Aliphatic hydrocarbons in the bitumens of the Puertollano oil shales

The aliphatic fraction of the bitumens from 3 bands of the Puertollano oil shale field have been studied by gas chromatography and gas chromatography/mass spectrometry. Previous work published over the last years reached different conclusions about the origin of the organic matter in the Puertollano oil shales. A reappraisal of some of the biomarkers was then needed for a better understanding of the aliphatic hydrocarbons assemblage. The identified compounds, mainly n-alkanes in the medium to high boiling point region, acyclic isoprenoids and hopane-related compounds point towards a predominant algal/bacterial input. No unequivocally higher plant derived biomarkers have been detected in significant amounts, suggesting that the bulk of the organic matter derive from authocthonous organisms living in the basin. The biomarker maturity ratios indicate that the 3 Puertollano oil shales have similar maturities and are close to the onset of oil generation, assessing a slightly higher maturity than that determined by petrographic techniques.     

Contribution to hydrological modelling of small Amazonian catchments: application of rainfall–runoff models to simulate flow duration curves

Abstract

The objective of this study is to analyse three rainfall–runoff hydrological models applied in two small catchments in the Amazon region to simulate flow duration curves (FDCs). The simple linear model (SLM) considers the rainfall–runoff process as an input–output time-invariant system. However, the rainfall–runoff process is nonlinear; thus, a modification is applied to the SLM based on the residual relationship between the simulated and observed discharges, generating the modified linear model (MLM). In the third model (SVM), the nonlinearity due to infiltration and evapotranspiration is incorporated into the system through the sigmoid variable gain factor. The performance criteria adopted were a distance metric (δ) and the Nash-Sutcliffe coefficient (R²) determined between simulated and observed flows. The good results of the models, mainly the MLM and SVM, showed that they could be applied to simulate FDCs in small catchments in the Amazon region.

Editor D. Koutsoyiannis; Associate editor A. Montanari

Citation Blanco, C.J.C., Santos, S.S.M., Quintas, M.C., Vinagre, M.V.A., and Mesquita, A.L.A., 2013. Contribution to hydrological modelling of small Amazonian catchments: application of rainfall–runoff models to simulate flow duration curves. Hydrological Sciences Journal, 58 (7), 1–11.     

Relative importance of dust inputs and aquatic biological production as sources of lake sediments in an oligotrophic lake in a semi‐arid area

The dustfall (DF) and mass sedimentation rate (MSR) in Lake Alchichica, Central Mexico, were studied from June 2006 to June 2008. DF ranged between 0·11–0·93 g m^?2 d^?1 in the warm and rainy season and 0·54–1·21 g m^?2 d^?1 in the cold and dry season. MSR varied from 0·52–2·40 g m^?2 d^?1 in the stratification season to 1·14–5·07 g m^?2 d^?1 in the circulation season. The timing of the highest fluxes of DF and MSR is most likely a product of several factors coinciding during the cold and dry season: (a) availability of dust sources and the presence of strong winds (>7·5 m s^?1) in the DF case, and (b) the circulation period of the lake and the availability of nutrients in the MSR case. As expected, the DF in Alchichica was high and similar to that found in other arid and semi‐arid areas. However, the MSR was higher than that reported for other oligotrophic lakes. Particles captured in the aerial traps consisted mainly of detrital minerals; in contrast, particles found in the water traps were mostly biogenic and, to a lesser extent, detrital minerals. The MSR was one to seven times higher than the DF. In spite of the oligotrophic status of Lake Alchichica, the large size of the settled phytoplankton (autochthonous, waterborne) is what leads to the high MSR, which surpasses the DF (allochthonous, airborne) derived from whirlwinds originating in easily eroded terrains that are characteristic of arid/semi‐arid areas. Our results indicate that caution must be taken in considering that the DF amount measured through DF collectors located at the lake shore does actually represent the DF entering into the lake. Copyright © 2010 John Wiley & Sons, Ltd.     

Interaction between magnetic clouds and the heliospheric current sheet at 1 AU as it is observed by one single observation point

The effect of Magnetic Clouds (MCs) on the Heliospheric Current Sheet (HCS) local structure is yet an open question. Although it is widely accepted that a magnetic cloud has an important effect on the HCS shape, their structural relation, either the MC is part of the HCS or not, is not completely solved. Moreover, the problem grows up when trying to investigate three dimensional structures using one single observation point. We propose an approach to the MC–HCS study using magnetic models for the MC and local HCS structures, which are able of determining their relative orientation from one single spacecraft data. Three events have been selected in which an MC passage was observed close to HCS crossings. The results confirm the strong effect of MC passage on the HCS local orientation and they seem to be consistent with MCs propagating out of the HCS at 1 AU.     

