bluuebunny/crossref_metadata_2025_split huggingface dataset:
Borehole‐guided AVO analysis of P‐P and P‐S reflections: Quantifying uncertainty on density estimates
Authors: Djikpesse Hugues A. , Meghirbi Wael , Nizkous Irina , Cao Di 
Published: September 2010

ABSTRACTSeismic properties of isotropic elastic formations are characterized by the three parameters: acoustic impedance, Poisson's ratio and density. Whilst the first two are usually well estimated by analysing the amplitude variation with angle (AVA) of reflected P‐P waves, density is known to be poorly resolved. However, density estimates would be useful in many situations encountered in oil and gas exploration, in particular, for minimizing risks in looking ahead while drilling. We design a borehole seismic experiment to investigate the reliability of AVA extracted density. Receivers are located downhole near the targeted reflectors and record reflected P‐P and converted P‐S waves. A non‐linear, wide‐angle‐based Bayesian inversion is then used to access the a posteriori probability distributions associated with the estimation of the three isotropic elastic parameters. The analysis of these distributions suggests that the angular variation of reflected P‐S amplitudes provides additional substantial information for estimating density, thus reducing the estimate uncertainty variance by more than one order of magnitude, compared to using only reflected P‐waves.
Path‐integral seismic imaging
Authors: Landa E. , Fomel S. , Moser T.J. 
Published: September 2010

ABSTRACTA new type of seismic imaging, based on Feynman path integrals for waveform modelling, is capable of producing accurate subsurface images without any need for a reference velocity model. Instead of the usual optimization for traveltime curves with maximal signal semblance, a weighted summation over all representative curves avoids the need for velocity analysis, with its common difficulties of subjective and time‐consuming manual picking. The summation over all curves includes the stationary one that plays a preferential role in classical imaging schemes, but also multiple stationary curves when they exist. Moreover, the weighted summation over all curves also accounts for non‐uniqueness and uncertainty in the stacking/migration velocities. The path‐integral imaging can be applied to stacking to zero‐offset and to time and depth migration. In all these cases, a properly defined weighting function plays a vital role: to emphasize contributions from traveltime curves close to the optimal one and to suppress contributions from unrealistic curves. The path‐integral method is an authentic macromodel‐independent technique in the sense that there is strictly no parameter optimization or estimation involved. Development is still in its initial stage, and several conceptual and implementation issues are yet to be solved. However, application to synthetic and real data examples shows that it has the potential for becoming a fully automatic imaging technique.
Non‐monotone spectral projected gradient method applied to full waveform inversion
Authors: Zeev Noam , Savasta Olga , Cores Debora 
Published: September 2010

ABSTRACTThe seismic inversion problem is a highly non‐linear problem that can be reduced to the minimization of the least‐squares criterion between the observed and the modelled data. It has been solved using different classical optimization strategies that require a monotone descent of the objective function. We propose solving the full‐waveform inversion problem using the non‐monotone spectral projected gradient method: a low‐cost and low‐storage optimization technique that maintains the velocity values in a feasible convex region by frequently projecting them on this convex set. The new methodology uses the gradient direction with a particular spectral step length that allows the objective function to increase at some iterations, guarantees convergence to a stationary point starting from any initial iterate, and greatly speeds up the convergence of gradient methods. We combine the new optimization scheme as a solver of the full‐waveform inversion with a multiscale approach and apply it to a modified version of the Marmousi data set. The results of this application show that the proposed method performs better than the classical gradient method by reducing the number of function evaluations and the residual values.
Non‐hyperbolic moveout inversion of wide‐azimuth P‐wave data for orthorhombic media
Authors: Vasconcelos Ivan , Tsvankin Ilya 
Published: September 2010

