The Experts below are selected from a list of 108 Experts worldwide ranked by ideXlab platform
Steven Constable - One of the best experts on this subject based on the ideXlab platform.
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first results from a marine controlled source Electromagnetic Survey to detect gas hydrates offshore oregon
Geophysical Research Letters, 2006Co-Authors: Karen Weitemeyer, Kerry Key, Steven Constable, James BehrensAbstract:[1] Submarine gas hydrate is a hazard to drilling, a potential hydrocarbon resource, and has been implicated as a factor in both submarine slope stability and climate change. Bulk in situ electrical resistivities evaluated from Electromagnetic Surveys have the potential to provide an estimate of the total hydrate volume fraction more directly than by using seismic and well log data. We conducted a marine controlled-source Electromagnetic sounding at Hydrate Ridge, Oregon, USA, in August, 2004. Electromagnetic fields transmitted by a deep-towed horizontal electric dipole source were measured by a linear array of 25 seafloor Electromagnetic receivers, positioned 600 m apart to produce a dense coverage in the recorded electric field data. Results are presented in simple form by apparent resistivity pseudosections, which produce an approximate image of lateral resistivity variations across the study region. Resistivity values are consistent with those from well logs collected in the area and pseudosection features are correlated with seismic reflectors. Archie's Law, based on pseudosection apparent resistivities, predicts volumetric hydrate concentrations vary from 0–30% across the ridge.
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electrical resistivity structure of the valu fa ridge lau basin from marine controlled source Electromagnetic sounding
Geophysical Journal International, 2001Co-Authors: Lucy Macgregor, Martin Sinha, Steven ConstableAbstract:SUMMARY In December 1995 we carried out a comprehensive controlled-source Electromagnetic Survey of the Valu Fa Ridge at 22u25k Si n the Lau Basin. The Valu Fa Ridge is ab ack-arc spreading centre of intermediate spreading rate and is a site of extensive hydrothermal activity. Seismic studies have imaged a melt lens at an average depth of 3.2 km below the seafloor, surrounded by a zone of lowered seismic velocity, interpreted as a region of partial melt in the crust. The Electromagnetic experiment was part of a multidisciplinary study which included wide-angle and reflection seismics, bathymetry and potential field measurements. Electromagnetic signals at frequencies between 0.25 and 40 Hz were transmitted from a horizontal electric dipole towed close to the seafloor and were recorded by an array of 11 sea-bottom receivers at ranges of up to 20 km from the source. Over 80 hr of data, consisting of the magnitude of the horizontal electric field at the seafloor, were collected. These data have extremely low scatter compared to similar data from previous Surveys. The data were interpreted using a combination of 1- and 2-D forward modelling and inversion. The vertical resistivity gradient in the upper crust at the Valu Fa Ridge is abnormally low, with resistivities of less than 10 V m observed throughout layer 2 of the crust to a depth of 3 km. This is significantly more conductive at depth than the axis of the slow-spreading Reykjanes Ridge at 57u45kN, and the fastspreading East Pacific Rise at 13uN, where similar data sets have been collected in the past. Although the structure of layer 2 is well constrained by the Electromagnetic data, its extremely low resistivity causes rapid attenuation of Electromagnetic signals diffusing through it, and hence the data are not sensitive to the structure in layer 3, in particular the structure of the melt lens or surrounding low-velocity zone. The seismic velocity structure of the Valu Fa Ridge, determined from the coincident wide-angle seismic study, is similar to that observed at other mid-ocean ridges, with a steep seismic velocity gradient through layer 2 (although overall velocities are slightly lower). The seismic velocity anomaly calculated relative to an average off-axis structure is also small. This suggests that the very low resistivities observed at the axis are not caused by an upper crust of abnormally high porosity. However, hot and/or saline fluids permeating the crust can explain the low resistivities without affecting the seismic velocity. Since the conductive region extends unbroken from 3 km depth to the seafloor, it is probable that these fluids circulate to (or close to) the magma chamber itself.
James Behrens - One of the best experts on this subject based on the ideXlab platform.
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first results from a marine controlled source Electromagnetic Survey to detect gas hydrates offshore oregon
Geophysical Research Letters, 2006Co-Authors: Karen Weitemeyer, Kerry Key, Steven Constable, James BehrensAbstract:[1] Submarine gas hydrate is a hazard to drilling, a potential hydrocarbon resource, and has been implicated as a factor in both submarine slope stability and climate change. Bulk in situ electrical resistivities evaluated from Electromagnetic Surveys have the potential to provide an estimate of the total hydrate volume fraction more directly than by using seismic and well log data. We conducted a marine controlled-source Electromagnetic sounding at Hydrate Ridge, Oregon, USA, in August, 2004. Electromagnetic fields transmitted by a deep-towed horizontal electric dipole source were measured by a linear array of 25 seafloor Electromagnetic receivers, positioned 600 m apart to produce a dense coverage in the recorded electric field data. Results are presented in simple form by apparent resistivity pseudosections, which produce an approximate image of lateral resistivity variations across the study region. Resistivity values are consistent with those from well logs collected in the area and pseudosection features are correlated with seismic reflectors. Archie's Law, based on pseudosection apparent resistivities, predicts volumetric hydrate concentrations vary from 0–30% across the ridge.
