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Sperimentazione di procedure specifiche per l’elaborazione di dati sismici a riflessione nell’ambito della risoluzione di geometrie e proprietà petrofisiche di acquiferi superficiali
No full textNegli ultimi anni il metodo sismico, e più in particolare la tecnica a riflessione, sono state raramente impiegate in campo ambientale per lo studio e la valutazione di stato di contaminazione o di vulnerabilità di siti.
Questo fatto è in parte legato ai costi di esecuzione di un indagine sismica a riflessione con metodologia CMP, notoriamente onerosa, e secondariamente nel limitato grado di conoscenza della potenzialità risolutiva delle diverse tecniche geofisiche, oggigiorno disponibili, da parte degli operatori del settore.
Solo recentemente la crescente domanda di indagini non-distruttive, fortemente caratterizzanti, in associazione con il notevole progresso tecnologico delle piattaforme di acquisizione ed elaborazione dati, hanno reso realizzabile l’impiego della sismica a riflessione in questo ambiti.
Il soggetto di studio di questa tesi si inquadra in un progetto di esplorazione a scala regionale per la definizione di geometrie e proprietà petrofisiche di acquiferi poco profondi, in adiacenza ad alcuni distretti minerari in depositi glacio-fluviali del Pleistocene nel Saskatchewan meridionale in Canada. La caratterizzazione di questi acquiferi di bassa profondità è finalizzata alla tutela ed alla salvaguardia degli stessi e di acquiferi più profondi. Questi acquiferi infatti, potenzialmente sfruttabili per l’approvvigionamento idropotabile, sono a rischio di contaminazione per effetto di una possibile infiltrazione e migrazione di salamoie inquinanti dalle zone di stoccaggio e di accumulo dei materiali di risulta dei processi di estrazione e lavorazione del minerale.
Il progetto prevede la sperimentazione di tecniche avanzate di acquisizione ed elaborazione dati per la sismica a riflessione superficiale, anche in una prospettiva di studio di diverse sorgenti acustiche per un analisi comparata dei costi di esecuzione. In via generale va infatti sottolineato come il costo per la predisposizione della sorgente possa superare il 40% del totale dell’intero budget di esplorazione.
Ciascun aspetto dell’esperimento è stato progettato in modo da tenere in considerazione i rapporti di sequenzialità tra le successive fasi di acquisizione ed elaborazione dei dati sismici.
Attraverso la modellizzazione delle equazioni d’onda mediante le tecniche del tracciamento dei raggi e della riflettività, sulla base dei dati litostratigrafici disponibili, provenienti da terebrazione di pozzi, è stato possibile valutare la qualità dei tests sismici condotti in campo, stimare la risposta sismica locale ed eseguire una reale calibrazione dei parametri di acquisizione.
L’acquisizione dei dati, avvenuta con tecnica CMP, ha seguito pertanto le linee guida ricavate dalla modellizzazione bidimensionale ed è stata mirata, differentemente dalla sismica a riflessione petrolifera “convenzionale”, all’ottenimento di un elevato grado di risoluzione vertico-laterale nell’offset vicino e nella parte iniziale della scala temporale.
L’analisi e l’elaborazione dei dati sono stati indirizzati ad un superamento delle procedure classiche utilizzate nelle investigazioni di sismica profonda. Sono state sviluppate delle tecniche di filtraggio mono e bidimensionale, idonee all’elaborazione dei dati sismici di bassa profondità dove le riverberazioni, il rumore incoerente e l’elevato livello di energia delle onde di superficie mascherano quasi completamente il segnale riflesso.
Sono stati sperimentati ed adattati diversi algoritmi computazionali (deconvoluzione, trasformata F-K e trasformata K-L) ed, in fase avanzata, è stata creata una piattaforma di elaborazione a supporto dello studio delle modificazioni di carattere dell’evento riflesso (ampiezza e fase) al variare dell’offset.
