bluuebunny/crossref_metadata_2025 huggingface dataset:
Flowering, growth and defence in the two sexes: consequences of herbivore exclusion for Salix polaris
Authors: Dormann Carsten F. , Skarpe Christina
Published: March 2003

For a long time, dioecious plants have been a model system for understanding the interactions between plants and herbivores. Differences in growth rate and, consequently, investment in defence between sexes may lead to skewed sex ratios due to differential herbivory.

In this study we evaluated the applicability of this idea to polar willow (Salix polaris), which in the study site, Svalbard, displays a female‐biased sex ratio.

Excluding reindeer for 3 years increased the abundance of male flowers in one of two vegetation types investigated. Growth rates differed only slightly between the sexes, with females investing more in inflorescences.

The concentration of chemical defence compounds (phenolics and condensed tannins) did not differ between the sexes.

On the basis of these findings, the idea that growth rate‐dependent herbivory caused the unbalanced sex ratio in S. polaris has to be rejected. Possibly an interaction of niche differentiation between male and female willows, in combination with reindeer grazing, produced the observed female‐biased sex ratio, but the mechanism remains unclear.
Species‐specific patterns of litter processing by terrestrial isopods (Isopoda: Oniscidea) in high intertidal salt marshes and coastal forests
Authors: Zimmer M. , Pennings S. C. , Buck T. L. , Carefoot T. H.
Published: March 2003

The species‐specificity of litter processing by three species of isopods at the interface between salt marsh and coastal forest habitats in the south‐eastern United States was examined.

To quantify isopod performance, measurements were taken of feeding, digestion and growth of isopods fed on three litter types (Juncus roemerianus, Quercus virginiana and Pinus palustris) and on artificial diets containing one of three classes of model phenolic compounds (simple phenolics and hydrolysable and condensed tannins).

To quantify the ecosystem impact of isopods, promotion of microbial respiration, changes in detritus chemistry, and the quantity of litter processed by isopod populations were measured.

The results support three hypotheses concerning isopod–litter interactions. (i) Isopod performance on different litter types can be predicted based on chemical litter traits, e.g. phenolic concentrations and C : N ratios. (ii) Fully terrestrial isopods are better adapted to the range of phenolics found in angiosperm litter than are semiterrestrial species inhabiting the supralittoral. (iii) Isopod species differ with respect to their impact on decomposition processes due to species‐specific digestive capabilities, different effects on microbial decomposition and different rates of net litter processing.

Because isopods are transitional between semiterrestrial and terrestrial habitats, unlike most other salt marsh detritivores, they are likely to play a unique role in decomposition processes and in the flux of materials between salt marsh and terrestrial habitats.
Effect of caudal autotomy on locomotor performance in a viviparous skink, Niveoscincus metallicus
Authors: Chapple D. G. , Swain R.
Published: March 2003

Tail loss has repeatedly been associated with the impairment of locomotor performance in lizards. However, although the immediate impact of tail loss may be severe, most previous studies have failed to establish how long such locomotory costs persist.

We investigated both the short‐term (24–36 h) and long‐term (4 and 12 weeks) consequences of tail loss on locomotor performance in a viviparous ground‐dwelling skink, Niveoscincus metallicus (O’Shaughnessy 1874). Sprint speed, climbing ability and stamina were measured as performance variables. The locomotor performance of both adult males and pregnant females were tested during the study.

Male and female lizards responded differently to tail loss, although the immediate impact of autotomy on performance was generally minor. In males caudal autotomy imposed a significant immediate impact on sprint speed, while in females stamina was reduced immediately after autotomy.

Females regenerated their tails significantly faster than males. Recovery of locomotor performance over the 3‐month duration of the study was observed in females (for endurance capacity) but not males (for sprint speed).

Overall, the impact of tail loss on locomotor performance was generally limited or short‐lived in N. metallicus.
Bacterial flagellation and cell division
Authors: Aizawa Shin‐Ichi , Kubori Tomoko
Published: March 2003

The peritrichous flagella of Salmonella are synthesized and function through many cell generations. There are two different aspects in the relationship between flagellar biogenesis and cell division. Filament growth is independent from the cell cycle and the length of filaments appear to be locally controlled at each flagellar base, whereas the number of filaments (or flagellar basal bodies) is dependent on cell cycle. We present a model to explain how the number of filaments is maintained through generations. We will also introduce a new direction for research that might directly connect flagellation and cell division; the global communication between flagellar genes and external factors of a complex regulatory network in a cell.
The Suppressor‐mutator element and the evolutionary riddle of transposons
Authors: Fedoroff Nina V.
Published: March 2003

This review focuses on the epigenetic control of the maize Suppressor‐mutator (Spm) transposon and the evolutionary origin of epigenetic mechanisms. Methylation of the Spm promoter prevents transcription and transposition, and the methylation of the adjacent GC‐rich sequence renders the inactive state heritable. Spm encodes an epigenetic activator, TnpA, one of the two Spm‐encoded transposition proteins. TnpA can reactivate an inactive, methylated Spm both transiently and heritably, and it is also a transcriptional repressor of the unmethylated Spm promoter. Features common to epigenetic mechanisms in general suggest that they originated as a means of decreasing the recombinogenicity of duplicated sequences.
Regulation of intrinsic terminator by translation in Escherichia coli: transcription termination at a distance downstream
Authors: Abe Hiroyuki , Abo Tatsuhiko , Aiba Hiroji
Published: March 2003

