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Calibration of a Vadose Zone Model Using Water Injection Monitored by GPR and Electrical Resistance Tomography

^a Dipartimento di Scienze Geologiche e Geotecnologie, Univ. of Milano-Bicocca, Milan, Italy
^b Dipartimento di Geoscienze, Univ. of Padua, Padova, Italy

View larger version (23K): [in this window] [in a new window]   FIG. 1. A conceptual scheme for the use of geophysical data to calibrate unsaturated water flow models. The key issues related to the process are highlighted on the right.

View larger version (30K): [in this window] [in a new window]   FIG. 2. Scheme of the Gorgonzola experimental site. All four boreholes were drilled to 20-m depth. Three boreholes (A, B, and C) were equipped with electrodes for electrical resistance tomography (ERT) measurements, while Borehole D was used for core retrieval only. The trench was dug to 2-m depth for the purpose of water injection during the tracer experiments. Cross-hole ERT and ground penetrating radar time-lapse data were collected between Boreholes A and B.

View larger version (12K): [in this window] [in a new window]   FIG. 3. Description of the core retrieved from Borehole D and results of the corresponding laboratory analyses for saturated hydraulic conductivity and grain size distribution (as 10, 50, and 90% mass fractions); b.g.l. = below ground level.

View larger version (13K): [in this window] [in a new window]   FIG. 4. Example of pressure–saturation curve (fitted with the chosen van Genuchten parameters) and resistivity-saturation curve (fitted with Archie's law = 138.9/S1.0667, where is bulk resistivity in m and S is volumetric water saturation) measured on core material from Borehole D, at 15-m depth.

View larger version (19K): [in this window] [in a new window]   FIG. 5. Moisture content profiles estimated at the Gorgonzola test site from January to April 2005, derived from zero-offset profile radar and the constitutive relationship by Topp et al. (1980). The moisture content curves clearly show the falling water table from January to April. The residual moisture content values (r) obtained in the laboratory on samples from the retrieved core (to the right) are shown on the profile relevant to Boreholes B to C (b.g.l. = below ground level).

View larger version (29K): [in this window] [in a new window]   FIG. 6. Measured water injection flow rate at the Gorgonzola site during the test in January 2006, and corresponding monitoring plan using cross-hole ground penetrating radar (zero-offset profile [ZOP] and multiple-offset gather [MOG]) and cross-hole electrical resistance tomography (ERT). The photo shows the trench and the flow control device during the injection experiment.

View larger version (53K): [in this window] [in a new window]   FIG. 7. Moisture content background images in January 2006 (before water injection), as derived from electrical resistance tomography (ERT) and multiple-offset gather (MOG) ground penetrating radar cross-hole imaging using a laboratory-calibrated Archie's law and the relationship of Topp et al. (1980).

View larger version (32K): [in this window] [in a new window]   FIG. 8. Moisture content changes with respect to background (Fig. 7) as imaged by zero-offset profile (ZOP) and multiple-offset gather (MOG) ground penetrating radar and electrical resistance tomography (ERT) at 3 h after injection start (b.g.l. = below ground level).

View larger version (31K): [in this window] [in a new window]   FIG. 9. Moisture content changes with respect to background (Fig. 7) as imaged by zero-offset profile (ZOP) and multiple-offset gather (MOG) ground penetrating radar and electrical resistance tomography (ERT) at 45 h after injection start (b.g.l. = below ground level).

View larger version (34K): [in this window] [in a new window]   FIG. 10. Evolution of moisture content changes () with respect to background (Fig. 7) as a function of time, imaged by zero-offset profile (ZOP) and electrical resistance tomography (ERT) at selected time steps.

View larger version (14K): [in this window] [in a new window]   FIG. 11. Moisture content changes () with respect to background imaged by zero-offset profile (ZOP) ground penetrating radar during (A) the 11 h of the injection phase, (B) 14 h after the injection end (early drainage phase), and (C) the late drainage phase. The arrows indicate the depth of the center of mass (b.g.l. = below ground level).

View larger version (18K): [in this window] [in a new window]   FIG. 12. Results of three-dimensional flow model calibration based on electrical resistance tomography (ERT), multiple-offset gather (MOG), and zero-offset profile (ZOP) estimates of the center of mass depth as a function of time (Ks = saturated hydraulic conductivity; b.g.l. = below ground level).

View larger version (22K): [in this window] [in a new window]   FIG. 13. Comparison between the moisture content changes () imaged by zero-offset profile (ZOP) ground penetrating radar (black lines) and the corresponding profiles obtained from the calibrated three-dimensional unsaturated flow modeling and averaging of three-dimensional moisture content distribution across the Fresnel volume (red lines). Note the obvious overestimation of water mass in the model results, in spite of the model being calibrated for the depth of the center of mass with time (see Fig. 12) (b.g.l. = below ground level).