217 research outputs found
Analysis of marine controlled source electromagnetic data for the assessment of gas hydrates in the Danube deep-sea fan, Black Sea
No full textMarine controlled source electromagnetic (CSEM) data have been analyzed as part of a larger interdisciplinary field study to reveal the distribution and concentration of gas hydrates and free gas in two working areas (WAs) in the offshore Danube fan in the western Black Sea. The areas are located in the Bulgarian sector in about 1500 m water depth (WA1) and in the Romanian sector in about 650 m water depth (WA2). Both areas are characterized by channel levee systems and wide spread occurrences of multiple bottom simulating reflections (BSRs) suggesting the presence of gas hydrates. Electrical resistivity models have been derived from two-dimensional (2D) inversions of inline CSEM data using a seafloor-towed electric dipole-dipole system. Comparing the resistivity models with coincident reflection seismic profiles reveals insight in the sediment stratigraphy of the gas hydrate stability zone (GHSZ). Gas hydrate and free gas saturation estimates have been derived with a stochastic approach of Archie's relationship considering uncertainties in the input parameters available from drilling with the MeBo-200 seafloor rig in WA2. The resistivity models generally reflect the transition of marine to lacustrine conditions expressed by a sharp decay of pore water salinities in the top 30–40 m below seafloor caused by freshwater phases of the Black Sea due to sea level low stands in the past. In WA1, we derived saturation estimates of 10–20% within a 100 m thick layer at around 50 m depth below the channel which compares well with estimates from seismic P-wave velocities. The layer extends below the western levee with even higher saturations of 20–30%, but high gas hydrate saturations are unlikely within the fine grained, clayey sediment section, and the high resistivities may reflect different lithologies of lower permeability and porosity. The resistive layer terminates below the eastern levee where increasing resistivities at depth towards a stack of multiple BSRs indicate gas hydrate and free gas concentrations in the order of 10% to locally 30%. WA2 is characterized by a major slope failure at the landward edge of the gas hydrate stability field next to the channel. Gas hydrate saturation estimates within the slump area are close to zero within the GHSZ which is in agreement with coring results of the nearby MeBo drill sites. Elevated resistivities below the steeply upward bending BSR lead to saturation estimates less than 10% of free gas that may have accumulated
Model-Based Identification of the Base of the Gas Hydrate Stability Zone in Multichannel Reflection Seismic Data, Offshore Costa Rica
No full textAlong the pacific margin offshore Costa Rica the Bottom Simulating Reflector (BSR) shows a patchy occurrence in 2-D seismic reflection profiles. The reason for this can be either lack of free gas beneath parts of the gas hydrate stability zone (GHSZ) or poor seismic imaging. We compare far to near offset stacked common midpoint sections to reduce imaging ambiguity utilizing the amplitude variation with offset effect and thus successfully distinguish BSRs from regular sediment reflections. In combination with 1-D modeling of the base of the GHSZ we disqualify or qualify reflections in the predicted depth range as BSR. Additionally we calculate the heat flow and compare it with an analytical solution to detect thermal anomalies, for example, at the frontal prism. The higher confidence in BSR depths based on the far offset stacks and heat flow calculations allows further analyses on gas hydrate concentration estimates and tectonic evolution of the margin
Seafloor massive sulphide exploration using deep-towed controlled source electromagnetics: Navigational uncertainties
No full textDeep-towed geophysical surveys require precise knowledge of navigational parameters such as instrument position and orientation because navigational uncertainties reflect in the data and therefore in the inferred geophysical properties of the subseafloor. We address this issue for the case of electrical conductivity inferred from controlled source electromagnetic data. We show that the data error is laterally variable due to irregular motion during deep towing, but also due to lateral variations in conductivity, including those resulting from topography. To address this variability and quantify the data error prior to inversion, we propose a 2-D perturbation study. Our workflow enables stable and geologically reliable results for multicomponent and multifrequency inversions. An error estimation workflow is presented, which comprises the assessment of navigational uncertainties, perturbation of navigational parameters, and forward modelling of electric field amplitudes for a homogeneous and then a heterogeneous subseafloor conductivity model. Some navigational uncertainties are estimated from variations of direct measurements. Other navigational parameters required for inversion are derived from the measured quantities and their error is calculated by means of error propagation. Some navigational parameters show direct correlation with the measured electric fields. For example, the antenna dip correlates with the vertical electric field and the depth correlates with the horizontal electric field. For the perturbation study each standard deviation is added to the navigational parameters. Forward models are run for each perturbation. Amplitude deviations are summed in quadrature with the stacking error for a total, laterally varying, data error. The error estimation is repeated for a heterogeneous subseafloor model due to the large conductivity range (several orders of magnitude), which affects the forward model. The approach enables us to utilize data from several components (multiple electric fields, frequencies and receivers) in the inversion to constrain the final model and reduce ambiguity. The final model is geologically reasonable, in this case enabling the identification of conductive metal sulphide deposits on the seafloor.</p
