bluuebunny/crossref_metadata_2025 huggingface dataset:
The yeast kinetochore protein Slk19 is required to prevent aberrant chromosome segregation in meiosis and mitosis
Authors: Pfiz Stefan , Zimmermann Johann , Hilt Wolfgang
Published: March 2003

Background Slk19 is a coiled‐coil protein, which locates to the kinetochores of S. cerevisiae. Most cells lacking Slk19 undergo incomplete meiosis and form dyads during sporulation. Endogenous chromosomes appeared to be predominantly divided in an equational manner during single‐division meiosis of slk19 null mutants.

Results: We have monitored the segregation of artificial chromosomes (YACs) in slk19 null mutants during both single‐division meiosis and complete meiosis. In contrast to the results obtained with endogenous chromosomes, YACs only rarely undergo equational segregation during single division meiosis, although high rates of aberrant segregation were detected. This accounts for the high frequency of lethal spores among dyads of slk19Δ null mutants. The fraction of slk19Δ cells that were able to form tetrads solely exhibited YAC segregation defects in meiosis II, whereas the segregation of YACs in meiosis I was normal in these cells. This result might indicate that correct chromosome division in meiosis I is a prerequisite for tetrad formation. slk19 null mutants also showed YAC instability in mitosis and reduced survival after the induction of mitotic spindle damage.

Conclusion: Slk19 is required to avoid aberrant segregation of chromosomes in meiosis I and II and in mitosis. We suggest that the absence of Slk19 leads to uncoupling of chromosome movement from completion of microtubule attachment and resolution of chromosome cohesion.
Mouse GSPT2, but not GSPT1, can substitute for yeast eRF3 in vivo
Authors: Le Goff Catherine , Zemlyanko Olga , Moskalenko Svetlana , Berkova Nadia , Inge‐Vechtomov Sergei , Philippe Michel , Zhouravleva Galina
Published: March 2003

Background The termination of protein synthesis in eukaryotes involves at least two polypeptide release factors (eRFs), eRF1 and eRF3. In mammals two genes encoding eRF3 structural homologues were identified and named GSPT1 and GSPT2.

Results: In the present study, we demonstrate that mouse mGSPT2 but not mGSPT1 could functionally substitute the essential yeast gene SUP35. However, we show that the complementation property of mGSPT1 protein is modified when NH2‐tagged by GST. Since mGSPT1 and mGSPT2 differ mainly in their N‐terminal regions, we developed a series of N‐terminal deleted constructs and tested them for complementation in yeast. We found that at least amino acids spanning 84–120 of mGSPT1 prevent the complementation of sup35 mutation. The fact that chimeras between mGSPT1, mGSPT2 and yeast Sup35 complement the disruption of the SUP35 gene indicates that the N‐terminal region of mGSPT1 is not sufficient by itself to prevent complementation. Complementation of the mutant with a double disruption of SUP35 and SUP45 genes is obtained when mGSPT2 and human eRF1 are co‐expressed but not by co‐expression of mGSPT1 and human eRF1.

Conclusions: Our results strongly suggest that the two proteins (mGSPT1 and mGSPT2) are different. We hypothesize that the full length mGSPT1 does not have the properties expected for eRF3.
Densin‐180, a synaptic protein, links to PSD‐95 through its direct interaction with MAGUIN‐1
Authors: Ohtakara Kazuhiro , Nishizawa Miwako , Izawa Ichiro , Hata Yutaka , Matsushima Satoshi , Taki Waro , Inada Hiroyasu , Takai Yoshimi , Inagaki Masaki
Published: March 2003

Background: Densin‐180, a brain‐specific protein highly concentrated at the postsynaptic density (PSD), belongs to the LAP [leucine‐rich repeats and PSD‐95/Dlg‐A/ZO‐1 (PDZ) domains] family of proteins, some of which play fundamental roles in the establishment of cell polarity.

Results: To identify new Densin‐180‐interacting proteins, we screened a yeast two‐hybrid library using the COOH‐terminal fragment of Densin‐180 containing the PDZ domain as bait, and we isolated MAGUIN‐1 as a Densin‐180‐binding protein. MAGUIN‐1, a mammalian homologue of Drosophila connector enhancer of KSR (CNK), is known to interact with PSD‐95 and has a short isoform, MAGUIN‐2. The Densin‐180 PDZ domain bound to the COOH‐terminal PDZ domain‐binding motif of MAGUIN‐1. Densin‐180 co‐immunoprecipitated with MAGUIN‐1 as well as with PSD‐95 from the rat brain. In dissociated hippocampal neurones Densin‐180 co‐localized with MAGUINs and PSD‐95, mainly at neuritic spines. In transfected cells, Densin‐180 formed a ternary complex with MAGUIN‐1 and PSD‐95, whereas no association was detected between Densin‐180 and PSD‐95 in the absence of MAGUIN‐1. MAGUIN‐1 formed a dimer or multimer via the COOH‐terminal leucine‐rich region which is present in MAGUIN‐1 but not in ‐2. Among the PDZ domains of PSD‐95, the first was sufficient for interaction with MAGUIN‐1.

