Generalized Born inversion of seismic reflection data

Summary . Born inverse methods give accurate and stable results when the source wavelet is impulsive. However, in many practical applications (reflection seismology) an impulsive source cannot be realized and the inversion needs to be generalized to include an arbitrary source function. In this paper, we present a Born solution to the seismic inverse problem which can accommodate an arbitrary source function and give accurate and stable results. It is shown that the form of the generalized inversion algorithm reduces to a Wiener shaping ***filter, which is solved efficiently using a Levinson recursion algorithm. Numerical examples of synthetic and real field data illustrate the validity of our method.     

Recovering magnetic susceptibility from electromagnetic data over a one-dimensional earth

While the inversion of electromagnetic data to recover electrical conductivity has received much attention, the inversion of those data to recover magnetic susceptibility has not been fully studied. In this paper we invert frequency-domain electromagnetic (EM) data from a horizontal coplanar system to recover a 1-D distribution of magnetic susceptibility under the assumption that the electrical conductivity is known. The inversion is carried out by dividing the earth into layers of constant susceptibility and minimizing an objective function of the susceptibility subject to fitting the data. An adjoint Green's function solution is used in the calculation of sensitivities, and it is apparent that the sensitivity problem is driven by three sources. One of the sources is the scaled electric field in the layer of interest, and the other two, related to effective magnetic charges, are located at the upper and lower boundaries of the layer. These charges give rise to a frequency-independent term in the sensitivities. Because different frequencies penetrate to different depths in the earth, the EM data contain inherent information about the depth distribution of susceptibility. This contrasts with static field measurements, which can be reproduced by a surface layer of magnetization. We illustrate the effectiveness of the inversion algorithm on synthetic and field data and show also the importance of knowing the background conductivity. In practical circumstances, where there is no a priori information about conductivity distribution, a simultaneous inversion of EM data to recover both electrical conductivity and susceptibility will be required.     

Tracking recent recovery from eutrophication in a high arctic lake (Meretta Lake, Cornwallis Island, Nunavut, Canada) using fossil diatom assemblages

Meretta Lake (Resolute Bay, Cornwallis Island, Nunavut, Canada) is a high arctic lake that received raw sewage for almost 50 years from the Canadian Department of Transport Base. The lake was sampled from 1968–72 during the International Biological Programme, as part of the Char Lake Project. As the number of users at the Transport Base declined throughout the 1990s, so too did the lake's nutrient levels, and Meretta Lake is now classified as oligotrophic. A previous diatom-based paleolimnological study revealed marked species assemblage shifts coincident with sewage inputs beginning in the late 1940s; however, because the core was taken at a time when nutrient levels were still relatively high (i.e., 1993), the diatom record did not yet track any signs of recovery. In this present study, we examined fossil diatom assemblages from a sediment core taken in 2001. Our results indicate a shift to the pre-impact diatom assemblages in the most recent sediments, indicating that the paleolimnological record is tracking the decreased nutrient inputs to this high arctic lake, and confirms that no significant lags exist in these largely ice-covered lakes.     

Organochlorine residues,eggshell thickness,and nest success in barn owls from the Chesapeake Bay

Eggs of barn owls (Tyto alba) were collected from 18 nests in offshore duck blinds on the Maryland side of the lower Potomac River estuary in 1972 and 1973 and analyzed for organochlorine residues. DDE was found in 100% of the clutches, PCBs in 89%, and dieldrin in 78%. Eggshell thickness was inversely correlated with concentrations of DDE, DDD, and dieldrin residues. Six of the 18 clutches had mean DDE residues above 5 ppm, and eggshell thickness in these six clutches was significantly less (P<0.001) than in the other 12 clutches. The owls produced 1.7 young per active nest in 1973. This rate is slightly below the reproductive rate needed to maintain a stable population. An estimated 15% of the population carried concentrations of organochlorine residues that may have been detrimental to their reproduction. Passerine birds, taken extensively as food by a small proportion of the population, are believed to have been the source of elevated concentrations of organochlorines in these barn owls.     

Line-of-sight magnetic flux imbalances caused by electric currents

Several physical and observational effects may contribute to the significant imbalances of magnetic flux that are often observed in active regions. We consider an effect not previously treated: the influence of electric currents in the photosphere. Electric currents can cause a line-of-sight flux imbalance because of the directionality of the magnetic field they produce. Currents associated with magnetic flux tubes produce larger imbalances than do smoothly-varying distributions of flux and current. We estimate the magnitude of this effect for current densities, total currents, and magnetic geometry consistent with observations. The expected imbalances lie approximately in the range 0–15%, depending on the character of the current-carrying fields and the angle from which they are viewed. Observationally, current-induced flux imbalances could be indicated by a statistical dependence of the imbalance on angular distance from disk center. A general study of magnetic flux balance in active regions is needed to determine the relative importance of other - probably larger -effects such as dilute flux (too weak to measure or rendered invisible by radiative transfer effects), merging with weak background fields, and long-range connections between active regions.Operated for the National Science Foundation by the Association of Universities for Research in Astronomy.     

