bluuebunny/crossref_metadata_2025_split huggingface dataset:
Seismic modelling study of a subglacial lake
Authors: Carcione José M. , Gei Davide 
Published: October 2003

ABSTRACTWe characterize the seismic response of Lake Vostok, an Antarctic subglacial lake located at nearly 4 km depth below the ice sheet. This study is relevant for the determination of the location and morphology of subglacial lakes. The characterization requires the design of a methodology based on rock physics and numerical modelling of wave propagation. The methodology involves rock‐physics models of the shallow layer (firn), the ice sheet and the lake sediments, numerical simulation of synthetic seismograms, ray tracing, τ–p transforms, and AVA analysis, based on the theoretical reflection coefficients. The modelled reflection seismograms show a set of straight events (refractions through the firn and top‐ice layer) and the two reflection events associated with the top and bottom of the lake. Theoretical AVA analysis of these reflections indicates that, at near offsets, the PP‐wave anomaly is negative for the ice/water interface and constant for the water/sediment interface. This behaviour is shown by AVA analysis of the synthetic data set. This study shows that subglacial lakes can be identified by using seismic methods. Moreover, the methodology provides a tool for designing suitable seismic surveys.
Petrophysical inversion of AVA data
Authors: Mazzotti A. , Zamboni E. 
Published: October 2003

ABSTRACTWe investigate the interactions between the elastic parameters,VP,VSand density, estimated by non‐linear inversion of AVA data, and the petrophysical parameters, depth (pressure), porosity, clay content and fluid saturation, of an actual gas‐bearing reservoir. In particular, we study how the ambiguous solutions derived from the non‐uniqueness of the seismic inversion affect the estimates of relevant rock properties. It results that the physically admissible values of the rock properties greatly reduce the range of possible seismic solutions and this range contains the actual values given by the well. By means of a statistical inversion, we analyse how approximatea prioriknowledge of the petrophysical properties and of their relationships with the seismic parameters can be of help in reducing the ambiguity of the inversion solutions and eventually in estimating the petrophysical properties of the specific target reservoir. This statistical inversion allows the determination of the most likely values of the sought rock properties along with their uncertainty ranges. The results show that the porosity is the best‐resolved rock property, with its most likely value closely approaching the actual value found by the well, even when we insert somewhat erroneousa prioriinformation. The hydrocarbon saturation is the second best‐resolved parameter, but its most likely value does not match the well data. The depth of the target interface is the least‐resolved parameter and its most likely value is strongly dependent ona prioriinformation. Although no general conclusions can be drawn from the results of this exercise, we envisage that the proposed AVA–petrophysical inversion and its possible extensions may be of use in reservoir characterization.
Joint AVO inversion, wavelet estimation and noise‐level estimation using a spatially coupled hierarchical Bayesian model
Authors: Buland Arild , Omre Henning 
Published: October 2003

ABSTRACTThe main objective of the AVO inversion is to obtain posterior distributions for P‐wave velocity, S‐wave velocity and density from specified prior distributions, seismic data and well‐log data. The inversion problem also involves estimation of a seismic wavelet and the seismic‐noise level. The noise model is represented by a zero mean Gaussian distribution specified by a covariance matrix. A method for joint AVO inversion, wavelet estimation and estimation of the noise level is developed in a Bayesian framework. The stochastic model includes uncertainty of both the elastic parameters, the wavelet, and the seismic and well‐log data. The posterior distribution is explored by Markov‐chain Monte‐Carlo simulation using the Gibbs' sampler algorithm. The inversion algorithm has been tested on a seismic line from the Heidrun Field with two wells located on the line. The use of a coloured seismic‐noise model resulted in about 10% lower uncertainties for the P‐wave velocity, S‐wave velocity and density compared with a white‐noise model. The uncertainty of the estimated wavelet is low. In the Heidrun example, the effect of including uncertainty of the wavelet and the noise level was marginal with respect to the AVO inversion results.
Computations of secondary potential for 3D DC resistivity modelling using an incomplete Choleski conjugate‐gradient method
Authors: Wu Xiaoping , Xiao Yifei , Qi Cheng , Wang Tongtong 
Published: October 2003