Conclusion: These results suggest that the potential to dimerize or multimerize allows MAGUIN‐1 to bind simultaneously to both Densin‐180 and PSD‐95, leading to the ternary complex assembly of these proteins at the postsynaptic membrane.
A minigene containing four discrete cis elements recapitulates GATA‐1 gene expression in vivo
Authors: Ohneda Kinuko , Shimizu Ritsuko , Nishimura Shigeko , Muraosa Yasushi , Takahashi Satoru , Engel James Douglas , Yamamoto Masayuki
Published: March 2003

Background:  The GATA‐1haematopoietic enhancer (G1HE), located between 3.9 and 2.6 kb 5′ to the haematopoietic first exon, is essential for GATA‐1 gene transcription in erythroid cells. However, G1HE is not sufficient to confer tissue specificity on the GATA‐1 gene in vivo, indicating that additional regulatory sequences are necessary.

Results: We demonstrate here that two other upstream promoter elements containing a double GATA motif or two CACCC boxes are also indispensable for reporter gene expression in erythroid cells in the transgenic mouse. The combination of these three cis‐acting regions was sufficient for reporter expression in primitive erythroid cells, as demonstrated by linking the elements together into a 659 bp artificial (GdC) minigene. The minigene activated the transcription of a reporter gene from either the endogenous or an exogenous thymidine kinase promoter, retaining cell type‐specificity. The addition of a 320 bp fragment in the first intron to the GdC minigene sustained reporter expression in the definitive stage. Moreover, a line of transgenic mouse that expressed GATA‐1 cDNA under the control of the complete 979 bp minigene rescued GATA‐1 germ line mutant mice from embryonic lethality.

Conclusions: A combination of four distinct sequence motifs co‐operatively serve as a fundamental functional unit for GATA‐1 erythroid transcription in vivo.
Taxilin; a novel syntaxin‐binding protein that is involved in Ca2+‐dependent exocytosis in neuroendocrine cells
Authors: Nogami Satoru , Satoh Sachie , Nakano Michiko , Shimizu Hiroaki , Fukushima Hiromichi , Maruyama Ayumi , Terano Akira , Shirataki Hiromichi
Published: March 2003

Background:  The syntaxin family is a central coordinator and participates in multiple protein–protein interactions in the soluble N‐ethyl maleimide‐sensitive factor attachment protein receptor machinery, which is involved in intracellular vesicle traffic. However, the molecular mechanism by which the syntaxin family regulates intracellular vesicle transport is not well known.

Results:  We have identified and purified a novel binding partner of syntaxin‐3 from rat lung, and isolated and sequenced the cDNA of its human homologue from a human brain cDNA library. The cDNA had an open reading frame encoding a protein of 546 amino acids with a calculated Mr of 61 890. We tentatively referred to this protein as taxilin. A structural analysis of taxilin revealed the existence of an extraordinarily long coiled‐coil domain in its C‐terminal half. Syntaxin‐1a and ‐4, as well as syntaxin‐3 interacted with taxilin, but syntaxin‐7 or ‐8 did not. Northern blot analysis showed that taxilin was ubiquitously expressed. Over‐expression of full‐length taxilin inhibited Ca2+‐dependent exocytosis in PC12 cells.

Conclusions:  These results indicate that taxilin is a novel binding partner of several syntaxin family members and suggest that taxilin is involved in Ca2+‐dependent exocytosis in neuroendocrine cells.
Mitogen‐activated protein kinase p38 defines the common senescence‐signalling pathway
Authors: Iwasa Hiroaki , Han Jiahuai , Ishikawa Fuyuki
Published: March 2003

Background: Cellular senescence is a state of irreversible growth arrest shown by normal cells, and has been most extensively studied in replicative senescence caused by telomere shortening. Several conditions, including oncogenic Ras over‐expression and inappropriate culture conditions, also induce senescence without telomere shortening. However, it remains unclear how a common set of senescence phenotypes is indistinguishably induced in various types of senescence.

Results: We demonstrate that p38 mitogen‐activated protein kinase (MAPK) plays important causative roles in senescent cells following telomere shortening, Ras‐Raf activation, oxidative stress or inappropriate culture conditions. By monitoring the kinetics of p38 activation, we suggest that p38 is activated not directly by the initial stimuli, but in response to unidentified cellular conditions caused by these stimuli. Importantly, this p38‐activating condition appears to be defined quantitatively as a sum of continuous and low‐level stresses, and remains even after the initial stimuli are withdrawn, which may explain the well‐known irreversible nature of cellular senescence. We also show that papilloma virus E7 abolishes the p38‐induced growth arrest but not other senescence‐associated phenotypes, indicating the differential role of pRb in the downstream of p38.

