E. Cardarelli

Evaluation of tunnel stability using integrated geophysical methods

Abstract An integrated interpretation was made of data, from ground penetrating radar (GPR), seismic refraction and seismic transmission tomography, collected inside the catchment tunnels of a potable water source in central Italy. Rock fracturing and obsolescence of the concrete lining in a tunnel led a landslide that caused structural instability in the catchment work structures. To assess the stability of the rock close to the landslide, geophysical surveys were preferred to boreholes and geotechnical tests in order to avoid water pollution and the risk of further landslides. The interpretation of integrated data from seismic tomography and 200 MHz antenna GPR resulted in an evaluation of some of the elastic characteristics and the detection of discontinuities in the rock. Note also that an analysis of the back-scattered energy was required for the GPR data interpretation. The integration of seismic refraction data and 450 MHz antenna allowed us to identify the loosened zone around the tunnel and the extent of the mass involved in the cave-in, while GPR data from 225 MHz were used to evaluate the quality of contact between concrete lining and massive rock.

Electrical resistivity tomography to detect buried cavities in Rome: a case study

Cavities located in shallow layers of the subsoil represent a major hazard especially in sites located in town centres. For this reason, it is necessary to determine their location and size in order to evaluate the risk of subsidence and to draw up plans for restoration and safety. In this study, electrical resistivity tomography was carried out, both to detect cavities and to test a new inversion algorithm that we developed. The cavities are located in a volcanic formation in a public recreation park where there are municipal buildings. The survey was performed using two different arrays: dipole-dipole and Wenner. A new approach to adopting an initial model and inequality constraints was used and the results from different algorithms were compared. The effective range of the dimensions of cavities was determined in order to make an approximate evaluation of ground stability and any potential subsidence hazard.

Two case studies of the determination of parameters of urban waste dumps

Abstract This paper describes a survey carried out on two wastes dumps using VES and the seismic refraction technique. The dumps have different geometrical and technical characteristics and are located on different geological formations. Anomalously low P waves velocities in the dumps allow us to identify the limits and the thickness of the dumps. The VES method contributed to the general reconstruction of the area and indicated the presence of pollution in the surrounding area.

Some considerations on shallow seismic reflection surveys

Abstract High-resolution shallow seismic reflection surveys require more attention to the choice of source and configuration, receivers and recording geometry for optimizing data acquisition than conventional oil exploration surveys. Moreover, some standard processing techniques to increase signal/noise (S/N) ratio need special accuracy (for example, surgically precise removal of early-time coherent noise and iterative, small time shift static corrections). This paper compares results obtained using different sources at two test sites: explosive, cap, shotgun, hammer and weight drop. Data from experiments using geophones with different natural frequencies and using various acquisition geometries are also compared. In data processing, it is demonstrated how increasing the S/N ratio for high-resolution results requires special consideration in some common processing steps (F–K filter, first arrivals muting, elimination of air wave and static corrections). The comparison, based on shot gathers and stack sections, shows that attenuation of high frequencies by the earth is the most significant influence on the spectral properties of the data, as expected the source itself also does have some influence on frequency content, depending to some extent on surface conditions. The high-velocity explosive sources produced the highest frequency reflections and best S/N ratio, because they have higher energy related to higher burn/blast velocity and source containment then the other sources and they are used in hole (i.e. below ground surface where the air wave energy is more attenuated) but the shotgun also an explosive source was reasonably comparable to high explosive when used in hole. Special care must be taken during processing to insure artifacts are distinguished from real reflection events.

Integrated geophysical surveys on waste dumps: evaluation of physical parameters to characterize an urban waste dump (four case studies in Italy)

Geophysical surveys were carried out on different waste dumps to evaluate key geometric and physical parameters. Depending on the dump dimensions and physical characteristics different geophysical techniques were used. Vertical electrical sounding, electrical resistivity tomography, induced polarization and seismic refraction techniques were integrated to eliminate the non-uniqueness of solutions and for a better understanding of the results. Physical parameters inside and outside the dumps were compared. The change of physical parameters such as resistivity, chargeability, and P-wave velocity allowed evaluation of waste dump geometry, leachate saturation levels, and thickness of waste. Furthermore, in illegal dumps, the size and waste type disposed could be evaluated. Calculated results were compared with plans and bookkeeping from the dumps investigated

Electrical resistivity and induced polarization tomography in identifying the plume of chlorinated hydrocarbons in sedimentary formation: a case study in Rho (Milan — Italy):

Resistivity and induced polarization surveying were originally developed for mineral exploration but are now finding new applications in the field of environmental and engineering geophysics. The present article reports the results of a geophysical survey performed with the aim of identifying a plume of chlorinated hydrocarbons in sedimentary formations of the Pandania plain. The tested site is characterized by three sand and gravel aquifers containing a quantity of clay particles which influence the overall bulk resistivity and chargeability. According to data obtained using shallow boreholes, mainly dense non-aqueous phase liquids were found as contaminants in the first and second aquifer. The aforementioned geo-electrical methods were applied in both two- and three-dimensional approaches. Steel and copper electrodes were used in the process of field data acquisition and the results of the survey were compared. The geophysical survey revealed some anomalies that could be explained by the presence of dense non-aqueous phase liquids in the soil medium. The concept of normalized chargeability facilitates the interpretation of detected induced polarization anomalies. The shape of the plume was inferred from maps of resistivity and chargeability to a depth of 25 m below the surface of the ground.