Background:

Rho‐independent terminators in Escherichia coli are DNA sequences of 30–50 bp consisting of a GC‐rich dyad symmetry sequence followed by a run of T residues in the nontemplate strand. The transcription termination at the Rho‐independent terminator occurs within the T‐tract in vitro. It has been believed that the transcription termination at the Rho‐independent terminator occurs within the T‐tract in vivo, as established in vitro, and therefore the 3′ ends of mRNAs are mostly generated as a direct result of transcription termination. However, how the transcription termination occurs and how the 3′ ends of mRNAs are formed in living cells remains to be studied.

Results:

We developed a double terminator system in which a second Rho‐independent terminator was placed downstream of the crp terminator. This system made it possible to detect transcripts that pass through the crp terminator by Northern blotting. We found that most of the crp transcripts extend beyond the crp terminator. The transcriptional read‐through at the crp terminator was reduced when the translation of crp mRNA was interrupted. The level of the read‐through transcript decreased with distance between the two terminators, suggesting that transcription termination occurs at multiple positions beyond the crp terminator.

Conclusion:

We conclude that most RNA polymerase reads through the crp terminator in the natural situation and terminates transcription over a wide region downstream of the crp terminator, resulting in heterogeneous primary transcripts that are subsequently processed back to the terminator hairpin. We propose that ribosome translation to the crp stop codon causes read‐through of the terminator. The regulatory effect of translation on Rho‐independent termination may be a general phenomenon at other operons.
Transposition of IS1 circles
Authors: Shiga Yasuyuki , Sekine Yasuhiko , Ohtsubo Eiichi
Published: March 2003

Background

IS1, the smallest active transposable element in bacteria, encodes transposase. IS1 transposase promotes transposition as well as production of miniplasmids from a plasmid carrying IS1 by deletion of the region adjacent to IS1. The IS1 transposase also promotes production of IS1 circles consisting of the entire IS1 sequence and a sequence, 6–9 bp in length, as a spacer between terminal inverted repeats of IS1. The biological significance of the generation of IS1 circles is not known.

Results

Plasmids carrying an IS1 circle with a spacer sequence 6–9 bp long transposed to target plasmids at a very high frequency when transposase was produced from a co‐resident plasmid. The products were target plasmids with the donor plasmid inserted at the ends of IS1 in the IS1 circle. This insertion accompanied the removal of the spacer sequence and duplication of the sequence at the target site. IS1 circles with a much longer spacer sequence transposed less frequently. The SOS response was induced in cells harbouring a plasmid with an IS1 circle owing to transposase. IS1 circles could transpose in the strain deficient in H‐NS, a nucleoid‐associated DNA‐binding protein known to be required for the transposition of IS1.

Conclusions

IS1 circles appear to act as intermediates for simple insertion into the target DNA via cleavage of the circles which induces the SOS response. H‐NS may function in promoting the assembly of an active IS1 DNA‐transposase complex at the terminal inverted repeats.
Presence of telomeric G‐strand tails in the telomerase catalytic subunit TERT knockout mice
Authors: Yuan Xunmei , Ishibashi Shun , Hatakeyama Shinji , Saito Motoki , Nakayama Jun‐ichi , Nikaido Rika , Haruyama Takahiro , Watanabe Yoshifumi , Iwata Hijiri , Iida Mari , Sugimura Haruhiko , Yamada Nobuhiro , Ishikawa Fuyuki
Published: March 2003

Background

Telomerase consists of two essential subunits, the template RNA (TR; telomerase RNA) and the catalytic subunit TERT (telomerase reverse transcriptase). Knockout mice with a mTR (mouse TR) deletion have been described and well characterized. However, mice with a mTERT (mouse TERT) deletion have not been reported.

Results

mTERT‐knockout mice have been constructed. The first generation mTERT −/− mice were fertile, and did not show any noticeable macroscopic or microscopic phenotypic change. All tissue cells derived from mTERT −/− mice that were examined lacked telomerase activity, indicating that mTERT is the only gene encoding the telomerase catalytic subunit. Pulse field gel electrophoresis (PFGE) and nondenaturing in‐gel hybridization analyses showed that mouse telomeric DNA has G‐strand 5′‐overhangs, as demonstrated for human and yeast cells. This telomeric single‐stranded G‐tail was also observed in MEF (mouse embryonic fibroblast) and liver cells derived from mTERT −/− mice.