ABSTRACTThe azimuthally varying non‐hyperbolic moveout of P‐waves in orthorhombic media can provide valuable information for characterization of fractured reservoirs and seismic processing. Here, we present a technique to invert long‐spread, wide‐azimuth P‐wave data for the orientation of the vertical symmetry planes and five key moveout parameters: the symmetry‐plane NMO velocities, V(1)nmo and V(2)nmo, and the anellipticity parameters, η(1), η(2) and η(3). The inversion algorithm is based on a coherence operator that computes the semblance for the full range of offsets and azimuths using a generalized version of the Alkhalifah–Tsvankin non‐hyperbolic moveout equation.The moveout equation provides a close approximation to the reflection traveltimes in layered anisotropic media with a uniform orientation of the vertical symmetry planes. Numerical tests on noise‐contaminated data for a single orthorhombic layer show that the best‐constrained parameters are the azimuth ϕ of one of the symmetry planes and the velocities V(1)nmo and V(2)nmo, while the resolution in η(1) and η(2) is somewhat compromised by the trade‐off between the quadratic and quartic moveout terms. The largest uncertainty is observed in the parameter η(3), which influences only long‐spread moveout in off‐symmetry directions. For stratified orthorhombic models with depth‐dependent symmetry‐plane azimuths, the moveout equation has to be modified by allowing the orientation of the effective NMO ellipse to differ from the principal azimuthal direction of the effective quartic moveout term.The algorithm was successfully tested on wide‐azimuth P‐wave reflections recorded at the Weyburn Field in Canada. Taking azimuthal anisotropy into account increased the semblance values for most long‐offset reflection events in the overburden, which indicates that fracturing is not limited to the reservoir level. The inverted symmetry‐plane directions are close to the azimuths of the off‐trend fracture sets determined from borehole data and shear‐wave splitting analysis. The effective moveout parameters estimated by our algorithm provide input for P‐wave time imaging and geometrical‐spreading correction in layered orthorhombic media.
Observation of azimuthal anisotropy from the seismic reflectivity of a Tertiary turbidite sand
Authors: MacBeth Colin , Shams Asghar 
Published: September 2010

ABSTRACTP‐wave data from a time‐lapse 3D OBC survey have been analysed to estimate and interpret azimuthal seismic anisotropy. This is achieved by careful processing to preserve the azimuthal signature. The survey images a major reservoir body in a channelized turbidite field in the Gulf of Mexico. Three distinct and significant anisotropy anomalies are discovered on or around this particular ‘4500‐ft sand’, all of which change intensity but not orientation with hydrocarbon production. These anomalies are distributed along the highest concentration of cumulative sand thickness, with their symmetry axes aligned with the main channel axis. We suspect that this time‐lapse anisotropy could be caused by the alignment of the depositional grain fabric. Theoretical calculation predicts that this mechanism, when combined with fluid‐saturation changes, can generate the observed pattern of behaviour. If further supported by other researchers, this result would indicate that appropriately designed seismic surveys could be a useful tool for palaeo‐direction studies in clastic reservoirs and also a useful constraint for directional permeability in the reservoir flow simulation model.
Refraction traveltime and amplitude corrections for very near‐ surface inhomogeneities
Authors: Palmer Derecke 
Published: September 2010

