INVERSION OF POST-STACK SEISMIC DATA USING SIMULATED ANNEALING1

Model-based inversion of seismic reflection data is a global optimization problem when prior information is sparse. We investigate the use of an efficient, global, stochastic optimization method, that of simulated annealing, for determining the two-way traveltimes and the reflection coefficients. We exploit the advantage of an ensemble approach to the inversion of full-scale target zones on 2D seismic sections. In our ensemble approach, several copies of the model-algorithm system are run in parallel. In this way, estimation of true ensemble statistics for the process is made possible, and improved annealing schedules can be produced. It is shown that the method can produce reliable results efficiently in the 2D case, even when prior information is sparse.     

Anomalous geochemical signals from phosphatic Middle Cambrian rocks in the southern Georgina Basin, Australia

Middle Cambrian rocks of the Georgina Basin contain both phosphatic and organic-rich sediments, the former often occurring as commercially viable phosphate deposits, and the latter as minor oil shale occurrences. Both kinds of rocks occur in the Hay River Formation penetrated by Bureau of Mineral Resources (BMR) stratigraphic corehole Tobermory 14. Detailed sampling and analysis of this core has been undertaken to assess the chemostratigraphy of the Hay River Formation; to examine the possible use, as a palaeoceanographic tool, of changes in δ¹³C values of carbonates and kerogens; and to gain an understanding of phosphate deposition in this part of the basin. The Upper Hay River Formation was deposited in a normal marine environment characterized by oxic bottom conditions. It contains several episodes of significant trace metal enrichment which appear to indicate periodic replacement of the water body. The Lower Hay River Formation, however, was deposited in an abnormal marine environment low in sulphate and Mo and enriched in P. Its carbonate phase has positive δ¹³C values 1–1.5%₀ enriched over the accepted Cambrian marine carbonate value of -0.57 ± 0.17%₀. The very low sulphate concentration, Mo-depletion and spiking of carbonate δ¹⁸O values suggests there was an influx of deeper ocean water of sufficient volume to maintain prolonged stable anoxic bottom waters. These data support a previous suggestion that phosphogenesis in the Georgina Basin was related to ‘Oceanic Anoxic Events'.     

Quaternary coastal lithofacies,sequence development and stratigraphy in a passive margin setting,North Carolina and Virginia,USA

Lithofacies analysis is fundamental to unravelling the succession of depositional environments associated with sea‐level fluctuations. These successions and their timing are often poorly understood. This report defines lithofacies encountered within the north‐eastern North Carolina and south‐eastern Virginia Quaternary section, interprets their depositional environments, presents a model for coastal depositional sequence development in a passive margin setting and uses this understanding to develop the stratigraphy and Quaternary evolutionary history of the region. Data were obtained from numerous drill cores and outcrops. Chronology was based on age estimates acquired using optically stimulated luminescence, amino acid racemization, Uranium series and radiocarbon dating techniques. Geomorphic patterns were identified and interpreted using light detection and ranging imagery. Since lithofacies occurrence, distribution and stratigraphic patterns are different on interfluves than in palaeo‐valleys, this study focused on interfluves to obtain a record of highstand sea‐level cycles with minimal alteration by fluvial processes during subsequent lowstands. Nine primary lithofacies and four diagenetic facies were identified in outcrops and cores. The uppermost depositional sequence on interfluves exhibits an upward succession from shelly marine lithofacies to tidal estuarine lithofacies and is bounded below by a marine ravinement surface and above by the modern land surface. Older depositional sequences in the subsurface are typically bounded above and below by marine ravinement surfaces. Portions of seven depositional sequences were recognized and interpreted to represent deposition from late middle Pleistocene to present. Erosional processes associated with each successive depositional sequence removed portions of older depositional sequences. The stratigraphic record of the most recent sea‐level highstands (Marine Isotope Stage 5a and Marine Isotope Stage 3) is best preserved. Glacio‐isostatic adjustment has influenced depositional patterns so that deposits associated with late Quaternary sea‐level highstands (Marine Isotope Stages 5c, 5a and 3), which did not reach as high as present sea‐level according to equatorial eustatic records, are uplifted and emergent within the study area.     

