Jacopo Boaga

Shear wave profiles from surface wave inversion: the impact of uncertainty on seismic site response analysis

Inversion is a critical step in all geophysical techniques, and is generally fraught with ill-posedness. In the case of seismic surface wave studies, the inverse problem can lead to different equivalent subsoil models and consequently to different local seismic response analyses. This can have a large impact on an earthquake engineering design. In this paper, we discuss the consequences of non-uniqueness of surface wave inversion on seismic responses, with both numerical and experimental data. Our goal is to evaluate the consequences on common seismic response analysis in the case of different impedance contrast conditions. We verify the implications of inversion uncertainty, and consequently of data information content, on realistic local site responses. A stochastic process is used to generate a set of 1D shear wave velocity profiles from several specific subsurface models. All these profiles are characterized as being equivalent, i.e. their responses, in terms of a dispersion curve, are compatible with the uncertainty in the same surface wave data. The generated 1D shear velocity models are then subjected to a conventional one-dimensional seismic ground response analysis using a realistic input motion. While recent analyses claim that the consequences of surface wave inversion uncertainties are very limited, our test points out that a relationship exists between inversion confidence and seismic responses in different subsoils. In the case of regular and relatively smooth increase of shear wave velocities with depth, as is usual in sedimentary plains, our results show that the choice of a specific model among equivalent solutions strongly influences the seismic response. On the other hand, when the shallow subsoil is characterized by a strong impedance contrast (thus revealing a characteristic soil resonance period), as is common in the presence of a shallow bedrock, equivalent solutions provide practically the same seismic amplification, especially in the frequency range of engineering interest.

Soil–plant interaction monitoring: Small scale example of an apple orchard in Trentino, North-Eastern Italy

Accurate monitoring and modeling of soil-plant systems are a key unresolved issue that currently limits the development of a comprehensive view of the interactions between soil and atmosphere, with a number of practical consequences including the difficulties in predicting climatic change patterns. This paper presents a case study where time-lapse minimal-invasive 3D micro-electrical tomography (ERT) is used to monitor rhizosphere eco-hydrological processes in an apple orchard in the Trentino region, Northern Italy. In particular we aimed at gaining a better understanding of the soil-vegetation water exchanges in the shallow critical zone, as part of a coordinated effort towards predicting climate-induced changes on the hydrology of Mediterranean basins (EU FP7 CLIMB project). The adopted strategy relied upon the installation of a 3D electrical tomography apparatus consisting of four mini-boreholes carrying 12 electrodes each plus 24 mini-electrodes on the ground surface, arranged in order to image roughly a cubic meter of soil surrounding a single apple tree. The monitoring program was initially tested with repeated measurements over about one year. Subsequently, we performed three controlled irrigation tests under different conditions, in order to evaluate the water redistribution under variable root activities and climatic conditions. Laboratory calibration on soil samples allowed us to translate electrical resistivity variations into moisture content changes, supported also by in-situ TDR measurements. Richards equation modeling was used also to explain the monitoring evidence. The results clearly identified the effect of root water uptake and the corresponding subsoil region where active roots are present, but also marked the need to consider the effects of different water salinity in the water infiltration process. We also gained significant insight about the need to measure quantitatively the plant evapotranspiration in order to close the water balance and separate soil structure effects (primarily, hydraulic conductivity) from water dynamics induced by living plants.

Plant-soil interactions in salt marsh environments: Experimental evidence from electrical resistivity tomography in the Venice Lagoon

The role of root water uptake in regulating soil water saturation in salt marshes is controversial. Modeling studies suggest that soil aeration is improved by transpiration, with implications for the distribution of vegetation species and of the associated topographic features controlling the hydraulic regime of the marshland and eventually its survival. Marsh vegetation plays a key role in the preservation of such critical environment, which represents unique marker for climatic change and impact studies. However, the direct quantification of space-time aeration patterns has remained elusive, in part, because of the limitations posed by high salinity to conventional observation techniques such as time or frequency domain reflectometry. Here we show that time-lapse microscale electrical resistivity tomography, coupled with tensiometric observations, allows the identification of variably saturated zones and the characterization of space-time soil moisture dynamics in a salt marsh in the Venice Lagoon (Italy). This is the first quantitative observational experiment which confirms that periodically flooded plants are capable of producing a persistently aerated layer below the flooded surface when transpiration proceeds at a sufficiently high rate. The experimental results are compared against previously published model predictions.

