@booklet {GarciaX.MonteysX.EvansR.LandSzpak2014,
	title = {{Near seabed geology using a multidisciplinary geophysical approach in a shallow gas environment in the Malin Sea (NW Ireland)}},
	year = {2014},
	keywords = {accepted, Geochemistry, Geophysics, Geosystems},
	author = {Garcia, X. and Monteys, X. and Evans, R.L and Szpak, M.T.}
}
@article {Neukirch2014,
	title = {{Nonstationary magnetotelluric data processing with instantaneous parameter}},
	journal = {Journal of Geophysical Research: Solid Earth},
	volume = {119},
	number = {3},
	year = {2014},
	pages = {1634{\textendash}1654},
	issn = {21699313},
	doi = {10.1002/2013JB010494},
	url = {http://doi.wiley.com/10.1002/2013JB010494},
	author = {Neukirch, M. and Garcia, X.}
}
@article {Roux2014,
	title = {{Optimizing an experimental design for a CSEM experiment: methodology and synthetic tests}},
	journal = {Geophysical Journal International},
	volume = {197},
	number = {1},
	year = {2014},
	month = {jan},
	pages = {135{\textendash}148},
	abstract = {Optimizing an experimental design is a compromise between maximizing information we get about the target and limiting the cost of the experiment, providing a wide range of constraints. We present a statistical algorithm for experiment design that combines the use of linearized inverse theory and stochastic optimization technique. Linearized inverse theory is used to quantify the quality of one given experiment design while genetic algorithm (GA) enables us to examine a wide range of possible surveys. The particularity of our algorithm is the use of the multi-objective GA NSGA II that searches designs that fit several objective functions (OFs) simultaneously. This ability of NSGA II is helping us in defining an experiment design that focuses on a specified target area. We present a test of our algorithm using a 1-D electrical subsurface structure. The model we use represents a simple but realistic scenario in the context of CO2 sequestration that motivates this study. Our first synthetic test using a single OF shows that a limited number of well-distributed observations from a chosen design have the potential to resolve the given model. This synthetic test also points out the importance of a well-chosen OF, depending on our target. In order to improve these results, we show how the combination of two OFs using a multi-objective GA enables us to determine an experimental design that maximizes information about the reservoir layer. Finally, we present several tests of our statistical algorithm in more challenging environments by exploring the influence of noise, specific site characteristics or its potential for reservoir monitoring. {\textcopyright} The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.},
	keywords = {Electrical properties, Inverse theory, Magnetotelluric},
	issn = {0956-540X},
	doi = {10.1093/gji/ggt525},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-84897848257\&partnerID=tZOtx3y1 http://gji.oxfordjournals.org/content/early/2014/01/29/gji.ggt525.short},
	author = {Roux, E. and Garcia, X.}
}
@article {Nagel2013,
	title = {{Coupled Numerical Evaluations of the Geomechanical Interactions Between a Hydraulic Fracture Stimulation and a Natural Fracture System in Shale Formations}},
	journal = {Rock Mechanics and Rock Engineering},
	volume = {46},
	number = {3},
	year = {2013},
	month = {mar},
	pages = {581{\textendash}609},
	abstract = {Due to the low permeability of many shale reservoirs, multi-stage hydraulic fracturing in horizontal wells is used to increase the productive, stimulated reservoir volume. However, each created hydraulic fracture alters the stress field around it, and subsequent fractures are affected by the stress field from previous fractures. The results of a numerical evaluation of the effect of stress field changes (stress shadowing), as a function of natural fracture and geomechanical properties, are presented, including a detailed evaluation of natural fracture shear failure (and, by analogy, the generated microseismicity) due to a created hydraulic fracture. The numerical simulations were performed using continuum and discrete element modeling approaches in both mechanical-only and fully coupled, hydro-mechanical modes. The results show the critical impacts that the stress field changes from a created hydraulic fracture have on the shear of the natural fracture system, which in-turn, significantly affects the success of the hydraulic fracture stimulation. Furthermore, the results provide important insight into: the role of completion design (stage spacing) and operational parameters (rate, viscosity, etc.) on the possibility of enhancing the stimulation of the natural fracture network ({\textquoteright}complexity{\textquoteright}); the mechanisms that generate the microseismicity that occurs during a hydraulic fracture stimulation; and the interpretation of the generated microseismicity in relation to the volume of stimulated reservoir formation. {\textcopyright} 2013 Springer-Verlag Wien.},
