The effect of topography on the determination of the geoid using analytical downward continuation

The method of analytical downward continuation has been used for solving Molodensky’s problem. This method can also be used to reduce the surface free air anomaly to the ellipsoid for the determination of the coefficients of the spherical harmonic expansion of the geopotential. In the reduction of airborne or satellite gradiometry data, if the sea level is chosen as reference surface, we will encounter the problem of the analytical downward continuation of the disturbing potential into the earth, too. The goal of this paper is to find out the topographic effect of solving Stoke’sboundary value problem (determination of the geoid) by using the method of analytical downward continuation. It is shown that the disturbing potential obtained by using the analytical downward continuation is different from the true disturbing potential on the sea level mostly by a −2πGρh 2/p. This correction is important and it is very easy to compute and add to the final results. A terrain effect (effect of the topography from the Bouguer plate) is found to be much smaller than the correction of the Bouguer plate and can be neglected in most cases. It is also shown that the geoid determined by using the Helmert’s second condensation (including the indirect effect) and using the analytical downward continuation procedure (including the topographic effect) are identical. They are different procedures and may be used in different environments, e.g., the analytical downward continuation procedure is also more convenient for processing the aerial gravity gradient data. A numerical test was completed in a rough mountain area, 35°<ϕ<38°, 240°<λ<243°. A digital height model in 30″×30″ point value was used. The test indicated that the terrain effect in the test area has theRMS value ±0.2−0.3 cm for geoid. The topographic effect on the deflections of the vertical is around1 arc second.     

Underice Operation of SJT-5 ROV

- The authors have used a low-cost ROV of observation type for underice surveying of the Nen-Jiang Cross-river oil pipe line in the Da Qing oil field. The operation method, process and the safety measures for severe cold areas are presented. The fruitful tries and experience may be useful to ROV operators in severe cold areas.     

Effect of a nodular segment on the dynamic response of a tubular pile subjected to longitudinal vibration

Acta Geotechnica - This paper investigates the effect of a nodular segment, commonly found in nodular piles, on the dynamic response of a vertical vibrating tubular pile. A developed model of a...     

Prediction of maximal daily average values of relativistic electron fluxes in geostationary orbit during the magnetic storm recovery phase

The relation of the maximal daily average values of the relativistic electron fluxes with an energy higher than 2 MeV, obtained from the measurements on GOES geostationary satellites, during the recovery phase of magnetic storms to the solar wind parameters and magnetospheric activity indices has been considered. The parameters of Pc5 and Pi1 geomagnetic pulsations and the relativistic electron fluxes during the prestorm period and the main phase of magnetic storms have been used together with the traditional indices of geomagnetic activity (A _E, K _p, D _st). A simple model for predicting relativistic electron fluxes has been proposed for the first three days of the magnetic storm recovery phase. The predicted fluxes of the outer radiation belt relativistic electrons well correlate with the observed values (R ∼ 0.8–0.9).     

High resolution facies record on late Holocene flood plain sediments from lower reaches of Narmada Valley,Western India

A high resolution quantitative granulometric record for site Uchediya [21°43′2.22″ N, 73° 6′26.22″ E; 10 m a. s. l.] gives understanding towards accretion history of the late Holocene flood plain in the lower reaches of Narmada River. Two sediment facies (sandy and muddy) and seven subfacies (sandy subfacies: St_MS+FS+CS, Sm_FS+MS, Sl_FS+VFS, and St_{MS + CS}; muddy subfacies: Fm_SILT+VFS+FS, Fm_SILT+VFS (O) and Fm_SILT+VFS (T)) are identified based on cluster analysis supplemented with sedimentary structures observed in field and other laboratory data. Changes in hydrodynamics are further deduced based on various sedimentological parameters and their ratios leading to arrive at a depositional model.     