Noninvasive characterization of the Trecate (Italy) crude-oil contaminated site: links between contamination and geophysical signals

The characterization of contaminated sites can benefit from the supplementation of direct investigations with a set of less invasive and more extensive measurements. A combination of geophysical methods and direct push techniques for contaminated land characterization has been proposed within the EU FP7 project ModelPROBE and the affiliated project SoilCAM. In this paper, we present results of the investigations conducted at the Trecate field site (NW Italy), which was affected in 1994 by crude oil contamination. The less invasive investigations include ground-penetrating radar (GPR), electrical resistivity tomography (ERT), and electromagnetic induction (EMI) surveys, together with direct push sampling and soil electrical conductivity (EC) logs. Many of the geophysical measurements were conducted in time-lapse mode in order to separate static and dynamic signals, the latter being linked to strong seasonal changes in water table elevations. The main challenge was to extract significant geophysical signals linked to contamination from the mix of geological and hydrological signals present at the site. The most significant aspects of this characterization are: (a) the geometrical link between the distribution of contamination and the site’s heterogeneity, with particular regard to the presence of less permeable layers, as evidenced by the extensive surface geophysical measurements; and (b) the link between contamination and specific geophysical signals, particularly evident from cross-hole measurements. The extensive work conducted at the Trecate site shows how a combination of direct (e.g., chemical) and indirect (e.g., geophysical) investigations can lead to a comprehensive and solid understanding of a contaminated site’s mechanisms.

The Influence of Subsoil Structure and Acquisition Parameters in MASW Mode Mis-identification

ABSTRACT Inversion of surface wave dispersion properties is commonly used to derive shear wave velocity depth profiles. However, one of the critical and yet rarely considered issues in this ill-posed inversion process is mode contamination. Rayleigh dispersion modes are the theoretically possible solutions of motion. Experimentally, we define Rayleigh dispersion properties from spectra energy maxima in some domain (as f–k), thus possibly producing only apparent experimental dispersion curves, where energy spreads onto several modes. If this phenomenon is not recognized, the inversion of an apparent dispersion curve can produce results unrelated to the actual subsurface structure. In this work, we present the results of synthetic tests that highlight the most common subsoil conditions and acquisition pitfalls that can give rise to surface wave mode contamination. In particular, we consider three typical subsoil structures that can produce this phenomenon: 1) a simple two-layer system with a strong impedanc...

Frequency-dependent multi-offset phase analysis of surface waves: an example of high-resolution characterization of a riparian aquifer

Multi-offset phase analysis of seismic surface waves is an established technique for the extraction of dispersion curves with high spatial resolution and, consequently, for the investigation of the subsurface in terms of shear wave velocity distribution. However, field applications are rarely documented in the published literature. In this paper, we discuss an implementation of the multi-offset phase analysis consisting of the estimation of the Rayleigh wave velocity by means of a moving window with a frequency-dependent length. This allows maximizing the lateral resolution at high frequencies while warranting stability at the lower frequencies. In this way, we can retrieve the shallow lateral variability with high accuracy and, at the same time, obtain a robust surface-wave velocity measurement at depth. In this paper, we apply this methodology to a dataset collected for hydrogeophysical purposes and compare the inversion results with those obtained by using refraction seismics and electrical resistivity tomography. The surface-wave results are in good agreement with those provided by the other methods and demonstrate a superior capability in retrieving both lateral and vertical velocity variations, including inversions. Our results are further corroborated by the lithological information from a borehole drilled on the acquisition line. The availability of multi-offset phase analysis data also allows disentangling a fairly complex interpretation of the other geophysical results.

An efficient tool for cultural heritage seismic soil classification: frequencytime analysis method in Venice historical center and its lagoon (Italy)

Invaluable historical heritage needs peculiar protections from natural hazards. Seismic risk is one of the most dangerous menaces for historical building, due to their critical seismic response. Venice historical center and its lagoon (Italy) is one of the most famous humanity assets. The city center is an urban complex grown in more than one thousand years, based on poor lagoon sediments. This urban environment is particularly fragile. From a technical point of view it presents several limitations to characterize subsoil properties, especially seismic velocities of soil. In this study it is proposed the single station frequency-time analysis (FTAN) as efficient shear wave velocity survey methodology. FTAN procedure has very simplified field logistics, if compared to multi-channels common methods. The work presents a case study of FTAN applications on several sites of the Venice historical center and its lagoon. The application of this geophysical technique allows an estimation of the seismic velocities of soil, necessary for seismic response evaluations. Thanks to the results of shear-wave velocity the soil was classified by the use of the seismic European classification and the Italian earthquake-proof regulation. Since the Venice subsoil is characterized from inhomogeneous deposits, even very near sites can present different seismic velocities. The work results enhance these different soil properties, in good agreement with the most recent lithostratigraphic reconstruction of the Venice subsoil. Different Vs conditions can lead to substantially different seismic behaviors, imposing particular characterizations for aseismic purposes and site response analyses. FTAN methodology efficiently meets geological reconstruction. This non invasive procedure seems to accurately answer the modern anti-seismic design requirements, even in such a difficult historical urban complex.

