non-invasive methodologies for the characterization...

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2014-2015

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NON-INVASIVE METHODOLOGIES FOR THE CHARACTERIZATION OF THE EARTH’S CRITICAL ZONE

Ph.D. candidate: LAURA BUSATO, II course

Tutor: Prof. GIORGIO CASSIANI Co-Tutor: Dott. JACOPO BOAGA

Cycle: XXIX Abstract

The Earth’s Critical Zone is the near-surface domain where the interactions between rock, soil, water, air, and living organisms take place. In this heterogeneous boundary layer, complex and interacting processes operate on second-to-eon timescales and atomic-to-global space-scales, and, mainly, determine the availability of life-sustaining resources. Although ECZ is strongly interconnected with human activity, its characterization is still in an early stage. Therefore, we combined different non-invasive geophysical techniques to investigate three ECZ subdomains: (i) a reconstructed river embankment, where concrete appears as a relatively conductive mean with some heterogeneities, (ii) the areas surrounding an alpine river, where we used heat as a natural tracer, and (iii) the root zone of two orange trees, irrigated with different techniques. For the second and third case studies, our time-lapse monitoring well highlighted the occurring processes, but only an appropriate flow and transport modelling will allow a deeper understanding of these complex domains. Introduction The Earth’s Critical Zone (ECZ) is the heterogeneous, near-surface environment in which complex interactions involving rock, soil, water, air, and living organisms regulate the natural habitat and determine the availability of life-sustaining resources (National Research Council, 2001). These interacting phenomena occur at small-to-large scale in terms of both time and space and have a strong interconnection with human activity. Therefore, several disciplines are currently studying this shallow boundary layer (e.g., geology, biology, hydrology, etc.) but, despite that, we are still far from a deep understanding of this complex domain (Anderson et al., 2008). A major step forward in the characterization of the ECZ can be provided by the application of non-invasive geophysical techniques combined with flow and transport models. These cost-effective and relatively fast methodologies allow the monitoring of the domain of interest at different scales both in terms of time and space and, moreover, the data thus obtained can be linked to the information supplied by other disciplines through an appropriate modelling process, so as to apply a more complete holistic approach (Binley et al., 2015). In order to characterize the ECZ, one of the most commonly employed techniques is the electrical resistivity tomography (ERT, Daily et al., 2004), both from surface (e.g., Nyquist et al., 2008; Perri et al., 2014) and in (micro-)boreholes (e.g., Crook et al., 2008; Boaga et al., 2013; Cassiani et., al 2014), since it allows the measurement of resistivity (ρ), a parameter strongly related to water content (θ). Conversely, a promising methodology is distributed temperature sensing (DTS, Selker et al., 2006). Its usage for these applications has been developing since the last two decades, revealing a wide adaptability to the typical issues of this field of study. DTS is based on Raman scattering spectroscopy (Gardiner, 1989) and employs heat as tracer (Anderson, 2005) using fiber-optic cables to acquire temperature (T) values (Westhoff et al., 2001). Another useful technique is multichannel analysis of surface waves (MASW, Park et al., 1999), which uses ground roll as a source of information in order to compute S-wave velocity (vs) profiles of the near surface. Also ground penetrating radar (GPR, e.g., Davis and Annan, 1989) is widely applied, since its constitutive parameter (dielectric constant, εr) can be related to water content using one of the various relationships available (e.g., Topp et al., 1980). Finally, self potential (SP, e.g., Jouniaux, 2009) is a passive technique to measure natural voltages (V), which can arise as a consequence of fluxes within the soil. All these methodologies are rather cost-effective, non-invasive and relatively fast to apply. During this second year we investigated four case studies representing three examples of ECZ subdomains particularly interconnected with human activity: (i) a reconstructed river embankment characterized by seepage phenomena, (ii) the hyporheic zone (HZ, Orghidan, 2010) of an alpine river and (iii and iv) the root zone of two orange trees, irrigated with different techniques. In the first case study we