Conclusion: These results indicate that p38 comprises the senescence‐executing pathway in response to diverse stimuli.
Synaptic localization of SAPAP1, a synaptic membrane‐associated protein
Authors: Yao Ikuko , Iida Junko , Nishimura Wataru , Hata Yutaka
Published: March 2003

Background: SAPAP1 was originally identified as a protein interacting with the guanylate kinase domain of PSD‐95. SAPAP1 also interacts with various proteins, including neurofilaments, synaptic scaffolding molecule (S‐SCAM), nArgBP2, dynein light chain and Shank through different regions.

Results: We expressed various regions of SAPAP1 in hippocampal neurones. The synaptic targeting of SAPAP1 was mediated by the N‐terminal region and did not depend on the interaction with PSD‐95 or S‐SCAM. SAPAP1 was not involved in the synaptic localization of PSD‐95 or S‐SCAM, but affected that of Shank. The synaptic targeting of SAPAP1 was not suppressed by blocking NMDA or AMPA receptors. Fluorescent recovery after a photobleaching study revealed that SAPAP1 was immobile at synapses.

Conclusion: SAPAP1 is a component of the static core of PSD, and its dynamics are different from those of the other PSD components, PSD‐95, S‐SCAM and BEGAIN.
Kailas: geology of a sacred mountain
Authors: WALTHAM TONY
Published: March 2003

Standing isolated on the barren Tibetan Plateau, the gleaming white snow‐clad pyramid of Mount Kailas is a remnant of massive conglomerates formed within the Himalayan orogeny. Reached by an approach walk through the Himalayas in western Nepal, the circuit of Kailas on the pilgrims' kora path offers some views of the impressively exposed geology.
Revisiting the Charnian Supergroup: new advances in understanding old rocks
Authors: Carney John N.
Published: March 2003

More than two centuries of study have culminated in work that promises to re‐write large parts of the stratigraphy, mode of origin, palaeontology, age and structure of the late Precambrian to early Cambrian ‘basement’ rocks in Charnwood Forest, UK.
Searching for the earliest record of life on Earth
Authors: Rollinson Hugh
Published: March 2003

Are the remains of the Earth's earliest life now to be found in the hostile wilderness of Greenland?
Non-linear inversion using general measures of data misfit and model structure
Authors: Farquharson Colin G. , Oldenburg Douglas W.
Published: March 2003

We investigate the use of general, non-l  2 measures of data misfit and model structure in the solution of the non-linear inverse problem. Of particular interest are robust measures of data misfit, and measures of model structure which enable piecewise-constant models to be constructed. General measures can be incorporated into traditional linearized, iterative solutions to the non-linear problem through the use of an iteratively reweighted least-squares (IRLS) algorithm. We show how such an algorithm can be used to solve the linear inverse problem when general measures of misfit and structure are considered. The magnetic stripe example of Parker (1994) is used as an illustration. This example also emphasizes the benefits of using a robust measure of misfit when outliers are present in the data. We then show how the IRLS algorithm can be used within a linearized, iterative solution to the non-linear problem. The relevant procedure contains two iterative loops which can be combined in a number of ways. We present two possibilities. The first involves a line search to determine the most appropriate value of the trade-off parameter and the complete solution, via the IRLS algorithm, of the linearized inverse problem for each value of the trade-off parameter. In the second approach, a schedule of prescribed values for the trade-off parameter is used and the iterations required by the IRLS algorithm are combined with those for the linearized, iterative inversion procedure. These two variations are then applied to the 1-D inversion of both synthetic and field time-domain electromagnetic data.
Bounds on the complex conductivity of geophysical mixtures
Authors: Tripp Alan C. , Cherkaeva Elena , Hulen Jeff
Published: March 2003

Following previous work on bounds for complex dielectrics, bounds on the complex conductivity of a mixture of two isotropic components can be developed which are independent of any special assumption concerning the geometry of the mixture. If certain broad restrictions are assumed, such as isotropy of the mixture, then the bounds can be made more restrictive. These bounds reveal the range of the induced polarization response which can be caused by a mixture of two materials of known complex conductivity. The bounds can also be generalized for spectral responses. The bounds are conservative lithologically in the sense that many of the special models corresponding to boundary responses have lithological counterparts.

The chief use for the given bounds is to gain insight into the nature of the induced polarization response. It is also possible to use the bounds to estimate the volume fractions of the components. We illustrate how this is done for the case of a general anisotropic medium.
Improvement to resistivity pseudosection modelling by removal of near‐surface inhomogeneity effects: application to a soil system in south Cameroon*
Authors: Ritz Michel , Robain Henri , Pervago Evgeni , Albouy Yves , Camerlynck Christian , Descloitres Marc , Mariko Adama
Published: March 2003

Near‐surface inhomogeneities (NSIs) can lead to severe problems in the interpretation of apparent resistivity pseudosections because their effects significantly complicate the image aspect. In order to carry out a more efficient and reliable interpretation process, these problematic features should be removed from field data. We describe a filtering scheme using two‐sided half‐Schlumberger array data. The scheme was tested on synthetic data, generated from a simple 2D resistivity model contaminated by NSIs, and is shown to be suitable for eliminating such contaminations from apparent resistivity data. Furthermore, the original model without NSIs can be recovered satisfactorily from the inversion of filtered apparent resistivity data. The algorithm is also applied efficiently to a real data set collected at Nsimi, in southern Cameroon, along a 200‐m shallow depth profile crossing a complex transitional zone. For this case, the filtering scheme provides accurate structural and behavioural interpretations of both the geometry of the major soil constituents and the groundwater partitioning.
Modelling of local velocity anomalies: a cookbook
Authors: Juliard Corinne , Thore Pierre D.
Published: March 2003