	keywords = {Discrete element modeling, Horizontal well, Hydraulic fracturing, Multi-stage fracturing, Natural fractures, Numerical simulation, Stimulation, Stimulation optimization, Well completion},
	issn = {0723-2632},
	doi = {10.1007/s00603-013-0391-x},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-84877873776\&partnerID=tZOtx3y1},
	author = {Nagel, N. B. and Sanchez-Nagel, M. A. and Zhang, F. and Garcia, X. and Lee, B.}
}
@article {Szpak2012,
	title = {{Geophysical and geochemical survey of a large marine pockmark on the Malin Shelf, Ireland}},
	journal = {Geochemistry, Geophysics, Geosystems},
	volume = {13},
	number = {1},
	year = {2012},
	month = {jan},
	pages = {n/a{\textendash}n/a},
	abstract = {Marine pockmarks are a specific type of seabed geological setting resembling craters or pits and are considered seabed surface expressions of fluid flow in the subsurface. A large composite pockmark on the Malin Shelf, off the northern coast of Ireland was surveyed and ground truthed to assess its activity and investigate fluid related processes in the subsurface. Geophysical (including acoustic and electromagnetic) data confirmed the subsurface presence of signatures typical of fluids within the sediment. Shallow seismic profiling revealed a large shallow gas pocket and typical gas related indicators such as acoustic blanking and enhanced reflectors present underneath and around the large pockmark. Sulphate profiles indicate that gas from the shallow reservoir has been migrating upwards, at least recently. However, there are no chimney structures observed in the sub-bottom data and the migration pathways are not apparent. Electromagnetic data show slightly elevated electrical conductivity on the edges of the pockmarks and a drop below regional levels within the confines of the pockmark, suggesting changes in physical properties of the sediment. Nuclear Magnetic Resonance (NMR) experiments were employed to characterize the organic component of sediments from selected depths. Very strong microbial signatures were evident in all NMR spectra but microbes outside the pockmark appear to be much more active than inside. These observations coincide with spikes in conductivity and the lateral gas bearing body suggesting that there is an increase in microbial activity and biomass when gas is present. Copyright 2012 by the American Geophysical Union.},
	keywords = {electromagnetic, Malin Shelf, microbial, NMR, organic matter, pockmark},
	issn = {15252027},
	doi = {10.1029/2011GC003787},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-84856247401\&partnerID=tZOtx3y1},
	author = {Szpak, M. T. and Monteys, X. and O{\textquoteright}Reilly, S. and Simpson, A. J. and Garcia, X. and Evans, R. L. and Allen, C. C. R. and McNally, D. J. and Courtier-Murias, D. and Kelleher, B. P.}
}
@conference {Nagel2012b,
	title = {{A numerical evaluation of the geomechanical interactions between a hydraulic fracture stimulation and a natural fracture system}},
	booktitle = {46th US Rock Mechanics / Geomechanics Symposium 2012},
	volume = {3},
	year = {2012},
	pages = {1882{\textendash}1898},
	abstract = {Due to the low permeability of many shale gas reservoirs, multi-stage hydraulic fracturing in horizontal wells is used to increase the stimulated volume. However, each created hydraulic fracture alters the stress field around it, and subsequent fractures are affected by the stress field from the previous fractures, which results in higher net pressures, smaller fracture widths, and diminished microseismic emissions. The results of a numerical evaluation of the effect of stress shadowing, as a function of natural fracture and geomechanical properties, are presented, including a detailed evaluation of natural fracture shear failure (and, by analogy, the microseismicity) due to a created hydraulic fracture using both continuum and discrete element modeling approaches. The results show the critical impact that a created hydraulic fracture has on the shear of the natural fracture system, which in-turn, significantly affects the success of the stimulation. Furthermore, the results provide important insight into the mechanisms that generate the microseismicity that occurs during a hydraulic fracture stimulation. Copyright 2012 ARMA, American Rock Mechanics Association.},