ABSTRACTAn accurate and efficient 3D finite‐difference (FD) forward algorithm for DC resistivity modelling is developed. In general, the most time‐consuming part of FD calculation is to solve large sets of linear equations: Ax=b, where A is a large sparse band symmetric matrix. The direct method using complete Choleski decomposition is quite slow and requires much more computer storage. We have introduced a row‐indexed sparse storage mode to store the coefficient matrix A and an incomplete Choleski conjugate‐gradient (ICCG) method to solve the large linear systems. By taking advantage of the matrix symmetry and sparsity, the ICCG method converges much more quickly and requires much less computer storage. It takes approximately 15 s on a 533 MHz Pentium computer for a grid with 46 020 nodes, which is approximately 700 times faster than the direct method and 2.5 times faster than the symmetric successive over‐relaxation (SSOR) conjugate‐gradient method. Compared with 3D finite‐element resistivity modelling with the improved ICCG solver, our algorithm is more efficient in terms of number of iterations and computer time. In addition, we solve for the secondary potential in 3D DC resistivity modelling by a simple manipulation of the FD equations. Two numerical examples of a two‐layered model and a vertical contact show that the method can achieve much higher accuracy than solving for the total potential directly with the same grid nodes. In addition, a 3D cubic body is simulated, for which the dipole–dipole apparent resistivities agree well with the results obtained with the finite‐element and integral‐equation methods. In conclusion, the combination of several techniques provides a rapid and accurate 3D FD forward modelling method which is fundamental to 3D resistivity inversion.
Velocity and attenuation in partially saturated rocks: poroelastic numerical experiments
Authors: Helle Hans B. , Pham Nam H. , Carcione José M. 
Published: October 2003

ABSTRACTWe use a poroelastic modelling algorithm to compute numerical experiments on wave propagation in a rock sample with partial saturation using realistic fluid distribution patterns from tomography scans. Frequencies are in the range 10 to 500 kHz. The rock is a homogeneous isotropic sandstone partially filled with gas and water, which are defined by their characteristic values of viscosity, compressibility and density. We assume no mixing and that the two different pore‐fills occupy different macroscopic regions. The von Kármán self‐similar correlation function is used, employing different fractal parameters to model uniform and patchy fluid distributions, respectively, where effective saturation is varied in steps from full gas to full water saturation.Without resorting to additional matrix–fluid interaction mechanisms, we are able to reproduce the main features of the variation in wave velocity and attenuation with effective saturation and frequency, as those of published laboratory experiments. Furthermore, the behaviour of the attenuation peaks versus water saturation and frequency is similar to that of White's model. The conversion of primary P‐wave energy into dissipating slow waves at the heterogeneities is shown to be the main mechanism for attenuating the primary wavefield. Fluid/gas patches are shown to affect attenuation more than equivalent patches in the permeability or solid‐grain properties.
Patterns in species richness and distribution of vascular epiphytes in Chiapas, Mexico
Authors: Wolf Jan H. D. , Alejandro Flamenco‐S 
Published: October 2003