The determination of small‐scale velocity anomalies (from tens to a few hundreds of metres) is a major problem in seismic exploration. The impact of such anomalies on a structural interpretation can be dramatic and conventional techniques such as tomographic inversion or migration velocity analysis are powerless to resolve the ambiguity between structural and velocity origins of anomalies. We propose an alternative approach based on stochastic modelling of numerous anomalies until a set of models is found which can explain the real data. This technique attempts to include as much a priori geological information as possible. It aims at providing the interpreter with a set of velocity anomalies which could possibly be responsible for the structural response. The interpreter can then choose one or several preferred models and pursue a more sophisticated analysis. The class of retained models are all equivalent in terms of data and therefore represent the uncertainty in the model space.

The procedure emulates the real processing sequence using a simplified scheme. Essentially, the technique consists of five steps:

1 Interpretation of a structural anomaly in terms of a velocity anomaly with its possible variations in terms of position, size and amplitude.

2 Drawing a model by choosing the parameters of the anomaly within the acceptable range.

3 Modelling the traveltimes in this model and producing the imaging of the reflected interface.

4 Comparing the synthetic data with the real data and keeping the model if it lies within the data uncertainty range.

5 Iterate from step 2.

In order to avoid the high computational cost inherent in using statistical determinations, simplistic assumptions have been made:

• The anomaly is embedded in a homogeneous medium: we assume that the refraction and the time shift due to the anomaly have a first‐order effect compared with ray bending in the intermediate layers.

• We model only the zero‐offset rays and therefore we restrict ourselves to structural problems.

• We simulate time migration and so address only models of limited structural complexity.

These approximations are justified in a synthetic model which includes strong lateral velocity variations, by comparing the result of a full processing sequence (prestack modelling, stack and depth migration) with the simplified processing. This model is then used in a blind test on the inversion scheme.
Compressional‐ and shear‐wave velocities as a function of confining stress in dry sandstones
Authors: Khaksar  , Griffiths  , McCann
Published: March 2003

A laboratory study was carried out to investigate the influence of confining stress on compressional‐ and shear‐wave velocities for a set of rock samples from gas‐producing sandstone reservoirs in the Cooper Basin, South Australia. The suite of samples consists of 22 consolidated sublitharenites with helium porosity ranging from 2.6% to 16.6%. We used a pulse‐echo technique to measure compressional‐ and shear‐wave velocities on dry samples (cylindrical 4.6 × 2 cm) at room temperature and at elevated confining stress (≤ 60 MPa). Compressional‐ and shear‐wave velocities in samples increase non‐linearly with confining stress. A regression equation of the form V = A − Be−DP gives a good fit to the measured velocities with improved prediction of velocities at high confining stresses compared with equations suggested by other studies. The predicted microcrack‐closure stresses of the samples show values ranging from 70 MPa to 95 MPa and insignificant correlation with porosity, permeability or clay content. There is a positive correlation between change in velocity with core porosity and permeability, but this association is weak and diminishes with increasing confining stress. Experimental results show that pore geometry, grain‐contact type, and distribution and location of clay particles may be more significant than total porosity and clay content in describing the stress sensitivity of sandstones at in situ reservoir effective stress. The stress dependence of Cooper Basin sandstones is very large compared with data from other studies. The implication of our study for hydrocarbon exploration is that where the in situ reservoir effective stress is much less than the microcrack‐closure stress of the reservoir rocks, the variation of reservoir effective stress could cause significant changes in velocity of the reservoir rocks. The velocity changes induced by effective stress in highly stress‐sensitive rocks can be detected at sonic‐log and probably surface‐seismic frequencies.
The 3D shear experiment over the Natih field in Oman. Reservoir geology, data acquisition and anisotropy analysis
Authors: Potters  , Groenendaal  , Oates  , Hake  , Kalden
Published: March 2003

This paper describes a large‐scale reservoir characterization experiment carried out in Oman in 1991 which comprised the acquisition, processing and interpretation of a 28.4 km2 3D multicomponent seismic experiment over the Natih field. The objective of the survey was to obtain information on the fracture network present in the Natih carbonates from shear‐wave anisotropy. Shear‐wave anisotropy in excess of 20% time splitting was encountered over a large part of the survey. The seismic results are confirmed by geological and well data but provide additional qualitative information on fracturing where this was not available before. Regions of stronger and weaker shear‐wave anisotropy appear to be fault‐bounded. The average well flow rates (which are fracture‐dominated) within such blocks correlate with the average anisotropy of the blocks. The further observation that the anisotropy is largest in the fracture gas cap of the reservoir suggests that shear waves can provide a direct hydrocarbon indicator for fractured rock.
Dip moveout of converted waves and parameter estimation in transversely isotropic media
Authors: Tsvankin Ilya , Grechka Vladimir
Published: March 2003

