325,967 research outputs found
The effect of hydrate content on seismic attenuation: A case study for Mallik 2L-38 well data, Mackenzie delta, Canada
No full textObservations of velocities in sediments containing gashydrates show that the strength of sediments increases with hydrate saturation. Hence it is expected that the attenuation of these sediments will decrease with increasing hydrate saturation. However, sonic log measurements in the Mallik 2L-38 well and cross hole tomography measurements in the Mallik field have shown that attenuation increases with hydrate saturation. We studied a range of mechanisms by which increasing hydrate saturation could cause increased attenuation. We found that a difference in permeability between the host sediment and the newly formed hydrate can produce the observed effect. We modelled attenuation in terms of Biot and squirt flow mechanisms in composite media. We have used our model to predict observed attenuations in the Mallik 2L-38 well, Mackenzie Delta, Canada
Estimation of gas hydrate concentration from multi-component seismic data at sites on the continental margins of NW Svalbard and the Storegga region of Norway
No full textHigh-resolution seismic experiments, employing arrays of closely spaced, four-component ocean-bottom seismic recorders, were conducted at a site off western Svalbard and a site on the northern margin of the Storegga slide, off Norway to investigate how well seismic data can be used to determine the concentration of methane hydrate beneath the seabed. Data from P-waves and from S-waves generated by P–S conversion on reflection were inverted for P- and S-wave velocity (Vp and Vs), using 3D travel-time tomography, 2D ray-tracing inversion and 1D waveform inversion. At the NW Svalbard site, positive Vp anomalies above a sea-bottom-simulating reflector (BSR) indicate the presence of gas hydrate. A zone containing free gas up to 150-m thick, lying immediately beneath the BSR, is indicated by a large reduction in Vp without significant reduction in Vs. At the Storegga site, the lateral and vertical variation in Vp and Vs and the variation in amplitude and polarity of reflectors indicate a heterogeneous distribution of hydrate that is related to a stratigraphically mediated distribution of free gas beneath the BSR. Derivation of hydrate content from Vp and Vs was evaluated, using different models for how hydrate affects the seismic properties of the sediment host and different approaches for estimating the background-velocity of the sediment host. The error in the average Vp of an interval of 20-m thickness is about 2.5%, at 95% confidence, and yields a resolution of hydrate concentration of about 3%, if hydrate forms a connected framework, or about 7%, if it is both pore-filling and framework-forming. At NW Svalbard, in a zone about 90-m thick above the BSR, a Biot-theory-based method predicts hydrate concentrations of up to 11% of pore space, and an effective-medium-based method predicts concentrations of up to 6%, if hydrate forms a connected framework, or 12%, if hydrate is both pore-filling and framework-forming. At Storegga, hydrate concentrations of up to 10% or 20% were predicted, depending on the hydrate model, in a zone about 120-m thick above a BSR. With seismic techniques alone, we can only estimate with any confidence the average hydrate content of broad intervals containing more than one layer, not only because of the uncertainty in the layer-by-layer variation in lithology, but also because of the negative correlation in the errors of estimation of velocity between adjacent layers. In this investigation, an interval of about 20-m thickness (equivalent to between 2 and 5 layers in the model used for waveform inversion) was the smallest within which one could sensibly estimate the hydrate content. If lithological layering much thinner than 20-m thickness controls hydrate content, then hydrate concentrations within layers could significantly exceed or fall below the average values derived from seismic data
The microstructure of sediment-hosted hydrates: evidence from effective medium modelling of laboratory and borehole seismic data
No full textMuch of our knowledge of hydrate distribution in the subsurface comes frominterpretations of remote seismic measurements. A key step in such interpretations isan effective medium theory that relates the seismic properties of a given sediment toits hydrate content. A variety of such theories have been developed; these theoriesgenerally give similar results if the same assumptions are made about the extent towhich hydrate contributes to the load-bearing sediment frame. We have furtherdeveloped and modified one such theory, the self-consistentapproximation/differential effective medium approach, to incorporate additionalempirical parameters describing the extent to which both the sediment matrix material(clay or quartz) and the hydrate are load-bearing. We find that a single choice ofthese parameters allows us to match well both P and S wave velocity measurementsfrom both laboratory and in situ datasets, and that the inferred proportion of hydratethat is load-bearing varies approximately linearly with hydrate saturation. Thisproportion appears to decrease with increasing hydrate saturation for gas-richlaboratory environments, but increase with hydrate saturation when hydrate is formedfrom solution and for an in situ example
A review of records of the Trimeresurus albolabris Gray, 1842 group from the Indian subcontinent: expanded description and range extension of Trimeresurus salazar, redescription of Trimeresurus septentrionalis and rediscovery of historical specimens of Trimeresurus davidi (Reptilia: Viperidae)
No full textVogel, Gernot, Mallik, Ashok Kumar, Chandramouli, S. R., Sharma, Vivek, Ganesh, S. R. (2022): A review of records of the Trimeresurus albolabris Gray, 1842 group from the Indian subcontinent: expanded description and range extension of Trimeresurus salazar, redescription of Trimeresurus septentrionalis and rediscovery of historical specimens of Trimeresurus davidi (Reptilia: Viperidae). Zootaxa 5175 (3): 343-366, DOI: https://doi.org/10.11646/zootaxa.5175.3.