Reply to comment on Shear wave profile from surface wave inversion: the impact of uncertainty on seismic site response analysis

Socco et al (2012 J. Geophys. Eng. 9 241) comment on our study about the effect of non-uniqueness of surface wave solutions on seismic site response analysis. In particular, they refer to the approach we adopted for the selection of equivalent shear wave velocity profiles and argue that it leads to overestimation of the uncertainty due to the inherent ill-posedness of the problem. Moreover, for one of the synthetic cases of our original paper, they calculate a different set of equivalent velocity profiles, retrieving the corresponding amplification spectra. From these results, Socco et al claim that their general conclusion that the impact of solution non-uniqueness on seismic response simulations is negligible. In this reply we demonstrate that (a) the uncertainty bounds used by Socco et al in their prediction analysis, as a consequence of their surface wave inversion procedure, are unreasonably narrow; (b) consequently, their shaking predictions appear to suffer no impact from their underestimated uncertainty; and (c) their presented case shows an amplification spectrum that is only the result of assuming the existence of a bedrock at 150 m that causes resonance of the overlying layer—practically independent of the details of the S-wave velocity distribution.

Insights into bedrock surface morphology using low-cost passive seismic surveys and integrated geostatistical analysis

The HVSR (Horizontal to Vertical Spectral Ratio) technique is very popular in the context of seismic microzonation and for the mapping of shallow seismic reflectors, such as the sediment/bedrock transition surface. This easy-to-deploy single station passive seismic technique permits the collection of a considerable amount of HVSR data in a cost-effective way. It is not surprising that some recent studies have adopted single station micro-tremor analyses in order to retrieve information on geological structures in 1D, 2D or even 3D reconstructions. However, the interpolation approaches followed in these studies for extending the punctual HVSR data spatially are not supported by a detailed spatial statistical analysis. Conversely, in order to exploit the informative content and quantify the related uncertainty of HVSR data it is necessary to utilize a deep spatial statistical analysis and objective interpolation approaches. Moreover, the interpolation approach should make it possible to use expert knowledge and auxiliary information. Accordingly, we present an integrated geostatistical approach applied to HVSR data, collected for retrieving information on the morphology of a buried bedrock surface. The geostatistical study is conducted on an experimental dataset of 116 HVSR data collected in a small thermal basin located in the Venetian Plain (Caldiero Basin, N-E Italy). The explorative geostatistical analysis of the data coupled with the use of interpolation kriging techniques permit the extraction of relevant information on the resonance properties of the subsoil. The utilized approach, based on kriging with external drift (or its extension, i.e. regression kriging), permits the researcher to take into account auxiliary information, evaluate the related prediction uncertainty, and highlight abrupt variations in subsoil resonance frequencies. The results of the analysis are discussed, also with reflections pertaining to the geo-engineering and geo-environmental context.

HVSR technique in near-surface thermal-basin characterization: the example of the Caldiero district (North-East Italy)

AbstractIn the present work, non-invasive microtremor survey is applied for the preliminary characterization of the thermal district of Caldiero, in the eastern Po Plain (Italy), a thermal resource known since the Roman times for the presence of warm springs. The work’s aim was to test the suitability of the microtremor passive technique for the preliminary characterization of a thermal basin, in terms of resonance properties between the alluvial deposit covers and the rocky bedrock. In particular, the horizontal-to-vertical spectral ratio (HVSR) single-station technique is adopted to estimate the resonance properties of the geothermal basin. More than 100 HVSR single-station measurements are collected and analyzed in the area studied. HVSR information is integrated with the findings of the analysis of the data relative to more than 20 deep wells. Such an integration permits the calibration for validation of the geophysical data. The correlation between the results from microtremor survey, LiDAR data analysis, and wells information highlighted the main geological structures of the site, which are potentially responsible for the thermal waters arising. HVSR results are then interpreted in terms of regional geological setting, textural, and hydrogeological properties of the superficial deposits and the underlying rocky bedrock. The results of this method confirm the existence of significant correlations between the resonance behavior of the basin, as determined by the HVSR technique, and the geological assessment. This suggests that the single-station technique could be a very promising tool for a rapid and preliminary non-invasive method for the near-surface general characterization of geothermal basins.