Geophysical quantification of porosity in the soda lakes of the Lake Neusiedl-Seewinkel Basin

Abstract

The Lake Neusiedl-Seewinkel Basin is home to a system of unique soda lakes, which harbor a variety of rare and endangered flora and fauna. The existence of these lakes is facilitated by a complex equilibrium between climate and surface-groundwater interactions, where capillary forces pull soda (NaHCO3) and clay from the shallow aquifer to the surface. The accumulation of clay develops an impermeable layer that acts as a hydraulic barrier near the surface allowing rain water to form the eponymous lakes. Assessing lateral and temporal variations in porosity, clay and salt content, in particular within this impermeable layer, is important to understand the surface-groundwater dynamics at site and address the impact of climate change and artificial drainage of groundwater on the lakes leading to their on-going degradation. We investigate here the applicability of seismic methods to quantify near surface variations in porosity, while information on clay content and salinity are resolved through electric methods. In particular, we conduct measurements with the multichannel analysis of surface waves (MASW), the P-wave seismic refraction tomography (SRT) and the induced polarization (IP) methods during dry (summer) and wet (winter) periods in three adjacent soda lakes: one considered active, one degrading, one degraded. Quantitative estimates of porosity and water saturation are inverted from MASW and P-wave SRT data sets based on the Biot-Gassmann fluid substitution theory and an extension of the Hertz-Mindlin contact theory accounting for capillary suction effects taking place in the vadose zone. The complementary IP measurements aid in the identification of the salt-bearing clay rich impermeable layer, associated with higher electrical conductivity values, to sustain the porosity estimation based on the seismic methods and gain information on the clay content and pore fluid salinity at each site. In August 2022, we installed a permanent IP monitoring profile within the active lake to observe temporal changes in electrical conductivity related to variations in soil moisture due to seasonal variations such as precipitation. Our results reveal different geophysical signatures in the three lakes corresponding to their presumed ecological state of degradation. In general, we observe higher P- and S-wave velocity values and lower Poisson’s ratio and electrical conductivity values for the degrading/degraded lake than in case of the intact lake. The impermeable layer of the intact lake is clearly distinguishable from MASW and IP images, whereas it is less well resolved and exhibits a higher porosity in the degraded lake. The joint inversion of SRT and MASW overall improves the subsurface characterization as it solves for shallow porosity variations within the impermeable layer, which were not detectable through the independent inversions, clearly revealing differences in porosity between the three sites.