Conclusions

mTERT‐knockout mice show phenotypes that are apparently normal at least during the early generations. This observation is similar to that obtained with the mTR‐knockout mice. The presence of the telomeric G‐strand tails in mTERT −/− mice suggests that these telomeric 5′‐overhangs are produced by telomerase‐independent mechanisms, as has been proposed for yeast and human.
LUCA15, a putative tumour suppressor gene encoding an RNA‐binding nuclear protein, is down‐regulated in ras‐transformed Rat‐1 cells
Authors: Edamatsu Hironori , Kaziro Yoshito , Itoh Hiroshi
Published: March 2003

Background

The proliferation of mammalian cells is controlled by various intracellular mitogenic signalling pathways. In the intracellular pathways, Ras is involved in the activation of proto‐oncogenes such as an immediate early gene c‐fos. The somatic mutations of ras genes that elicit the constitutive activation of Ras have been found in tumours. Although these findings suggest that the constitutive activation of Ras‐mediated pathways alters the expression of a set of genes involving tumorigenesis, these genes have not yet fully been studied.

Results

To study the up‐ or down‐regulated genes in ras‐transformed cells, we analysed Rat‐1 transfectants expressing Ras(G12V) mutant protein in response to isopropyl‐1‐β‐thio‐
D‐galactoside using a differential display. We found that the mRNA level of rat homologue of LUCA15, which has been cloned initially as a putative tumour suppressor gene mapped on human chromosome 3, was down‐regulated by the expression of Ras(G12V). Epitope‐tagged LUCA15 protein was localized in nuclei and had the ability to bind poly(G) RNA homopolymers in vitro. Moreover, ectopic expression of LUCA15 in human fibrosarcoma HT1080 cells suppressed the cell growth.

Conclusion

These results demonstrate that LUCA15 is one of the down‐regulated genes in ras‐transformed cells, and suggests that LUCA15 may function as a negative regulator of cell proliferation by the alteration of its mRNA level.
Genetic dissection of SecA: suppressor mutations against the secY205 translocase defect
Authors: Matsumoto Gen , Nakatogawa Hitoshi , Mori Hiroyuki , Ito Koreaki
Published: March 2003

The driving force for protein translocation across the bacterial plasma membrane is provided by SecA ATPase, which undergoes striking conformational changes characterized by the membrane insertion and deinsertion cycle. This action of SecA requires the membrane‐embedded SecYEG complex. Previously, we have identified a cold‐sensitive secY mutation (secY205), affecting the most carboxy‐terminal cytosolic domain, that did not allow an ATP‐dependent insertion of a SecA‐preprotein complex. Thus, this mutant provides an excellent system for genetic analysis of the SecY–SecA interaction.

We carried out a systematic isolation of secA mutations that suppressed secY205 cold‐sensitivity. A total of 40 independent suppressor mutations were classified into: (i) allele‐specific suppressors, acting only against secY205, and (ii) ‘super active’ suppressors, acting against almost any sec defects. The former class of mutations, presumably with specific effects on the SecY–SecA interaction, clustered in two regions close to the Walker motif A sequences of the two ATP‐binding domains. The latter mutations, enhancing general SecA activities, were mostly in or around the minor ATP‐binding domain.

The Walker motif A regions of SecA are important for the SecA–SecY interaction that leads to the SecA conformational changes required for insertion into the SecYEG channel. The minor ATP‐binding domain is important for the down‐regulation of SecA activities.
Fission yeast homologues of the B′ subunit of protein phosphatase 2A: multiple roles in mitotic cell division and functional interaction with calcineurin
Authors: Tanabe Osamu , Hirata Dai , Usui Hirofumi , Nishito Yasumasa , Miyakawa Tokichi , Igarashi Kazuhiko , Takeda Masao
Published: March 2003

Background

Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase distributed in eukaryotes from yeast to human, and plays pivotal roles in diverse cellular functions such as metabolism, cell cycle progression, gene expression and development. PP2A holoenzyme is a heterodimer of a catalytic subunit C and a regulatory subunit A, or a heterotrimer of C, A and a variable regulatory subunit consisting of three families; B, B′, and PR72. Specific functions for each variable subunit are not well understood.

Results

Two fission yeast genes pbp1+ and pbp2+ homologous to the regulatory subunit B′ were isolated. Physical in vivo interaction of the gene products with the catalytic subunit was demonstrated. A double disruption haploid mutant (Δpbp1Δpbp2) showed growth defect, cell shape and size abnormality, multiseptation and anucleated cell formation due to abnormality in septum positioning. These phenotypes were suppressed by human B′ cDNA, indicating the striking conservation of the B′ function from yeast to human. Over‐expression of fission yeast B′ led to growth defects, a loss of cell shape polarity, septal abnormality and anucleated cell formation. Δpbp1Δpbp2 and pbp1 null haploids were hypersensitive to calcineurin inhibitors, cyclosporin A and FK506, with which the mutants underwent arrest at post‐anaphase and cell lysis. Double disruption of calcineurin and pbp1+, but not pbp2+, genes led to synthetic lethality.

