G. Morelli

Preliminary use of ground-penetrating radar and electrical resistivity tomography to study tree roots in pine forests and poplar plantations

In this study, we assess the possibility of using ground penetrating radar (GPR) and electrical resistivity tomography (ERT) as indirect non-destructive techniques for root detection. Two experimental sites were investigated: a poplar plantation [mean height of plants 25.7 m, diameter at breast height (dbh) 33 cm] and a pinewood forest mainly composed of Pinus pinea L. and Pinus pinaster Ait. (mean height 17 m, dbh 29 cm). GPR measures were taken using antennas of 900 and 1500 MHz applied in square and circular grids. ERT was previously tested along 2-D lines, compared with GPR sections and direct observation of the roots, and then using a complete 3-D acquisition technique. Three-dimensional reconstructions using grids of electrodes centred and evenly spaced around the tree were used in all cases (poplar and pine), and repeated in different periods in the pine forest (April, June and September) to investigate the influence of water saturation on the results obtainable. The investigated roots systems were entirely excavated using AIR-SPADE Series 2000. In order to acquire morphological information on the root system, to be compared with the GPR and ERT, poplar and pine roots were scanned using a portable on ground scanning LIDAR. In test sections analysed around the poplar trees, GPR with a high frequency antenna proved to be able to detect roots with very small diameters and different angles, with the geometry of survey lines ruling the intensity of individual reflectors. The comparison between 3-D images of the extracted roots obtained with a laser scan data point cloud and the GPR profile proved the potential of high density 3-D GPR in mapping the entire system in unsaturated soil, with a preference for sandy and silty terrain, with problems arising when clay is predominant. Clutter produced by gravel and pebbles, mixed with the presence of roots, can also be sources of noise for the GPR signals. The work performed on the pine trees shows that the shape, distribution and volume of roots system, can be coupled to the 3-D electrical resistivity variation of the soil model map. Geophysical surveys can be a useful approach to root investigation in describing both the shape and behaviour of the roots in the subsoil.

Integrated geophysical surveys to investigate the Scarsella vault of St. John's Baptistery in Florence

A geophysical study of the Scarsella vault, the high altar of St. Johns Baptistery in Florence, was conducted to determine the internal structural features and thus ensure its proper restoration. The Scarsellas crossvault is covered with precious mosaics dating back to the Middle Ages. The vault is 10 m long and 5 m wide, varying in thickness 5–2.5 m. To determine the extent of some fractures known to exist on the internal surface and to define the structural features of the vault itself, three different geophysical techniques were applied: electrical resistivity tomography (ERT), seismic transmission tomography, and ground-penetrating radar (GPR). Data reduction involved both 2D processing and inversion of the ERT and GPR sections, 3D inversion of seismic tomography, and 3D rendering of ERT and GPR images. An overall interpretation was obtained by correlating the different results, leading to a more detailed knowledge of the vaults internal structure. Data acquisition was done with extreme care to prevent any possible damage to the mosaics while still ensuring high-quality measurements. A high-precision topographic survey of the vault was necessary to process the data, especially in the case of seismic tomography, where a precise location of sensors and shotpoints is crucial. The 3D image of the investigated structure is shown in Figure 1. Figure 1. Perspective view of the internal vault and roof coverage. Electrical resistivity data were acquired using slim 2.5-cm long needle-shaped, AgCl-coated silver electrodes, which were carefully placed in the joints between the tesserae of the mosaic. The contact resistance between the electrodes and the vault material was minimized by using medical conductive gel. ERT was carried out along seven profiles from R1 to R7 (Figure 2) located on the internal surface of the vault, both longitudinally (R1 to R5) and transversely (R6 and R7). The survey resulted in about 2500 …

4D cross-borehole electrical resistivity tomography to control resin injection for ground stabilization: a case history in Venice (Italy)

Settlements of building foundations are generally due to water content changes in the shallow subsurface, both by natural and man-made causes. Although resin injection is revealed to be a satisfactory solution for ground consolidation, a continuous monitoring of the process is needed to achieve optimal results. In order to control the injection of expanding resins, a field procedure is developed, based on the use of time-lapse three-dimensional (4D) Electrical Resistivity Tomography (ERT). The choice of electrical resistivity, as a parameter for designing and monitoring the consolidation work, is based on the key assumption that this physical property is the most sensitive to water content changes in soils. During the injection stage, repeated ERT acquisitions allow the injection process to be controlled and the injection schedule and its parameters to be modified, whenever necessary. In this paper the procedure and its results are illustrated, through a case history in Venice (Italy), where salt-water bearing soils also had to be taken into account. Careful analysis of electrode array configurations and parameters had therefore to be performed in advance. Horizontal and vertical sections from the resulting 3D resistivity models show, through a noticeable local increase of the resistivity at and nearby the injection points, that re-homogenization of soil is successfully achieved. Repeated 3D ERT measurements, carried out three and a half years after the consolidation work, show that stabilization of the subsoil below and around settled foundations is achieved, as also confirmed by comparing suitable extensimeter measurements on overlying structures, carried out before and after the treatment.