ABSTRACTThe performance of refraction inversion methods that employ the principle of refraction migration, whereby traveltimes are laterally migrated by the offset distance (which is the horizontal separation between the point of refraction and the point of detection on the surface), can be adversely affected by very near‐surface inhomogeneities. Even inhomogeneities at single receivers can limit the lateral resolution of detailed seismic velocities in the refractor.The generalized reciprocal method ‘statics’ smoothing method (GRM SSM) is a smoothing rather than a deterministic method for correcting very near‐surface inhomogeneities of limited lateral extent. It is based on the observation that there are only relatively minor differences in the time‐depths to the target refractor computed for a range of XY distances, which is the separation between the reverse and forward traveltimes used to compute the time‐depth. However, any traveltime anomalies, which originate in the near‐surface, migrate laterally with increasing XY distance. Therefore, an average of the time‐depths over a range of XY values preserves the architecture of the refractor, but significantly minimizes the traveltime anomalies originating in the near‐surface. The GRM statics smoothing corrections are obtained by subtracting the average time‐depth values from those computed with a zero XY value. In turn, the corrections are subtracted from the traveltimes, and the GRM algorithms are then re‐applied to the corrected data. Although a single application is generally adequate for most sets of field data, model studies have indicated that several applications of the GRM SSM can be required with severe topographic features, such as escarpments.In addition, very near‐surface inhomogeneities produce anomalous head‐wave amplitudes. An analogous process, using geometric means, can largely correct amplitude anomalies. Furthermore, the coincidence of traveltime and amplitude anomalies indicates that variations in the near‐surface geology, rather than variations in the coupling of the receivers, are a more likely source of the anomalies.The application of the GRM SSM, together with the averaging of the refractor velocity analysis function over a range of XY values, significantly minimizes the generation of artefacts, and facilitates the computation of detailed seismic velocities in the refractor at each receiver. These detailed seismic velocities, together with the GRM SSM‐corrected amplitude products, can facilitate the computation of the ratio of the density in the bedrock to that in the weathered layer. The accuracy of the computed density ratio improves where lateral variations in the seismic velocities in the weathered layer are known.
A time‐domain induced‐polarization method for estimating permeability in a shaly sand reservoir
Authors: Tong Maosong , Li Li , Wang Weinan , Jiang Yizhong 
Published: September 2010

ABSTRACTIt is known that the time‐domain induced‐polarization decay curve for a shaly sand reservoir depends on the pore structure of the reservoir, and this curve can be used to estimate permeability, which is a determining factor in making production decisions in the petroleum industry. Compared with NMR logging tools, induced polarization has several advantages, such as a deep depth of investigation and a high signal‐to‐noise ratio. The purpose of this paper is to establish an appropriate model using induced polarization to estimate the permeability. The curve can be modelled as a weighted superposition of exponential relaxations. The plot of weight versus the relaxation time constant is defined as the relaxation time spectrum.Induced‐polarization decay‐curve measurements were performed on 123 samples from the Daqing oilfield using a four‐electrode technique. A singular‐value decomposition method was used to transform the induced‐polarization decay data into a spectrum. Different models to estimate the permeability were discussed. The results of the research indicate that the induced‐polarization measurements greatly improve the statistical significance of permeability correlations. Compared with the traditional forms, AφC and AFC, the forms, ATBφC and ATBFC, have lower error factors, where T, Φ and F are the geometric mean time constant of the induced‐polarization relaxation time spectrum, the porosity and the resistivity formation factor, respectively, and A, B and C are constants. The mean time constant is the decisive parameter in the permeability estimation and it is not completely independent of the resistivity formation factor. The additional use of the porosity and the resistivity formation factor leads to an appreciable improvement. It is concluded that this new model will make it possible to estimate the permeability of a shaly sand reservoir downhole.
3D Fourier finite‐difference migration by alternating‐direction‐implicit plus interpolation
Authors: Zhang Jin‐Hai , Wang Wei‐Min , Fu Li‐Yun , Yao Zhen‐Xing 
Published: September 2010

ABSTRACTConventional two‐way splitting Fourier finite‐difference migration for 3D complex media yields azimuthal anisotropy where an additional phase correction is needed with much increase of computational cost. We incorporate the alternating‐direction‐implicit plus interpolation scheme into the conventional Fourier finite‐difference method to reduce azimuthal anisotropy. This scheme retains the high‐order remnants ignored by the two‐way splitting in the form of a wavefield interpolation in the wavenumber domain. The wavefield interpolation for each step of downward extrapolation is implemented between the wavefields before and after the conventional Fourier finite‐difference extrapolation. As the Fourier finite‐difference migration is implemented in the space and wavenumber dual space, the Fourier transforms between space and wavenumber domain that were needed for the alternating‐direction‐implicit plus interpolation in frequency domain (FD) migration are saved in Fourier finite‐difference migration. Since the azimuth anisotropy in Fourier finite‐difference is much less than that in FD, the application of the alternating‐direction‐implicit plus interpolation scheme in Fourier finite‐difference migration is superior to that in FD migration in handling complex media with large velocity contrasts and steep dips. Impulse responses show that the presented method reduces the azimuthal anisotropy at almost no extra cost.
A global perspective of ground level, ‘ambient’ carbon dioxide for assessing the response of plants to atmospheric CO2
Authors: Ziska L. H. , Ghannoum O. , Baker J. T. , Conroy J. , Bunce J. A. , Kobayashi K. , Okada M. 
Published: September 2010