Can liquefied debris flows deposit clean sand over large areas of sea floor? Field evidence from the Marnoso‐arenacea Formation,Italian Apennines

The Marnoso‐arenacea Formation in the Italian Apennines is the only ancient rock sequence where individual submarine sediment density flow deposits have been mapped out in detail for over 100 km. Bed correlations provide new insight into how submarine flows deposit sand, because bed architecture and sandstone shape provide an independent test of depositional process models. This test is important because it can be difficult or impossible to infer depositional process unambiguously from characteristics seen at just one outcrop, especially for massive clean‐sandstone intervals whose origin has been controversial. Beds have three different types of geometries (facies tracts) in downflow oriented transects. Facies tracts 1 and 2 contain clean graded and ungraded massive sandstone deposited incrementally by turbidity currents, and these intervals taper relatively gradually downflow. Mud‐rich sand deposited by cohesive debris flow occurs in the distal part of Facies tract 2. Facies tract 3 contains clean sandstone with a distinctive swirly fabric formed by patches of coarser and better‐sorted grains that most likely records pervasive liquefaction. This type of clean sandstone can extend for up to 30 km before pinching out relatively abruptly. This abrupt pinch out suggests that this clean sand was deposited by debris flow. In some beds there are downflow transitions from turbidite sandstone into clean debrite sandstone, suggesting that debris flows formed by transformation from high‐density turbidity currents. However, outsize clasts in one particular debrite are too large and dense to have been carried by an initial turbidity current, suggesting that this debris flow ran out for at least 15 km. Field data indicate that liquefied debris flows can sometimes deposit clean sand over large (10 to 30 km) expanses of sea floor, and that these clean debrite sand layers can terminate abruptly.     

Modelling soil map-unit inclusions by Monte Carlo simulation

Abstract

Error and uncertainty in spatial databases have gained considerable attention in recent years. The concern is that, as in other computer applications and, indeed, all analyses, poor quality input data will yield even worse output. Various methods for analysis of uncertainty have been developed, but none has been shown to be directly applicable to an actual geographical information system application in the area of natural resources. In spatial data on natural resources in general, and in soils data in particular, a major cause of error is the inclusion of unmapped units within areas delineated on the map as uniform. In this paper, two alternative algorithms for simulating inclusions in categorical natural resource maps are detailed. Their usefulness is shown by a simplified Monte Carlo testing to evaluate the accuracy of agricultural land valuation using land use and the soil information. Using two test areas it is possible to show that errors of as much as 6 per cent may result in the process of land valuation, with simulated valuations both above and below the actual values. Thus, although an actual monetary cost of the error term is estimated here, it is not found to be large.     

THE CORE OF THE QUADRANTID METEOROID STREAM IS TWO HUNDRED YEARS OLD

The Quadrantids are one of the most active annual meteor showers and have a number of unusual features. One is a sharp brief maximum, 12–14 h in length. A second is the Quadrantids, relatively recent appearance in our skies, the first observation having likely been made in 1835. Until recently no likely parent with a similar orbit had been observed and previous investigators concluded that the stream was quite old, with the stream’s recent appearance and sharp peak attributed to a recent fortuitous convergence of meteoroid orbits. The recent discovery of the near-Earth asteroid 2003 EH1 on an orbit very similar to that of the Quadrantids has almost certainly uncovered the parent body of this stream. From the simulations of the orbit of this body and of meteoroids released at intervals from it in the past, we find that both the sharp peak and recent appearance of the Quadrantids can most easily be explained assuming meteoroids were ejected in substantial numbers near 1800 AD.     

Vertical profiling of Saharan dust with Raman lidars and airborne HSRL in southern Morocco during SAMUM