Provenance of the Bosnian Flysch

Sandwiched between the Adriatic Carbonate Platform and the Dinaride Ophiolite Zone, the Bosnian Flysch forms a c. 3000 m thick, intensely folded stack of Upper Jurassic to Cretaceous mixed carbonate and siliciclastic sediments in the Dinarides. New petrographic, heavy mineral, zircon U/Pb and fission-track data as well as biostratigraphic evidence allow us to reconstruct the palaeogeology of the source areas of the Bosnian Flysch basin in late Mesozoic times. Middle Jurassic intraoceanic subduction of the Neotethys was shortly followed by exhumation of the overriding oceanic plate. Trench sedimentation was controlled by a dual sediment supply from the sub-ophiolitic high-grade metamorphic soles and from the distal continental margin of the Adriatic plate. Following obduction onto Adria, from the Jurassic–Cretaceous transition onwards a vast clastic wedge (Vranduk Formation) was developed in front of the leading edge, fed by continental basement units of Adria that experienced Early Cretaceous synsedimentary cooling, by the overlying ophiolitic thrust sheets and by redeposited elements of coeval Urgonian facies reefs grown on the thrust wedge complex. Following mid-Cretaceous deformation and thermal overprint of the Vranduk Formation, the depozone migrated further towards SW and received increasing amounts of redeposited carbonate detritus released from the Adriatic Carbonate Platform margin (Ugar Formation). Subordinate siliciclastic source components indicate changing source rocks on the upper plate, with ophiolites becoming subordinate. The zone of the continental basement previously affected by the Late Jurassic–Early Cretaceous thermal imprint has been removed; instead, the basement mostly supplied detritus with a wide range of pre-Jurassic cooling ages. However, a c. 80 Ma, largely synsedimentary cooling event is also recorded by the Ugar Formation, that contrasts the predominantly Early Cretaceous cooling of the Adriatic basement and suggests, at least locally, a fast exhumation.     

Comparison of Two Methods Used to Model Shape Parameters of Pareto Distributions

Two methods are compared for estimating the shape parameters of Pareto field-size (or pool-size) distributions for petroleum resource assessment. Both methods assume mature exploration in which most of the larger fields have been discovered. Both methods use the sizes of larger discovered fields to estimate the numbers and sizes of smaller fields: (1) the tail-truncated method uses a plot of field size versus size rank, and (2) the log–geometric method uses data binned in field-size classes and the ratios of adjacent bin counts. Simulation experiments were conducted using discovered oil and gas pool-size distributions from four petroleum systems in Alberta, Canada and using Pareto distributions generated by Monte Carlo simulation. The estimates of the shape parameters of the Pareto distributions, calculated by both the tail-truncated and log–geometric methods, generally stabilize where discovered pool numbers are greater than 100. However, with fewer than 100 discoveries, these estimates can vary greatly with each new discovery. The estimated shape parameters of the tail-truncated method are more stable and larger than those of the log–geometric method where the number of discovered pools is more than 100. Both methods, however, tend to underestimate the shape parameter. Monte Carlo simulation was also used to create sequences of discovered pool sizes by sampling from a Pareto distribution with a discovery process model using a defined exploration efficiency (in order to show how biased the sampling was in favor of larger fields being discovered first). A higher (more biased) exploration efficiency gives better estimates of the Pareto shape parameters.     

Pockmarks in the inner Oslofjord,Norway

Multibeam bathymetric surveys of the Inner Oslofjord, Norway have revealed a high density of pockmarks in the 179-km² inner fjord area, which contains over 500 pockmarks of varying size, typically 20–50 m in diameter and 2–10 m deep. These pockmarks have been investigated with a variety of techniques, including acoustic subbottom profiling, sedimentological and geochemical analyses of cores, remotely operated vehicle observation, and morphometry. Both the distribution and shapes of the pockmarks suggest that they are related to structures in the bedrock underlying relatively thin (<50 m) unconsolidated glacial and postglacial sediments. The data provide no direct indication of a particular mode of pockmark formation, but release of large amounts of biogenic, shallow methane seems unlikely. Several lines of evidence point to a continuous process of pockmark formation followed by inactivity, with some pockmarks recently active whereas others have been inactive for a considerable time. Some pockmarks are characterised by coarse sediment in their centres. The density, variety and easy access make this pockmark field an ideal model area for pockmark research. John S. Gray is deceased.     

Fishing for diatoms: fish gut analysis reveals water quality changes over a 75-year period

Defining the reference state of some perturbed streams and rivers is challenging since their non-altered state is undocumented. Despite the near-absence of pristine sites in highly anthropogenic areas, information about aquatic communities that existed prior to human disturbance can be obtained from historic sources. Sediment coring is commonly used in paleolimnology to estimate past environmental conditions of lakes, but this technique is often not appropriate for studies of lotic systems. An alternative is to use diatom assemblages present in the guts of fish that were captured in the streams prior to significant human disturbance. The purpose of the present study was to evaluate the biological integrity of several streams in Ontario and Québec during the early twentieth century based on “paleo” diatom assemblages extracted from the guts of fish stored in museums. The Eastern Canadian diatom index (IDEC: Indice Diatomées de l’Est du Canada) was used to evaluate the biological status of “paleo” and “modern” diatom assemblages. The IDEC shows the position of diatom assemblages on a general pollution gradient. The comparison of IDEC values calculated for the 1925–1948 and the 2003–2007 periods showed that several streams were severely polluted in the early 1900s. In general, present water quality has declined compared to the early 1900s. The biological integrity of only three of the 22 sites has increased. IDEC values were not influenced by the species of fish studied.