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Ottenere sezioni stack da dati di sismica a rifrazione: un tentativo di migliorare la conoscenza sulla geometria del sottosuolo
No full textShallow refraction seismic survey is a very common and useful subsurface investigation method. However,
traditional shallow seismic refraction processing methods show long-standing limitations due to the almost complete
reliance on the first arrival signal. On the other hand, full wave form processing of data can by-pass inherent limitations
and improve the imaging of the subsurface. In this sense, better results can be achieved through digital processing that
has been successfully developed in reflection seismology. In particular, a time cross-section, similar to the well known
reflection cross-section, can be generated through processing of reflected signals, using tools like CMP stacking that
improves the signal-to-noise ratio. Other tools imply deconvolution and migration for improvement of, respectively,
vertical and lateral resolution. Muting and dip filtering for deletion of coherent noise. This note presents a discussion on
the feasibility of this kind of approach. Different time cross-sections from different processing tests were compared with
a HR reflection seismic section used as calibration: particularly, triangular mute windows and the exclusion of the traces
relative to the external shots enable the elimination of the strong ringing due to the first arrivals and this tends to be the
main problem; furthermore, the external shots prove to be useful for the reconstruction of the deeper structures (with
arrival times greater than 400 ms) but not as useful for the reconstruction of the superficial structures
Resistivity variations affecting ert detectability and imaging of a thin conductive layer
No full textModelling the influence of the starting model on ert detectability and imaging of a thin conductive layer
No full textTHE USE OF SYNTHETIC DATASET MODELLING TO ASSESS THE DETECTABILITY OF ERT SURVEYS WITH DIFFERENT ARRAYS WHEN IDENTIFYING KARST INTERVALS IN THE HES OF POITIERS (FRANCE)
No full textElectrical Resistivity Tomography (ERT) surveys were undertaken at the Hydrogeological Experimental Site (HES) of Poitiers, France to identify karst intervals at depth of 35-40 m and 85-87 m host in the Dogger Limestone fractured-karst aquifer. Dogger Limestone occurs at a depth ranging between 30 and 120 m and is overlain by argillaceous limestone. Three-dimensional resistivity imaging was obtained from full inversion of combined 2D ERT data collected along five parallel 470 m long profiles with 50 m line spacing. Data were collected using both Wenner-Schlumberger (WS) and Pole-Dipole (PD) arrays. A 3D block measuring 481 × 203 m in size with a maximum depth of 100 m was surveyed. The hybrid array sequence (the combination of WS and PD array sequences) showed better imaging, due to the combination of the large vertical resolution of WS, large lateral resolution and penetration depth of PD. First, resistivity soundings have been sorted from ERT apparent resistivity data in order to verify the ERT detectability when identifying karst intervals. Then, synthetic dataset modelling was performed to assess the detectability of the ERT resistivity block when identifying karst intervals. Different starting models were implemented to perform forward modelling. The use of a very heterogeneous and anisotropic starting model implemented by using all information available from resistivity and stratigraphic logs and imagery of borehole walls provided unrealistic results. This is due to the too high level of detail of the starting model respect to the ERT system detectability and resolving ability. Then, a simpler, layered starting model comprising argillaceous limestone, low porosity limestone layers and karst intervals was implemented. Inversion of the synthetic dataset obtained by forward modelling was performed. Modelling allowed addressing different issues limiting the karst intervals detectability: an excessively low thickness with respect to the system's ability; an excessively large resistivity gap when compared to the shallow argillaceous limestone that also reduced the investigation depth and led to a severe underestimate of the limestone resistivity; variation of the layers resistivity with depth
Resistivity imaging of salt paleo-water contaminations: Comparing 1D, different array 2D and 3D methods at the San Re test site (Northern Italy)
No full textFROM INVERSE RESISTIVITY MODEL TO RAW DATA: ASSESSING THE DETECTABILITY OF A THIN CLAY LAYER WITHIN THE ALLUVIAL AQUIFER OF THE SUNCERI TEST SITE (HONDURAS)
