An integrated study of the hydrogeology of volcanic islands using helicopter borne transient electromagnetic: Application in the Galápagos Archipelago

Abstract

The hydrology of volcanic islands is poorly characterized due to their complex internal structure and challenging access. The Galápagos Islands, an isolated basaltic environment with unique ecosystems and growing anthropogenic pressure, suffer from scarcity of freshwater resources and from the lack of fundamental knowledge on their hydrology. To overtake these constraints and provide fresh water to the population, a geophysical survey and a hydroclimatologic network at watershed scale have been performed over Santa Cruz. An innovative helicopter borne transient electromagnetic method permitted a quasi-3D (an inversion scheme allowing for a 3D model description but with a local 1D forward algorithm) resistivity mapping of the massif and the identification of its hydrogeological potential. The latter is composed of a perched aquifer on the windward mountainside and a basal aquifer. Our hydroclimatologic network provided useful data records and allowed us to characterize the hydrodynamic properties of the basal aquifer. The geophysical data allow us to precise its extension. The conceptual hydrogeological model proposed for Santa Cruz confirms one of the models proposed for Le Piton de la Fournaise (Réunion Island). It has now to be confirmed with boreholes and a detailed geochemical study including noble gas analysis. To cite this article: E.

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

“…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.…”

“…The data filtering is designed to enhance near-surface resistivity variations by avoiding any smoothing of the early-time data, while late-time data are more severely filtered to obtain as much penetration depth as possible (Auken et al, 2009a).…”

“…The bulk of volcanic edifices is composed by lava flows and tephra deposits, which are intruded by dykes in vent zones (Bardintzeff and McBirney, 2006). Dykes can form impervious sub-vertical barriers to groundwater flow, while red "baked" soils and tuff deposits may form impervious layers sub-parallel to the topography (Custodio, 2004;Singhal and Gupta, 2010).…”

Improved Geoarchaeological Mapping with Electromagnetic Induction Instruments from Dedicated Processing and Inversion

“…The 1D forward response is very fast with the computation of roughly 1000 model After processing, the soundings were inverted. The backbone of the inversion routine is the fully non-linear AarhusInv code [39], which is used extensively for ground-based EM data, as well as airborne [40][41][42]. The forward response is 1D without approximations, such as the LIN approximation.…”

3D resistivity gridding of large AEM datasets: A step toward enhanced geological interpretation

“…Due to the nature of airborne surveys, the distribution of soundings is inhomogeneous, with a high density of soundings along flight lines (one sounding every 10 meters), and gaps of data in between (usually 250 meters wide). Results revealed interesting buried low-resistivity bodies in Santa Cruz, presented by d 'Ozouville et al (2008a) and Auken et al (2009). The soundings have been newly processed and inverted using the Spatially Constrained Inversion scheme (SCI) (Viezzoli et al, 2008) to a 19 layer "smooth" resistivity model, where the layer thicknesses are distributed logarithmically from the surface down to 250 m below topography.…”