A WATER INJECTION EXPERIMENT IN THE VADOSE ZONE: THE USE AND VALUE OF NON INVASIVE CROSS-HOLE DATA FOR MODEL CALIBRATION
Creators: name: Cassiani, Giorgio
name: Deiana, Rita
name: Cassiani, Giorgio
Publisher: name: XVI International Conference on Computational Methods in Water Resources
Year: 2006

description: The characterization of the deep vadose zone is traditionally a difficult task, since
access without disturbance of samples or modification of in-situ conditions is
practically impossible. On the other hand, the presence and flow of water in the
vadose zone controls a number of phenomena of great environmental interests, such as
contamination of water resources, catchment hydrology, floods and slope stability. As
a consequence, non invasive – geophysical - techniques, such as electrical
resistivity tomography (ERT) and ground-penetrating radar (GPR), are being
increasingly used to investigate the vadose zone. The use of cross-hole methods is
particularly effective and increasingly applied. The dependence of the geophysical
response on changes in soil moisture content, e.g. via changes in electrical
resistivity or dielectric properties, is the key mechanism that permits the use of
non-invasive techniques to monitor the vadose zone in time-lapse mode, i.e. via
repeated measurements over time. The data from non-invasive techniques can be
subsequently used to calibrate physical-mathematical models of water flow in the
unsaturated zone. In this paper, we present the results of a forced infiltration
experiment conducted at the research site in Gorgonzola, close to Milan, Italy. The
site has been equipped to monitor the unsaturated flow dynamics in the local alluvial
quaternary sediments of the Po river plain, characterized by a fairly coarse
sandy-gravel grain size distribution. Four 20 m deep boreholes were drilled at the
site; three of them are permanently equipped with stainless steel borehole electrodes
for ERT imaging. Each borehole has a set of 24 electrodes, spaced 0.8 m vertically.
The same boreholes are also  being used for cross-hole GPR tomography. Both MOG
(multiple offset gathers) and ZOP (zero offset profiles) have been adopted as
configurations for GPR surveys. Conversion from GPR traveltimes to moisture content
values has been provisionally performed using the semi-empirical relationships. ERT
has been used so far only to provide qualitative images of changes in resistivity due
to water saturation changes. Inversion of ERT data has been performed using the
CRTomo 2D code (Kemna, 2000), while the code Migratom (Jackson and Tweeton, 1994) was
used for the inversion of GRP MOG data. ZOP profiles have been analyzed under the
assumption of straight ray horizontal propagation. Monitoring of natural infiltration
processes started in January 2005. In July 2005 an artificial water injection
experiment was performed using a 2 m deep, 2.60 m long trench. A total of 3 cubic
meters of tap water infiltrated in about 2 hour time. A number of ERT and GPR MOG
datasets were acquired from July 5 (date of water injection) to July 11, 2005. The
data were used to calibrate a 3D unsaturated flow model, thus helping the
identification of field-saturated hydraulic conductivity and other  unsaturated soil
parameters. Strengths and limitations of the methodology are analysed and discussed.