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combined ERT, MASW, GPR, and SP to characterize the inner and outer structure of a concrete embankment prone to seepage phenomena, while for the alpine river we took advantage of the joint use of ERT and DTS measurements, employing an innovative instrumentation deployment (i.e., both the multicore cable and the fibre-optic are located within the hyporheic zone, under the riverbed and perpendicular to it). Finally, the surveys on the third and fourth case studies were carried out using 3D micro-electrical tomography (3D micro-ERT), with the application of 9 micro-boreholes per three combined with superficial electrodes. Case studies and methodologies Frassine river This case study consists in a reconstructed river embankment located within the municipality of Megliadino San Fidenzio (Padova, northern Italy), in the Prà di Botte area (Lat. 45°15’18.3’’ N, Long. 11°32’35.8’’ E). From a geological point of view, this site lies in the Adige Alluvial Plain, which is part of the Venetian-Friulian Plain, and therefore is characterized by Tertiary to Quaternary sediments. The entire domain has been strongly influenced by the Last Glacial Maximum (LGM, 30,000-17,000 years B.P.) and the following post-LGM phase, which led to paleosols (e.g., caranto paleosol) and fluvial terraces (Fontana et al., 2008). More in detail, the field site area consists of ancient fluvial ridges now evolved into silty-clayey soils with a low sand content (ARPAV, 2013). The Frassine river, to which the reconstructed embankment belongs, is one of the six segments into which the Guà river is divided. It flows between Borgo Frassine (Montagnana, PD) and Brancaglia (Este, PD), with a length of 13.42 km, and is part of the Brenta-Bacchiglione catchment. From a hydrological point of view, this river has a mixed regime that implies higher flow rates during spring and autumn and lower flow rates on the other seasons (Bondesan, 2001). The flooding event that caused the levee rupture took place in the early afternoon of November 1st 2010. The collapse occurred on the right levee and had spread up to a total length of 100 m when a preliminary barrier with boulders of different diameters was built. Then, the positioning of several reinforced concrete bars on the levee outer side increased the structure stability, while the installation of sheet piles on the inner side allowed the stopping of the water flow. Finally, three days after the flood event, the whole preliminary structure was covered with aggregates (tout-venant), thus ending the first step of the embankment restoration. In order to provide a long-term solution, new reinforcement works from the embankment crest took place in 2012 (second step). They can be divided into three phases: (i) concrete injection along the whole preliminary structure (110.0 m length, 9.5 m depth), (ii) jet-grouting diaphragm construction, extended also to the undamaged levee (170.0 m total length, 22.0 m total depth) and (iii) Tube a Manchette grouting to further waterproof the whole structure. Despite these actions to improve the embankment’s functionality, several water infiltration phenomena occurred during the following months, thus highlighting a lack of homogeneity within the new structure. Therefore, nine geotechnical soundings were carried out in 2013: Six of them analysed the reconstructed part, showing a rather complex domain, where the different reinforcing interventions overlap. Since they provide only punctual information, these soundings dot not allow a complete evaluation of the embankment and lack in identifying the (probable) discontinuities within this segment. The remaining three soundings examined the earthen levee, which is homogeneously composed of silty-clayey sand, as expected. Furthermore, they show a clayey level between 11 m and 13 m depth. Given the need of dealing with these infiltration events, new interventions modified the outer face of the reconstructed embankment in 2014 (third step): A stability bank with a 0.5 m thick gravel drainage mattress was built, with a seepage berm and a collector ditch to avoid water stagnation. A section of the resulting geometry is outlined in Fig. 1: The embankment total height from ground level is circa 8.7 m, with a crown 4 m wide, and both inner and upper outer faces have a ratio of width to height of 3:2. The stability bank is 5 m high from ground level, has a ratio of width to height of 2:1 and a 4 m wide crown. The characterization of this site is performed through the application of four different methodologies: electrical resistivity tomography (ERT), multichannel analysis of surface waves (MASW), ground penetrating radar (GPR), and self-potential (SP). The first methodology, ERT, is applied with different