No full textERT surveys were undertaken at the Sunceri test site located in the city of San Pedro Sula (Honduras). Four deep pumping wells are located at the site, together with 26 shallow and 12 deep piezometers drilled through alluvial deposits. Piezometric levels, hydro-chemical features of groundwater and borehole logs indicate that a clay layer is located at a depth of approximately 24 m and separates an upper unconfined aquifer from a deeper confined aquifer. This layer is widespread across the entire area and shows a variable thickness (6 m on average) and an average resistivity of 17 Qm. The inverse resistivity sections reveal the presence of the layer. However, they fail to show that it is discontinuous across the area. In particular, the layer doesn't appear in an area affected by a severe drawdown of the piezometric level due to the interference between the cones of depression of the abstraction wells. Elsewhere, the inverse resistivity sections show a local overestimation of the layer thickness when compared to borehole logs. To explain these inconsistencies and assess the layer detectability it was necessary to look for the clay layer within the raw data. One dimensional resistivity soundings have been sorted from ERT apparent resistivity data. Then, inverse resistivity sections were compared to raw data resistivity soundings. Furthermore, to assess the influence of the shallow resistivity heterogeneities and the severe drawdown of the piezometric level on the apparent resistivity data, 2D synthetic dataset modelling and sensitivity analysis were performed. The results indicated that in the area affected by a severe drawdown of the piezometric level, the clay layer is undetectable where its thickness falls below 4.9 m. Elsewhere, the apparent thickness variation of the clay layer is due to the variation of the resistivity with depth and the occurrence of shallow lateral heterogeneities as saturated and unsaturated zones, depression cones and the drawdown of the piezometric level affecting the sensitivity and also the investigation depth along the profile
VSP processed down-hole data within local seismic response assessment
No full textLocal effects evaluation may be solved using different methodological ways. VEL Project of Tuscany district
approach, foresees the evaluation of the surface effects in a specified zone of a "forecasted" earthquake, through geological,
geomorphologic, geotechnical, geophysical and numerical modelling. The knowledge of bedrock depth and geometry is a
fundamental issue in such a multidisciplinary integrated approach. Down-hole tests carried out in not sufficiently deep
borehole to reach bedrock, have been processed as VSP (Vertical Seismic Profiling) method, allowing the knowledge of
bedrock depth. This has been obtained developing a processing aimed to the study of reflected signals that provided
important information on discontinuities occurring below the borehole bottom. Results obtained by VSP processing from
conventional SH waves down-hole data, have been calibrated by comparison with an high-resolution shear wave reflection
line carried out in the same site. This comparison provided new cognitive elements and work prospects
Application of high resolution shear wave seismic methods to a geotechnical problem
No full textApplied geophysical techniques in
underground exploration can be useful in defining
the subsoil geometry and the spatial relationships of
the physico–mechanical properties of the materials.
An example is presented to demonstrate how highresolution
seismic techniques using shear waves can
assist with a geotechnical issue. The survey has been
carried out to elucidate the subsoil geometry and
characterize lithotypes in terms of physicomechanical
parameters, useful in the geotechnical
study of an historic monument in which fractures
developed following a downslope excavation
COMPARISON BETWEEN VLF-EM AND RESISTIVITY ANOMALIES ASSOCIATED WITH SALT PALEO-WATERS CONTAMINATIONS IN AN ALLUVIAL AQUIFER: THE SAN RE TEST SITE (NORTHERN ITALY)
No full textThe alluvial aquifer of the Oltrepò Pavese plain sector (Po Valley, Northern Italy) is contaminated by Na-Cl paleo-waters, rising from the tertiary substratum and mixing with the shallow groundwater. This phenomenon is localized along the Vogherese Fault, a buried tectonic discontinuity. Geophysical surveys were undertaken in two separate phases to map and characterize the contaminations. The first phase involved resistivity depth soundings undertaken along a cross section of the Vogherese Fault and VLF-EM surveys carried out over 150 km2 for a rapid assessment of the distribution of saline waters, even where no wells for sampling are available. NE-SW trends of high conductivity anomalies were revealed. These trends can be correlated to the occurrence of the Vogherese Fault trace and secondary sub-parallel discontinuities along which paleo-waters uprise. San Re test site was chosen as representative of the entire study area to carry out a more detailed phase of investigations which included: four resistivity depth soundings, five 2D ERT surveys (470 m long) that were undertaken along an approximately 2600 m long profile crossing the fault zone and overlapping a significant length of VLF-EM surveys and a resistivity profiling; another ERT survey was undertaken at a transversal angle to the profile; four 3D ERT surveys and a short spread 2D ERT were undertaken to achieve a detailed investigation of the salt water plumes. The overlapping of different surveys allowed a comparison between electromagnetic and resistivity anomalies associated with the occurrence of saltwater contaminations. The detectability of VLF-EM and resistivity profiling surveys when identifying saltwater contaminations was defined through a comparison with the saltwater plumes imaging obtained by 2D and 3D ERTs. On a total of 14 saltwater contaminations (steeply-dipping and well coupled with the transmitter) with a length ranging between 8 and 195 m, 50 % have been detected as conductivity boundary, while 50 % as conductivity body by VLF-EM surveys
Time-lapse 3D ERT imaging of a brackish paleo-water contamination at the alluvial aquifer of the San Re test site (Northern Italy)
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