Anne M Trehu - One of the best experts on this subject based on the ideXlab platform.
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a marine Electromagnetic Survey to detect gas hydrate at hydrate ridge oregon
Geophysical Journal International, 2011Co-Authors: Kare Weitemeye, Steve Constable, Anne M TrehuAbstract:Gas hydrates are a potential energy resource and hazard for drilling and infrastructure, yet estimates of global volume vary by over three orders of magnitude. Hydrates are electrically resistive compared to water saturated sediment and so Electromagnetic methods provide an additional tool to seismic Surveys and drilling for determining hydrate saturations. A marine Electromagnetic Survey was carried out at Hydrate Ridge, Oregon, USA, with the aim of testing the use of controlled source Electromagnetic (CSEM) and magnetotelluric (MT) methods to map gas hydrate and free gas below the gas hydrate stability zone. A 2-D CSEM inversion supports the scenario deduced from previous seismic and drilling results, which indicate two mechanisms of hydrate emplacement: a transport-dominated and reaction-dominated regime. A prominent resistive region of 2.5–4 ?m at a depth of about 130 mbsf, near the seismic bottom simulating reflector (BSR), suggests that 27 to 46 per cent of the bulk volume is filled with hydrate, depending on whether Archie's Law or the Hashin-Strikman bounds are used. This is representative of a reaction-dominated regime for hydrate emplacement, and where a significant low velocity zone exists based on a seismic tomography inversion, suggests large quantities of free gas below the BSR. Electrical resistivity logging while drilling (LWD) data show general agreement with the CSEM inversion model except for a CSEM-derived resistive region at seismic horizon A, known to transport free gas into the gas hydrate stability zone. Inversion of MT data collected simultaneously during the CSEM Survey provides a complimentary low-resolution image of the shallow sediments and shows folding in the accretionary complex sediments similar to that imaged by a tomographic seismic velocity model.
Karen Weitemeyer - One of the best experts on this subject based on the ideXlab platform.
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first results from a marine controlled source Electromagnetic Survey to detect gas hydrates offshore oregon
Geophysical Research Letters, 2006Co-Authors: Karen Weitemeyer, Kerry Key, Steven Constable, James BehrensAbstract:[1] Submarine gas hydrate is a hazard to drilling, a potential hydrocarbon resource, and has been implicated as a factor in both submarine slope stability and climate change. Bulk in situ electrical resistivities evaluated from Electromagnetic Surveys have the potential to provide an estimate of the total hydrate volume fraction more directly than by using seismic and well log data. We conducted a marine controlled-source Electromagnetic sounding at Hydrate Ridge, Oregon, USA, in August, 2004. Electromagnetic fields transmitted by a deep-towed horizontal electric dipole source were measured by a linear array of 25 seafloor Electromagnetic receivers, positioned 600 m apart to produce a dense coverage in the recorded electric field data. Results are presented in simple form by apparent resistivity pseudosections, which produce an approximate image of lateral resistivity variations across the study region. Resistivity values are consistent with those from well logs collected in the area and pseudosection features are correlated with seismic reflectors. Archie's Law, based on pseudosection apparent resistivities, predicts volumetric hydrate concentrations vary from 0–30% across the ridge.
Noriko Tada - One of the best experts on this subject based on the ideXlab platform.
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electrical conductivity imaging of the philippine sea upper mantle using seafloor magnetotelluric data
Physics of the Earth and Planetary Interiors, 2010Co-Authors: Kiyoshi Baba, Tadanori Goto, Hisashi Utada, Takafumi Kasaya, Hisayoshi Shimizu, Noriko TadaAbstract:Abstract We performed a three-year seafloor Electromagnetic Survey in the Philippine Sea, including the western edge of the Pacific Ocean, to image electrical features of a deep mantle slab stagnating in the transition zone and the surrounding mantle in three dimensions (3-D). The project iterated one-year deployment of ocean bottom electromagnetometers (OBEMs) using a total of 37 instruments installed at 18 sites. The data obtained have been analyzed in the order of their recovery based on a magnetotelluric (MT) method. In this study, we attempt to obtain a one-dimensional (1-D) electrical conductivity model beneath the Philippine Sea and the Pacific region separately that can be used as a reference model in the first step toward the 3-D analysis. The resultant 1-D models show three main features: (1) The conductivity in the shallower 200 km of the upper mantle depths of the two regions contrasts sharply, which is qualitatively consistent with the large difference in lithospheric age. (2) The conductivity at 200–300 km depth in both regions is more or less the same at approximately 0.3 S m −1 . (3) The conductivity just below 400 km depth is higher for the Philippine Sea mantle than for the Pacific mantle. The conductivity structure can be interpreted in terms of the thermal structure, mantle hydration, and existence of partial melt using experimental results for the conductivity of mantle minerals. If the conductivity is interpreted simply as the effect of temperature, the mantle beneath the Philippine Sea could be hotter than the dry solidus of mantle peridotite and thus partially molten. However, beneath the Pacific region, the present analysis suggests that the partial melting is not required under the assumed peridotitic composition even if we consider mantle hydration.