For transverse isotropy with a vertical symmetry axis (VTI media), P‐wave reflection data alone are insufficient for building velocity models in depth. Here, we show that all parameters of VTI media responsible for propagation of P‐ and SV‐waves (the P‐wave and S‐wave vertical velocities VP0 and VS0 and the anisotropic parameters ε and δ) can be obtained by combining P‐wave traveltimes with the moveout of PS‐waves converted at a horizontal and dipping interface. Using converted modes, rather than pure S‐waves, avoids the need for expensive shear‐wave excitation on land and makes the method suitable for offshore exploration.

The inversion algorithm is based on a new analytic description of the dip moveout of PS‐waves developed for symmetry planes of anisotropic media (and for any vertical plane in models with weak azimuthal anisotropy). The common‐midpoint (CMP) traveltime–offset relationship, derived in a parametric form and represented through the components of the slowness vector of the P‐ and S‐waves, makes it possible to compute the moveout curve of the PS‐wave without two‐point ray tracing. This formalism also leads to closed‐form solutions for moveout attributes, such as the coordinates (xmin, tmin) of the traveltime minimum, the normal‐moveout (NMO) velocity defined at x = xmin and the slope of the moveout curve (apparent slowness) at zero offset.

The parameter‐estimation algorithm operates with reflection moveout of P‐ and PS‐waves from a horizontal and dipping reflector. The NMO velocities of P‐ and PS‐waves from horizontal events and the ratio of the corresponding zero‐offset traveltimes yield three equations for the four unknown medium parameters. The remaining parameter is found from an overdetermined system of equations that includes the P‐wave NMO velocity and moveout attributes of the PS‐wave for a dipping event. Numerical analysis shows that the PS‐wave dip‐moveout signature plays a crucial role in obtaining accurate estimates of the anisotropic parameters. The joint inversion of P and PS data provides the necessary information not only for P‐wave depth imaging in VTI media, but also for the processing of PS‐waves, including re‐sorting of PS traces into common‐reflection‐point gathers and transformation to zero offset (TZO).
Magnetic field transforms with low sensitivity to the direction of source magnetization and high centricity
Authors: Stavrev Petar , Gerovska Daniela
Published: March 2003

Magnetic data interpretation faces difficulties due to the various shapes of magnetic anomalies and the positions of their extrema with respect to the causative bodies for different directions of the source magnetization. The well‐known transforms — reduction to the pole, pseudogravity field, and analytic signal (total gradient) — help in reducing the problem. Another way to achieve the required effect is the transformation of magnetic data, ΔT or Z, into values of the anomalous magnetic intensity T. In this respect, we have found some transforms based on differential operators such as the gradient of T and its modulus R = |∇T|, the Laplacian L = ∇2T, the product T ∇2T and its square root Q, and the Laplacian ∇2(T2) and its square root E, to be useful. They are slightly sensitive to the magnetization orientation and their extrema occur above the sources.

For a 2D anomaly of a homogeneous causative body, the proposed transforms do not depend on the inclination of magnetization. In the 3D case, such independence does not exist even for the elementary field of a point dipole. The influence of the magnetization direction is estimated by an integral coefficient of sensitivity. This coefficient takes values of up to 2.0 for ΔT or Z anomalies, while their transforms T, R, E, Q and L have values of less than 0.28, 0.29, 0.24, 0.16 and 0.07, respectively, i.e. on average, 10 times less. The estimation of the centricity is carried out using the relative deviation of the principal extremum of the anomaly or its transforms from the epicentre of the model body at a depth equal to 100 units. For a ΔT anomaly this deviation is up to 67%; for the L transform it is less than 8%; for Q, E, R and T it is less than 10%, 15%, 20% and 25%, respectively.

The proposed transforms take only non‐negative values. With respect to their shape, the peripheral magnetic extrema are removed, the anomalous configuration is simplified and the resolution of complicated interference patterns is improved. Their calculation does not require additional data for the direction of magnetization, which is an essential advantage over the reduction‐to‐the‐pole and pseudogravity‐field transforms. A joint analysis of the measured field and its transforms T, E and L offers possibilities for more confident separation of the anomalous effects and direct correlation to their sources. The model tests performed and the 3D field applications to real magnetic data confirm the useful properties of the transforms suggested here.
Removal of DC power‐line magnetic‐field effects from airborne total magnetic‐field measurements
Authors: Gharibi Mehran , Pedersen Laust B.
Published: March 2003