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set-ups, in order to vary depth of investigation, resolution, and section of the embankment. The surveys consisted of:

1. Lengthwise profiles: 48 stainless-steel electrodes were placed along the river crest, in correspondence to the reconstructed part. We used two different spacing, 1 m and 2 m, so as to vary both total length (47 m and 96 m, respectively) and depth of investigation (about 10 m and 20 m, respectively). All these surveys share the position of the 23rd electrode, which was fixed to guarantee the overlapping of the profiles. Furthermore, we employed both Wenner-Schlumberger and dipole-dipole skip zero acquisition schemes;

2. Cross-embankment profiles: We used two different lines (one upstream and one downstream, labelled T2 and T1 respectively), with 48 stainless-steel electrodes spaced 0.75 m each. In both cases the acquisition scheme chosen was dipole-dipole skip 0. They covered the outline of the embankment (from the seepage berm to the water level, via the embankment crest) in two cross-sections, selected according to the results of the lengthwise surveys;

3. Cross-river profiles: We placed 72 stainless-electrodes spaced 1 m along a line covering the outline of the reconstructed embankment, the riverbed bed, and the profile of the left bank. Also here, we employed both Wenner-Schlumberger and dipole-dipole skip zero acquisition schemes.

Each ERT sounding is performed with an IRIS Instruments SYSCAL Pro resistivity meter measuring both direct and reciprocal resistance values (Daily et al., 2004), necessary to assess the measurement error and obtained exchanging the current electrodes with the potential ones. Then, we used the MASW technique to compare the reconstructed sector to a natural adjacent one, given the difference in terms of mechanical properties that should arise as a consequence of the different materials involved. More in detail, we took advantage of a seismic streamer, a specific tool composed of several geophones linked together that allows the dragging of the whole equipment along the embankment crest. We carried out 25 measurements, whereof 19 on the reconstructed sector (in correspondence to the ERT lengthwise profiles) and 6 on the earthen levee. For each MASW sounding we used a Geometrics Geode seismograph with sampling rate equal to 250 ns, acquisition time equal to 2 s, and 24 geophones with 4.5 Hz frequency spaced 2 m (i.e., total length of the line equal to 46 m) placed lengthwise on the embankment’s crest. The application of GPR took place in correspondence of the cross-embankment ERT lines (T1 and T2), thus resulting in two radargrams, G1 (downstream) and G2 (upstream). We chose a zero offset profile (ZOP) approach, where both the transmitter and the receiver are lowered along the inner and outer embankment faces, respectively. We used a PulsEKKO borehole system with two 100 MHz antennas. The last technique, SP, was used to monitor the variations in terms of potential along the seepage berm and the lower part of the embankment’s outer face. Fig. 1 Topographic profile of the reconstructed embankment. Picture facing downstream. The blue line represents the water level. Vermigliana creek The Vermigliana creek is the main watercourse of the Upper Val di Sole (northern Italy), originates from the Presena Glacier and has a nivo-glacial regime. Moreover, discharge variation can be identified also on a daily basis. The whole Upper Val di Sole bottom is filled with heterogeneous glacial till and quaternary slope deposits, whose granulometry ranges from clays to boulders (Dal Piaz et al., 2007). Our site is located near the small village of Vermiglio (TN). Here, we combined two different methodologies, electrical resistivity tomography (ERT) and distributed temperature sensing (DTS), taking advantage of two horizontal oriented boreholes drilled below the river itself. More in detail, the first technique is


