Extensive perched aquifer and structural implications revealed by 3D resistivity mapping in a Galapagos volcano

d’Ozouville, Noemí; Auken, Esben; Sørensen, Karsten Engsig; Violette, Sophie; Marsily, Ghislain de; Deffontaines, Benoı̂t; Merlen, Godfrey

doi:10.1016/j.epsl.2008.03.011

Cited by 62 publications

(36 citation statements)

References 21 publications

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“…Although we have few constraints on groundwater hydrology of Sierra Negra, subsurface resistivity mapping of the upper ~ 2 km of Santa Cruz Island to the east reveals perched aquifers and saline incursions into the edifice, demonstrating the variable porosities and permeabilities of the layers. These studies also indicate that faults and/or impermeable dike zones control the structure of the edifice [ d ' Ozouville et al ., ]. By analogy, perched aquifers may occur on the southern, drip‐forest side of Sierra Negra and Cerro Volcanoes.…”

Section: Discussionmentioning

confidence: 99%

Imaging rapidly deforming ocean island volcanoes in the western Galápagos archipelago, Ecuador

et al. 2014

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Using local body wave arrival-time tomography methods to determine 3-D seismic velocity structure, we imaged the plumbing system of Sierra Negra Volcano, Galápagos. This hot spot volcanic chain includes some of the fastest deforming volcanoes in the world, making this an ideal location to study shield volcano plumbing systems. We inverted P and S wave arrivals recorded on a 15-station temporary array between July 2009 and June 2011 using an a priori 1-D velocity model constrained by offshore refraction studies. With local seismicity from nearby volcanoes as well as the ring fault system, the model resolution is good between depths of 3 and 15.5 km. The propagation of S waves throughout this volume argues against any large high-melt accumulations, although a shallow melt sill may exist above 5 km. We image a broad low-velocity region (>25 km laterally) below Sierra Negra at depths~8-15 km. No large, regional velocity increase is found within the limits of good resolution, suggesting that crust is thicker than 15 km beneath the western Galápagos archipelago. Our results are consistent with crustal accretion of mafic cumulates from a large-volume magma chamber that may span the boundary between preplume and accreted crust. The similarity between our results and those of Hawaii leave open the possibility that the crust has also been thickened by under-plating.

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Section: Discussionmentioning

confidence: 99%

Imaging rapidly deforming ocean island volcanoes in the western Galápagos archipelago, Ecuador

et al. 2014

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“…Several systems have been developed, measuring in the frequency-domain such as Resolve and DigHEM by Fugro Airborne Surveys (e.g. Mullen et al, 2007 andSiemon, 2009), and in the time-domain, such as SkyTEM (Sørensen and Auken, 2004) or VTEM by Geotech Airborne Inc. AEM surveys have been successfully conducted during the past decades for risk assessment and hydrogeological prospection in various geological contexts (Bosch et al, 2009;Mogi et al, 2009;Steuer et al, 2009;Supper et al, 2009;Viezzoli et al, 2010b), including volcanic (Auken et al, 2009b;Finn et al, 2001Finn et al, , 2007d'Ozouville et al, 2008). In most of the above-mentioned studies, geophysical data have been interpreted with a substantial set of drill hole and geological data available prior to the survey.…”

Section: A Pryet Et Al: Hydrogeological Settings Of a Volcanic Islamentioning

confidence: 99%

Hydrogeological settings of a volcanic island (San Cristóbal, Galapagos) from joint interpretation of airborne electromagnetics and geomorphological observations

Pryet

d’Ozouville

Violette

et al. 2012

Hydrol. Earth Syst. Sci.

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Abstract. Many volcanic islands face freshwater stress and the situation may worsen with climate change and sea level rise. In this context, an optimum management of freshwater resources becomes crucial, but is often impeded by the lack of data. With the aim of investigating the hydrogeological settings of southern San Cristóbal Island (Galapagos), we conducted a helicopter-borne, transient electromagnetic survey with the SkyTEM system. It provided unprecedented insights into the 3-D resistivity structure of this extinct basaltic shield. Combined with remote sensing and fieldwork, it allowed the definition of the first hydrogeological conceptual model of the island. Springs are fed by a series of perched aquifers overlying a regional basal aquifer subject to seawater intrusion. Dykes, evidenced by alignments of eruptive cones at the surface, correspond to sharp sub-vertical contrasts in resistivity in the subsurface, and impound groundwater in a summit channel. Combined with geomorphological observations, airborne electromagnetics are shown to be a useful for hydrogeological exploratory studies in complex, poorly known environments. They allow optimal development of land-based geophysical surveys and drilling campaigns.

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“…This is even more puzzling given the very long tradition of inductive methods in groundwater resources evaluations (e.g., Fitterman and Stewart, 1986;Tezkan, 1999;d'Ozouville et al, 2008). One possible reason relates to the success and flexibility of ERT for this type of applications (e.g., Kemna et al, 2002).…”

Section: Introductionmentioning

confidence: 99%

Focused time-lapse inversion of radio and audio magnetotelluric data

Rosas‐Carbajal¹,

Linde²,

Kalscheuer³

2012

Journal of Applied Geophysics

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Geoelectrical techniques are widely used to monitor groundwater processes, while surprisingly few studies have considered audio (AMT) and radio (RMT) magnetotellurics for such purposes. In this numerical investigation, we analyze to what extent inversion results based on AMT and RMT monitoring data can be improved by (1) time-lapse difference inversion; (2) incorporation of statistical information about the expected model update (i.e., the model regularization is based on a geostatistical model); (3) using alternative model norms to quantify temporal changes (i.e., approximations of l 1 and Cauchy norms using iteratively reweighted least-squares), (4) constraining model updates to predefined ranges (i.e., using Lagrange Multipliers to only allow either increases or decreases of electrical resistivity with respect to background conditions). To do so, we consider a simple illustrative model and a more realistic test case related to seawater intrusion. The results are encouraging and show significant improvements when using time-lapse difference inversion with non l 2 model norms. Artifacts that may arise when imposing compactness of regions with temporal changes can be suppressed through inequality constraints to yield models without oscillations outside the true region of temporal changes. Based on these results, we recommend approximate l 1 -norm solutions as they can resolve both sharp and smooth interfaces within the same model.

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Extensive perched aquifer and structural implications revealed by 3D resistivity mapping in a Galapagos volcano

Cited by 62 publications

References 21 publications

Imaging rapidly deforming ocean island volcanoes in the western Galápagos archipelago, Ecuador

Imaging rapidly deforming ocean island volcanoes in the western Galápagos archipelago, Ecuador

Hydrogeological settings of a volcanic island (San Cristóbal, Galapagos) from joint interpretation of airborne electromagnetics and geomorphological observations

Focused time-lapse inversion of radio and audio magnetotelluric data

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