AbstractAim  We aim to assess regional patterns in the distribution and species richness of vascular epiphytes with an emphasis on forests that differ in altitude and the amount of rainfall.Location  Tropical America, in particularly the 75,000 km2 large state of Chiapas in southern Mexico at 14.5–18.0°N. Chiapas is diverse in habitats with forests from sea‐level to the tree‐line at c. 3800 m altitude and with annual amounts of rainfall ranging from 800 to over 5000 mm. It is also one of the botanical best‐explored regions in the tropics.Methods  First we give an overview of epiphyte inventories to date. Such epiphyte surveys were mostly carried out on the basis of surface area or individual trees and we discuss their problematic comparison. Applying a different methodological approach, we then used 12,276 unique vascular epiphyte plant collections from Chiapas that are deposited in various botanical collections. The locality data were georeferenced and compiled in a relational data base that was analysed using a geographical information system. To compare the number of species between inventories that differed in the numbers of records, we estimated the total richness, SChao, at each.Results  We recorded 1173 vascular epiphyte species in thirty‐nine families (twenty‐three angiosperms), comprising c. 14% of all confirmed plant species in the state. About half of all species were orchids (568). Ferns and bromeliads were the next species‐rich groups with 244 and 101 species, respectively. Most species were found in the Montane Rain Forest and in the Central Plateau. Trees of different forest formations, rainfall regimes, altitudes and physiographical regions supported a characteristic epiphyte flora.Main conclusions  We were able to confirm the presumed presence of a belt of high diversity at mid‐elevations (500–2000 m) in neotropical mountains. In contrast to predictions, however, we observed a decrease in diversity when the annual amount of rainfall exceeded 2500 mm. The decrease is attributed to wind‐dispersed orchids, bromeliads and Pteridophyta that may find establishment problematical under frequent downpours. In the wet but seasonal forests in Chiapas, this decrease is not compensated by plants in the animal‐dispersed Araceae that are abundant elsewhere. We presume that in addition to the annual amount of rainfall, its distribution in time determines the composition of the epiphyte community.
Phytogeographical relations of the Andean dry valleys of Bolivia
Authors: López Ramiro Pablo 
Published: October 2003

AbstractAim The objective of this study is to examine the phytogeographical affinities of the Andean dry valleys of Bolivia in order to contribute to a better understanding of the Andean dry flora's distribution, origin and diversity. Particular emphasis is given to the analysis of the floristic connections of this flora with more austral parts of South America.Location The dry valleys of Bolivia are located in the Andes of the southern half of the country, at elevations between 1300 and 3200 m.Methods An extensive floristic list compiled by the author to evaluate plant diversity in these Andean regions was used as the base for this study. To accomplish this, all recorded genera and species were assigned, respectively, to 11 and 12 phytogeographical elements established previously by the author. Two phytogeographical spectra were thus obtained and analysed.Results At the genus level, the Andean dry valleys of Bolivia are clearly dominated by genera that have widespread distributions (cosmoplitan and subtropical genera). Many of these reached the Andes from the lowland region of the Chaco. At species level, Andean elements constitute more than 60% of the species total, most of which are restricted to the central‐southern Andes. This suggests that Chaco‐related and Andean genera had considerable levels of speciation in these valleys. Many genera and more than half the species have their northernmost distribution in the dry valleys of Bolivia, thereby underlining strong relationships with central‐southern South America (mainly Argentina, Paraguay and southern Brazil). The data supports the belief of the existence, in central‐southern Peru, of a floristic disjunction in dry to arid environments that separates a tropical dry flora north of this limit from a dry subtropical/warm temperate flora south of it.Main conclusions The Andean dry valleys of Bolivia are diverse plant communities with high levels of endemism (c. 18% of the species). The species of this region are more related to those present in central‐southern South America than to the flora of northern South America that ranges southwards to Peru. Many of the species have restricted distributions in the dry Andes of Bolivia and Argentina, and many genera of these dry valleys have their northernmost distribution in Bolivia/southern Peru, too. The data point to high levels of speciation also in the central Andes.
Correlates of geographic range size in New Zealand Chionochloa (Poaceae) species
Authors: Lloyd Kelvin M. , Bastow Wilson J. , Lee William G. 
Published: October 2003

