydrazide has a phenyl (11c) or a hydrogen (11b) group as substituent, rather than CF3, the equilibrium is shifted to the ionic side. Replacement of the chloride counterion by triflate, using trimethylsilyl triflate, shifts the 10a/11a mixture to the ionic side. Low-temperature NMR monitoring of the stepwise formation of 10/11 was carried out and provided insight into the reaction mechanism. In an attempt to grow crystals of 11c, the pentacoordinate tautomer analogue, an unprecedented chloride-bridged disiloxane complex, with two pentacoordinate silicons sharing a common axial chloro ligand, crystallized and was characterized and described.

Journal Article↗