Mapping of complex hydrocarbons contaminant using geoelectric and electromagnetic methods

A combined geophysical survey was carried out in a oil refinery to study the complex hydrocarbon contamination of the soil and the near surf ace aquifer, resulting from several years of leakage of hydrocarbon products. A near surf ace level characterized by a strong reflector was weIl detected using Ground Penetrating Radar; lateral changes of the strength of the reflection and the subsequent drilling confirmed the existence of a free phase of hydrocarbon in the vadose zone above the water tabIe. Because of the limited thickness of the layer embedded with the residual oil, the geoelectrical resistivity measurements were not effective for detecting this level. The resistivity measurements pointed out very low values in the upper zone of the aquifer; this effect can be associated to complex chemical reactions between the dissolved hydrocarbon phase and the aquifer system. These effects were investigated using high resolution geoelectrical tomography. Soil data resulting from several boreholes confirmed the reliability of the geophysical response. Geochemical and physical analysis of the soil and groundwater samples, combined with the geophysical investigation, allowed the definition of a reliable model ofthe polluted area.

Near Foundation Soil Stiffening Evaluation after Resins Injection by a Novel 3D Interpretation of Surface Waves Data

The characterization of soil settlements below foundations of buildings using surface waves methods represent an extreme challenging problem because of the presence of localized shear waves variation and shallow heterogeneity. To tackle the near-foundation shear waves distribution reconstruction challenge a method for the elaboration of surface waves recordings unbound from both rigid field geometry and from the classic layered subsurface assumption would be highly desirable. We introduce a novel elaboration strategy of surface waves data, based on the Direct Interpretation of Phase Lags (DIPL-3D)among pairs of seismic signals which aims to retrieve the three dimensional shear waves velocity subsurface distribution without using inversion. As an example application, the method is used for the evaluation of near foundation soil stiffening after an expanding resins injection intervention

Integrated geophysical survey for the detection of hydrocarbon pollution

Geophysical methods provide useful tools to detect and map large areas potentially subjected to hydrocarbon pollution; many technical papers on the use of different geophysical techniques are reported in literature (Atenkwana, 1998, Godio and Morelli, 1998). Electromagnetic mapping, GPR investigation and electrical methods have proven to be effective for detecting polluted areas. An extensive survey has been undertaken on contaminated sites in Northern Italy over the last two years. The site assessment using the hydro-geological approach and destructive test (drillings) was completed with integrated geophysical investigations. The survey involved the characterisation of the subsoil assessment in some test sites, where oil recovering wells, tanks and pipelines were located. The main goal of the combined investigations was to find the presence of hydrocarbon pollution due to the leakage from the old tanks and pipelines. Because of the geological setting of the region, the main aim was to localise the free hydrocarbon phase above the shallow aquifer (2-3 meters in depth) and the presence of contamination in the vadose zone. Due to the long permanence of hydrocarbons in the soil (probably more than 20 years), chemical and biological reactions could have occurred in the polluted zones; these phenomena could complicate the interpretation of the geophysical response. A short description of the results of the geophysical tests is here reported: the selected examples relates to the survey performed on an area of about 50 x 100 m, previously employed to collect gas and oil. Most of the cultural features, such as tanks and pipelines, were removed from the site. The soil and water sampling confirmed the suspicion of the existence of a high contamination. The main goal of the geophysical survey was to verify the extension of the polluted zone, in order to plan a subsequent recovering of the area. Different methods were employed: a preliminary fast mapping was carried out using the low induction technique (LIN); the CM031 (Geophyzica) instrument in a dipole vertical mode was used. Ground probing radar (GPR) acquisition, which operates in the range between 100 MHz up to 500 MHz, was performed. Finally, electrical resistivity tomography (ERT) was carried out to confirm the response of the former geophysical investigations. Electrical measurements were performed employing the geo-electric intelligent nodes scanning system Syscal R2 (IRIS instrument). An electrode spacing of 2 meters was adopted in the dipole-dipole and Wenner array configuration for a total of 32 electrodes. The data processing involved the mapping of the in-phase and conductivity (quadrature component) of the electromagnetic response of the LIN acquisition; the GPR data were processed in order to obtain the time-slices of the reflection amplitude. ERT data were processed using a least square inversion (2D) with smoothness constrains, according to the procedure described by Morelli and Labrecque (1996).