AbstractAim To test for correlations between plant traits and geographic range size. Location: New Zealand.Methods Trait data were derived from comparative experiments, in which plants were grown in pots or in a common garden, that tested for intrinsic differences among the species in traits relating to growth, reproduction and dispersal. Controlled experiments were used to test for differences in responses to drought and waterlogging stress. Geographic range size was measured as the number of 10 km grid squares in the New Zealand region containing at least one occurrence of the species.Results Growth rate, dispersal capacity and environmental tolerance were all positively related to geographic range size. Geographically restricted species tended to have more variable flowering between years. Flowering intensity, reproductive allocation, seed set, diaspore size, and responses to single environmental factors were not related to geographic range size.Main conclusions The differences between range‐restricted and widespread Chionochloa species appear to represent alternative strategies of coping with environmental change in a dynamic landscape. Range‐restricted species are specialized to temporally persistent habitats that are of limited geographic extent. As a consequence, they have evolved traits that conflict with persistence in widespread habitats. The implication for conservation management is that the conservation of rare plants will frequently depend on protection of their habitats. The widespread Chionochloa species possess traits that enable them to disperse to and occupy a greater range of habitats. These traits have allowed some of these species to expand their ranges following environmental changes that favoured an increase in grassland extent.
The northern limit of Pinus banksiana Lamb. in Canada: explaining the difference between the eastern and western distributions
Authors: Asselin Hugo , Payette Serge , Fortin Marie‐Josée , Vallée Sheila 
Published: October 2003

AbstractAim Present northern distribution limit of jack pine (Pinus banksiana Lamb.) follows the northern limit of continuous open boreal forest in western Canada, but not in eastern Canada where it is located further south. We tested the hypothesis that fire plays a more important role than climate in explaining the present position of the northern distribution limit of jack pine.Location An experimental jack pine plantation was set up in 1992, c. 300 km north of the present distribution limit of the species, in the Boniface river area of northern Québec (57°43′ N, 76°05′ W).Methods Climate and fire data were used to compare sites at and north of the present distribution limit of jack pine. In 2001, surviving individuals from the plantation were measured (total height, annual shoot elongation, basal diameter, and presence/absence of cones).Results Climate data from the ten weather stations used in this study did not show major differences. The northern limit of jack pine distribution is closely associated with the occurrence of fires larger than 200 ha. Survival of the planted jack pines was 31%. About 25% of the surviving pines qualified as normal, single‐stem individuals; the others were slightly uprooted and/or showed marks of erosion or foraging. Cones were produced, although no viable seeds were found.Main conclusions The low number of degree‐days above 5 °C at the plantation site could explain why the seeds were not viable. However, such climate conditions are not sufficient to prevent growth, as was shown by annual shoot elongation measurements. Most of the surviving jack pines from the Boniface river plantation are relatively healthy and follow a normal developmental programme. Low fire frequency and small fire size are amongst the main factors that prevented P. banksiana from migrating further north or east following deglaciation in northern Québec and Labrador.
Biogeography of temperate Australasian Polystichum ferns as inferred from chloroplast sequence and AFLP
Authors: Perrie Leon R. , Brownsey Patrick J. , Lockhart Peter J. , Brown Elizabeth A. , Large Mark F. 
Published: October 2003

AbstractAim and location New Zealand began to separate from Gondwana c. 85 Ma, and has been isolated from the nearest large landmass, Australia, by some 2000 km of the Tasman Sea since c. 60 Ma. Given New Zealand's long geographical isolation, there has been considerable interest in explaining the origins of its different biotic elements. Here we investigate the biogeography of the fern genus Polystichum from temperate Australasia. Six species are found in New Zealand, four in Australia, and two on Lord Howe Island.Methods The evolutionary relationships between the twelve Polystichum species found in temperate Australasia were inferred from phylogenetic analyses of two molecular data sets: DNA sequence from the chloroplast rps4–trnS spacer locus; and AFLP DNA‐fingerprinting. The timing of the separation between Australian and New Zealand Polystichum was estimated by using the fossil record to temporally calibrate the rbcL sequence differentiation between representative species from these regions.Results Species of Polystichum from New Zealand appear to comprise a monophyletic group. This suggests that Polystichum crossed the Tasman only once. Temporal calibration of the rbcL sequence differentiation between Australian and New Zealand Polystichum indicates that a vicariant explanation for their separation can be rejected in favour of trans‐oceanic dispersal.Main conclusions The extant diversity within New Zealand Polystichum appears to have been derived from a single, trans‐oceanic dispersal event (within the last c. 20 Myr), followed by a relatively extensive in situ ecological radiation.
Phylogeny, biogeography and adaptive radiation of Pachycladon (Brassicaceae) in the mountains of South Island, New Zealand
Authors: Heenan P. B. , Mitchell A. D. 
Published: October 2003