Roles of B′ and its functional interaction with calcineurin in fission yeast. This hypothetical summary scheme is depicted based on the observations in this study and others (Jiang & Hallberg 2000; Sugiura et al. 1998; Yoshida et al. 1994). CAB′ is the trimeric holoenzyme of PP2A containing B′ as a variable regulatory subunit.

image

Conclusion

The fission yeast B′ subunit of PP2A plays critical roles in cell shape control and septum formation, and shares essential functions with calcineurin for viability, possibly through their roles in cytokinesis and cell wall integrity.
Roles of Toll‐like receptors in innate immune responses
Authors: Takeda Kiyoshi , Akira Shizuo
Published: March 2003

Innate immunity recognizes invading micro‐organisms and triggers a host defence response. However, the molecular mechanism for innate immune recognition was unclear. Recently, a family of Toll‐like receptors (TLRs) was identified, and crucial roles for these receptors in the recognition of microbial components have been elucidated. The TLR family consists of 10 members and will be expanding. Each TLR distinguishes between specific patterns of microbial components to provoke innate immune responses. The activation of innate immunity then leads to the development of antigen‐specific adaptive immunity. Thus, TLRs control both innate and adaptive immune responses.
Over‐expression of DREF in the Drosophila wing imaginal disc induces apoptosis and a notching wing phenotype
Authors: Yoshida Hideki , Inoue Yoshihiro H. , Hirose Fumiko , Sakaguchi Kengo , Matsukage Akio , Yamaguchi Masamitsu
Published: March 2003

Background DNA replication‐related element binding factor (DREF) has been suggested to be involved in regulation of DNA replication‐ and proliferation‐related genes in Drosophila. While the effects on the mutation in the DNA replication‐related element (DRE) in cultured cells have been studied extensively, the consequences of elevating wild‐type DREF activity in developing tissues have hitherto remained unclear.

Results We over‐expressed DREF in the wing imaginal disc using a GAL4‐UAS targeted expression system in Drosophila. Over‐expression of DREF induced a notching wing phenotype, which was associated with ectopic apoptosis. A half reduction of the reaper, head involution defective and grim gene dose suppressed this DREF‐induced notching wing phenotype. Furthermore, this was also the case with co‐expression of baculovirus P35, a caspase inhibitor. In addition, over‐expression of the 32 kDa boundary element‐associated factor (BEAF‐32), thought to compete against DREF for common binding sites in genomic regions, rescued the DREF‐induced notching wing phenotype, while a half reduction of the genomic region, including the BEAF‐32 gene, exerted enhancing effects. To our knowledge, this is the first evidence for a genetic interaction between DREF and BEAF‐32.

Conclusion The DREF‐induced notching wing phenotype is caused by induction of apoptosis in the Drosophila wing imaginal disc.
Seeking the source of the gold used for Irish Bronze‐Age goldwork
Authors:   THE PREHISTORIC GOLD RESEARCH GROUP
Published: March 2003

The Group describes how the characterization of alluvial gold from Britain and Ireland is shedding light on an important archaeological problem.
A country full of holes
Authors: FORD TREVOR D.
Published: March 2003

Mining voids exit througout Britain
Man as geological agent
Authors: Waltham Tony , Forster Alan
Published: March 2003

A Yorkshire landslide has been reactivated by man' activities.
Caveat emptor– fake fossils from the Far East
Authors: Milner Angela C. , Forey Peter L. , Greenwood Terry , Williams C. Terry
Published: March 2003

Business is booming in the ancient Far Eastern (mainly Chinese) art of fossil faking. Some of the fakes don't even use fossil materials.
The geology of the Plateau d'Aigurande, France
Authors: Nowell D. A. G.
Published: March 2003

Off the beaten track in central France.
Komatiites: new information on the type locality (Barberton), and some new ideas
Authors: Jones A. P.
Published: March 2003

Archean serpentinites in the Komati river valley, South Africa, have distinctive crystal textures and volcanic features which distinguish them from normal coarse‐grained serpentinites. They are interpreted to represent peridotite lavas erupted into a deep submarine environment.
Generalized Dix equation and analytic treatment of normal‐moveout velocity for anisotropic media*
Authors: Grechka Vladimir , Tsvankin Ilya , Cohen Jack K.
Published: March 2003

Despite the complexity of wave propagation in anisotropic media, reflection moveout on conventional common‐midpoint (CMP) spreads is usually well described by the normal‐moveout (NMO) velocity defined in the zero‐offset limit. In their recent work, Grechka and Tsvankin showed that the azimuthal variation of NMO velocity around a fixed CMP location generally has an elliptical form (i.e. plotting the NMO velocity in each azimuthal direction produces an ellipse) and is determined by the spatial derivatives of the slowness vector evaluated at the CMP location. This formalism is used here to develop exact solutions for the NMO velocity in anisotropic media of arbitrary symmetry.

For the model of a single homogeneous layer above a dipping reflector, we obtain an explicit NMO expression valid for all pure modes and any orientation of the CMP line with respect to the reflector strike. The contribution of anisotropy to NMO velocity is contained in the slowness components of the zero‐offset ray (along with the derivatives of the vertical slowness with respect to the horizontal slownesses) — quantities that can be found in a straightforward way from the Christoffel equation. If the medium above a dipping reflector is horizontally stratified, the effective NMO velocity is determined through a Dix‐type average of the matrices responsible for the ‘interval’ NMO ellipses in the individual layers. This generalized Dix equation provides an analytic basis for moveout inversion in vertically inhomogeneous, arbitrarily anisotropic media. For models with a throughgoing vertical symmetry plane (i.e. if the dip plane of the reflector coincides with a symmetry plane of the overburden), the semi‐axes of the NMO ellipse are found by the more conventional rms averaging of the interval NMO velocities in the dip and strike directions.