Power lines carrying DC current can strongly affect total magnetic‐field measurements. A simple algorithm using Biot–Savart's law was made to remove magnetic‐field components due to a DC power line from airborne total magnetic‐field measurements in the Gävle area, Sweden. The power‐line location was estimated from observed data and then split into short line segments. The magnetic‐field components due to each segment were calculated and summed together to give the total magnetic effect due to the power line at each observation point. The corrected total magnetic field was calculated by subtracting the power‐line magnetic‐field vector, projected on to the direction of the main field, from the measured total field. The results show a successful removal of the power‐line magnetic effect from the total magnetic‐field measurements. However, an error in the estimation of the power‐line location can result in a magnetic‐field residual after correction. A non‐linear median filtering was used to remove this residual when needed.
Structural–formational interpretation tools for seismic stratigraphy
Authors: Mushin I.A. , Makarov V.V. , Kozlov E.A. , Lowrie A.
Published: March 2003

In the framework of a structural‐and‐formational interpretation (SFI) approach to seismic data processing and geological interpretation, specific software/technology tools were created to facilitate sequence stratigraphy analysis and reservoir characterization. The most important tools are two types of time–frequency representation of seismic data: the first is spectrum‐time analysis (STAN), which presents a seismic trace as a series of very narrow frequency band traces, while the second tool converts an initial seismic section into a set of sections ranked with preselected frequency bands, narrower than those of the initial section, but wider than those related to STAN traces. Jointly, these two representations reflect both general trends and local temporal and spatial variations of seismic data frequency content. The use of these tools, developed in Russia in the 1980s, facilitates detection of sedimentation cycles and their depositional environments, identification of hierarchies within faulting patterns, and delineation of geological anomalies on seismic sections. This, in turn, provides reliable starting points for palaeotectonic restoration and basin modelling. In many regions, these tools have helped to clarify obscure formation structures under study and to estimate the hydrocarbon potential of these formations.
An apparent‐resistivity concept for low‐frequency electromagnetic sounding techniques
Authors: Van Der Kruk J. , Meekes J.A.C. , Van Den Berg P.M. , Fokkema J.T.
Published: March 2003

Apparent resistivity is a useful concept for initial quickscan interpretation and quality checks in the field, because it represents the resistivity properties of the subsurface better than the raw data. For frequency‐domain soundings several apparent‐resistivity definitions exist. One definition uses an asymptote for the field of a magnetic dipole in a homogeneous half‐space and is useful only for low induction numbers. Another definition uses only the amplitude information of the total magnetic field, although this results in a non‐unique apparent resistivity. To overcome this non‐uniqueness, a complex derivation using two different source–receiver configurations and several magnetic field values for different frequencies or different offsets is derived in another definition. Using the latter theory, in practice, this means that a wide range of measurements have to be carried out, while commercial systems are not able to measure this wide range. In this paper, an apparent‐resistivity concept is applied beyond the low‐induction zone, for which the use of different source–receiver configurations is not needed. This apparent‐resistivity concept was formerly used to interpret the electromagnetic transients that are associated with the turn‐off of the transmitter current. The concept uses both amplitude and phase information and can be applied for a wide range of frequencies and offsets, resulting in a unique apparent resistivity for each individual (offset, frequency) combination. It is based on the projection of the electromagnetic field data on to the curve of the field of a magnetic dipole on a homogeneous half‐space and implemented using a non‐linear optimization scheme. This results in a fast and efficient estimation of apparent resistivity versus frequency or offset for electromagnetic sounding, and also gives a new perspective on electromagnetic profiling. Numerical results and two case studies are presented. In each case study the results are found to be comparable with those from other existing exploration systems, such as EM31 and EM34. They are obtained with a slight increase of effort in the field but contain more information, especially about the vertical resistivity distribution of the subsurface.
Identification and classification of multiple reflections with self‐organizing maps
Authors: Essenreiter Robert , Karrenbach Martin , Treitel Sven
Published: March 2003

Artificial neural networks can be used effectively to identify and classify multiple events in a seismic data set. We use a specialized neural network, a self‐organizing map (SOM), that tries to establish rules for the characterization of the physical problem. Selected seismic data attributes from CMP gathers are used as input patterns, such that the SOM arranges the data to form clusters in an abstract space. We show with synthetic and real data how the SOM can identify and classify primaries and multiples, and how it can classify the various types of multiple corresponding to a certain generating mechanism in the subsurface.
On removing the primary field from fixed‐wing time‐domain airborne electromagnetic data: some consequences for quantitative modelling, estimating bird position and detecting perfect conductors
Authors: Smith Richard
Published: March 2003