Boulder 1, Station 2, Apollo 17: Petrology and petrogenesis

Boulder 1, Station 2, Apollo 17 is a stratified boulder containing dark clasts and dark-rimmed light clasts set in a light-gray friable matrix. The gray to black clasts (GCBx and BCBx) are multigenerational, competent, high-grade metamorphic, and partially melted breccias. They contain a diverse suite of lithic clasts which are mainly ANT varieties, but include granites, basaltic-textured olivine basalts, troctolitic and spinel troctolitic basalts, and unusual lithologies such as KREEP norite, ilmenite (KREEP) microgabbro, and the Civet Cat norite, which is believed to be a plutonic differentiate. The GCBxs and BCBxs are variable in composition, averaging a moderately KREEPy olivine norite. The matrix consists of mineral fragments derived from the observed lithologies plus variable amounts of a component, unobserved as a clast-type, that approximates a KREEP basalt in composition, as well as mineral fragments of unknown derivation. The high-temperature GCBxs cooled substantially before their incorporation into the friable matrix of Boulder 1. The light friable matrix (LFBx) is texturally distinct from the competent breccia clasts and, apart from the abundant ANT clasts, contains clasts of a KREEPy basalt that is not observed in the competent breccias. The LFBx lacks such lithologies as the granites and the Civet Cat norite observed in the competent breccias and in detail is a distinct chemical as well as textural entity. We interpret the LFBx matrix as Serenitatis ejecta deposited in the South Massif, and the GCBx clasts as remnants of an ejecta blanket produced by an earlier impact. The source terrain for the Serenitatis impact consisted of the competent breccias, crustal ANT lithologies, and the KREEPy basalts, attesting to substantial lunar activity prior to the impact. The age of the older breccias suggests that the Serenitatis event is younger than 4.01±0.03 b.y.     

Vitrification darkening of rock powders: implications for optical properties of the lunar surface

Laboratory experiments show that albedoes as low as those on the Moon can be produced by vacuum vitrification and associated chemical fractionation of ordinary terrestrial basaltic material. Vitrification is established as an unequivocal process that can account for the low albedo and apparent local darkening with age of the lunar surface. The spectral reflectance curves of glass powders are significantly different than those of the parent rock mineralogy; thus, the presence of ubiquitous glass in lunar surface material complicates compositional determinations by interpretation of spectral reflectance curves. Vitrification of rocks on the Moon may highly modify the chemical composition of the resulting glass; thus, glass fragments found in lunar fines cannot be assumed to represent bulk parent rock material. Progressive impact vitrification of lunar surface material throughout the Moon's history may have led to a fine-grain, opaque, refractory-rich material we call ultimate glass. This unidentified and, at this point, hypothetical component may exist in dark regolith material; if found, it may be a useful indicator of regolith maturity.Paper dedicated to Prof. Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.     

Characterization and molecular analysis of hepatic microsomal UDP-glucuronosyltransferases in the marine fish, Pleuronectes platessa

Here we report the results of the first molecular enzymological study of fish UDP-glucuronosyltransferase (UDPGT). UDPGT activities of plaice liver microsomes were greatest for planar phenols and there was low but measurable conjugation of bilirubin, testosterone and androsterone. A highly purified preparation was isolated which possessed activities towards 1-naphthol, bilirubin and testosterone, containing several molecular weight species of M_r 52–56 kDa. On immunoblot analysis these proteins cross-reacted with a polyspecific anti-rat UDPGT antibody, suggesting that a number of plaice UDPGT isoforms with epitopes in common with the corresponding rat enzymes had been purified.     

Irregular Satellites of Jupiter: a study of the capture direction

The irregular satellites of Jupiter are believed to be captured asteroids or planetesimals. In the present work is studied the direction of capture of these objects as a function of their orbital inclination. We performed numerical simulations of the restricted three-body problem, Sun-Jupiter-particle, taking into account the growth of Jupiter. The integration was made backward in time. Initially, the particles have orbits as satellites of Jupiter, which has its present mass. Then, the system evolved with Jupiter losing mass and the satellites escaping from the planet. The reverse of the escape direction corresponds to the capture direction. The results show that the Lagrangian points L1 and L2 mainly guide the direction of capture. Prograde satellites are captured through these two gates with very narrow amplitude angles. In the case of retrograde satellites, these two gates are wider. The capture region increases as the orbital inclination increases. In the case of planar retrograde satellites the directions of capture cover the whole 360° around Jupiter. We also verified that prograde satellites are captured earlier in actual time than retrograde ones. This paper was presented at the Asteriods, Comets and Meteors meeting held at Búzios, Rio de Janeiro, Brazil in August 2005 and could not be included in the special issue related to that conference.     

Fragmentation model of meteoroid motion,mass loss,and radiation in the atmosphere

Abstract— We present the basic differential equations of meteor physics (the single body equations). We solve them numerically including two possible types of fragmentation: into large pieces and into a cluster of small fragments. We have written a Fortran code that computes the motion, ablation and light intensity of a meteoroid at chosen heights, and allows for the ablation and shape density coefficients σ and K, as well as the luminous efficiency τ, to be variable with height/time. We calibrated our fragmentation model (FM) by the best fit to observational values for the motion, ablation, radiation, fragmentation and the terminal masses (recovered meteorites) for the Lost City bolide. The FM can also handle multiple and overlapping meteor flares. We separately define both the apparent and intrinsic values of σ, K, and τ. We present in this paper values of the intrinsic luminous efficiency as function of velocity, mass, and normalized air density. Detailed results from the successful application of the FM to the Lost City, Innisfree, and Benesov bolides are also presented. Results of applying the FM to 15 bolides with very precise observational data are presented in a survey mode (Table 7). Standard deviations of applying our FM to all these events correspond to the precision of the observed values. Typical values of the intrinsic ablation coefficient are low, mostly in the range from 0.004 to 0.008 s² km^?2, and do not depend on the bolide type. The apparent ablation coefficients reflect the process of fragmentation. The bolide types indicate severity of the fragmentation process. The large differences of the “dynamic” and “photometric” mass from numerous earlier studies are completely explained by our FM. The fragmentation processes cannot be modeled simply by large values of the apparent ablation coefficient and of the apparent luminous efficiency. Moreover, our new FM can also well explain the radiation and full dynamics of very fast meteoroids at heights from 200 km to 130 km.