Detecting fracture networks and karst features alignments similarities in the aquifer system of Tsanfleuron, Swiss Alps.

The role that fractures play for karst development has been widely described, most particularly for the generation of preferential flow pathways. The alignments between fractures and karst features, such as conduits and dolines, occur in various regions where the rock is prone to diaclase or where the tectonic history imprints fracture patterns that facilitate rock dissolution. However, most studies specifically linking the orientation of fractures and karst features are largely descriptive. A quantitative analysis of the geometries and their interconnection is required before modeling any discrete karst network model (DKN).

In order to analyze the relationship between structural geology and karst development, we propose a novel quantitative approach, in between description and modeling. This approach is applied on the karst aquifer system of Tsanfleuron in the Helvetic domain of the western Alps. We check if the fracture and conduit orientation is similar, by statistically comparing their azimuths. The aim is to quantify and identify which fracture families have the highest influence in the development of karst features.

We interpret fracture alignments on a scale 1:2.500, based on a set of data acquired from an uncrewed aerial vehicle with 10 cm/px resolution, consisting of a digital elevation model and an orthomosaic image. Fracture interpretation was verified in the field. Karst surveys were previously acquired by the Groupe de Spéléologie Rhodanien, Société Spéléologique Genevoise, Spéléo-Club Jura, and Groupe Spéléo Lausanne. We calculate the azimuths from the fracture interpretation and karst surveys. We identify the dolines by the circular and ellipsoidal shape as depressions on the DEM, and their alignments are detected with the application of the Hough transform.

The superposition of structural and karstic features on maps and the plot of their direction in rose diagrams show that some fracture sets coincide with the orientation of most of the conduits in the study area. We apply the chi-square test to verify how similar are the distributions of the fractures and conduit azimuths. The null hypothesis (H0) is that the distributions are similar, and the alternative hypothesis (H1) is that the distributions are significantly different. If the chi-square test yields a value under the selected p-value = 0.05, we reject the null hypothesis.

Comparison of all azimuths ranging from 0° to 180° shows that hypothesis H0 cannot be rejected; therefore, the distributions of fractures and karst conduits are similar. Testing more specific direction ranges indicates that fractures and conduits are strongly aligned especially in the NE-SW, ENE-WSW, and E-W directions (with high p-values). The alignments of the dolines are based on fewer measurements, but show a preferential NE-SW orientation.

We conclude that the fracture and karst conduit azimuth distributions are similar. Therefore, fractures, helped by the gentle dip of the bedding, controlled the development of the karst in Tsanfleuron, mainly in the NE-SW, ENE-WSW, and E-W directions. This result will be used for the construction of a DKN model in future work.