Radar interferometry techniques for the study of ground subsidence phenomena: a review of practical issues through cases in Spain

Subsidence related to multiple natural and human-induced processes affects an increasing number of areas worldwide. Although this phenomenon may involve surface deformation with 3D displacement components, negative vertical movement, either progressive or episodic, tends to dominate. Over the last decades, differential SAR interferometry (DInSAR) has become a very useful remote sensing tool for accurately measuring the spatial and temporal evolution of surface displacements over broad areas. This work discusses the main advantages and limitations of addressing active subsidence phenomena by means of DInSAR techniques from an end-user point of view. Special attention is paid to the spatial and temporal resolution, the precision of the measurements, and the usefulness of the data. The presented analysis is focused on DInSAR results exploitation of various ground subsidence phenomena (groundwater withdrawal, soil compaction, mining subsidence, evaporite dissolution subsidence, and volcanic deformation) with different displacement patterns in a selection of subsidence areas in Spain. Finally, a cost comparative study is performed for the different techniques applied.     

A modified rock physics model for analysis of seismic signatures of low gas-saturated rocks

In seismic applications, the bulk modulus of porous media saturated with liquid and gas phases is often estimated using Gassmann's fluid substitution formula, in which the effective bulk modulus of the two-phase fluid is the Reuss average of the gas and liquid bulk moduli. This averaging procedure, referred to as Wood's approximation, holds if the liquid and gas phases are homogeneously distributed within the pore space down to sizes well below the seismic wavelength and if the phase transfer processes between liquid and gas domains induced by the pressure variations of the seismic wave are negligible over the timescale of the wave period. Using existing theoretical results and low-frequency acoustic measurements in bubbly liquids, we argue that the latter assumption of “frozen” phases, valid for large enough frequencies, is likely to fail in the seismic frequency range where lower effective bulk modulus and velocity, together with dispersion and attenuation effects, are expected. We provide a simple method, which extends to reservoir fluids a classical result by Landau and Lifshitz valid for pure fluids, to compute the effective bulk modulus of thermodynamically equilibrated liquid and gas phases. For low gas saturation, this modulus is significantly lower than its Wood's counterpart, especially at the crossing of bubble point conditions. A seismic reflector associated to a phase transition between a monophasic and a two-phase fluid thus will appear. We discuss the consequences of these results for various seismic applications including fizz water discrimination and hydrocarbon reservoir depletion and CO₂ geological storage monitoring.     

On the Flux-Rope Topology of Ejecta Observed in the Period 1997 – 2006

In the following study our aim is to analyse the magnetic flux-rope topology of some events observed in the interplanetary medium related to ejecta. The magnetic field structures associated with interplanetary coronal mass ejections are globally classified in magnetic clouds and ejecta. One of the main questions regarding these phenomena concerns their flux-rope or non-flux-rope magnetic field line configuration. From the experimental measurements the only way to elucidate such a question is analysing the corresponding data by means of a flux-rope physical model. After selecting the ejecta events observed during the period 1997?–?2006, we have analysed them in light of an analytical model with that topology for the magnetic field components, initially developed for magnetic clouds, and with a non-force-free character; then, incorporating the expansion of the magnetic structure during their evolution in the interplanetary medium. Different parameters obtained from the fitting of the model are related to the orientation of the axis of the magnetic flux-rope structure and, additionally, the closest distance approach of the spacecraft to its axis. One of the main conclusions achieved concerns the fact that the axes of most of those structures are close to the Sun–Earth line, which implies that the passage of the spacecraft through the corresponding ejecta event is by its flank. In general, we show a rough procedure for the analysis and classification of ejecta in terms of their magnetic field topology.     

Observable Effects of Interplanetary Coronal Mass Ejections on Ground Level Neutron Monitor Count Rates

In this work, non-recurrent Forbush decreases (FDs) triggered by the passage of shock-driving interplanetary coronal mass ejections (ICMEs) have been analyzed. Fifty-nine ICMEs have been studied, but only 25 % of them were associated to a FD. We find that shock-driving magnetic clouds (MCs) produce deeper FDs than shock-driving ejecta. This fact can be explained regarding the observed growing trends between decreases in neutron monitor (NM) count rate and MC/ejecta speed and its associated rigidity. MCs are faster and have higher associated rigidities than ejecta. Also the deceleration of ICMEs seems to be a cause for producing FDs, as can be inferred from the decreasing trend between NM count rate and deceleration. This probably implies that the interaction between the ICME traveling from the corona to the Earth and the solar wind can play an important role in producing deeper FDs. Finally, we conclude that ejecta without flux rope topology are the ones less effective in unchaining FDs.     

Variability of Circumstellar Emission from Dust Envelopes Around Carbon Stars

The main constituents of the dust, produced around late-type carbon stars, are thought to be carbon and silicon carbide (SiC), although their exact nature is not yet well established. This subject has been addressed by several authors and good fits of a large number of IRAS sources (chosen among the carbon stars exhibiting the SiC feature at about 11.3 μm) were obtained. In this work we use the same procedure on a limited number of objects, of the same type, taking into account the variability of such sources and the changes induced in their IR spectra. For this purpose, the chosen stars have been observed with a spectral resolution higher than that available for the IRAS data (λ/Δλ ≅ 50), using the CGS3 instrument of the UKIRT telescope, both in the low (λ/Δλ ≅ 160) and in the high (λ/Δλ ≅ 500) resolution configuration. The results are discussed and some preliminary conclusions are drawn. This revised version was published online in September 2006 with corrections to the Cover Date.