Use of atmospheric tides to estimate the hydraulic conductivity of confined and semi-confined aquifers

International audienceCharacterizing groundwater responses to natural drivers is cost effective and offers great potential in hydrogeological investigations. However, there is a lack of method development and evaluation, for example by comparing results with those derived from using conventional methods. This paper presents a modified method to calculate the hydraulic conductivity (K) of confined aquifers using the well water response to atmospheric tides. The approach separates the Earth and atmospheric tide influences on filtered well water-level records in the time domain. The resulting ill-posed regression deconvolution problem can be overcome by constraining the well water response to atmospheric tides in order to follow a physically realistic semi-diurnal barometric response function (S2-BRF), or to follow directly a modified hydraulic model (BE-Hvorslev) similar to a slug test evaluation. An analysis with synthetic data shows that K up to 10-4 m/s can be estimated when pressure records with short sampling intervals are available. Application to a field dataset from Cambodia and Benin, with 20-minute to 60-minute sampling intervals, respectively, results in K values of 5.82∙10-7 m/s and 2.9.10-7 m/s. This agrees with results independently derived from pumping tests for both confined sediments and semi-confined hard-rock conditions. This method offers a promising and low-cost approach to derive K solely from monitoring datasets in confined aquifers. This is especially advantageous for low-conductivity formations where hydraulic testing takes time

Fluid-driven seismic swarms in the Gripp valley (Haute-Pyrénées, France)

SUMMARY An unusual seismic activity has recently occurred in the Gripp valley, located in the central part of the French Pyrenees. Since spring 2020, two new swarms appeared, clearly outside the usual location of the seismicity in this area. On 20 September 2020, almost concomitantly with the activation of the second seismic swarm, a hole suddenly opened in the bed of a local river, the Adour de Payolle. This hole drained the water from the river, which dried up over 500 m. We follow and study the spatial and temporal evolution of these clusters, using four temporary stations deployed a few days or months after the beginning of the crisis to complete the regional network. These additional data lead to the construction of a comprehensive catalogue of more than 4900 earthquakes, using both a template matching approach and a deep-learning based phase picking method to complete and improve the initial catalogue available from the French seismological agency. This allows highlighting a slow and clear migration of the seismicity during 1 yr. Precise absolute and relative event locations reveal a dipping faulting structure, confirmed by the focal mechanism estimated for the highest magnitude event of the sequence (ML 3). We propose to explain the observed migration of the seismicity by deep fluids going up through a newly discovered faulting structure

Seismic responses to fluid pressure perturbations in a slipping fault,

International audienceSeismicity induced by fluid injection in a natural fault is investigated in situ in the near field of the source. We present synchronous seismic and hydromechanical measurements directly recorded in the decametric injection zone. The three main types of seismic events were recorded during injection and shut-in: high-amplitude and short duration seismic events (SE) (i.e., microearthquakes), low to constant amplitude and 5 to 17 s long tremor-like signals (TLS), and long period events (LP) with a narrow-frequency band content. Seismicity first initiates with a sequence of SE and TLS, when pressure is high (~3.5 MPa), slip is activated on the fault, which experiences a twentyfold increase of permeability. Then LP events appear to be associated to fluid leakage in the fault caused by dilation during slip. During shut-in, residual pressures as low as 0.6 MPa still trigger SE events. We show that the initial TLS sequence triggers when a progressive transition occurs from rupture controlled by effective stress variations close to the injection source to a large friction weakening-dominated slip on the fault. We conclude that the combination of these different seismic signal types may be a proxy to monitor fault instability associated to fluid pressure perturbations