	isbn = {9781622765140},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-84873277033\&partnerID=tZOtx3y1},
	author = {Nagel, N. B. and Sanchez-Nagel, M. A. and Garcia, X. and Lee, B.}
}
@article {Miensopust2011,
	title = {{Lithospheric structures and Precambrian terrane boundaries in northeastern Botswana revealed through magnetotelluric profiling as part of the Southern African Magnetotelluric Experiment}},
	journal = {Journal of Geophysical Research},
	volume = {116},
	number = {B2},
	year = {2011},
	month = {feb},
	pages = {B02401},
	abstract = {Within the framework of the Southern African Magnetotelluric Experiment a focused study was undertaken to gain improved knowledge of the lithospheric geometries and structures of the westerly extension of the Zimbabwe craton (ZIM) into Botswana, with the overarching aim of increasing our understanding of southern African tectonics. The area of interest is located in northeastern Botswana, where Kalahari sands cover most of the geological terranes and very little is known about lithospheric structures and thicknesses. Some of the regional-scale terrane boundary locations, defined based on potential field data, are not sufficiently accurate for local-scale studies. Investigation of the NNW-SSE orientated, 600 km long ZIM line profile crossing the Zimbabwe craton, Magondi mobile belt, and Ghanzi-Chobe belt showed that the Zimbabwe craton is characterized by thick (\~{}220 km) resistive lithosphere, consistent with geochemical and geothermal estimates from kimberlite samples of the nearby Orapa and Letlhakane pipes (\~{}175 km west of the profile). The lithospheric mantle of the Ghanzi-Chobe belt is resistive, but its lithosphere is only about 180 km thick. At crustal depths a northward dipping boundary between the Ghanzi-Chobe and the Magondi belts is identified, and two middle to lower crustal conductors are discovered in the Magondi belt. The crustal terrane boundary between the Magondi and Ghanzi-Chobe belts is found to be located further to the north, and the southwestern boundary of the Zimbabwe craton might be further to the west, than previously inferred from the regional potential field data. Copyright 2011 by the American Geophysical Union.},
	issn = {0148-0227},
	doi = {10.1029/2010JB007740},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-79951472921\&partnerID=tZOtx3y1},
	author = {Miensopust, M. P. and Jones, A. G. and Muller, M. R. and Garcia, X. and Evans, R. L.}
}
@article {Garcia2010b,
	title = {{Internal structure of the western flank of the Cumbre Vieja volcano, La Palma, Canary Islands, from land magnetotelluric imaging}},
	journal = {Journal of Geophysical Research},
	volume = {115},
	number = {B7},
	year = {2010},
	month = {jul},
	pages = {B07104},
	abstract = {Large-scale mass wasting is a natural part of the evolution of volcanic islands, where deformation and indications of flank instability, such as large-scale faulting and seismic and aseismic slip are common. The Cumbre Vieja volcano on the island of La Palma (Canary Islands) provides an ideal setting to address fundamental questions about the structure, evolution and stability of island volcanoes. The island of La Palma is still in a shield-building stage, and it has been postulated that the western side of the island lies over a pre-existing zone of weakness that can nucleate ruptures. We undertook an audiomagnetotelluric (AMT) survey over the proposed unstable western flank to try to image structures that may be associated with the zone of weakness. The magnetotelluric method (MT) is a geophysical technique used to map the presence of fluids or image important structural contrasts. The goals of this study were (1) to delineate the unstable flank, (2) to map the structures underneath and (3) to determine the presence and geometry of fluids. The results show a 1 km thick top resistive layer overlaying an area of reduced resistivity (enhanced conductivity), interpreted as a layer consisting of an alteration zone and also fluids. Our results confirm previous studies that suggested the existence of a western flank lying over collapse debris material and hyaloclastites, and also they allow us to map part of the subaerial southern extent of the Cumbre Nueva units that lie beneath the more recent Cumbre Vieja rocks. In addition, dimensionality analysis maps the rotation of the dike emplacement off ridge, along the western flank in an en echelon fashion.Copyright 2010 by the American Geophysical Union.},