AbstractAim To report analyses and propose hypotheses of adaptive radiation that explain distributional patterns of the alpine genus Pachycladon Hook.f. – a morphologically diverse genus from New Zealand closely related to Arabidopsis thaliana.Location South Island, New Zealand.Methods Morphological and nrDNA ITS sequence phylogenies were generated for Pachycladon. An analysis is presented of species distributional patterns and attributes.Results Phylogenetic analyses of morphological characters and nrDNA ITS sequence data were found to be congruent in supporting three New Zealand clades for Pachycladon. Monophyletic groups identified within the genus are geographically distinct and are associated with different geological parent materials. Distribution maps, latitude and altitude range, and data on geological parent material are presented for the nine named and one unnamed species of Pachycladon from New Zealand.Main conclusions (a) Panbiogeographic hypotheses accounting for the origin and present‐day distribution of Pachycladon in New Zealand are not supported.

(b) Species diversity and distributions of Pachycladon are explained by a Late Tertiary–Quaternary adaptive radiation associated with increasing specialization to geological substrates. Pachycladon cheesemanii Heenan & A.D.Mitch. is morphologically similar to the closest overseas relatives. It is a geological generalist and has wide latitudinal and altitudinal ranges, and we suggest it resembles the ancestral form of the genus in New Zealand. Pachycladon novae‐zelandiae (Hook.f.) Hook.f. and P. wallii (Carse) Heenan & A.D.Mitch. are a southern South Island group that predominantly occurs on Haast Schist, are polycarpic, have lobed leaves, and lateral inflorescences. Pachycladon enysii (Cheeseman) Heenan & A.D.Mitch., P. fastigiata (Hook.f.) Heenan & A.D.Mitch., and P. stellata (Allan) Heenan & A.D.Mitch. are restricted to greywacke in the eastern South Island, and are facultatively monocarpic, have serrate leaves, and stout terminal inflorescenes.

(c) Present distributions of Pachycladon species may relate to Pleistocene climate change. Pachycladon enysii reaches the highest altitude of New Zealand species of Pachycladon and is most common in the Southern Alps in Canterbury. We propose that this species survived on nunataks at the height of the last glaciation. In contrast, P. fastigiata grows at a lower altitude and is absent from the high mountains of the Southern Alps. We suggest it was extirpated from this area during the last glaciation.
Geographical sampling bias and its implications for conservation priorities in Africa
Authors: Reddy Sushma , Dávalos Liliana M. 
Published: October 2003

AbstractAim To design and apply statistical tests for measuring sampling bias in the raw data used to the determine priority areas for conservation, and to discuss their impact on conservation analyses for the region.Location Sub‐Saharan Africa.Methods An extensive data set comprising 78,083 vouchered locality records for 1068 passerine birds in sub‐Saharan Africa has been assembled. Using geographical information systems, we designed and applied two tests to determine if sampling of these taxa was biased. First, we detected possible biases because of accessibility by measuring the proximity of each record to cities, rivers and roads. Second, we quantified the intensity of sampling of each species inside and surrounding proposed conservation priority areas and compared it with sampling intensity in non‐priority areas. We applied statistical tests to determine if the distribution of these sampling records deviated significantly from random distributions.Results The analyses show that the location and intensity of collecting have historically been heavily influenced by accessibility. Sampling localities show dense, significant aggregation around city limits, and along rivers and roads. When examining the collecting sites of each individual species, the pattern of sampling has been significantly concentrated within and immediately surrounding areas now designated as conservation priorities.Main conclusions Assessment of patterns of species richness and endemicity at the scale useful for establishing conservation priorities, below the continental level, undoubtedly reflects biases in taxonomic sampling. This is especially problematic for priorities established using the criterion of complementarity because the estimated spatial costs of this approach are highly sensitive to sampling artefacts. Hence such conservation priorities should be interpreted with caution proportional to the bias found. We argue that conservation priority setting analyses require (1) statistical tests to detect these biases, and (2) data treatment to reflect species distribution rather than patterns of collecting effort.