AbstractFor most studies involving the response of plants to future concentrations of atmospheric carbon dioxide (CO2), a current concentration of 360–370 μatm is assumed, based on recent data obtained from the Mauna Loa observatory. In the present study, average seasonal diurnal values of ambient CO2 obtained at ground level from three global locations (Australia, Japan and the USA) indicated that the average CO2 (at canopy height) can vary from over 500 μatm at night to 350 μatm during the day with average 24‐h values ranging from 390 to 465 μatm. At all sites sampled, ambient CO2 rose to a maximum value during the pre‐dawn period (03.00–06.00 hours); at sunrise, CO2 remained elevated for several hours before declining to a steady‐state concentration between 350 and 400 μatm by mid‐morning (08.00–10.00 hours). Responses of plant growth to simulations of the observed variation of in situ CO2 were compared to growth at a constant CO2 concentration in controlled environment chambers. Three diurnal patterns were used (constant 370 μatm CO2, constant 370 during the day (07.00–19.00 hours), high CO2 (500 μatm) at night; or, high CO2 (500 μatm) at night and during the early morning (07.00–09.00 hours) decreasing to 370 μatm by 10.00 hours). Three plant species − soybean (Glycine max, L (Merr.), velvetleaf (Abutilon theophrasti L.) and tomato (Lycopersicon esculentum L.) − were grown in each of these environments. For soybean, high night‐time CO2 resulted in a significant increase in net assimilation rate (NAR), plant growth, leaf area and biomass relative to a constant ambient value of CO2 by 29 days after sowing. Significant increases in NAR for all three species, and significant increases in leaf area, growth and total biomass for two of the three C3 species tested (velvetleaf and soybean) were also observed after 29 days post sowing for the high night/early morning diurnal pattern of CO2. Data from these experiments suggest that the ambient CO2 concentration experienced by some plants is higher than the Mauna Loa average, and that growth of some agricultural species at in situ CO2 levels can differ significantly from the constant CO2 value used as a control in many CO2 experiments. This suggests that a reassessment of control conditions used to quantify the response of plants to future, elevated CO2 may be required.
Distribution of aboveground live biomass in the Amazon basin
Authors: SAATCHI S. S. , HOUGHTON R. A. , DOS SANTOS ALVALÁ R. C. , SOARES J. V. , YU Y. 
Published: September 2010

AbstractThe amount and spatial distribution of forest biomass in the Amazon basin is a major source of uncertainty in estimating the flux of carbon released from land‐cover and land‐use change. Direct measurements of aboveground live biomass (AGLB) are limited to small areas of forest inventory plots and site‐specific allometric equations that cannot be readily generalized for the entire basin. Furthermore, there is no spaceborne remote sensing instrument that can measure tropical forest biomass directly. To determine the spatial distribution of forest biomass of the Amazon basin, we report a method based on remote sensing metrics representing various forest structural parameters and environmental variables, and more than 500 plot measurements of forest biomass distributed over the basin. A decision tree approach was used to develop the spatial distribution of AGLB for seven distinct biomass classes of lowland old‐growth forests with more than 80% accuracy. AGLB for other vegetation types, such as the woody and herbaceous savanna and secondary forests, was directly estimated with a regression based on satellite data. Results show that AGLB is highest in Central Amazonia and in regions to the east and north, including the Guyanas. Biomass is generally above 300 Mg ha−1 here except in areas of intense logging or open floodplains. In Western Amazonia, from the lowlands of Peru, Ecuador, and Colombia to the Andean mountains, biomass ranges from 150 to 300 Mg ha−1. Most transitional and seasonal forests at the southern and northwestern edges of the basin have biomass ranging from 100 to 200 Mg ha−1. The AGLB distribution has a significant correlation with the length of the dry season. We estimate that the total carbon in forest biomass of the Amazon basin, including the dead and belowground biomass, is 86 Pg C with ±20% uncertainty.
Laboratory incubations reveal potential responses of soil nitrogen cycling to changes in soil C and N availability in Mojave Desert soils exposed to elevated atmospheric CO2
Authors: SCHAEFFER SEAN M. , BILLINGS SHARON A. , DAVE EVANS R. 
Published: September 2010

