How Close are the Edges of a Closed Fracture?

A laboratory experiment is peresented where the theory of amplitude reflection tuning in thin beds is tested. The results show a very good agreement with the theory previsio

Non-Seismic Methods on Shallow Water Environments

Characterization of inland water is a topic of great interest due to the broad spectrum of potential applications. Applied geophysic boasts different techniques adapted to retrieve useful information about these kinds of environments. Certainly, the most common geophysical techniques used in shallow waters are the seismic methods. However, there are some situations in which seismic methods could fail. Although nowadays it does not exist a method able to solve completely this task, electromagnetic techniques are a cost efficient tool to provide useful information. Thanks to their versatility, we concentrated our attention on the possibility of the Ground Penetrating Radar (GPR) and of the low induction number electromagnetic multi-frequency soundings measurements, carried from the water surface. We started from acquisitions performed in controlled settings. We described how we reproduced the field condition of a riverine GPR survey in laboratory experimentation. We selected a 1500 MHz GPR antenna, and we studied five types of riverine bottom sediments. We developed two different approaches to interpret the GPR responses of the sediments: the velocity and the amplitude analysis. The amplitude analysis developed is particularly innovative and fit very well the field requirements. We tried to estimate the sediments porosities by some mixing rules by the electromagnetic properties founded with both the analysis performed. The comparison among the porosities provided by the GPR measurements and the porosities measured by direct methods confirm the accuracy of the velocity analysis and it highlights the poor reliability of the amplitude analysis. Successively, we tested our methodology in survey condition. We conducted an integrated geophysical campaign on a stretch of the river Po in order to check the GPR ability to discriminate the variability of riverbed sediments through an analysis of the bottom reflection amplitudes. We conducted continuous profiles with a 200MHz GPR system and a handheld broadband electromagnetic sensor. A conductivity meter and a TDR provided punctual measurements of the water conductivity, permittivity and temperature. The processing and the interpretation of both the GEM-2 and GPR data were enhanced by the reciprocal results and by integration with the punctual measurements of the electromagnetic properties of the water. The GPR measurements provided maps of the bathymetry and of the bottom reflection amplitude. The high reflectivity of the riverbed, derived from the GPR interpretation, agreed with the results of the direct sampling campaign that followed the geophysical survey. The variability of the bottom reflection amplitudes map, which was not confirmed by the direct sampling, could also have been caused by scattering phenomena due to the riverbed clasts which are dimensionally comparable to the wavelength of the radar pulse. About the multi-frequency electromagnetic sensor, we analyzed the induction number, the depth of investigation (DOI) and the sensitivity of our experimental setup by forward modeling varying the water depth, the frequency and the bottom sediment resistivity. The simulations led to an optimization of the choice of the frequencies that could be reliably used for the interpretation. The 3406 Hz signal had a DOI in the PO water (27 Ωm) of 2.5m and provided sediment resistivities higher than 100 Ωm. We applied a bathymetric correction to the conductivity data using the water depths obtained from the GPR data. We plotted a map of the river bottom resistivity and compared this map to the results of a direct sediment sampling campaign. The resistivity values (from 120 to 240Ωm) were compatible with the saturated gravel with pebbles in a sandy matrix that resulted from the direct sampling, and with the known geology

Waterborne GPR survey for estimating bottom-sediment variability: A survey on the Po River, Turin, Italy

We conducted an integrated geophysical survey on a stretch of the river Po in order to check the GPR ability to discriminate the variability of riverbed sediments through an analysis of the bottom reflection amplitudes. We conducted continuous profiles with a 200-MHzGPR system and a handheld broadband EM sensor.Aconductivity meter and a TDR provided punctual measurements of water conductivity, permittivity, and temperature. The processing and interpretation of the GEM-2 and GPR data were enhanced by reciprocal results and by integration with the punctual measurements of the EM properties of the water. We used a processing flow that improved the radargram images and preserved the amplitude ratios among the different profiles and the frequency content at the bottom reflection signal.We derived the water attenuation coefficient both from the punctual measurements using the Maxwell formulas and from the interpretation of the GPR data, finding an optimal matching between the two values. The GPR measurements provided maps of the bathymetry and of the bottom reflection amplitude. The high reflectivity of the riverbed, derived from the GPR interpretation, agreed with the results of the direct sampling campaign that followed the geophysical survey. The variability of the bottom-reflection-amplitudes map, which was not confirmed by the direct sampling, could also have been caused by scattering phenomena due to the riverbed clasts which are dimensionally comparable to the wavelength of the radar pulse

Misure elettromagnetiche a basso numero di induzione e radar, da natante, per lo studio dei fondi fluviali

Sono presentati i risultati di una campagna di misure elettromagnetiche a basso numero di induzione condotte da natante su un tratto del fiume Po a Torino. I risultati, verificati con una campionamento diretto, hanno permesso di stimare la natura dei sedimenti di fondo del fium

Esperienze di laboratorio sulla possibilità di discriminare porosità di sedimenti fluviali con misure radar dalla superficie dell'acqua.

In laboratorio, in una vasca con diverse altezze d'acqua che copriva sedimenti a diversa granulometria, sono state egffettuate misure GPR per valutare la possibilità di discriminare i sedimenti in base all'ampiezza di riflessione del segnale radar

Estimation of thin fracture aperture in a marble block by GPR sounding

In order to optimize the cutting of the marble blocks to be used in the restoration of the dome of the Holy Shroud Chapel of Turin cathedral, some GPR tests have been done in a laboratory to detect thin fractures in a marble block. We made some preliminary measurements to estimate the GPR wave velocity in an intact block and to calculate the permittivity of the marble. Then, the block was cut and GPR measurements with different fracture fillings and fracture apertures were performed. The results demonstrate that with at 2 GHz in antenna a fracture, a millimeter open, can be detected. An analysis carried out on the experimental reflection coefficients with the thin layer theory, allowed us to estimate the different fracture apertures. We found the possibility of estimating fracture apertures ranging from 1/10 to 1/5 of the wavelength at 2 GHz in air. The phase and the amplitude of the reflected signals with different fillings demonstrated the possibility of detecting thin fractures and discriminating between dry- or water -saturated filling

Joint Inversion of Surface-wave Dispersion and P-wave Refraction Data for Laterally Varying Layered Models

We present here a joint inversion method to build P- and S-wave velocity models from surface-wave and P-wave refraction data, specifically designed to deal with laterally varying layered environments which can present strong velocity contrasts with depth. In this case, a smooth minimum-structure inversion produces smooth models even for geological models, which are overall layered. The proposed algorithm is also able to incorporate a-priori information available over the site and any physical law to link model parameters. This method presents advantages with respect to individual surface wave analysis and refraction tomography since it imposes internal consistency for all the model parameters, reducing the required apriori assumptions and the ill-ness of the two methods. We describe the algorithm and we show its application to synthetic and field dataset