Marine dipole–dipole controlled source electromagnetic and coincident-loop transient electromagnetic experiments to detect seafloor massive sulphides: effects of three-dimensional bathymetry
No full textSeafloor massive sulphides (SMSs) are regarded as a potential future resource to satisfy the growing global demand of metals including copper, zinc and gold. Aside from mining and retrieving profitable amounts of massive sulphides from the seafloor, the present challenge is to detect and delineate significant SMS accumulations, which are generally located near mid-ocean ridges and along submarine volcanic arc and backarc spreading centres. Currently, several geophysical technologies are being developed to detect and quantify SMS occurrences that often exhibit measurable contrasts in their physical parameters compared to the surrounding host rock. Here, we use a short, fixed-offset controlled source electromagnetic (CSEM) system and a coincident-loop transient electromagnetic (TEM) system, which in theory allow the detection of SMS in the shallow seafloor due to a significant electrical conductivity contrast to their surroundings. In 2016, CSEM and TEM experiments were carried out at several locations near the Trans- Atlantic Geotraverse hydrothermal field to investigate shallow occurrences of massive sulphides below the seafloor. Measurements were conducted in an area that contains distinct SMS sites located several kilometres off-axis from the Mid-Atlantic ridge, some of which are still connected to hydrothermal activity and others where hydrothermal activity has ceased. Based on the quality of the acquired data, both experiments were operationally successful. However, the data analysis indicates bias caused by three-dimensional (3D) effects of the rough bathymetry in the study area and, thus, data interpretation remains challenging. Therefore, we study the influence of 3D bathymetry for marine CSEM and TEM experiments, focusing on shallow 3D conductors located beneath mound-like structures.We analyse synthetic inversion models for attributes associated with 3D distortions of CSEM and TEM data that are not sufficiently accounted for in conventional 1D (TEM) and 2D (CSEM) interpretation schemes. Before an adequate quantification of SMS in the region is feasible, these 3D effects need to be studied to avoid over/underestimation of SMS using the acquired EM data. The sensitivity of CSEM and TEM to bathymetry is investigated by means of 3D forward modelling, followed by 1D (TEM) and 2D (CSEM) inversion of the synthetic data using realistic error conditions. Subsequently, inversion models of the synthetic 3D data are analysed and compared to models derived from the measured data to illustrate that 3D distortions are evident in the recorded data sets
Combined interpretation of marine controlled source electromagnetic and reflection seismic data in the German North Sea: a case study
No full textThis paper focuses on the combined analysis and interpretation of
controlled source electromagnetic (CSEM) and multichannel reflection
seismic (MCS) data along one profile in the German North Sea with the
goal to reduce ambiguities in interpretation. The shallow water
environment of the North Sea is characterized by a complex geological
development which includes rifting, several ice age cycles, a
propagating shelf margin, mass-transport deposits and salt dome
formation. Seismic and electromagnetic methods are sensitive to
different physical properties of the seabed and therefore complement
each other. We analyse the MCS data with a migration velocity tomography
and an amplitude variation with offset analysis and discuss seismic
velocities and densities. For true amplitude recovery the amplitude
distortions are calibrated with in situ logging data. The CSEM data are
analysed in 2-D, for which, for the first time, data were included that
were acquired while the instrument was towed on the seafloor in addition
to the stationary sites. The CSEM inversions are constrained by seismic
horizons. The joint interpretation focuses on two seismic reflectors:
One can be interpreted as an unconformity marking a lithological change
from fresh water-bearing glacial deposits to compacted sediments below,
and the other one as a layer of fine-grained deposits potentially
capping patchy shallow gas occurrences. This exemplary case study shows
how the combination of both methods can benefit by interpreting complex
geology and eliminating ambiguous explanations
Trans-dimensional Bayesian inversion of controlled-source electromagnetic data in the German North Sea
No full textThis paper presents the first controlled-source electromagnetic survey carried out in the German North Sea with a recently developed seafloor-towed electrical dipole–dipole system, i.e., HYDRA II. Controlled-source electromagnetic data are measured, processed, and inverted in the time domain to estimate an electrical resistivity model of the sub-seafloor. The controlled-source electromagnetic survey targeted a shallow, phase-reversed, seismic reflector, which potentially indicates free gas. To compare the resistivity model to reflection seismic data and draw a combined interpretation, we apply a trans-dimensional Bayesian inversion that estimates model parameters and uncertainties, and samples probabilistically over the number of layers of the resistivity model. The controlled-source electromagnetic data errors show time-varying correlations, and we therefore apply a non-Toeplitz data covariance matrix in the inversion that is estimated from residual analysis. The geological interpretation drawn from controlled-source electromagnetic inversion results and borehole and reflection seismic data yield resistivities of ?1 ?m at the seafloor, which are typical for fine-grained marine deposits, whereas resistivities below ?20 mbsf increase to 2–4 ?m and can be related to a transition from fine-grained (Holocene age) to unsorted, coarse-grained, and compacted glacial sediments (Pleistocene age). Interface depths from controlled-source electromagnetic inversion generally match the seismic reflector related to the contrast between the different depositional environments. Resistivities decrease again