In the process of removing the primary field from fixed‐wing time‐domain airborne EM data, the response is decomposed into two parts, which are referred to here as the time‐domain ‘in‐phase’ and ‘quadrature’ components. The time‐domain in‐phase component is dominated by the primary field, which varies significantly as the transmitter–receiver separation changes. The time‐domain quadrature component comes solely from the secondary response associated with currents induced in the ground and this is the component that has traditionally been used in the interpretation of data from fixed‐wing towed‐bird time‐domain EM systems. In the off‐time, the quadrature response is very similar to the total secondary response. However, there are large differences in the on‐time and even some small differences in the off‐time.One consequence of these differences is that when airborne EM data are to be interpreted using a synthetic mathematical model, the synthetic data calculated should also be the quadrature component. A second consequence relates to the time‐domain in‐phase component which is sometimes used to estimate the receiver‐sensor (bird) position. The bird‐position estimation process assumes there is no secondary field in the in‐phase component. If the ground is resistive, the secondary contained in the in‐phase component is small, so the bird‐position estimate is accurate to about 30 cm, but in highly conductive areas the secondary contribution can be large and the position estimate can be out by as much as 5 m. A third consequence arises for highly conductive bodies, the response of which is predominantly in‐phase. This means that any response from these types of body is lost in the component that has been removed in the primary‐field extraction procedure. However, if the bird position is measured very accurately, the actual free‐space primary field can be estimated. If this is then subtracted from the estimated primary (actually free‐space primary plus secondary in‐phase response), then the residual is the secondary in‐phase response of the ground. Using this methodology, very conductive ore bodies could be detected. However, a sensitivity analysis shows that detection of a large vertically dipping very conductive body at 150 m depth would require that the bird position be measured to an accuracy of about 1.4 cm and the aircraft attitude to within about 0.01°. Such tolerances are very stringent and not easily attainable with current technology.
Inversion of DC resistivity data using neural networks
Authors: El‐Qady Gad , Ushijima Keisuke
Published: March 2003

The inversion of geoelectrical resistivity data is a difficult task due to its non‐linear nature. In this work, the neural network (NN) approach is studied to solve both 1D and 2D resistivity inverse problems. The efficiency of a widespread, supervised training network, the back‐propagation technique and its applicability to the resistivity problem, is investigated. Several NN paradigms have been tried on a basis of trial‐and‐error for two types of data set. In the 1D problem, the batch back‐propagation paradigm was efficient while another paradigm, called resilient propagation, was used in the 2D problem. The network was trained with synthetic examples and tested on another set of synthetic data as well as on the field data. The neural network gave a result highly correlated with that of conventional serial algorithms. It proved to be a fast, accurate and objective method for depth and resistivity estimation of both 1D and 2D DC resistivity data. The main advantage of using NN for resistivity inversion is that once the network has been trained it can perform the inversion of any vertical electrical sounding data set very rapidly.
Poisson's ratio at high pore pressure
Authors: Carcione José M. , Cavallini Fabio
Published: March 2003

Laboratory investigations suggest that a precise relationship exists between Poisson's ratio, pore pressure and fluid type. Values of Poisson's ratio for dry samples are significantly smaller than those for fluid‐saturated samples. The values are anomalously high for high pore pressure, with the possibility of differentiating between gas‐saturated, brine‐saturated and oil‐saturated porous rocks.

The present study considers two overpressure models, based on oil/gas conversion and disequilibrium compaction, to obtain Poisson's ratio versus differential pressure (confining pressure minus pore pressure). The model results are in good agreement with experiments. Poisson's ratio is approximately constant at high differential pressures and increases (decreases) for saturated (dry) rocks at low differential pressures. Fluid type can be determined at all differential pressures from Poisson's ratio. The analysis is extended to the anisotropic case by computing the three Poisson's ratios of a transversely isotropic rock versus differential pressure. While one of them is practically independent of effective pressure, the others increase with increasing pore pressure. Experiments performed on cores under different pressure conditions, and calibration of the models with these data, provide a tool for inverting pore pressure from seismic data.
Fluid‐dependent shear‐wave splitting in fractured media
Authors: Sayers Colin M.
Published: March 2003

Azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock. A shear wave propagating in an azimuthally anisotropic medium splits into two components with different polarizations if the source polarization is not aligned with the principal axes of the medium. For vertical propagation of shear waves in a horizontally layered medium containing vertical fractures, the shear‐wave splitting depends on the shear compliance of the fractures, but is independent of their normal compliance. If the fractures are not perfectly vertical, the shear‐wave splitting also depends on the normal compliance of the fractures. The normal compliance of a fluid‐filled fracture decreases with increasing fluid bulk modulus. For dipping fractures, this results in a decrease in shear‐wave splitting and an increase in shear‐wave velocity with increasing fluid bulk modulus. The sensitivity of the shear‐wave splitting to fluid bulk modulus depends on the interconnectivity of the fracture network, the permeability of the background medium and on whether the fracture is fully or partially saturated.
Analysis of the resolution function in seismic prestack depth imaging
Authors: Gelius L.‐J. , Lecomte I. , Tabti H.
Published: March 2003

We consider the problem of estimating subsurface quantities such as velocity or reflectivity from seismic measurements. Because of a limited aperture and band‐limited signals, the output from a seismic prestack reconstruction method is a distorted or blurred image. This distortion can be computed using the concept of resolution function, which is a quantity readily accessible in the Fourier space of the model. The key parameter is the scattering wavenumber, which at a particular image point is defined by the incident and scattered ray directions in a given background model. Any location in any background model can be considered. In general, the resolution function will depend on the following four quantities: the background velocity model, the frequency bandwidth, the wavefield type and the acquisition geometry.