Shear-wave velocity profiling at sites with high stiffness contrasts: a comparison between invasive and non-invasive methods

Three sites of the Italian Strong Motion Network (RAN) have been selected for detailed S-wave profiling, using both borehole and surface wave seismic methods. At these sites, the presence of stiffness contrasts within the soil column is found to influence the surface wave propagation profoundly. Advanced aspects in surface wave inversion such as resolution, accuracy and higher-mode interpretation must be properly taken into account to obtain realistic results from the surface wave dispersion observations. The possibility of mode misidentification and the loss of resolution with depth in surface wave interpretation are explored using synthetic modelling together with active and passive seismic data sets. With high stiffness contrasts, the possibility of mode jumps and higher mode dominance over specific frequency ranges is very probable. This is true also for normally dispersive sites, where the shear velocity increases with depth, though higher mode dominance is recognized as more common in the case of a shear-wave velocity inversion within the soil column and the sensitivity of the dispersion curves with respect to those layers beneath the low-velocity zone may be significantly reduced. Pitfalls in the inversion resulting from mode misidentification can be avoided by investigating the effective phase velocity distribution, using active data sets and full waveform seismic modelling. When an unambiguous modal identification is achieved, the results obtained by surface wave inversion are very satisfactorily consistent with borehole data.

Variable damping factors in travel time tomography

Abstract In travel time tomography, the evaluation of the problems regarding the introduction of priori information requires special attention. In this article, we analyze the importance of data measurement errors and the ‘damping factors’ in the solution of a system of linear equations. The inversion method considered in this study minimizes the squares of the difference between the slowness in each single cell and a mean slowness. We discuss the different behavior of high and low velocity in cells caused by the damping factor. Considering that the problem is not linear, the errors introduced by the damping or by the data errors will affect the solutions in a different way. Synthetic models show that damping factors introduce significant distortions around the lower velocity cells and low distortions around the higher velocity cells. Furthermore we note that the low velocity cells, for any value of damping factor, are ‘well detected’. On the contrary, the high velocity cells are in general masked by smoothing generated by the damping factor. To avoid these distortions we introduce different damping factors smaller damping factors corresponding to lower velocity cells and higher ones for higher velocities, the results obtained indicate that in this way we approach the true field velocity. Finally we show a field example in which a wrong anomalous trend is eliminated with the introduction of the variable damping factors into the system. We conclude that the variable damping factors can be introduced when strong variations exist in the survey area.

Detection and imaging of piping sinkholes by integrated geophysical methods

Piping sinkholes may naturally develop in the case of a thick overburden overlying calcareous bedrock. Their detection and imaging is a challenging task for geophysical methods, not only because of the required resolution and depth of penetration, but also because major pitfalls may arise, in such geologically complex areas, from the speculative interpretation of geophysical anomalies as geological features. Data integration from different geophysical methods is essential to remove these interpretation ambiguities, caused by large near-surface gradients and heterogeneities in the soil properties, as well as by oscillations of the water table and anomalous water circulation. We present an investigation procedure consisting of the sequential application and integrated interpretation of Electrical Resistivity Tomography (ERT), Seismic Refraction Tomography (SRT) and Self Potential (SP) measurements for locating and monitoring piping sinkholes with application to a site in Central Italy. This approach is a compromise between resolution and cost-effectiveness, and it is designed to be economically affordable by the private end user. In complex geological scenarios, it is usually not possible to rate a single geophysical technique as superior to all the others in terms of resolution, cost-effectiveness and diagnostic capability. The independent information coming from the different geophysical methods is the key to removing interpretation ambiguity when evaluating the position and the development over time of the piping sinkholes. The application of the proposed investigation procedure allowed us to individuate a small area subject to the formation of a piping sinkhole. The geophysical results were confirmed about one year after the execution of the geophysical measurements, as the site exhibited surface evidence of a piping sinkhole, with the formation of a small pond filled with sulphurous water and gases coming from below.

Tomographic inversion of time-domain resistivity and chargeability data for the investigation of landfills using a priori information

In this paper, we present a new code for the modelling and inversion of resistivity and chargeability data using a priori information to improve the accuracy of the reconstructed model for landfill. When a priori information is available in the study area, we can insert them by means of inequality constraints on the whole model or on a single layer or assigning weighting factors for enhancing anomalies elongated in the horizontal or vertical directions. However, when we have to face a multilayered scenario with numerous resistive to conductive transitions (the case of controlled landfills), the effective thickness of the layers can be biased. The presented code includes a model-tuning scheme, which is applied after the inversion of field data, where the inversion of the synthetic data is performed based on an initial guess, and the absolute difference between the field and synthetic inverted models is minimized. The reliability of the proposed approach has been supported in two real-world examples; we were able to identify an unauthorized landfill and to reconstruct the geometrical and physical layout of an old waste dump. The combined analysis of the resistivity and chargeability (normalised) models help us to remove ambiguity due to the presence of the waste mass. Nevertheless, the presence of certain layers can remain hidden without using a priori information, as demonstrated by a comparison of the constrained inversion with a standard inversion. The robustness of the above-cited method (using a priori information in combination with model tuning) has been validated with the cross-section from the construction plans, where the reconstructed model is in agreement with the original design.