Three ground-based Raman lidars and an airborne high-spectral-resolution lidar (HSRL) were operated during SAMUM 2006 in southern Morocco to measure height profiles of the volume extinction coefficient, the extinction-to-backscatter ratio and the depolarization ratio of dust particles in the Saharan dust layer at several wavelengths. Aerosol Robotic Network (AERONET) Sun photometer observations and radiosoundings of meteorological parameters complemented the ground-based activities at the SAMUM station of Ouarzazate. Four case studies are presented. Two case studies deal with the comparison of observations of the three ground-based lidars during a heavy dust outbreak and of the ground-based lidars with the airborne lidar. Two further cases show profile observations during satellite overpasses on 19 May and 4 June 2006. The height resolved statistical analysis reveals that the dust layer top typically reaches 4–6 km height above sea level (a.s.l.), sometimes even 7 km a.s.l.. Usually, a vertically inhomogeneous dust plume with internal dust layers was observed in the morning before the evolution of the boundary layer started. The Saharan dust layer was well mixed in the early evening. The 500 nm dust optical depth ranged from 0.2–0.8 at the field site south of the High Atlas mountains, Ångström exponents derived from photometer and lidar data were between 0–0.4. The volume extinction coefficients (355, 532 nm) varied from 30–300 Mm⁻¹ with a mean value of 100 Mm⁻¹ in the lowest 4 km a.s.l.. On average, extinction-to-backscatter ratios of 53–55 sr (±7–13 sr) were obtained at 355, 532 and 1064 nm.     

The anthropogenic influence on carbonaceous aerosol in the European background

To constrain the relatively uncertain anthropogenic impact on the organic aerosol load, radiocarbon analyses were performed on aerosol samples, collected year-round, at six non-urban sites including a maritime background and three remote mountain stations, lying on a west-east transect over Western Europe. From a crude three component model supported by TOC and levoglucosan filter data, the fossil fuel, biomass burning and biogenic TOC fraction are estimated, showing at all stations year-round, a relatively constant fossil fuel fraction of around  (26 ± 6)%  , a dominant biogenic contribution of on average  (73 ± 7)%  in summer and the continental as well as the maritime background TOC to be only about 50% biogenic. Assuming biomass burning as completely anthropogenic, the carbonaceous aerosol concentration at the mountain sites was found to have increased by a factor of up to  (1.4 ± 0.2)  in summer and up to  (2.5 ± 1.0)  in winter. This figure is significantly lower, however, than the respective TOC change since pre-industrial times seen in an Alpine ice core. Reconciling both observations would require an increase, since pre-industrial times, of the background biogenic aerosol load, which is estimated at a factor of 1.3–1.7.     

On resonant Rossby–Haurwitz triads

The dynamics of non-divergent flow on a rotating sphere are described by the conservation of absolute vorticity. The analytical study of the non-linear barotropic vorticity equation is greatly facilitated by the expansion of the solution in spherical harmonics and truncation at low order. The normal modes are the well-known Rossby–Haurwitz (RH) waves, which represent the natural oscillations of the system. Triads of RH waves, which satisfy conditions for resonance, are of critical importance for the distribution of energy in the atmosphere.
We show how non-linear interactions of resonant RH triads may result in dynamic instability of large-scale components. We also demonstrate a mathematical equivalence between the equations for an orographically forced triad and a simple mechanical system, the forced-damped swinging spring. This equivalence yields insight concerning the bounded response to a constant forcing in the absence of damping. An examination of triad interactions in atmospheric reanalysis data would be of great interest.     

Vertical profiling of convective dust plumes in southern Morocco during SAMUM

Lifting of dust particles by dust devils and convective plumes may significantly contribute to the global mineral dust budget. During the Saharan Mineral Dust Experiment (SAMUM) in May–June 2006 vertical profiling of dusty plumes was performed for the first time. Polarization lidar observations taken at Ouarzazate (30.9°N, 6.9°W, 1133 m height above sea level) are analyzed. Two cases with typical and vigorous formation of convective plumes and statistical results of 5 d are discussed. The majority of observed convective plumes have diameters on order of 100–400 m. Most of the plumes (typically 50–95%) show top heights <1 km or 0.3DLH with the Saharan dust layer height DLH of typically 3–4 km. Height-to-diameter ratio is mostly 2–10. Maximum plume top height ranges from 1.1 to 2.9 km on the 5 d. 5–26 isolated plumes and clusters of plumes per hour were detected. A low dust optical depth (<0.3) favours plume evolution. Observed surface, 1 and 2–m air temperatures indicate that a difference of 17–20 K between surface and 2-m air temperature and of 0.9–1 K between the 1 and 2-m temperatures are required before convective plumes develop. Favourable horizontal wind speeds are 2–7 m s⁻¹.