Modelling of normal moveout in general heterogeneous anisotropic media requires dynamic ray tracing of only one (zero‐offset) ray. Remarkably, the expressions for geometrical spreading along the zero‐offset ray contain all the components necessary to build the NMO ellipse. This method is orders of magnitude faster than multi‐azimuth, multi‐offset ray tracing and, therefore, can be used efficiently in traveltime inversion and in devising fast dip‐moveout (DMO) processing algorithms for anisotropic media. This technique becomes especially efficient if the model consists of homogeneous layers or blocks separated by smooth interfaces.

The high accuracy of our NMO expressions is illustrated by comparison with ray‐traced reflection traveltimes in piecewise‐homogeneous, azimuthally anisotropic models. We also apply the generalized Dix equation to field data collected over a fractured reservoir and show that P‐wave moveout can be used to find the depth‐dependent fracture orientation and to evaluate the magnitude of azimuthal anisotropy.
Inversion of band‐limited TEM responses
Authors: Effersø Flemming , Auken Esben , Ingvard Sørensen Kurt
Published: March 2003

It is shown that the electromagnetic (EM) spectrum is characterized by strong amplitude‐modulated transmitters operating in the target bandwidth of transient electromagnetic (TEM) measurements. As these transmitters cause significant noise in TEM soundings, it is mandatory to band‐limit the input signals to improve the signal‐to‐noise ratio and thereby the depth of exploration. Band‐limitation will distort the TEM responses, which leads to erroneous inversion results if the applied low‐pass filters are not accounted for in the inversion scheme. We incorporate the low‐pass filters in the inversion scheme and test the inversion approach on theoretical and field data. Inversion of band‐limited theoretical responses results in recovery of erroneous resistivity models if the filters are not included in the inversion scheme. By contrast, inversion of band‐limited theoretical and field data, for which the applied low‐pass filters are included in the inversion scheme, leads to recovery of similar resistivity models, independent of the applied cut‐off frequencies.
Grounded‐source TEM modelling of some deep‐seated 3D resistivity structures
Authors: Poddar
Published: March 2003

Long‐offset transient electromagnetics (LOTEM) is now regarded as a suitable electrical method for deep exploration along with magnetotellurics (MT). In this method, the vertical magnetic‐field impulse response and, occasionally, the horizontal electric‐field step response of a grounded‐wire source on the surface of the earth are measured. Here, these two responses are computed for 3D models of three deep resistivity structures of interest in hydrocarbon exploration: (i) a faulted graben in a resistive basement rock at a depth of 4 km beneath a conductive overburden; (ii) a facies change in a resistive layer buried at a depth of 2 km in the conductive overburden above a resistive basement; and (iii) an anticlinal uplift of a resistive layer at a depth of 1 km in the conductive overburden above a resistive basement. The results show that the sensitivity of the electric‐field response to model perturbation is generally greater than that of the magnetic‐voltage response. Further, the electric‐field sensitivity is confined to early and intermediate times while that of the magnetic‐voltage response is confined to intermediate and late times. Hence it is recommended that both electric and magnetic recordings are made in a LOTEM survey so that the final results can be presented as apparent‐resistivity curves derived from the two responses jointly as well as separately.
Local and controlled prestack depth migration in complex areas
Authors: Lecomte Isabelle
Published: March 2003

Prestack depth migrations based on wavefield extrapolation may be computationally expensive, especially in 3D. They are also very dependent on the acquisition geometry and are not flexible regarding the geometry of the imaging zone. Moreover, they do not deal with all types of wave, considering only primary reflection events through the model. Integral approaches using precalculated Green's functions, such as Kirchhoff migration and Born‐based imaging, may overcome these problems. In the present paper, both finite‐difference traveltimes and wavefront construction are used to obtain asymptotic Green's functions, and a generalized diffraction tomography is applied as an example of Born‐based acoustic imaging. Target‐orientated imaging is easy to perform, from any type of survey and subselection of shot/receiver pairs. Multifield imaging is possible using Green's functions that take into account, for instance, reflections at model boundaries. This may help to recover parts of complex structures which would be missing using a paraxial wave equation approach. Finally, a numerical evaluation of the resolution, or point‐spread, function at any point of the depth‐migrated section provides valuable information, either at the survey planning stage or for the interpretation.
A platform for Kirchhoff data mapping in scalar models of data acquisition
Authors: Bleistein  , Jaramillo
Published: March 2003