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carried out using a waterproof multicore cable with 48 brass electrodes, spaced 1 m, placed inside the upstream borehole. To ensure a higher data reliability, we deployed also a superficial survey line of 24 stainless steel electrodes (12 on each river levee) spaced 1 m, in vertical correspondence to the horizontal borehole. Furthermore, 4 take-outs allowed the connection of the 4 electrodes nearest to the watercourse to the cable itself, which was partly located over the bridge upstream. Each ERT survey is performed with an IRIS Instruments SYSCAL Pro resistivity meter using all 72 electrodes, with a skip 0 dipole-dipole scheme. The final dataset of each acquisition is made up of 4885 values of both direct and reciprocal measurements. The 200 m long fibre-optic for the DTS measurements is located in the downstream horizontal borehole, parallel to the ERT perforation. Since the DTS perforation is 70 m long, the fibre-optic has been folded, creating a “double-ended” configuration (i.e., both ends are connected to the DTS instrument). For every DTS survey we used the AP Sensing N4386A Distributed Sensing system setting both the sampling interval and the spatial resolution equal to 1 m. In each survey, we acquired three single traces with update time and measurement time both equal to 30 s (i.e., every 30 seconds a new trace acquisition begins and lasts 30 seconds) and then averaged the three temperature values thus obtained for every sampling point. In spring 2015 our site has been equipped with a piezometer (diameter equal to 127 mm with a 101 mm diameter PVC case, completely screened and provided with a probe to measure head, temperature, and electrical conductivity) and with an Aqua TROLL probe (In-Situ Inc.) to create an hourly record of level, conductivity, and temperature of the river water. We carried out two time-lapse monitoring: (i) Seasonal monitoring, between April and August 2015 (measuring approximately once a month) and (ii) Intensive monitoring, with measurements carried out every two hours between 30th June 2015 and 1st July 2015. Root zone (Palazzelli site)

Fig. 2 a) Map representing the Palazzelli field site set-up. The position of the micro-boreholes (red dots) identifies four quadrants, one of which is centred on the plant (green dot). Each quadrant comprises 24 stainless steel electrodes (blue dots) creating a superficial grid with 0.26 m spacing. b) Example of parallelepiped defined by four micro-boreholes and 24 superficial electrodes (plant quadrant). Each micro-borehole hosts 12 stainless steel electrodes (i.e., 48 electrodes for each parallelepiped). The set-up is the same for both plants. Our last case study comprises two orange trees located in an orange orchard in eastern Sicily (Palazzelli, Lentini, SR) belonging to the Citrus Mediterranean Crops Research Center (CRA-ACM). This orchard is composed of 8-year old [Citrus sinensis (L.) Osbeck] cv “Tarocco Sciara” C1882 grafted on Carizo citrange rootstock [Poncirus trifoliata (L.) Raf. × C. sinensis (L.) Osbeck] trees, while the soil is classified as sandy-loam (69.7% sand, 10.5% clay, and 19.8% silt). Given the constant water scarcity affecting Sicily and the need of improving water use efficiency, four different irrigation techniques are currently employed in this orchard in a randomized block design (i.e., 12 blocks with 24 plants each, each


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block irrigated with one the four irrigation techniques available): (i) Full irrigation, where trees are watered with 100% of total evapotranspiration (ETc), (ii) Sustained deficit irrigation (75% ETc), (iii) Regulated deficit irrigation (50% ETc), and (iv) Partial root drying (PRD), where the water supply is equal to 50% ETc and is provided alternatively on each side of the root-zone, while the other side is kept dry. The irrigation takes place thanks to two surface lateral pipes per tree row, in order to apply a drip irrigation technique. Moreover, this orchard is also equipped with an automated meteorological station (Consoli et al., 2014). Our time-lapse monitoring focuses on two orange trees, one fully irrigated (control tree) and one irrigated with the PRD treatment. For each tree we applied the configuration represented in Fig. 2: 9 micro-boreholes (with 12 stainless steel electrodes each, spaced 0.10 m) pinpoint the edges of 4 parallelepipeds, one of which is centred on the orange tree. Then, each superficial quadrant is equipped with 24 stainless steel electrodes covering a superficial grid of 0.26 m spacing. Each survey was carried out using a SYSCAL Pro resistivity meter (IRIS Instruments) with a dipole-dipole “skip 0” scheme and measuring both direct and reciprocal resistance (R) values (each dataset is then made up of 4885 values). We carried out our measurements in June, July, and September and each time comprises a background measurement (i.e., prior to irrigation), two during-irrigation surveys for the tree quadrant, and a final measurement on each quadrant after the end of the scheduled irrigation.

