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  • 1
    Article
    Article
    1999
    ISSN: 0028-0836 
    Language: English
    In: Nature (London), 1999-10-21, Vol.401 (6755), p.782-785
    Description: Mount Etna, the largest volcano in Europe, lies close to the subduction-related Aeolian magmatic arc but shows no trace of subducted material in its magmas. Mount Etna is also situated on continental crust yet shows oceanic basalt affinities, with isotopic ratios of helium and carbon suggesting that it is fed by the same type of mantle source as are mid-ocean ridge basalts. Here we propose that although this giant volcano is not subduction-related-in the sense that it is not part of the magmatic arc-its formation is strongly related to the nearby subduction process. Based on a three-dimensional model of the tectonic plates in this region, we propose that the voluminous melting under Mount Etna results from 'suction' of asthenospheric material from under the neighbouring African plate. Such lateral flow is expected when descending slabs migrate backwards in the mantle (rollback) leaving low-pressure regions behind them. This was previously identified at the northern end of the Tonga arc (southwest Pacific Ocean) where such flow feeds arc or backarc magmatism. Here we show that in the south Tyrrhenian subduction zone, slab rollback pulls asthenospheric material much farther along the plate contact, reaching the base of the crust in the forearc region. This explains the voluminous melting under Mount Etna and also the recent uplift of the forearc region (the Calabrian peninsula).
    Subject(s): Earth, ocean, space ; Tectonics. Structural geology. Plate tectonics ; Earth sciences ; Crystalline rocks ; Igneous and metamorphic rocks petrology, volcanic processes, magmas ; Exact sciences and technology
    ISSN: 0028-0836
    E-ISSN: 1476-4687
    Source: Academic Search Ultimate
    Source: Nature Journals Online
    Source: Alma/SFX Local Collection
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  • 2
    Language: English
    In: Geophysical research letters, 2004-02-20, Vol.31 (4), p.L04606-n/a
    Description: We present results and methodology for predicting permeability from thin sections. The method consists of two key components–reconstruction of 3D porous media from 2D thin sections, and 3D flow simulation using the Lattice‐Boltzmann (LB) technique. We construct 3D porous media using sequential indicator simulation (SIS), a geostatistical method, with conditional data and input statistical parameters from thin sections. Permeability is then estimated through flow simulation on the reconstructed porous media. The LB flow simulation successfully handles very complex reconstructed 3D pore geometries. Computed permeabilities from seven thin section samples show good agreement with laboratory measurements over a wide range of permeability. We compare our method to one that uses only thin sections without 3D reconstruction. The comparison shows that our method gives better prediction of permeability, and is less sensitive to statistical errors from discrepancy between thin sections and core samples.
    Subject(s): Hydrology ; Groundwater transport ; Permeability and porosity ; Transport properties ; Numerical solutions ; Physical Properties of Rocks ; Mathematical Geophysics ; Modeling
    ISSN: 0094-8276
    E-ISSN: 1944-8007
    Source: Wiley Online Library All Backfiles
    Source: Alma/SFX Local Collection
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  • 3
    Language: English
    In: Geophysics, 2011-10, Vol.76 (5), p.O23-O33
    Description: The fundamental concept of time-lapse seismic monitoring is that changes in physical parameters--such as saturation, pore fluid pressure, temperature, and stress--affect rock and fluid properties, which in turn alter the seismic velocity and density. Increasingly, however, time-lapse seismic monitoring is called upon to quantify subsurface changes due in part to chemical reactions between injected fluids and the host rocks. This study springs from a series of laboratory experiments and high-resolution images assessing the changes in microstructure, transport, and seismic properties of fluid-saturated sandstones and carbonates injected with CO2. Results show that injecting CO2 into a brine-rock system induces chemo-mechanical mechanisms that permanently change the rock frame. Injecting CO2 into brine-saturated-sandstones induces salt precipitation primarily at grain contacts and within small pore throats. In rocks with porosity lower than 10%, salt precipitation reduces permeability and increases P- and S-wave velocities of the dry rock frame. On the other hand, injecting CO2-rich water into micritic carbonates induces dissolution of the microcrystalline matrix, leading to porosity enhancement and chemo-mechanical compaction under pressure. In this situation, the elastic moduli of the dry rock frame decrease. The results in these two scenarios illustrate that the time-lapse seismic response of chemically stimulated systems cannot be modeled as a pure fluid-substitution problem. A first set of empirical relationships links the time-variant effects of injection to the elastic properties of the rock frame using laboratory velocity measurements and advanced imaging.
