Time-lapse surface to depth gravity measurements on a karst system reveal the dominant role of the epikarst as a water storage entity

Abstract

International audienceIn this study we attempt to understand the water storage variations in a karst aquifer on the Larzac Plateau (South of France) using ground-based gravimetry. Surface to 60 m depth gravity measurements are performed three times a year since 2006 down a pothole, in complement to monthly absolute gravity (AG) measurements at three sites. The time variations of the surface to depth (STD) gravity differences are compared with the AG variations. Using a simple Bouguer plate model, we find that the STD gravity differences are very similar to the AG variations. The STD gravity differences are then used to determine apparent density values. These integrative density values are compared with measured grain densities from core samples to obtain an apparent porosity and saturation change representative of the investigated depth. The apparent porosity ranges from 4.8 to 7.3 per cent. We then discuss on the repartition of the apparent physical properties with respect to the epikarst and infiltration zone karst structures. We argue that AG and STD differences monitor epikarst water storage variations. Within this scope, we discuss the fact that seasonal scale water storage variation occurs predominantly in the epikarst

Natural Water Treatment Systems for Safe and Sustainable Water Supply in the Indian Context: Saph Pani

“…Viewing sub-surface for an effective managed aquifer recharge from a geophysical perspective (to locate water content), mise-à-la-masse (to trace water filled cavities), self-potential (to map preferential pathways) are employed with specific success (Jacob et al, 2009;Guérin et al, 2009;Jardani et al, 2006;Meyerhoff et al, 2012). Very few studies focused on the use of geophysical microgravimetric and gravity gradient techniques to target sub-surface cavities and karst features (Colley, 1963;Neumann, 1965;Butler, 1984;Blizkovsky, 1979).…”

Application of Microgravimetry to Assessing Collapse Risk in Carbonate Sinkhole Areas, Cheria Basin, Northeast of Algeria

“…The second derivative map shows essentially the same information as the residual gravity map. The first derivative profile, however, clearly resolves the anomaly into two negative anomalies centered at the (22,13) and (35,28) profiles lines. The results of interpretations confirmed the presence of a significant cavity feature at this location which varied in dimension and depth laterally.…”

“…4) were chosen due to the interesting negative anomaly centered at (22,13) and because it is representative of areas at the site about which nothing was known prior to verification. All eighteen profiles show the negative anomaly feature between profile line (22,13) and (35,28) (Fig. 4).…”

“…Euler deconvolution and system of source equivalent [34,35] provided approx. positions and depths to the anomaly source.…”

The benefits of gravimeter observations for modelling water storage changes at the field scale

“…The gravity time series thus primarily reflect WSC on the field scale, but the exact sampling volume is difficult to define. Consequently, the question arises: How can we use temporal gravity measurements for hydrological applications? Different studies focus on the interpretation of the gravity signal by single storage components (e.g., surface water (Lampitelli and Francis, 2010;Bonatz and Sperling, 1995), snow (Breili and Pettersen, 2009), soil moisture (Van Camp et al, 2006), or groundwater (Takemoto et al, 2002;Harnisch and Harnisch, 2006)), or by estimation of different subsurface properties (e.g., porosity (Jacob et al, 2009), fractures (Hokkanen et al, 2007), block content (Van Camp et al, 2006) or specific yield (Pool and Eychaner, 1995)). The unambiguous identification of the exact source of the gravimeter signal is difficult or even impossible if no additional information is available implying that the estimation of single parameters on the storages or properties is associated with a high uncertainty (Pool, 2008;Creutzfeldt et al, 2010a).…”