Data processing, preliminary results and discussions Frassine river Electrical resistivity tomography. We performed the inversion of the ERT data at our disposal thanks to two different software, both provided by Lancaster University (UK): Profiler (for lengthwise and cross-embankment profiles) and R2 (for the cross-river profiles). The error threshold for the resistance measurements and the error for the inversions are both fixed at 5%. The lengthwise resistivity cross-sections well identify the presence of the reconstructed part within the river embankment. This domain is highlighted by relatively higher resistivity values (150-600 Ohm m), with respect to the natural levee, which, on the contrary, is characterised by relatively lower resistivity values (50-100 Ohm m). These values well agree with the materials forming these domains (i.e., reconstructions materials and clayey sand, respectively). The lower part of this cross-section shows rather low resistivity values (50 Ohm m on average), which may be related to either natural sediments or grouting. Moreover, the cross-sections with 1 m electrode spacing helped us distinguishing two subdomains within the reconstructed part, thus showing some heterogeneity in this structure: The first, downstream, has an average resistivity of 500 Ohm m; the second, upstream, has an average resistivity of 300 Ohm m. Fig. 3 T1: resistivity cross-embankment profile. The black dots represent the position of the stainless steel electrodes. We can identify two subdomains: (i) Lateral parts, with a higher average resistivity due to the ballasts covering and (ii) Central part, with a lower average resistivity related to the concrete septum (affected by heterogeneity in the middle part). The high resistivity domain between 25 m and 30 m locates the drainage mattress. Picture facing downstream. All distances are

Resistivity (Ohm m)


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expressed in meters. These differences led us to the two cross-embankment soundings, T1 and T2 (downstream and upstream, respectively), aimed at identifying the nature of this variation. First of all, T1 has an average higher resistivity, if compared to T2. More in detail, each cross-section can be divided into two subdomains (Fig. 3):

- Lateral parts: Both inner and outer parts have higher resistivity values because of the ballasts covering the embankment faces. Moreover, these cross-sections well identify also the drainage mattress within the stability bank;

- Central part: In both cases, this subdomain has lower average resistivity values (<150 Ohm m), likely due to the cement septum, in agreement with other examples in literature (e.g., Sjödahl et al., 2009). T1 shows a horizontal zone with resistivity of 150 Ohm m that does not appear in T2, thus underlying the absence of homogeneity within the reconstructed embankment. These differences may also be the reason behind different seepage phenomena.

Finally, we decided to combine both dipole-dipole and Wenner-Schlumberger resistance values to obtain the cross-river profile, in order to take advantage of both surveys into a unique resistivity cross-section. The result, shown in Fig. 4, well represents the difference between the reconstructed embankment (on the left) and the natural one (on the right): The left side shares the same features described for the cross-embankment profiles, but has a lower resolution, given the higher electrodes spacing (0.75 m vs. 1 m); the right side shows the natural levee, marked by a more conductive and homogeneous core (likely clayey sand), covered with more resistive gravel. A longer profile allows also to reach a deeper investigation depth, thus extending the information provided. Unfortunately, the data at our disposal do not permit us to identify the embankment’s foundations, since they only show a conductive domain. Fig. 4 Cross-river resistivity profile. The black dots represent the position of the stainless steel electrodes. This cross-section well highlights the differences between the reconstructed embankment (on the left) and the earthen one (on the right). The reconstructed embankment part shares the same features described of T1 (Fig. 3). Unfortunately, this cross-section does not provide information regarding the embankment foundations. Picture facing upstream. All distances are expressed in meters. Multichannel analysis of surface waves. Also the MASW data obtained well distinguish the reconstructed embankment from the natural levee. The vs model of the natural part is characterized by rather low values slightly increasing with depth (from 150 to 260 m/s) that are likely related to the presence of clayey sand, in agreement with both the ERT results and the geotechnical soundings. On the contrary, the frequency-phase velocity diagrams of the reconstructed sector display large energy dispersion, since the propagation of superficial waves is probably strongly influenced by the heterogeneities within this part. Moreover, the stiffening due to the concrete septum is represented by the higher energy in correspondence to velocities close to 1000 m/s (especially for frequencies > 25 Hz). Surface Waves Analysis Wizard, Matlab, and SWAMI allowed the analysis of the MASW data.