Kirchhoff data mapping (KDM) is a procedure for transforming data from a given input source/receiver configuration and background earth model to data corresponding to a different output source/receiver configuration and background model. The generalization of NMO/DMO, datuming and offset continuation are three examples of KDM applications. This paper describes a ‘platform’ for KDM for scalar wavefields. The word, platform, indicates that no calculations are carried out in this paper that would adapt the derived formula to any one of a list of KDMs that are presented in the text. Platform formulae are presented in 3D and in 2.5D. For the latter, the validity of the platform equation is verified — within the constraints of high‐frequency asymptotics — by applying it to a Kirchhoff approximate representation of the upward scattered data from a single reflector and for an arbitrary source/receiver configuration. The KDM formalism is shown to map this Kirchhoff model data in the input source/receiver configuration to Kirchhoff data in the output source/receiver configuration, with one exception. The method does not map the reflection coefficient. Thus, we verify that, asymptotically, the ray theoretical geometrical spreading effects due to propagation and reflection (including reflector curvature) are mapped by this formalism, consistent with the input and output modelling parameters, while the input reflection coefficient is preserved. In this sense, this is a ‘true‐amplitude’ formalism. As with earlier Kirchhoff inversion, a slight modification of the kernel of KDM provides alternative integral operators for estimating the specular reflection angle, both in the input configuration and in the output configuration, thereby providing a basis for amplitude‐versus‐angle analysis of the data.
Short note: Applications of third‐order statistics for the automatic time picking of seismic events
Authors: Srinivasan Karthik , Ikelle Luc T.
Published: March 2003

We present a new automatic time‐picking method based on third‐order statistics, namely bicoherence correlation. Contrary to conventional methods, which are based on second‐order statistics (i.e. cross‐correlation or neural‐network trainings), our method is less sensitive to coloured noise as well as the bandwidth of the signal. Bicoherence correlation can also be used for autotracking events in seismic data for an interpretation.
The feasibility of electromagnetic gradiometer measurements
Authors: Sattel Daniel , Macnae James
Published: March 2003

The quantities measured in transient electromagnetic (TEM) surveys are usually either magnetic field components or their time derivatives. Alternatively it might be advantageous to measure the spatial derivatives of these quantities. Such gradiometer measurements are expected to have lower noise levels due to the negative interference of ambient noise recorded by the two receiver coils. Error propagation models are used to compare quantitatively the noise sensitivities of conventional and gradiometer TEM data. To achieve this, eigenvalue decomposition is applied on synthetic data to derive the parameter uncertainties of layered‐earth models. The results indicate that near‐surface gradient measurements give a superior definition of the shallow conductivity structure, provided noise levels are 20–40 times smaller than those recorded by conventional EM instruments. For a fixed‐wing towed‐bird gradiometer system to be feasible, a noise reduction factor of at least 50–100 is required. One field test showed that noise reduction factors in excess of 60 are achievable with gradiometer measurements. However, other collected data indicate that the effectiveness of noise reduction can be hampered by the spatial variability of noise such as that encountered in built‐up areas. Synthetic data calculated for a vertical plate model confirm the limited depth of detection of vertical gradient data but also indicate some spatial derivatives which offer better lateral resolution than conventional EM data. This high sensitivity to the near‐surface conductivity structure suggests the application of EM gradiometers in areas such as environmental and archaeological mapping.
AVO investigations of shallow marine sediments
Authors: Riedel M. , Theilen F.
Published: March 2003

Amplitude‐variation‐with‐offset (AVO) analysis is based on the Zoeppritz equations, which enable the computation of reflection and transmission coefficients as a function of offset or angle of incidence. High‐frequency (up to 700 Hz) AVO studies, presented here, have been used to determine the physical properties of sediments in a shallow marine environment (20 m water depth). The properties that can be constrained are P‐ and S‐wave velocities, bulk density and acoustic attenuation. The use of higher frequencies requires special analysis including careful geometry and source and receiver directivity corrections. In the past, marine sediments have been modelled as elastic materials. However, viscoelastic models which include absorption are more realistic. At angles of incidence greater than 40°, AVO functions derived from viscoelastic models differ from those with purely elastic properties in the absence of a critical angle of incidence. The influence of S‐wave velocity on the reflection coefficient is small (especially for low S‐wave velocities encountered at the sea‐floor). Thus, it is difficult to extract the S‐wave parameter from AVO trends. On the other hand, P‐wave velocity and density show a considerably stronger effect. Attenuation (described by the quality factor Q) influences the reflection coefficient but could not be determined uniquely from the AVO functions. In order to measure the reflection coefficient in a seismogram, the amplitudes of the direct wave and the sea‐floor reflection in a common‐midpoint (CMP) gather are determined and corrected for spherical divergence as well as source and streamer directivity. At CMP locations showing the different AVO characteristics of a mud and a boulder clay, the sediment physical properties are determined by using a sequential‐quadratic‐programming (SQP) inversion technique. The inverted sediment physical properties for the mud are: P‐wave velocity α=1450±25 m/s, S‐wave velocity β=90±35 m/s, density ρ=1220±45 kg/m3, quality factor for P‐wave QP=15±200, quality factor for S‐wave QS=10±30. The inverted sediment physical properties for the boulder clay are: α=1620±45 m/s,β=360±200 m/s,ρ=1380±85 kg/m3,QP=790±660,QS=25±10.
Reference ellipsoids for anisotropic media
Authors: Ettrich N. , Gajewski D. , Kashtan B.
Published: March 2003