AbstractElevated atmospheric carbon dioxide (CO2) has the potential to alter soil carbon (C) and nitrogen (N) cycling in arid ecosystems through changes in net primary productivity. However, an associated feedback exists because any sustained increases in plant productivity will depend upon the continued availability of soil N. We took soils from under the canopies of major shrubs, grasses, and plant interspaces in a Mojave Desert ecosystem exposed to elevated atmospheric CO2 and incubated them in the laboratory with amendments of labile C and N to determine if elevated CO2 altered the mechanistic controls of soil C and N on microbial N cycling. Net ammonification increased under shrubs exposed to elevated CO2, while net nitrification decreased. Elevated CO2 treatments exhibited greater fluxes of N2O–N under Lycium spp., but not other microsites. The proportion of microbial/extractable organic N increased under shrubs exposed to elevated CO2. Heterotrophic N2‐fixation and C mineralization increased with C addition, while denitrification enzyme activity and N2O–N fluxes increased when C and N were added in combination. Laboratory results demonstrated the potential for elevated CO2 to affect soil N cycling under shrubs and supports the hypothesis that energy limited microbes may increase net inorganic N cycling rates as the amount of soil‐available C increases under elevated CO2. The effect of CO2 enrichment on N‐cycling processes is mediated by its effect on the plants, particularly shrubs. The potential for elevated atmospheric CO2 to lead to accumulation of NH4+ under shrubs and the subsequent volatilization of NH3 may result in greater losses of N from this system, leading to changes in the form and amount of plant‐available inorganic N. This introduces the potential for a negative feedback mechanism that could act to constrain the degree to which plants can increase productivity in the face of elevated atmospheric CO2.
Spatial and temporal variability of net ecosystem production in a tropical forest: testing the hypothesis of a significant carbon sink
Authors: SIERRA CARLOS A. , HARMON MARK E. , MORENO FLAVIO H. , ORREGO SERGIO A. , Del VALLE JORGE I. 
Published: September 2010

AbstractTropical forest ecosystems play an important role in the global carbon balance. Depending on age and land use, they can act as carbon sources, sinks, or be in approximate balance, but it is uncertain if global environmental changes are forcing these ecosystems outside their natural range of variation. We asked the question of whether or not the net carbon flux of a tropical primary forest, which should be in balance over the long term, is within the expected range of natural variation. A simple Bayesian hypothesis testing method was used to address this question for primary forests in the Porce region of Colombia. Net ecosystem production (NEP) was measured in this forest in a set of 33 permanent plots from 2000 to 2002 in 2, 1‐year intervals. Our estimate of NEP ranged between −4.03 and 2.22 Mg C ha−1 yr−1for the two intervals. This range was compared witha prioridefined range of natural variation estimated from the ecosystem model STANDCARB, which estimated spatial and temporal variation due to gap dynamics. The prior range of variation was estimated between −1.5 and 1.5 Mg C ha−1 yr−1. The observed data on NEP did not provide sufficient evidence to reject the null hypothesis that these forests are in C balance. We concluded that the ecosystem is likely behaving within its range of natural variation, but measurement uncertainties were a major limitation to finding evidence to reject the null hypothesis. A literature review of C flux studies in the tropics revealed that about half of the observations could be explained by gap dynamics alone, while significant C sinks have only been observed during La Niña years, with contrasting results in other tropical forests. In conclusion, observational data of carbon fluxes do not appear to provide direct evidence for a significant carbon sink in some sites in the tropics.