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Ground penetrating radar and self-potential. The analysis of the GPR data was realized thanks to the REFLEX software and to the application of the empirical relation proposed by Topp et al. (1980). The results well agree with the ERT data, since G2 and T2 have higher average water content and lower average resistivity, respectively. It is important to underline that the water content distribution within the artificial part is likely irregular, with higher values in the cemented domain. Moreover, the signal to noise ratio is lower in G2 with respect to G1, thus agreeing, at least from a qualitative point of view, with more conductive materials. On the other hand, the drainage mattress compromised the SP data interpretation, since its position has been communicated only after the acquisitions and then confirmed by the ERT cross-embankment acquisitions. Thus, although the SP profiles show some voltage variations, they are likely due to changes in terms of materials within the mattress itself rather than to infiltration phenomena. Vermigliana creek The ERT data at our disposal call for two different types of inversion. The first one leads to the representation of the absolute resistivity cross-section, which allows the description of the state of the domain at the survey time. After removing all the measurements pairs (direct and reciprocal) with an error higher than the fixed threshold, we performed the ERT data inversion thanks to the R2 code (Lancaster University, UK). All cross-sections show the same features highlighted during the monitoring carried out in 2014 (i.e., a very low resistivity domain - 50 Ωm - beneath the Vermigliana creek till a depth of 4 m below ground level and a difference between the left and the right bank, with the former slightly more conductive than the latter). The second inversion technique aims at highlighting how resistivity varies over time. This approach allows expressing these variations as percentage, with respect to the background survey (100% indicates no changes, higher values imply an increase in resistivity, and lower values are related to a decrease). In 2015 we carried out two different time-lapse monitoring: (i) Long-term time-lapse monitoring and (ii) short-term time-lapse monitoring. The former provides information regarding the variations occurred between April and August 2015 and highlights a complex domain, where the different behaviours of the subdomains (i.e., under the riverbed and left and right banks) are still evident (e.g., the influence on the left bank of an effluent of two small lakes upstream). Despite that, these results do not completely resemble last year long-term time-lapse monitoring likely because of the different climatic conditions occurred. On the other hand, the short-term time-lapse monitoring well highlights the small ρ variations arising on an hourly basis. Our measurements and data analyses pinpoint a constant superficial resistivity increase during the second day, due to the strong solar radiation and high temperatures that characterized Vermiglio on July 1st 2015. The left bank is characterized by a decrease in average resistivity with respect to the right bank (once again because of the effluent of two small lakes upstream), while the domain below the riverbed shows a constant decrease in resistivity (up to 6% in 18 hours). This decrease, at a first glance, may not be related to the infiltration of new glacial water, since it is characterized by a low ions content and therefore by higher resistivity values (290 Ohm m on average, during the short-term monitoring days). Together with the ERT technique, we also applied the DTS methodology to carry out our two different time-lapse monitoring. The long-term time-lapse monitoring shows the same pattern highlighted in 2014, where we can identify four subdomains: (i) is characterized by the highest temperature values, since here the fibre-optic cable is rather shallow and influenced by surface water coming from the two small lakes upstream, (ii) represents the segments corresponding to the sub-riverbed, where the trend is more variable and sharpens over time, given the complexity of the processes taking place, (iii) shows the traces relative to the right bank, which are clearly characterized by the lowest temperature values (probably because of a flux of glacial water), and finally, (iv) represents the part of the fibre-optic cable in excess, coiled and located at a depth of 0.5 m below ground level. The short-term monitoring provides interesting results, as it allows to represent the small T variations occurring in the hyporheic zone. As depicted in Fig. 5, between D0 (30th June, 9.30 am) and D9 (1st July, 3.24 am), the average temperature increases along the whole fibre-optic, except for the coiled part (which can be considered as reference). This increase is more pronounced in part (ii) (from 8.34 °C to 9.87 °C), likely because of the infiltration of hotter superficial water, while part (i) and (iii) show a slightly lower average T increase over time (from 9.45 °C to 10.27 °C and from 7.23 °C to 7.96 °C, respectively). This first trend is then followed by an average temperature drop until D12 (1st July, 11.01 am), which,