at greater depths to ?1 ?m with a minimum resistivity at ?300 mbsf where a seismic reflector (that marks a major flooding surface of late Miocene age) correlates with an increased gamma-ray count, indicating an increased amount of fine-grained sediments. We suggest that the grain size may have a major impact on the electrical resistivity of the sediment with lower resistivities for fine-grained sediments. Concerning the phase-reversed seismic reflector that was targeted by the survey, controlled-source electromagnetic inversion results yield no indication for free gas below it as resistivities are generally elevated above the reflector. We suggest that the elevated resistivities are caused by an overall decrease in porosity in the glacial sediments and that the seismic reflector could be caused by an impedance contrast at a thin low-velocity layer. Controlled-source electromagnetic interface depths near the reflector are quite uncertain and variable. We conclude that the seismic interface cannot be resolved with the controlled-source electromagnetic data, but the thickness of the corresponding resistive layer follows the trend of the reflector that is inclined towards the west
Educación Jurídica Internacional
No full text"Originally published as International Legal Education, in 26 Harv. Int'l L. J. 361 (1985). Translation by Public Translator Romina Redondo. The editors gratefully acknowledge the generosity of the author and the authorities of the Harvard International Law Journal in allowing the translation and reproduction of the article."Fil: Kennedy, David. Harvard University. Law School. Institute for Global Law and Policy. Cambridge, Estados Unidos"Publicado originalmente como International Legal Education, en 26 Harv. Int'l L. J. 361 (1985). Traducción a cargo de la Trad. Públ. Romina Redondo. Los editores agradecen la generosidad del autor y de las autoridades del Harvard International Law Journal al permitir la traducción y reproducción del artículo"
Marine mineral exploration with controlled‐source electromagnetics at the TAG hydrothermal field, 26°N Mid‐Atlantic Ridge
No full textSeafloor massive sulphide (SMS) deposits are of increasing economic interest in order to satisfy the relentless growth in worldwide metal demand. The Trans‐Atlantic Geotraverse (TAG) hydrothermal field at 26°N on the Mid‐Atlantic Ridge hosts several such deposits. This study presents new controlled source electromagnetic (CSEM), bathymetric and magnetic results from the TAG field. Potential SMS targets were selected based on their surface expressions in high‐resolution bathymetric data. High‐resolution reduced‐to‐the‐pole magnetic data show negative anomalies beneath and surrounding the SMS deposits, revealing large areas of hydrothermal alteration. CSEM data, sensitive to the electrical conductivity of SMS mineralization, further reveal a maximum thickness of up to 80 m and conductivities of up to 5 S/m. SMS samples have conductivities of up to a few thousand S/m, suggesting that remotely inferred conductivities represent an average of metal sulphide ores combined with silicified and altered host basalt that likely dominates at greater depths
Pathways of Peace. The Philosophy and Sociology of Peace and Nonviolence.
No full textThis book explores the close interconnection that exists between sociological and philosophical scholarship in relation to peace studies. Through an examination of the thought of nine leading philosophers and sociologists in their historical and geographical context, the author considers notions of nonviolent resistance, pacifism and reverse strike, as well as violence theories of conflict, theories of conflict resolution, the problem of war, and political transitions towards democratization. Engaging with the work of Thoreau, Gandhi, Ghaffar Khan, Capitini, Dolci, Bobbio, Galtung, Sharp and Weil, and considering the institutionalisation of peace research, this volume will appeal to scholars and students of sociology, politics and philosophy with interest in peace and security studies, and conflict
Marine CSEM synthetic study to assess the detection of CO2 escape and saturation changes within a submarine chimney connected to a CO2 storage site.
No full textCarbon capture and storage (CCS) within sealed geologic formations is an essential strategy to reduce global greenhouse gas emissions, the primary goal of the 2015 United Nations Paris Agreement. Large-scale commercial development of geological CO2 storage requires high-resolution remote sensing methods to monitor CO2 migration during/after injection. A geologic formation containing a CO2 phase in its pore space commonly exhibits higher electrical resistivity than brine-saturated (background) sediments. Here, we explore the added value of the marine controlled-source electromagnetic (CSEM) method as an additional and relevant geophysical tool to monitor moderate to significant changes in CO2 saturation within a fluid conduit breaking through the seal of a CCS injection reservoir, using a suite of synthetic studies. Our 2D CSEM synthetic models simulate various geologic scenarios incorporating the main structural features and stratigraphy of two North Sea sites, the Scanner Pockmark and the Sleipner CCS site. Our results show significant differentiation of leakage through the seal with CO2 saturation (SCO2 ) ranging between 20 and 50 per cent, while our rock physics model predicts that detection below 20 per cent would be challenging for CSEM alone. However, we are able to detect with our 2D inversion models the effects of saturation with 10 and 20 per cent CO2 within a chimney with 10 per cent porosity.
We demonstrate that simultaneous inversion of Ey and Ez synthetic electric field data facilitates a sharper delineation of a CO2 saturated chimney structure within the seal, whereas Ez synthetic data present higher sensitivity than Ey to SCO2 variation, demonstrating the importance of acquiring the whole 3D electric field. This study illustrates the value of incorporating CSEM into measurement, monitoring, and verification (MMV) strategies for operating marine CCS sites optimally
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