Initial geophysical and geological assessment of an industry 3D seismic survey covering the JAPEX/JNOC/GSC Mallik 5L-38 gas hydrate research well
No full textIndustry 3D seismic covers the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate well. Interpretation supported by biostratigraphy indicates a normally-faulted anticline with 5L-38 occupying the northern downthrown crest. This EW-striking fault, near 5L-38, has 800 m throw interpreted 700 m below gas hydrate levels. Fault offset at 5L-38 gas hydrate levels is probably several hundred metres less, because syn-depositional fault movement during or after Late Eocene time is suggested. Down hole velocity surveys at Imperial Mallik wells, P-59, J-37 and A-06, infer 105, 225, and 135 m gas hydrate, respectively, compared to 116 m known at 5L-38. Gas hydrate seismic characterization includes: multiple, generally weak, near-horizontal events; amplitude features on geologic reflectors; amplitude “blanking” and; chaotic vertical reflective zones surrounding faults. Recognized data contamination include amplitude-frequency degradation beneath lakes and reverberated energy. Noted are spatial correspondence of some subsurface faults with position/orientation of surface lakes, suggesting a possible genetic link
Review of ground temperatures in the Mallik field area: a constraint to the methane hydrate stability
No full textAnalysis of data from 32 industrial exploration wells in the Mallik field and surrounding area in the Mackenzie Delta-Beaufort Sea region allowed construction of temperature-depth profiles using regional heat-flow values, temperature at the base of ice-bearing
permafrost, and models of thermal conductivity with depth. An analysis of the stability conditions for methane hydrate showed that it is stable in the Mallik field area and that the depth to the base of the methane hydrate stability zone can be as deep as 1500 ± 100 m in areas of thick
permafrost. The depth to the base of the methane hydrate stability zone, calculated in this study using reconstructed temperature-depth profiles, was found in a majority of the wells to be 50-150 m deeper than that previously determined using linear temperature profiles and a constant thermal
conductivity with depth.</jats:p
Diffusive author(s), cohesive author: Analysis of S/N (1994)
No full textThis study indicates the ways in which various aspects of the author(s) are brought forth in Dumb type’s performance art, the S/N production. Previous research has suggested a non-hierarchical organization of Dumb type and the absence of a “privileged author” in Dumb type’s collaborative work, S/N. However, the results that I have investigated from member’s interviews on the creative process of S/N along with my analysis of the recorded images of S/N, indicate a different aspect of the author(s). First, S/N was created through, so to speak, the collective ideas of the members of Dumb type. Further, S/N has at least nine quotations from previous performances, installations, and printed writings, besides the work-in-progress technique. Explicating one of the “author functions” as given by Michel Foucault, each text has plural subjects of the author. However, it has been revealed from members’ interviews that Teiji Furuhashi had a decision-making role in selecting the members’ ideas within the performance. Since then, S/N has had plural subjects of creation; however, Furuhashi is one of the subjects of creation along with the “privileged author.” S/N has plural authors (diffusive authors) yet at the same time, it has a “privileged author,” Teiji Furuhashi (cohesive author)
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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