Deep Sensing of Transient Electrokinetic Response of Aquifer-Aquitard
System to Pumping
Abstract
We consider the transient electrokinetic response of an aquifer-aquitard
system to groundwater abstraction from the aquifer. The system was
instrumented with 18 non-polarizable copper/copper sulphate electrodes
installed at three different depths in the aquitard above the aquifer.
The sensing electrodes were installed at depth of 1, 2, and 4 m below
ground surface along three overlapping transects. The differential
voltages relative to a single permanent electrode were measured with a
Campbell Scientific CR1000 datalogger with a single multiplexer.
Additionally, six piezometers screened in the top 1.5 m of the confined
aquifer, were installed by direct-push. All the piezometers were
instrumented with pressure transducers to measure directly the hydraulic
response of the aquifer. The vertical variation of resistivity in the
aquitard was measured on sediment cores recovered from one of the
boreholes used to install the deepest piezometer. The resistivity
distribution at antecedent sediment wetness (moisture content) was
measured using the MC Miller resistivity boxes and meter, with wetness
measured gravimetrically. Previous exploratory drilling and sampling
activities at the site indicate that the aquifer is fractured greywacke
sandstone overlain with clayey aquitard of semi-consolidated alluvial
sediment, with the aquifer-aquitard contact at a depth of 10.3 m below
the ground surface. We report the results of the site instrumentation,
monitoring, characterization, hydraulic testing, and compare the results
of parameter estimation using streaming potential and hydraulic data
separately and jointly. We explore the effect of depth of installation
of the electrodes in the aquitard on signal strength and quality and
compare this to model predicted behavior using semi-analytical models
from the literature. The results suggest the need for deep sensing of
electrokinetic signals generated by groundwater flow to improve
signal-to-noise ratios and the usefulness of self-potential data for
hydrogeophysical characterization of aquifer-aquitard systems.