I temi di approfondimento specifico e conseguentemente gli aspetti innovativi hanno pertanto riguardato parte di questi aspetti.
Gli algoritmi di calcolo sui quali è stata condotta la sperimentazione sono stati il filtraggio F-K, in combinazione recursiva con la modellizzazione del campo velocità-profondità, ed il filtraggio K-L ad alta percentuale di ricostruzione del segnale. Queste applicazioni sono state confermate nella loro efficacia per la risoluzione di eventi riflessi in vicinanza della sorgente dove il rapporto “segnale/rumore coerente” è notoriamente molto basso.
La sperimentazione di una applicazione di analisi di variazione di ampiezza secondo l’offset (AVO), è stata condotta in corrispondenza ad un pozzo stratigrafico, su orizzonti riflettenti di bassa profondità, correlati, in sede di analisi ed interpretazione della sezione stack, con taluni limiti litologici.
Sono stati calcolati una serie di modelli teorici di variazione del rapporto di Poisson, attraverso un interfaccia acustica, ad alta riflettività, modificando opportunamente il campo di velocità intervallare nelle diverse unità litostratigrafiche.
I valori teorici dei coefficienti di riflessione, così calcolati, sono stati confrontati prima con le ampiezze delle fasi riflesse osservate su gathers sintetici ad offset variabile e poi con le ampiezze delle fasi riflesse misurate su un set di dieci shot gathers. E’ stato in questo modo possibile stimare il responso della dipendenza tra proprietà petrofisiche di unità litologico-acustiche di bassa profondità e comportamento AVO, e soprattutto come questo comportamento AVO sia “leggibile” in un dataset sismico di bassa profondità
G.P.R. imaging capabilities: the experience in the "Laurentina" archaeological test site
No full textA G.P.R. survey was recently carried out in the «Laurentina - Acqua acetosa» archaeological site near Rome. The aim of the survey was two fold: assessing the potential capabilities of this geophysical technique as related to data acquisition, processing and interpretation; developing a strategy to investigate archaeological sites.
According to an existing map of the buried ruins, derived from the site excavation, it was possible to evaluate the real imaging power of G.P.R. As a matter of fact all the archaeological structures were clearly outlined by radar images in the G.P.R. response. In addition other strong reflectors, due to excavation activity, were accurately mapped.
Data acquisition was designed to develop a strict comparison of various sets of instruments. 300-400 MHz single antennas and 500 MHz antenna arrays were carefully tested in the field to optil1lize the acquisition parameters and estimate an initial e.m. response.
A strong data processing was an heavy but necessary step in order to properly enhance the e.m. response of walls and pavements. The effectiveness of the processing algorithms results in a definite improvement of the radar image quality, expecially of the excavated
trench bottom signatures. Clutter frequency of the heterogeneous subsurface were worked out utilizing multi-frequency antennas. All these G.P.R. examples clearly exhibit a difference of resolving power between raw and processed data. A 3D arrangement and display can further enhance radar results and improve the final section interpretability
Shallow reflection imaging of complex stratigraphic features in shallow deposits
No full textA high resolution P-wave seismic reflection survey was conducted in the vicinity of a Potash Mine in southern Saskatchewan, Canada. The objectives of the survey were to image structure and stratigraphy of shallow Pleistocene glacial deposits and to compare the resolution of two different acoustic sources in this geological setting (explosive and elastic wave generator-EWG) .
Field records exhibit poor signal to noise ratios due to strong surface waves and reverberative disturbance contaminations. Optimized FK and KL transforms were utilized to attenuate these coherent noises and to enhance the primary reflections from the very shallow horizons . The data analysis was also assisted by forward modeling to guide spacial and temporal selection of processing parameters .