    Subject(s): reservoir rocks ; algorithms ; carbon dioxide ; applied (geophysical surveys & methods) ; geochemistry ; rock mechanics ; host rocks ; Geophysics ; Engineering geology ; technology ; sandstone ; saturation ; brines ; sedimentary rocks ; seismic methods ; geophysical methods ; petroleum ; four-dimensional models ; injection ; petroleum exploration ; clastic rocks ; Internal geophysics ; Earth, ocean, space ; Earth sciences ; Exact sciences and technology
    ISSN: 0016-8033
    E-ISSN: 1942-2156
    Source: SEG Digital Library
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  • 4
    Language: English
    In: Geophysical journal international, 2003-10, Vol.155 (1), p.319-326
    Description: SUMMARY The presence of clay minerals can alter the elastic behaviour of rocks significantly. Although clay minerals are common in sedimentary formations and seismic measurements are our main tools for studying subsurface lithologies, measurements of elastic properties of clay minerals have proven difficult. Theoretical values for the bulk modulus of clay are reported between 20 and 50 GPa. The only published experimental measurement of Young's modulus in a clay mineral using atomic force acoustic microscopy (AFAM) gave a much lower value of 6.2 GPa. This study has concentrated on using independent experimental methods to measure the elastic moduli of clay minerals as functions of pressure and saturation. First, ultrasonic P‐ and S‐wave velocities were measured as functions of hydrostatic pressure in cold‐pressed clay aggregates with porosity and grain density ranging from 4 to 43 per cent and 2.13 to 2.83 g cm−3, respectively. In the second experiment, P‐ and S‐wave velocities in clay powders were measured under uniaxial stresses compaction. In the third experiment, P‐wave velocity and attenuation in a kaolinite–water suspension with clay concentrations between 0 and 60 per cent were measured at ambient conditions. Our elastic moduli measurements of kaolinite, montmorillonite and smectite are consistent for all experiments and with reported AFAM measurements on a nanometre scale. The bulk modulus values of the solid clay phase (Ks) lie between 6 and 12 GPa and shear (μs) modulus values vary between 4 and 6 GPa. A comparison is made between the accuracy of velocity prediction in shaley sandstones and clay–water and clay–sand mixtures using the values measured in this study and those from theoretical models. Using Ks= 12 GPa and μs= 6 GPa from this study, the models give a much better prediction both of experimental velocity reduction due to increase in clay content in sandstones and velocity measurements in a kaolinite–water suspension.
    Subject(s): elastic moduli ; clay minerals ; acoustic velocities ; porosity
    ISSN: 0956-540X
    E-ISSN: 1365-246X
    Source: Alma/SFX Local Collection
    Source: Oxford Journals 2016 Current and Archive A-Z Collection
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  • 5
    Language: English
    In: Geophysics, 2007-01, Vol.72 (1), p.E1-E13
    Description: Knowledge of the pressure dependences of seismic velocities in unconsolidated sands is necessary for the remote prediction of effective pressures and for the projection of velocities to unsampled locations within shallow sand layers. We have measured the compressional- and shear-wave velocities and bulk, shear, and P-wave moduli at pressures from 0.1 to 20 MPa in a series of unconsolidated granular samples including dry and water-saturated natural sands and dry synthetic sand and glass-bead samples. The shear-wave velocities in these samples demonstrate an average pressure dependence approximately proportional to the fourth root of the effective pressure (VS∝p'1/4, as commonly observed at lower pressures. For the compressional-wave velocities, the exponent in the pressure dependence of individual dry samples is consistently less than the exponent for the shear-wave velocity of the same sample, averaging 0.23 for the dry sands and 0.20 for the glass-bead samples. These pressure dependences are generally consistent over the entire pressure range measured. A comparison of the empirical results to theoretical predictions based on Hertz-Mindlin effective-medium models demonstrates that the theoretical models vastly overpredict the shear moduli of the dry granular frame unless the contacts are assumed to have no tangential stiffness. The models also predict a lower pressure exponent for the moduli and velocities (V∝p'1/6) than is generally observed in the data. We attribute this discrepancy in part to the inability of the models to account for decreases in the amount of slip or grain rotation occurring at grain-to-grain contacts with increasing pressure.
