Giovanni Santarato

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 …

Geometric Seismic-Wave Inversion by the Boundary Element Method

Abstract Surface‐wave methods are widely used in earth sciences and engineering for the geometric characterization of geological bodies and tectonic structures of the subsurface. These techniques exploit the dispersive nature of Rayleigh waves to indirectly estimate shear wave velocity profiles from surface‐wave measurements; however, they are limited to parallel‐layered geometries. To overcome such limitations, we present a new class of geometric inverse models for a full waveform inversion (FWI) based on the boundary element method (BEM). The proposed approach enables an effective identification of two dimensional (2D) subsurface geometries by directly estimating the shape of laterally varying interfaces from raw measurements. It thus aims at filling the gap between the standard simplistic parallel‐layered‐based inversion and that of more complex three‐dimensional (3D) geometries based on finite element methods (FEMs). Numerical tests on synthetic data unveil the effectiveness of the inverse algorithm, and its applicability to field measurements is finally presented.

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.

Telluric and magnetotelluric study of the Ferrara High (northern Italy)

Abstract A telluric survey and several magnetotelluric (MT) soundings have been carried out across the Ferrara High (northern Italy), in a geothermal area previously explored in detail during a search for oil. The aim was to compare telluric, MT and gravimetric data with a structural model derived from a seismic investigation in a region of fairly well-known geology. The comparison between observed and predicted telluric pseudo-sections reveals a much better agreement than expected, especially since the gravimetric field data clearly show that the structure is effectively 3-D whereas the comparison was restricted to a single profile with 2-D interpretations. The 3-D character seems to appear most clearly in the telluric parameters at periods > 10 s. In the search for a 2-D interpretation of the telluric and MT data, an attempt was made to reduce the family of possible models by imposing external constraints. It was gratifying to find that the best fit with the seismic 2-D model was obtained when a uniform resistivity was required for each of the separate formations revealed by the seismic sounding.

On the correspondence between resistivity and texture of loose sediments, saturated with salt water

Electrical resistivity tomography (ERT) has been applied to characterize coastal aquifers heavily affected by the problem of groundwater salinization. With the help of a number of resistivity data sets and control boreholes, separate resistivity ranges were related to sand-grained sediments, saturated with salt water, and fine-grained sediments, silts and clays, deposited in a sub-marine environment. Frequency-domain electromagnetic soundings (FDEM) were then used to map the spatial resistivity variations in a selected site of the study area. One-dimensional inversion of FDEM real andimaginary parts provided a quantitative resistivity–depth model that was consistent with the ERT model section and direct borehole observations. The successful application of the FDEM technique supports its use for mapping shallow fluid pathways of salt-water intrusion over large areas.

Monitoring Creep Movements by Seismic Refraction

The Italian Apenninic area is a well known example of hydrogeological and landslide hazard, due to recent largely incoherent shallow formations like clay, sandstone, flysch. When artifacts must be built as roads, bridges, dams..., a very careful study of buried geology and lithological properties must be performed in advance, resorting to intensive geotechnical and geophysical surveys, to evaluate both geometry and mechanical properties of the shallowest formations.

Thickness Variations in Layered Subsurface Models - Effects on Simulated MASW

SUMMARY Seismic surface wave methods allow to retrieve the shallow subsurface shear wave velocity. Among these, the “Multi-channel Analysis of Surface Waves” is to date one of the most widely adopted non-invasive active-source approaches in the professional world for the evaluation of the stiffness properties of the ground for geotechnical engineering purposes. The method utilizes the dispersive nature of surface waves by constructing the dispersion curve which is then inverted to obtain the shear wave velocity profile. Dispersion curves generation typically requires the transformation of the recorded seismograms into the frequency-velocity (f-V) domain. Commercially available inversion algorithms assume the subsurface model as a stack of homogeneous parallel layers, but unfortunately this may lead to misleading results if the actual soil profile is far from the assumed 1D geometry. We investigate the effects of lateral heterogeneities due to variations of layer thicknesses on the f-V spectrum to assess the limitations of the 1D approach and thus to judge the reliability of such surface waves interpretation

A new attempt at automatic data processing in magnetotellurics

Abstract Considering the magnetotelluric (MT) signal in its physical reality as a stochastic process, we present here a new methodology for the search and analysis of the wave packets which are most compatible with theoretical calculation requirements. In adapting the length of the physical data blocks to a small number of oscillations of the signal, we are able to identify those data ensembles which show particularly high rotated coherence values (higher than 0.9). An example of such calculations is supplied by means of the analysis of an MT sounding which demonstrates the validity of this procedure.

Unusual geophysical techniques in Archaeology: HVSR and Induced Polarization. A case history

Conventionally employed geophysical methods in archaeological investigations , i.e. ground penetrating radar(GPR), magnetometry, geoelectric (electrical resistivity tomography: ERT) and geo-electromagnetic (FDEM) techniques, can be hampered by specific site conditions. In the case history here presented, i) magnetometry was ruled out due to the presence of iron structures used to support nearby fences and greenhouses partially covering part of the site; ii) GPR employment was tested but not employed due to low penetration depth (i.e. presence of superficial clay sediments) which was much less than the depth of the target, iii) ERT, unfortunately, could not detect the paleo-surface(s) layer(s) as their thickness is below the resolving power of the method. For these reasons and keeping in mind that the paleo-surfaces could have been stiffened due to trampling of human activity over the centuries along which the settlement existed, we employed the Horizontal-to-Vertical Spectral Ratio method (HVSR) which is sensitive to the presence of appreciable contrasts of acoustic impedance of such paleo-surfaces. The obtained results have been proved by direct excavations. Likewise, the Induced Polarization tomography (IPT) which was acquired during the ERT survey, shed light on the presence of increased and well-organized chargeability values that inferred the presence of a paleo-riverbed.

Two-Dimensional Seismic Wave Modeling and Inversion by the Boundary Element Method

Surface wave methods (SWM) are widely used for the geophysical characterization of geological bodies and tectonic structures in both Earth Sciences and Engineering. SWMs exploit the dispersive nature of Rayleigh waves to indirectly estimate shear wave velocity profiles from surface wave measurements, but they are limited to parallel-layered geometries. To overcome such limitations, we exploit the Boundary Element Method (BEM) to define a new class of geometric inversion models that allows to go directly from raw measurements to estimating the shape of laterally varying soil interfaces. The proposed approach enables a robust identification of the subsurface geometry and it aims at filling the gap between the standard simplistic parallel-layered-based SWM and the more complex three-dimensional Full Wave Inversion (FWI) based on Finite Element Methods. Numerical tests on synthetic data unveil the effectiveness of the inverse algorithm and its applicability to wave measurements. An application to field data is finally presented.