We first establish the resolution function for a general scattering model assuming local reaction. We then adapt this result for two well‐known scattering models: Born and Kirchhoff. For each of these approximations the corresponding resolution function is derived and discussed. Finally, by employing a simple synthetic data example we demonstrate the ability of the resolution function to predict the image distortions.
Prediction of reproductive success and failure in lesser snow geese based on early season climatic variables
Authors: Skinner W. R. , Jefferies R. L. , Carleton T. J. , Abraham R. F. Rockwell&dagger K. F.
Published: March 2003

The North American mid‐continent population of lesser snow geese (Anser caerulescens caerulescens) breeds in coastal areas of the Hudson Bay region. Breeding success is highly variable, particularly during recent decades. The availability of long‐term data sets of weather and the breeding success of geese allowed us to determine whether climatic variables in spring and early summer (May–June) are reliable predictors of different attributes of the reproductive biology of snow geese. A large region of strong anomalous cooling in north‐eastern North America has been the dominant anomalous climatic feature since the mid‐1970s. The cooling which becomes established during winter persists into spring and early summer when migration, nesting and hatching of geese are occurring. Redundancy analysis (RDA) of the data sets was made to identify dominant correlations and regression relationships between climatic and goose variables. Individual goose response variables were further explored with stepwise multiple regression and bipartial regression.

96.7% of year‐to‐year variance in the goose data was explained by the selected climatic data. The first four orthogonal axes out of seven possible axes explained 92.2% of the total variance. Date of last snow on the ground and mean daily temperature from 6 to 20 May formed the lowest and highest predictor scores, respectively. Initiation date and hatching date at the low end and total clutch size and clutch size at hatch at the high end were associated with these extremes, particularly in certain years. Days of freezing rain in May and total rainfall were correlated with nest failure. Bivariate correlation/regression showed that the most parsimonious model for nest initiation day was based on four climatic predictors, for hatching day four predictors, and for clutch size at hatch, nine predictors.

Both the multiple regression analyses and the redundancy analyses confirm the high degree of predictability of goose reproductive variables from selected climatic variables. As discussed, the correlations reflect both direct and indirect effects of climate on the reproductive biology of geese. The correlations are strongest in the early season and weaken by early summer.
Elevated CO2 concentrations and stomatal density: observations from 17 plant species growing in a CO2 spring in central Italy
Authors: Bettarini Isabella , Vaccari Francesco P. , Miglietta Franco
Published: March 2003

Stomatal density (SD) and stomatal conductance (gs) can be affected by an increase of atmospheric CO2 concentration. This study was conducted on 17 species growing in a naturally enriched CO2 spring and belonging to three plant communities. Stomatal conductance, stomatal density and stomatal index (SI) of plants from the spring, which were assumed to have been exposed for generations to elevated [CO2], and of plants of the same species collected in a nearby control site, were compared. Stomatal conductance was significantly lower in most of the species collected in the CO2 spring and this indicated that CO2 effects on gs are not of a transitory nature but persist in the long term and through plant generations. Such a decrease was, however, not associated with changes in the anatomy of leaves: SD was unaffected in the majority of species (the decrease was only significant in three out of the 17 species examined), and also SI values did not vary between the two sites with the exception of two species that showed increased SI in plants grown in the CO2‐enriched area. These results did not support the hypothesis that long‐term exposure to elevated [CO2] may cause adaptive modification in stomatal number and in their distribution.
State and change in carbon pools in the forests of tropical Africa
Authors: GASTON GREG , BROWN SANDRA , LORENZINI MASSIMILIANO , SINGH K. D.
Published: March 2003

To improve estimates of the state and change in C pools due to changes in land use in tropical forests of Africa, we combined spatially explicit estimates of biomass C density, obtained by modelling in a geographical information system (GIS), with new data on the area of forests (woody formations with a minimum of 10% crown cover) reported at subnational units for 1980 and 1990 by the Food and Agriculture Organization (FAO). Estimates of the biomass C densities for grass/shrub savannas were also included using a simple model based on precipitation. The total C pool in above– and below–ground forests and grass/shrub savannas of Africa for 1980 was 50.8 Pg (1015g), with aboveground forest biomass accounting for 75% of the total, below–ground forest biomass for 21%, and grass/shrub savannas for 4%. Area weighted mean biomass C densities were about 180 Mg ha–1 for lowland moist forests, 82 Mg ha–1 for all forests, and 6 Mg ha–1 for grass savannas. The total change in the aboveground forest C pool for the decade 1980–90 due to changes in land cover and use was estimated to be a decrease of 6.6 Pg C. Of this total, 43% was due to deforestation and 57% due to biomass reduction by other human activities. Six countries, mostly in central Africa, accounted for more than 73% of the total change in the C pool. The difference between state and change of C pool estimates made at the subnational scale and those made at the national scale proved to be insignificant across the region as a whole (2% for pools and – 1% for change in pool) but potentially important to individual countries (from + 36% to – 39% for pools and from + 43% to – 57% for change in pool). The differences between the two approaches may reflect a better match of the areas being deforested with the biomass C density of forests being cleared.