    Subject(s): reservoir rocks ; models ; applied (geophysical surveys & methods) ; Hertz-Mindlin model ; United States ; Geophysics ; elastic waves ; S-waves ; clastic sediments ; pressure ; velocity ; samples ; mineral composition ; sand ; seismic methods ; unconsolidated materials ; mathematical methods ; body waves ; geophysical methods ; petrography ; petroleum ; petroleum exploration ; sediments ; seismic waves ; Internal geophysics ; Earth, ocean, space ; Earth sciences ; Exact sciences and technology
    ISSN: 0016-8033
    E-ISSN: 1942-2156
    Source: SEG Digital Library
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  • 6
    Language: English
    In: Earth and planetary science letters, 2001, Vol.187 (1), p.117-130
    Description: The central Mediterranean comprises subduction, collision, and backarc extension, all in a relatively small area. In this paper, we analyze the topography of this region, examining the influence of subducting slabs on overriding plates. We take the observed surface elevation, remove the contribution of the crust to it and use the residual topography to identify regions that are unusually low, unusually high, or normal. Then, we calculate lithospheric thickness where local isostasy applies. The results provide information about the structure of the lithosphere in this complex region and about the vertical tectonics. In particular, we found a difference between the south Tyrrhenian subduction zone and the Apennines, consistent with the processes of slab rollback and slab break-off, respectively. In the south Tyrrhenian the edge of the oceanic plate is strongly pulled down whereas the edge of the overriding plate (the Calabrian Peninsula) is uplifted. This opposite trend indicates weak plate coupling – that is, the subducting slab, which is rapidly rolling back, hangs almost entirely on the oceanic plate and the overriding plate is free. In contrast, in the Apennines high residual topography and uplifting are detected on both sides of the mountain belt, consistent with complete break-off of the subducted lithosphere.
    Subject(s): lithosphere ; subduction ; Mediterranean region ; slabs ; topography ; plate collision
    ISSN: 0012-821X
    E-ISSN: 1385-013X
    Source: Backfile Package - All of Back Files EBS [ALLOFBCKF]
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  • 7
    Language: English
    In: Geophysics, 1996-10, Vol.61 (5), p.1363-1370
    Description: We have analyzed two laboratory data sets obtained on high-porosity rock samples from the North Sea. The velocities observed are unusual in that they seem to disagree with some simple models based on porosity. On the other hand, the rocks are unusually poorly-cemented (for laboratory studies, at least), and we investigate the likelihood that this is the cause of the disagreement. One set of rocks, from the Oseberg Field, is made of slightly cemented quartz sands. We find that we can model their dry-rock velocities using a cementation theory where the grains mechanically interact through cement at the grain boundaries. This model does not allow for pressure dependence. The other set of rocks, from the Troll Field, is almost completely uncemented. The grains are held together by the applied confining pressure. In this case, a lower bound for the velocities can be found by using the Hertz-Mindlin contact theory (interaction of uncemented spheres) to predict velocities at a critical porosity, combined with the modified Hashin-Strikman lower bound for other porosities. This model, which allows for pressure-dependence, also predicts fairly large Poisson's ratios for saturated rocks, such as those observed in the measurements. The usefulness of these theories may be in estimating the nature of cement in rocks from measurements such as sonic logs. The theories could help indicate sand strength in poorly consolidated formations and predict the likelihood of sand production. Both theoretical methods have analytical expressions and are ready for practical use.
    Subject(s): applied (geophysical surveys & methods) ; well-logging ; Atlantic Ocean ; elastic constants ; North Sea ; Poisson's ratio ; sandstone ; reservoir properties ; seismic logging ; North Atlantic ; petroleum ; Hertz-Mindlin contact theory ; clastic rocks ; diagenesis ; energy sources ; cementation ; shear modulus ; petroleum engineering ; porosity ; Geophysics ; velocity ; Oseberg Field ; sedimentary rocks ; oil and gas fields ; Economic geology ; mathematical methods ; Troll Field ; elastic properties ; Internal geophysics ; Physical properties of sedimentary rocks ; Sedimentary rocks ; Exact sciences and technology ; Earth, ocean, space ; Hydrocarbons ; Applied geophysics ; Earth sciences
    ISSN: 0016-8033
    E-ISSN: 1942-2156
    Source: SEG Digital Library
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  • 8
    Article
    Article
    1997
    ISSN: 0016-8033 
    Language: English
    In: Geophysics, 1997-10, Vol.62 (5), p.1480-1482
    Subject(s): Kozeny-Carman relation ; percolation ; Hydrogeology ; hydrology ; percolation threshold ; porosity ; permeability ; sorting ; specific surface area ; geometry ; clay mineralogy ; Crude oil, natural gas, oil shales producing equipements and methods ; Crude oil, natural gas and petroleum products ; Energy ; Exact sciences and technology ; Applied sciences ; Prospecting and production of crude oil, natural gas, oil shales and tar sands ; Fuels ; Characteristics of producing layers. Reservoir geology. In situ fluids
    ISSN: 0016-8033
    E-ISSN: 1942-2156
    Source: SEG Digital Library
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  • 9
    Language: English