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however, leads to an average temperature decrease in part (ii) equal to 0.66 °C (i.e., the average temperature decrease does not bring the system to the conditions measured the day before at the same time). Finally, the average temperature starts increasing again, up to the end of the short-term monitoring (D15 - 1st July, 4.57 pm). The trend thus identified, at least from a preliminary and simplistic point of view, resembles the T variation trend characterizing the river water temperature (as highlighted by the data at our disposal). Moreover, we can hypothesize that the left and right riverbanks behave in two different ways, or, more likely, that are subject to different phenomena (i.e., flux of lacustrine and glacial water, respectively). Nevertheless, a quantitative analysis is still necessary, in order to properly compare and combine all the different information available, since the interconnections existing between these parameters (e.g., between T and ρ) cannot be properly understood with a preliminary approach.

Fig. 5 DTS temperature profiles measured during the Vermigliana short-term monitoring (30th June 2015-1st July 2015). In each profile four segments can be identified: (i) left bank of the Vermigliana creek, (ii) Vermigliana creek, (iii) right bank of the Vermigliana creek, and (iv) fibre-optic cable rolled at 0.5 m below ground level. All the temperature profiles show the same trend, with higher temperatures in the left bank and lower values in the right one, while the hyporheic zone is characterized by a variable trend that sharpens over time. The short-term monitoring is characterized by: a) – c) an average temperature increase between D0 and D9, d) a decrease until D12, and e) a last increase until the end of the monitoring (D15).

a b

c d

e


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Root zone (Palazzelli site) The analysis of the 3D micro-ERT data is based on the same approach described for the Vermigliana case study, since it is independent from the site geometry. So, also in this case, we can consider both absolute and time-lapse inversions for both plants. Even if the data at our disposal are referred to a bigger volume (2.6×2.6×1.2 m3), we are going to focus only on the plant quadrants (1.3×1.3×1.2 m3, Fig. 2), given their higher importance. In both cases and for all the measurement times (i.e., June, July, and September 2015), the domain depicted by the absolute sections is relatively complex, thus leading to a rather difficult understanding of the main features (Fig. 6). Nevertheless, given the absence of evident soil texture variations, we may attribute the resistivity pattern to the root system distribution. The time-lapse inversions highlight, at all three measurement times, the proceeding of the scheduled irrigation, which is represented by a resistivity decrease in correspondence of the superficial drip irrigators. Also within the 3D domain, variations in terms of resistivity are represented, thus indicating probable water content changes over time. An in-depth analysis of these data is currently taking place, thanks to new inversions with combined datasets (i.e., all quadrants at the same time instead of each quadrant individually), which should reduce the heterogeneities arising among quadrants.

