 |
| Titel |
Relating bulk electrical conduction to litho-textural properties and pore-fluid conductivity within porous alluvial aquifers |
| VerfasserIn |
M. Mele, M. Giudici, S. Inzoli, E. Cavalli, R. Bersezio |
| Konferenz |
EGU General Assembly 2012
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063111
|
|
|
|
|
|
| Zusammenfassung |
| The estimate of hydraulic conductivity from Direct Current methods represents a powerful
tool in aquifer characterization as both electrical and hydraulic conductivities depend on
connected pore volumes and connected pore surface areas. A crucial, intermediate stage of
this process is the assessment of sediments’ textures and lithology from DC electrical
conductivity as the electrical response of the aquifers’ basic building blocks (i.e., hydrofacies)
is controlled by the prevailing process of electrical conduction, electrolytic (ÏăEL;
pore-volume dominated) vs. “shale” (ÏăSH; pore-surface dominated), determined
by pore-space structure, clay distribution and electrical properties of pore fluids
(ÏăW).
In this work laboratory experiments were conducted and the results were interpreted
through the analysis i) of a volume-averaged, macroscopic litho-textural property of
alluvial hydrofacies’, the coarse-to-fine ratio (C/F), as a “proxy” of the process of
electrical conduction within each samples on the basis of the volume proportion
between nonconductive, coarse-grained and conductive, shaly textures and ii) of the
surface conduction component, produced in fresh-to-salt water environment by clay
materials.
8 hydrofacies’ samples were collected with an hand-auger within the outcropping alluvial
aquifers of the Quaternary meander river belt of the southernmost Lodi plain (northern Italy),
represented by loose gravelly-sands to sands (6 samples), fine and sandy-silty clays (2
samples).
As a first step, laboratory measurements of the bulk electrical conductivity (ÏăB) of
representative sub-samples, totally saturated with water with different salinity (ÏăW from 125
to 1100 μs/cm), were performed. The experimental apparatus was made up by a series of
polycarbonate, cylindrical cells (9cm x 12cm) equipped with external, copper plates as
current electrodes and internal, copper squared-grids as potential electrodes. Electrical
conductivity of each sample was obtained averaging time-repeated measures during
48 h after the samples’ assemblage with a DC resistivity meter. As a second step,
texture analysis was performed in order to obtain the textures’ volume fractions of
each hydrofacies subsamples; C/F threshold equal to 1 identifies coarse-grained
litho-textural association (gravelly-sands to sands samples) and fine-grained litho-textural
association.
Plot of ÏăB vs. C/F generally shows an increasing conductivity with decreasing
C/F ratio and increasing ÏăW, that is consistent with previous studies perform on
field-scale electrical conductivity datasets obtained through DC resistivity soundings
calibrated on sediments outcropping the alluvial basin. The distributions of ÏăB vs. C/F
are fitted with a power-law regressions, showing a decreasing R2 with increasing
ÏăW.
A conduction model which takes into account C/F and ÏăW and considers ÏăB as the sum
of two terms, ÏăEL (pore-volume dominated) and ÏăSH (pore-surface dominated), where
surface conduction is treated as an equivalent shale volume conduction, was adopted.
Values of ÏăEL and ÏăSH were computed for each sample and for increasing ÏăW. |
|
|
|
|
|
|