Perturbation methods are common tools for describing wave propagation in weakly anisotropic media. The anisotropic medium is replaced by an average isotropic medium where wave propagation can be treated analytically and the correction for the effect of anisotropy is computed by perturbation techniques. This works well for anisotropies of up to 10%. Some materials (e.g. shales), however, can exhibit a much stronger anisotropy. In this case a background is required which still can be treated analytically but is applicable to stronger P‐wave anisotropy. We present an averaging technique to compute a best‐fitting ellipsoidal medium to an arbitrary anisotropic medium. Ellipsoidal media are sufficiently simple for analytical expressions to be available for many applications and allow consideration of strong P‐wave anisotropy. The averaging of the arbitrary anisotropic medium can be carried out globally (i.e. for the whole sphere) or sectorially (e.g. for seismic waves propagating predominantly in the vertical direction). We derive linear relationships for the coefficients of the ellipsoid which depend on the elastic coefficients of the anisotropic medium. We also provide specifications for best‐fitting elliptical and best‐fitting isotropic media. Numerical examples for different rocks demonstrate the improved approximation of the anisotropic model obtained using the formulae derived, compared with the conventionally used average isotropic medium.
1D inversion of DC resistivity data using a quality‐based truncated SVD
Authors: Muiuane Elonio A. , Pedersen Laust B.
Published: March 2003

Many DC resistivity inversion schemes use a combination of standard iterative least‐squares and truncated singular value decomposition (SVD) to optimize the solution to the inverse problem. However, until quite recently, the truncation was done arbitrarily or by a trial‐and‐error procedure, due to the lack of workable guidance criteria for discarding small singular values. In this paper we present an inversion scheme which adopts a truncation criterion based on the optimization of the total model variance. This consists of two terms: (i) the term associated with the variance of statistically significant principal components, i.e. the standard model estimate variance, and (ii) the term associated with statistically insignificant principal components of the solution, i.e. the variance of the bias term. As an initial model for the start of iterations, we use a multilayered homogeneous half‐space whose layer thicknesses increase logarithmically with depth to take into account the decrease of the resolution of the DC resistivity technique with depth. The present inversion scheme has been tested on synthetic and field data. The results of the tests show that the procedure works well and the convergence process is stable even in the most complicated cases. The fact that the truncation level in the SVD is determined intrinsically in the course of inversion proves to be a major advantage over other inversion schemes where it is set by the user.
Avoiding pitfalls of least‐squares inversion by using the energy‐flux error criterion
Authors: Causse Emmanuel
Published: March 2003

Seismic inversion by modelling and data fitting depends on the criterion chosen to measure the misfit between observed and modelled data. The popular least‐squares error criterion has an important drawback: it is sensitive both to the shape of the recording surface and to velocity variations along this surface. Tests on synthetic seismic reflection data show that least‐squares inversion may work surprisingly poorly in situations where (i) the range of angles between reflected rays and the acquisition surface is large, (ii) the velocity varies significantly along this surface, or (iii) a compensation for the effects of dissipation is applied to the gradients. In these situations, the gradients may contain important artefacts and have incorrect amplitudes. The outgoing flux of energy of the residual wavefield across the acquisition surface provides an alternative measure of the data misfit which is independent of the recording surface, provided this surface is closed, and which is only sensitive to the aperture in the practical situation of an open surface or line of receivers. Energy‐flux inversion presents a strong resemblance to reverse‐time migration, but with the additional possibility of iteratively improving the images. In all the tests, energy‐flux inversion provided better images than least‐squares inversion.
Seismic endoscopy: multi‐offset multi‐azimuth imaging around boreholes – data processing and experimental results
Authors: Nicollin Florence , Conil Frédéric , Gibert Dominique
Published: March 2003

An acoustic method, called seismic endoscopy, able to perform 3D imaging around shallow‐depth boreholes is presented. A probe, composed of an isotropic source and a directional receiver working in the 20–100 kHz frequency range, provides images of cylindrical volumes having radii of a few metres, with an accuracy of centimetres and 25° azimuthal directivity. In order to obtain clear images of the medium discontinuities, multi‐offset and multi‐azimuth data acquisition allows specific algorithms to be used to determine vertical directivity correction, azimuthal focusing and reflected wave enhancement by cancellation of the tube waves. The method is tested with data acquired in an acoustic tank and with synthetic data. Initial experimental results at a test site demonstrate the performance of the seismic endoscopy probe.
Modelling forest‐growth response to increasing CO2 concentration in relation to various factors affecting nutrient supply
Authors: Kirschbaum M. U. F. , Medlyn B. E. , King D. A. , Pongracic S. , Murty D. , Keith H. , Khanna P. K. , Snowdon P. , Raison R. J.
Published: March 2003

It is well recognized that photosynthesis of C3 plants is highly responsive to CO2 concentration. However, in natural ecosystems, plants are subject to a range of feed‐back effects that can interact with increased photosynthetic carbon gain in different ways so that it is not clear to what extent increased photosynthesis will translate into increased growth. To assess the probable growth response of nutrient‐limited forests to increasing CO2 concentration, we use a previously developed modelling framework and apply it under conditions where the supply of nutrients is affected by a range of different factors.

