ERT monitoring of the vadose zone of a karst system sounds like a challenge!

Geoelectrical monito
ring has proved its efficiency to provide valuable information in multiple
contexts such as active landslides, landfills pollution, dam infiltration or permafrost melting. In
other environments, such as karsts, ERT monitoring has not been broadly used.
In
karst systems, the vadose zone plays an important role in the water dynamics. In particular,
temporary perched aquifers can appear in the subsurface due to changes of weather conditions,
reduced evapotranspiration and the vertical gradients of porosity and
permeability. Seasonal
water variations in the infiltration zone of a karst system should then be observable with ERT.
Monitoring these variations may help separate the hydrological signature of the vadose zone from
that of the saturated zone. This inform
ation is thus required for understanding hydrological
processes that occur in a whole karst system. However, such hard rock environments can be very
heterogeneous, which makes ERT monitoring quite challenging.
We present a case study where ERT monitoring
is being used to track groundwater changes at the
Rochefort Cave Laboratory (RCL), located in the Variscan fold
-
and
-
thrust belt (Belgium), a region
that has many karstic networks within limestone units. Our investigations cover two years of
hydrogeophysica
l monitoring.
The permanent ERT monitoring takes place on a daily basis and is composed of an unconventional
combination of a 2D ERT profile with a pronounced topography, and a vertical borehole
crosscutting the vadose zone. Recent work on ERT survey opti
mization applied to this specific case
is discussed, and conventional surveys (e.g. dipole
-
dipoles) are compared with the optimized
surveys. Data inversion is performed using BERT.
For validating the assumptions based on ERT data analyses of the RCL site
monitoring, we are
especially focusing on groundwater changes that can be correlated with microgravimetric
monitoring installed at the same site, integrating both vadose and saturated zone signatures.
Correlation with direct measurements of dripping within
the underlying cave network is also
investigated.