	issn = {0148-0227},
	doi = {10.1029/2009JB006445},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-77955204988\&partnerID=tZOtx3y1},
	author = {Garcia, X. and Jones, A. G.}
}
@article {Munjiza2010,
	title = {{The Virtual Geoscience Workbench, VGW: Open Source tools for discontinuous systems}},
	journal = {Particuology},
	volume = {8},
	number = {2},
	year = {2010},
	month = {apr},
	pages = {100{\textendash}105},
	abstract = {In 1989 work began on merging finite and discrete elements. During 1990 algorithmic solutions for the new combined method were developed, together with a C-based implementation known as the Y code. In 2004 these developments and implementations were published as a textbook. Queen Mary University of London (QMUL) and Imperial College are currently engaged in a research project to make Y codes in both 2D and 3D available in Open Source format and with mainly geoscience problems in mind. The result is a set of Open Source tools for Geoscience (VGW), key components of which are summarised in this paper, together with illustrative range of simulation results. {\textcopyright} 2009 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences.},
	keywords = {Combined finite-discrete element, Geoscience, Modelling, Workbench},
	issn = {16742001},
	doi = {10.1016/j.partic.2009.04.008},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-77949485463\&partnerID=tZOtx3y1},
	author = {Munjiza, A. and XIANG, J. and Garcia, X. and Latham, J.P. and D{\textquoteright}Albano, G.G. Schiava and John, N.W.M.}
}
@article {GARCIA2009,
	title = {{A clustered overlapping sphere algorithm to represent real particles in discrete element modelling}},
	journal = {G{\'e}otechnique},
	volume = {59},
	number = {9},
	year = {2009},
	month = {jan},
	pages = {779{\textendash}784},
	abstract = {An algorithm is presented for representing irregular particles as clusters of overlapping spheres, for use within discrete element method (DEM) simulations of particulates. The algorithm is sufficiently fast to be implemented on desktop computers. Although volume and moment of inertia comparisons of resulting clusters with prototypical geometric data show that in the region of 50 to 100 spheres may be needed to represent a particle, actual DEM simulations of an unstable pile of simulated particles show that only 10 or so spheres may be sufficient to capture the mechanics of the system.},
	keywords = {Numerical modelling},
	issn = {0016-8505},
	doi = {10.1680/geot.8.T.037},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-77949702642\&partnerID=tZOtx3y1},
	author = {Garcia, X. and XIANG, J. and LATHAM, J.-P. and HARRISON, J.P.}
}
@article {Muller2009,
	title = {{Lithospheric structure, evolution and diamond prospectivity of the Rehoboth Terrane and western Kaapvaal Craton, southern Africa: Constraints from broadband magnetotellurics}},
	journal = {Lithos},
	volume = {112},
	year = {2009},
	month = {nov},
	pages = {93{\textendash}105},
	abstract = {A 1400~km-long, 2-D magnetotelluric (MT) profile, consisting of 69 sites at 20~km intervals, across the western part of the Archaean Kaapvaal Craton, the Proterozoic Rehoboth Terrane and the Late Proterozoic/Early Phanerozoic Ghanzi-Chobe/Damara Belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern African lithosphere. The lithospheric structures of the Rehoboth Terrane and Ghanzi-Chobe/Damara Belt have not been imaged previously by geophysical methods. Temperature is the primary control on the resistivity of mantle minerals, and the MT derived lithospheric thicknesses therefore provide a very reasonable proxy for the "thermal" thickness of the lithosphere (i.e., the thickness defined by the intersection of a conductive geotherm with the mantle adiabat), allowing approximate present-day geotherms to be calculated. The work indicates the following present-day average lithospheric thicknesses, to a precision of about {\textpm} 20~km, for each of the terranes traversed (inferred geotherms in brackets): Eastern Kimberley Block