Our analysis indicates that forest growth is likely to be highly stimulated by increasing CO2 concentration in forests with high fertility, in forests with nitrogen fixing plants, in those subject to fire or where nitrogen in wood is effectively removed from the biologically active cycle either through physical removal of stems in harvesting or through continued stem growth over long time periods. Forest growth is likely to be stimulated by CO2 concentration in both phosphorus‐ and sulphur‐limited forests provided nutrients in heartwood of trees are removed from the active nutrient cycle. Without this removal from the cycling system, however, sulphur‐limited forests should show little response to increasing CO2. In phosphorus‐limited forests without phosphorus removal, the response to increasing CO2 depends further on the equilibration state of the large pool of unavailable secondary phosphorus. Considered over periods of centuries during which the secondary pool has equilibrated, growth of phosphorus‐limited forests is likely to be only weakly stimulated by increasing CO2 concentration. However, over shorter periods, increasing CO2 concentration should lead to a substantial increase in productivity.

In general, it can be concluded that systems that are more open with respect to nutrient gains and losses are likely to be more responsive to increasing CO2 concentration than systems where the amount of available nutrients is less variable. In more open systems, operation at a lower internal nutrient concentration as a result of increasing atmospheric CO2 concentration can lead to reduced nutrient losses per unit carbon gain. Our analysis shows that the effect of increasing CO2 on forest growth can differ substantially between forests due to interactions with a range of factors that affect nutrient supply. The response of a particular forest to increasing CO2 concentration can only be predicted if the main factors controlling nutrient supply and growth in that forest are understood and incorporated into an assessment.
The effects of water table manipulation and elevated temperature on the net CO2 flux of wet sedge tundra ecosystems
Authors: Oechel Walter C. , Vourlitis George L. , Hastings Steven J. , Ault Richard P. , Bryant Pablo
Published: March 2003

In situ manipulations were conducted in a naturally drained lake on the arctic coastal plain near Prudhoe Bay, Alaska (70 °21.98′ N, 148 °33.72′ W) to assess the potential short‐term effects of decreased water table and elevated temperature on net ecosystem CO2 flux. The experiments were conducted over a 2‐year period, and during that time, water table depth of drained plots was maintained on average 7 cm lower than the ambient water table, and surface temperatures of plots exposed to elevated temperature were increased on average 0.5 °C. Water table drainage, and to a lesser extent elevated temperature, resulted in significant increases in ecosystem respiration (ER) rates, and only small and variable changes in gross ecosystem productivity (GEP). As a result, drained plots were net sources of ≈ 40 gC m–2 season–1 over both years of manipulation, while control plots were net sinks of atmospheric CO2 of about 10 gC m–2 season–1 (growing season length was an estimated 125 days). Control plots exposed to elevated temperatures accumulated slightly more carbon than control plots exposed to ambient temperatures. The direct effects of elevated temperature on net CO2 flux, ER, and GEP were small, however, elevated temperature appeared to interact with drainage to exacerbate the amount of net carbon loss. These data suggest that many currently saturated or nearly saturated wet sedge ecosystems of the north slope of Alaska may become significant sources of CO2 to the atmosphere if climate change predictions of increased evapotranspiration and reduced soil water status are realized. There is ample evidence that this may be already occurring in arctic Alaska, as a change in net carbon balance has been observed for both tussock and wet‐sedge tundra ecosystems over the last 2–3 decades, which coincides with a recent increase in surface temperature and an associated decrease in soil water content. In contrast, if precipitation increases relatively more than evapotranspiration, then increases in soil moisture content will likely result in greater carbon accumulation.
Preliminary estimates of the potential for carbon mitigation in European soils through no‐till farming
Authors: SMITH PETE. , POWLSON DAVID S. , GLENDINING MARGARET J. , SMITH JO. U.
Published: March 2003

In this paper we estimate the European potential for carbon mitigation of no‐till farming using results from European tillage experiments. Our calculations suggest some potential in terms of (a) reduced agricultural fossil fuel emissions, and (b) increased soil carbon sequestration. We estimate that 100% conversion to no‐till farming would be likely to sequester about 23 Tg C y–1 in the European Union or about 43 Tg C y–1 in the wider Europe (excluding the former Soviet Union). In addition, up to 3.2 Tg C y–1 could be saved in agricultural fossil fuel emissions. Compared to estimates of the potential for carbon sequestration of other carbon mitigation options, no‐till agriculture shows nearly twice the potential of scenarios whereby soils are amended with organic materials. Our calculations suggest that 100% conversion to no‐till agriculture in Europe could mitigate all fossil fuel‐carbon emissions from agriculture in Europe. However, this is equivalent to only about 4.1% of total anthropogenic CO2‐carbon produced annually in Europe (excluding the former Soviet Union) which in turn is equivalent to about 0.8% of global annual anthropogenic CO2‐carbon emissions.