Innovative downhole geophysical methods for high frequency seawater intrusion dynamics monitoring

The detailed characterization of salt water intrusion is a key to understand both submarine groundwater discharge and manage often intensively exploited groundwater resources in coastal areas. With the objective to study the response of a coastal aquifer to a series of boundary conditions, a new experimental site has been developed through a clastic aquifer located north of Barcelona (Spain). This hectometer scale site is located 50 m from the seashore and equipped with 17 nearby shallow holes, with depths ranging from 15 to 28 m. In order to study not only the sedimentary structure but also the response of the aquifer to a set of natural boundary conditions, downhole geophysical measurements have been deployed over the past 3 years in an innovative manner, either in a time-lapse or stationary manner. The downhole measurements are complicated by the unconsolidated nature of the sediment, obliging to perform all measurements through PVC. Also, the granitic nature of the sediment prevents clays identification from a direct use of gamma ray profiles. For this, constituting minerals (quartz, albite, feldspar, microcline, illite) were identified from X-ray diffraction on cores, and spectral gamma logs used to determine the illite fractions from Th/K ratios. In time lapse, high frequency electrical resistivity induction measurements show that preferential flow paths through the aquifer can be identified in a fast and reliable manner. Also, changes in depth of the fresh to salt water interface (FSWI) are precisely described, either in response to marine tides, or to a short but intense mediterranean rain event. Changes on the order of than 1.70 m are obtain in less than a day of heavy rain. Overnight as well as seasonal changes such as months of dryness are also illustrated due to the variability of pore fluid salinity and temperature, even over short periods of time such as tens of minutes. In stationary mode, the spectral natural gamma sensor located in front of the FSWI fluctuation zones records changes in front of all radioactive peaks (from K, Tl, Bi, but also Ra with Rn) during intense rain events such as that of October 18-19, 2017. This places constraints on Ra and Rn production rate during such an event, leading to trace fresh water outpour into the sea.Peer reviewe

Hydrodynamics and hydrogeochemical changes in the mixing zone of a coastal aquifer during a heavy rain event

The coastal aquifers are the place where meet fresh water and seawater. This confluence between both of them is called mixing zone which is in equilibrium due to the different density of the two masses and where various biogeochemical reactions takes place. We know that this equilibrium can be canceled during an important recharge event. Within the aquifer, such events displace the mixing zone over a short time period. This study is aims at the identification and description at high frequency of spatial and temporal response of the mixing zone and its impact on geochemical processes during one such fast water inflow event. We choose the experimental site of Argentona as the place of this study. Located in the northeast of Spain this site is subjected to a Mediterranean climate characterized by heavy rainfall amount recorded during Fall and early Spring period, concentrated during a few hours. Furthermore, this site is located on an alluvial aquifer, equipped with 16 shallow boreholes over a 100 m scale (Fig.1). The geological cross section presented in Fig.2 shows that the aquifer is multilayered and we suspect an effect of a clay/silt layer located at around 12 m.This work was funded by the projects CGL2013-48869-C2-1 y CGL2013-48869-C2-2-R of the Spanish Government. We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell Comarcal del Maresme in the construction of the research site.Peer reviewe

Quantifying fracture geometry with X-ray tomography: Technique of Iterative Local Thresholding (TILT) for 3D image segmentation

This paper presents a new method—the Technique of Iterative Local Thresholding (TILT)—for processing 3D X-ray computed tomography (xCT) images for visualization and quantification of rock fractures. The TILT method includes the following advancements. First, custom masks are generated by a fracture-dilation procedure, which significantly amplifies the fracture signal on the intensity histogram used for local thresholding. Second, TILT is particularly well suited for fracture characterization in granular rocks because the multi-scale Hessian fracture (MHF) filter has been incorporated to distinguish fractures from pores in the rock matrix. Third, TILT wraps the thresholding and fracture isolation steps in an optimized iterative routine for binary segmentation, minimizing human intervention and enabling automated processing of large 3D datasets. As an illustrative example, we applied TILT to 3D xCT images of reacted and unreacted fractured limestone cores. Other segmentation methods were also applied to provide insights regarding variability in image processing. The results show that TILT significantly enhanced separability of grayscale intensities, outperformed the other methods in automation, and was successful in isolating fractures from the porous rock matrix. Because the other methods are more likely to misclassify fracture edges as void and/or have limited capacity in distinguishing fractures from pores, those methods estimated larger fracture volumes (up to 80 %), surface areas (up to 60 %), and roughness (up to a factor of 2). These differences in fracture geometry would lead to significant disparities in hydraulic permeability predictions, as determined by 2D flow simulations

Role of mineralogical, structural and hydrodynamic rock properties in conduits formation in three distinct carbonate rock types

International audienceAdopting sustainable strategies to manage water resources of karst reservoirs requires to understand the carbonate rock reactivity that is responsible for the formation of these reservoirs, as well as the role of the rock mineralogy in the conduit formation. To this end, three carbonate rocks (chalk, crinoidal limestone and dolomite) with different mineralogy and internal structure are submitted to laboratory dissolution experiments by injecting an acid solution under atmospheric conditions and various hydrodynamic conditions with a homemade experimental device. The core samples are characterized by petrophysical investigations with laboratory and imagery techniques before and after the experiments, and the changes in chemical and hydraulic properties are recorded during the experiment. The resulting carbonate dissolution leads to the formation of preferential conduits and the increase in porosity and permeability of the samples. For the three rock types, the dissolution rate mainly depends on the mineral composition, the flow conditions and the initial structural properties. In addition, we observe that (i) for each rock type, the lower Péclet conditions applied to the samples, the lower global dissolution rate, (ii) for each hydrodynamic condition, chalk and crinoidal limestone have the highest and smallest dissolution rates, respectively, (iii) for a similar amount of injected acid, the dissolution is lower in dolomite than in calcite, and (iv) high rock heterogeneities in the chalk samples are responsible for high renewal rates that induce high dissolution rates