of the Kaapvaal Craton 220~km (41~mW m- 2), Western Kimberley Block 190~km (44~mW m- 2), Rehoboth Terrane 180~km (45~mW m- 2) and Ghanzi-Chobe/Damara Belt 160~km (48~mW m- 2). A clear relationship between the electrical resistivity structure of the lithosphere and the tectonic stabilisation-age of the terrane is evident. Good agreement between the inferred present-day lithospheric geotherms and surface heat flow measurements indicate the latter are strongly controlled by variations in lithospheric thickness. A significant difference in lithospheric thickness is observed between the Eastern and Western Kimberley blocks, and is consistent with previous seismic tomography images of the Kaapvaal Craton. The present-day lithospheric thickness, and reduced depth extent into the diamond stability field, accounts for the absence of diamondiferous kimberlites in the Gibeon and Gordonia kimberlite fields in the Rehoboth Terrane. Previously published mantle xenolith P-T arrays from the Gibeon, Gordonia and Kimberley fields, however, suggest that the Rehoboth Terrane had equilibrated to a cooler conductive palaeo-geotherm (40-42~mW m- 2 ), very similar to that of Eastern Kimberley Block of the Kaapvaal Craton, at some time prior to the Mesozoic eruption of the kimberlites. The timing and nature of both the thermal equilibration of the Rehoboth Terrane, and the subsequent lithospheric heating/thinning event required to account for its present-day lithospheric structure, are not well constrained. A model consisting of the penetration of heat transporting magmas into the lithosphere, with associated chemical refertilisation, at an early stage of Mesozoic thermalism appears to be the most plausible model at present to account for both the present-day lithospheric structure of the Rehoboth Terrane and an earlier, cooler palaeo-geotherm. Some problems, however, remain unresolved in terms of the isostatic response of the model. Based on a compilation of xenocryst Cr/Ca-in-pyrope barometry observations, the extent of depleted mantle in the Rehoboth Terrane is found to be significantly reduced with respect to the Eastern Kimberley Block: 117~km versus 138-167~km. It appears most likely that the depletion depth in both terranes, at least in the vicinity of kimberlite eruption, is explained by refertilisation of the lower lithospheric mantle. {\textcopyright} 2009.},
	keywords = {Diamond, Geotherm, Lithosphere, Magnetotellurics, Southern Africa, Xenolith},
	issn = {00244937},
	doi = {10.1016/j.lithos.2009.06.023},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-72049130282\&partnerID=tZOtx3y1},
	author = {Muller, M.R. and Jones, A.G. and Evans, R.L. and Gr{\"u}tter, H.S. and Hatton, C. and Garcia, X. and Hamilton, M.P. and Miensopust, M.P. and Cole, P. and Ngwisanyi, T. and Hutchins, D. and Fourie, C.J. and Jelsma, H.A. and Evans, S.F. and Aravanis, T. and Pettit, W. and Webb, S.J. and Wasborg, J.}
}
@conference {Monteys2009,
	title = {{Shallow geophysical characterization and fluid flow processes in two large pockmarks on the Malin Shelf, NW Ireland}},
	booktitle = {Near Surface 2009 - 15th European Meeting of Environmental and Engineering Geophysics},
	year = {2009},
	publisher = {European Association of Geoscientists and Engineers, EAGE},
	organization = {European Association of Geoscientists and Engineers, EAGE},
	abstract = {Shallow geophysical datasets and ground-truthing have been used in this research to characterize in detail two large gas related depressions in a recently discovered pockmark field on the Malin Shelf, northwest Ireland. Pockmarks are aligned to the main deep fault of the region, the SW-NE Skerryvore Fault. High resolution multibeam bathymetry and backscatter reveal the surface morphology of these seabed features to the meter scale. They appear as subcircular depressions and present several units within a generally smooth seabed. Shallow seismic and single-beam echosounder records reveal evidence of gas related activity within the subsurface strata. Electromagnetic results present a number of anomalies in conductivity associated with inhomogenities in the upper strata. These anomalies consist in a decrease in conductivity and in general, correlate well with acoustic anomalies.},
	isbn = {9073781728},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-84912149526\&partnerID=tZOtx3y1},
	author = {Monteys, X. and Garcia, X. and Evans, R. and Szpak, M. and Kelleher, B. and Hardy, D.}
}