A number of stratigraphically well defined horizons generated detectable signals between the surface and the Cretaceous bedrock. In addition, the seismic images permitted to outline fine structural details within the different depositional sequences. The final section revealed that reflection technique provides a very powerful method to characterize aquifers within Pleistocene glacial stratigraphy
A fast and integrated geophysical imaging system for large scale levee monitoring
No full textDuring recent floods in Northern and Central Italy as well as in the entire European Region several levee failures occurred and caused severe damages to the agriculture, to the nearby buildings and to the infrastructures. In most of the cases the collapse was due to high permeability anomalies within and below the levee body and frequently to recent development of small to medium size cavities caused by biological activity. The conditions of river embankments, especially when the structures are very old, turn then into a social and vital issue as a failure could cost up to hundreds of millions of Euro of damages and in extreme cases could also result in several casualties. Visual inspection of the surface along with sparse geotechnical testing has been the standard for several years for levee safety assessment but this approach is not fully satisfactory. Cone penetration tests and boreholes provide a detailed stratigraphy of the levee itself and of the underlying deposits but unfortunately it provides local information and no data are available in between tested spots. Geophysical methods could then play a major role in filling these gaps. Resistivity profiling (ERT), among the various geophysical techniques, is probably the most effective but it is fairly slow and expensive to become a practical tool for extensive monitoring of thousands of kilometres of levees. An integrated geophysical imaging system (namely EMAR), based on multifrequency electromagnetic (FDEM) induction and multichannel radar (GPR), has been developed, tested and validated along more than 100 kilometres of river embankments in North-eastern Italy. Using this approach it was possible to investigate up to 20 km of embankment per day showing interesting potentials for regional surveying and large scale monitoring. The EMAR anomalies (an average of two per kilometre) were further investigated with ERT profiles and, when confirmed, a small trench was opened to visually inspect the inner portion of the levee. A percentage of more than 90% of the first phase anomalies resulted in ERT anomalies caused by sand layers in the levee structure. The FDEM techniques are known to be affected by environmental noise and often also by sudden changes in the field background. To avoid the complexity of frequent FDEM site calibration and of the daily magnetic drift of the sensors, we developed a novel approach for EM data processing. Each “homogeneous” levee segment was processed separately and the resistive anomalies (high permeability sand layer or cavity), were identified as large negative deviation from a mean value typical of that levee segment. This procedure was repeated for all the various spacings and frequencies used during the survey. GPR data, after a semi-automatic check of the cart trajectories (15 channels), were also processed automatically to map anomalous reflectors spanned in 12 time planes, calculated every 5 ns in the two-way traveltime interval 0 ns — 60 ns. A GIS multivariate analysis algorithm was finally adapted to compare and weight the different signals (typically 3 EM frequencies and 12 radar reflectivity planes) and assign the levee heterogeneity to different classes of failure risk
High-resolution images of shallow aquifers—A challenge in near-surface seismology
No full textA near-surface multifold high-resolution seismic reflection experiment was conducted in the vicinity of the waste management area of a potash mine in western Canada. A buried channel was identified in the data, and the stratigraphy of the Quaternary infill of this structure was mapped. The spatial extent of several prominent gravel-sandy aquifers, which represent the hydrogeologic framework of the region, was outlined by the survey. The seismic signatures also established the hydraulic independence of three major aquifers along the survey line.
The complex heterogeneous lithology of the surface cover limited effective elastic-wave generation to surface sources. This geologic framework also caused propagation of strong diverse coherent-noise patterns which severely degraded reflected signal. The suppression of those overhelming interfering events required the design of noise-specific filters and their sequential multistep implementations. Results of forward modeling of background geologic information were crucial factors in the design of the data acquisition program and preliminary choices of the processing parameters, and (along with borehole data) were the primary guidance in the geologic interpretation of the final seismic section.