    In: Geophysics, 2000-04, Vol.65 (2), p.565-573
    Description: Marine seismic data and well-log measurements at the Blake Ridge offshore South Carolina show that prominent seismic bottom-simulating reflectors (BSRs) are caused by sediment layers with gas hydrate overlying sediments with free gas. We apply a theoretical rock-physics model to 2-D Blake Ridge marine seismic data to determine gas-hydrate and free-gas saturation. High-porosity marine sediment is modeled as a granular system where the elastic wave velocities are linked to porosity; effective pressure; mineralogy; elastic properties of the pore-filling material; and water, gas, and gas-hydrate saturation of the pore space. To apply this model to seismic data, we first obtain interval velocity using stacking velocity analysis. Next, all input parameters to the rock-physics model, except porosity and water, gas, and gas hydrate saturation, are estimated from geologic information. To estimate porosity and saturation from interval velocity, we first assume that the entire sediment does not contain gas hydrate or free gas. Then we use the rock-physics model to calculate porosity directly from the interval velocity. Such porosity profiles appear to have anomalies where gas hydrate and free gas are present (as compared to typical profiles expected and obtained in sediment without gas hydrate or gas). Porosity is underestimated in the hydrate region and is overestimated in the free-gas region. We calculate the porosity residuals by subtracting a typical porosity profile (without gas hydrate and gas) from that with anomalies. Next we use the rock-physics model to eliminate these anomalies by introducing gas-hydrate or gas saturation. As a result, we obtain the desired 2-D saturation map. The maximum gas-hydrate saturation thus obtained is between 13% and 18% of the pore space (depending on the version of the model used). These saturation values are consistent with those measured in the Blake Ridge wells (away from the seismic line), which are about 12%. Free-gas saturation varies between 1% and 2%. The saturation estimates are extremely sensitive to the input velocity values. Therefore, accurate velocity determination is crucial for correct reservoir characterization.
    Subject(s): physical properties ; applied (geophysical surveys & methods) ; well-logging ; Atlantic Ocean ; surveys ; accuracy ; saturation ; reservoir properties ; rocks ; North Atlantic ; models ; energy sources ; petroleum engineering ; porosity ; Geophysics ; pressure ; velocity ; marine methods ; seismic methods ; bottom-simulating reflectors ; marine environment ; Economic geology ; gas hydrates ; Blake-Bahama Outer Ridge ; geophysical methods ; geophysical surveys ; Internal geophysics ; Sedimentary rocks ; Exact sciences and technology ; Earth, ocean, space ; Hydrocarbons ; Applied geophysics ; Earth sciences ; Marine geology ; SEISMIC SURVEYS ; RESOURCE ASSESSMENT ; SEA BED ; SEISMIC DETECTION ; NATURAL GAS DEPOSITS ; GAS HYDRATES ; GAS SATURATION ; 03 NATURAL GAS ; SOUTH CAROLINA
    ISSN: 0016-8033
    E-ISSN: 1942-2156
    Source: SEG Digital Library
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  • 10
    Language: English
    In: Geophysics, 1998-10, Vol.63 (5), p.1659-1669
    Description: We interpret amplitude variation with offset (AVO) data from a bottom simulating reflector (BSR) offshore Florida by using rock-physics-based synthetic seismic models. A previously conducted velocity and AVO analysis of the in-situ seismic data showed that the BSR separates hydrate-bearing sediments from sediments containing free methane. The amplitudes at the BSR are increasingly negative with increasing offset. This behavior was explained by P-wave velocity above the BSR being larger than that below the BSR, and S-wave velocity above the BSR being smaller than that below the BSR. We use these AVO and velocity results to infer the internal structure of the hydrated sediment. To do so, we examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space: (1) the hydrate cements grain contacts and strongly reinforces the sediment, and (2) the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame. Only the second model can qualitatively reproduce the observed AVO response. Thus inferred internal structure of the hydrate-bearing sediment means that (1) the sediment above the BSR is uncemented and, thereby, mechanically weak, and (2) its permeability is very low because the hydrate clogs large pore-space conduits. The latter explains why free gas is trapped underneath the BSR. The seismic data also indicate the absence of strong reflections at the top of the hydrate layer. This fact suggests that the high concentration of hydrates in the sediment just above the BSR gradually decreases with decreasing depth. This effect is consistent with the fact that the low-permeability hydrated sediments above the BSR prevent free methane from migrating upwards.
    Subject(s): physical properties ; applied (geophysical surveys & methods) ; United States ; elastic waves ; reservoir properties ; seismograms ; seismic waves ; diagenesis ; models ; energy sources ; cementation ; velocity structure ; petroleum engineering ; Geophysics ; P-waves ; velocity ; consolidation ; Florida ; offshore ; seismic methods ; grain size ; bottom-simulating reflectors ; Economic geology ; gas hydrates ; AVO methods ; body waves ; geophysical methods ; sediments ; Internal geophysics ; Earth, ocean, space ; Applied geophysics ; Earth sciences ; Exact sciences and technology
    ISSN: 0016-8033
    E-ISSN: 1942-2156
    Source: SEG Digital Library
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