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

Abstract

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.

Assessing subsoil void hazards along a road system using H/V measurements, ERTs and IPTs to support local decision makers

“…In the second step, considering the limited investigation depth and taking into account that the 2D‐ERT profiles were not as high performing in a complex environment with variations of water content and porosity changes (Fischanger et al ), 12 3D‐ERTs/IPTs were designed (Figure a). Using the 3D‐ERT acquisitions (Loke and Barker , Santarato et al ), it was possible (i) to arrange the electrodes along nonlinear profiles around buildings, residential lot boundaries, and all the other obstacle; (ii) to increase the investigation depth up to 30 –40 m; and (iii) to optimise the acquisition sequence to enhance the sensitivity inside the 3D‐ERT boundaries.…”

“…The commercial software ErtLab TM (Geostudi Astier srl, Italy, and Multi‐Phase Technologies, USA) was employed to invert the ERT data. This software (i) employs finite‐element discretisation; (ii) implements an optimal data noise management (Cardarelli and Fischanger ; Fischanger et al ) for Occam’s regularisation (Constable, Parker and Constable ); (iii) has the ability to model the true remote electrode position by accounting for the effects of the B electrode when it is not placed at an appropriate “infinite” distance from the M‐N dipole (Santarato et al ); (iv) allows the exploration of the best electrode arrangements (i.e., the rotation of the electrode coordinates to achieve an electrode arrangement with a side as parallel as possible to the axis) to reduce mesh parametrisation; (v) calculates the quality of an inversion while accounting for every data point affected by the Gaussian noise (i.e., the error that affects the field data) and minimizing the misfit function between the field and modelled data, as defined in Santarato et al (); and (vi) allows setting a robust inversion to appropriately manage any noise during inversion by means of a data‐weighting algorithm that allows the adaptive changes of the variance matrix after each iteration for those data points that are poorly fit by the model. Therefore, the reliability of the model is not influenced by the shape of the loop.…”

A Review of the Advantages and Limitations of Geophysical Investigations in Landslide Studies

“…Therefore, in our analysis, we also have not considered it as a 4D technique, and the results show that in 85% of the works (Figure 3), 4D geophysical imaging is not discussed. In general, 4D ERT has been more frequently employed thanks to the development of ER multichannel measuring systems that significantly reduced the acquisition time [20,140]. These systems [such as those employed in [141,142]], in fact, (i) are able to simultaneously acquire a number of potential measurements for a single pair of current electrodes and (ii) can be set up to provide ERT at specific times during the day.…”

“…Nevertheless, even though tl-ERTs could be helpfully employed in landslide monitoring, since they could provide information about the water content changes (i.e., the data could be related to pore water pressure variations and, therefore, to landslide triggering mechanisms), there are still few examples of 4D ERTs in landslide areas [60,65,92]. Moreover, it is still needed to improve software such that it is able to (i) continuously (or very frequently) process acquired data (e.g., ErtLab by Geostudy Astier, [140]), (ii) to link ER variations with hydrological parameter changes, and (ii) to take into account that the positions of the electrodes could change over the time because of the landslide movement [38,65].…”

Deep electrical resistivity tomography for the prospection of low- to medium-enthalpy geothermal resources

“…Post-processing and data quality control of the data has been performed using PROSYS software from Iris Instruments and ERT-Lab (Fischanger et al 2013). From the 2318 points, a total of 1746 data values have been selected to perform the inversion.…”