Fig. 6 Example of volumetric absolute resistivity section at the Palazzelli site (quadrant with plant irrigated with PRD technique, acquisition made after the end of the scheduled irrigation). The relatively more conductive domain on the volume surface is due to the irrigation that occurred prior to this measurement. The black dots represent the electrodes positions (both on the surface and in the micro-boreholes). Conclusions and future work One of the main problems in the ECZ characterization is the obvious need of investigating a complex domain constantly interconnected with human activity, where several phenomena are occurring simultaneously and at different space scales. These preliminary results show the applicability and versatility of many geophysical techniques (ERT, DTS, MASW, GPR, and SP), which allow rather fast, cost-effective and, most of all, non-invasive investigations of different shallow domains. Although our data provided useful information regarding both structures and processes, their complete exploitation can be carried out only through an appropriate modelling process, where all the available information are


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combined. The first future goal will be the identification of the root distribution on the basis of the ERT data collected. Given the complexity and, above all, the novelty of the proposed method, a synthetic case will be developed, in order to assess the applicability of this approach. This will be done thanks to the CATHY (CATchment HYdrology) model (Camporese et al., 2010). Once the validity of this modus operandi will be confirmed, we will apply it to both Bulgherano and Palazzelli field sites, in order to assess the real root distribution assimilating the ERT data within our models. The same approach will be applied also for the Vermigliana field site, where both ERT and DTS data will be assimilated in order to better understand the interactions between surface water and groundwater within the hyporheic zone. References

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SUMMARY OF ACTIVITY IN THIS YEAR Courses: GLOBAQUA EU FP7: “A short course on hydrological modelling”, Dipartimento di Ingegneria Civile, Ambientale e Meccanica, Università degli Studi di Trento ANDREW BINLEY: “Groundwater resources and protection”, Lancaster Environment Centre, Lancaster University Workshops: “Soil-Plant-Atmosphere Continuum 2015”, Rothamsted Conference Centre, Harpenden, 13th-14th October 2015. Communications: BUSATO, L., BOAGA, J., PERUZZO, L., ASTA, G., COLA, S., SIMONINI, P. and CASSIANI, G. 2015. Study of a

reconstructed river embankment through a combination of non-invasive geophysical methodologies. GNGTS, 34° convegno, Trieste, 17-19 November 2015.

VANELLA, D., BUSATO, L., BOAGA, J., CASSIANI, G., BINLEY, A.M., PUTTI, M. and CONSOLI, S. 2015. Micro 3D ERT tomography for data assimilation modelling of active root zone. AGU Fall Meeting, San Francisco, 14-18 December 2015.

Posters: BUSATO, L., VANELLA, D., BOAGA, J., MANOLI, G., MARANI, M., PUTTI, M., CONSOLI, S., BINLEY, A.M. and

CASSIANI, G. 2015. Identification of active root zone by data assimilation techniques: monitoring and modelling of irrigation experiments. EGU General Assembly 2015, Vienna, 12-17 April 2015.

CASSIANI, G., BOAGA, J., BUSATO, L., PERRI, M.T., PUTTI, M., PASETTO, M., MAJONE, B. and BELLIN, A. 2015. Time-lapse electrical resistivity tomography and distributed temperature measurements in the hyphoreic zone of an alpine river. EGU General Assembly 2015, Vienna, 12-17 April 2015.

Teaching activities: PERUZZO L. 2015. Caratterizzazione geofisica di un tratto d’argine ricostruito in seguito a rotta. Master thesis, Dipartimento di Geoscienze, Università degli Studi di Padova. Advisor: Prof. CASSIANI, G., Co-advisor: Dott. BOAGA, J., Dott.ssa BUSATO, L. Other: Ph.D. visiting student at the Lancaster Environment Centre (Lancaster University, Lancaster, UK) under the supervision of professor A. Binley (October-December 2015). Field activities:

− Saletto (PD): Electrical resistivity tomography, multichannel analysis of surface waves, ground penetrating radar, and self-potential for the characterization of the right embankment of Frassine river after its rupture and reconstruction in 2010;

− Vermiglio (TN): Electrical resistivity tomography and distributed temperature sensing measurements in order to characterize the hyporheic and the riparian zones of the Vermigliana creek. Seasonal and intensive monitoring;

− Bologna (BO): Electrical resistivity tomography for time-lapse monitoring of the aquifer remediation.

non-invasive methodologies for the characterization...

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