Fundamental procedures were developed for mapping of glacial tills in theWestern Canadian Basin, techniques
that can be applied in other regions with similar near surface glacial stratigraphy. The experiment revealed that even closely spaced borehole information could never duplicate the detail of the subsurface images of the seismic data
Integrated Refraction Seismics and Tomographic Study of a Gravitational Collapse Phenomenon
No full textThe aim of this work is to create and test an automatic procedure to define an accurate
subsurface velocity model by using the first arrivals in the case of seismic acquisition
of a large quantity of data. We developed an integrated procedure involving
conventional refraction analysis and joint tomography of the direct (diving waves) and
refracted arrivals (head waves). We can thus obtain 3D velocity models which yield
detailed images of subsoil structure, and allow to define reliable geologicalgeophysical
models of use to mitigate landslide phenomena. We tested the procedure
in an area affected by gravitational collapse located in north-eastern Italy, in the
Dolomites near the city of Belluno; the application of the proposed method allowed
the definition of the depth and shape of the bedrock horizon
Assessment of channel side-walls with GPR techniques
No full textA Ground Penetrating Radar (G.P.R.) survey was recently carried out in the Murano island near Venice. The aim of the survey was two fold: outline the potential capabilities of this geophysical technique as related to assessing physical properties of channel side walls before and after cement infiltration; achieve an initial understanding of electro-magnetic wave propagation in fairly conductive layers and estimating the depth of buildings' foundations to optimize ground consolidation activities.
The attention to this matter has grown up in the last years in relation to the necessity of excavating the bottom of some channels to remove natural and artificial sediments for boat traffic resto ring and to re-design the channel profile to improve the side-wall stability.
The water. pumping and the mud excavation, prior to the maintenance intervention, dramatically increase the stress on the channel-side walls and could cause, especially in presence of fractures and cavities, a collapse of the wall structure with serious damages to the near buildings' foundations.
In order to address all these issues the problem was structured in some different steps: c1early imaging the sub surf ace of the "fondamenta" (passagewalk on the side of the building facing the channel) inc1uding a rough estimation of buildings' foundations and technical utilities location, detecting weak zones, such as cavities and fractures in the wall, and estimating the efficiency of cement mixture infiltration
Venice channel side-wall assessment with GPR technique - A case study
No full textA Ground Penetrating Radar survey was recently conducted in the Murano Island in the Venice Lagoon, to assist an ongoing channel and bank refurbishment activity. The major survey targets were detected and resolved down to the depth of the salty water table. The brine saturated environment limited effective propagation of electromagnetic waves. The results of some test records, conducted in a drained bank, have been obtained after a crucial trial phase of the optimum transducers and data processing choices. Radar penetration capability was suddenly increased after concrete reinforcement injection into bank's brine filled fractures and cavities. This occurrence has suggested the use of the radar system as a sort of reflectometer to map temporal changes in the dielectric properties of the investigated structure. Some field procedures have been developed with non-invasive and cost- effective radar imaging techniques to assess the quality and the amount of the grout injection. GPR results, confirmed by drilling cores, have been useful to calibrate the best concrete mixture. The detail of the radar images and its capability to provide comprehensive information proved to be an essential complement to direct investigations in engineering quality test of wall assessment. © 2004 Elsevier B.V. All rights reserved
High-resolution geophysical investigation of the archaeological site “Le Pozze” in the surroundings of the town of Lonato (Brescia, Northern Italy)
No full textDuring the summer 2004 a high-resolution GPR survey was conducted in an archaeological area located within a short distance from the town of Lonato in northern Italy by a new GSSI Terravision system. The aims of the survey were to sketch the geometry and the nature of the buried structures and to experiment a new approach in surveying large surfaces. An area of approximately 70.000 sqm was surveyed in less than 16 working hours with a inline trace spacing of 0.025 m and an average crossline spacing of 0.2 m for a total of about 490 km of GPR profiling. Spatial positioning was controlled with a separate on-board differential GPS system. The large amount of collected data was processed on a workstation-based environment under the Sun Solaris architecture with the seismic-unix package. Specific codes have been developed to integrate navigation and radar data. A number of well-defined reflectors are clearly visible in the final data volume. The framework of the buried structures was revealed by the geophysical images proving the potentials of the new radar system to assist large scale archaeological exploration
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