Patrizio Signanini

3D ERT Imaging Of The Fractured-Karst Aquifer Underlying The Experimental Site Of Poitiers (France): Comparing Wenner-Schlumberger, Pole-Dipole And Hybrid Arrays

Electrical Resistivity Tomography (ERT) surveys were undertaken to investigate the Dogger Limestone fractured-karst aquifer at the Hydrogeological Experimental Site (HES) of Poitiers, France. Three-dimensional resistivity imaging was obtained from full inversion of combined 2D ERT data collected along five parallel 470 m long profiles with a 50 m line spacing. A 3D block measuring 515 x 203 m in size with a maximum depth of 100 m was surveyed. Dogger Limestone occurs at a depth ranging between 30 and 120 m and is overlain by argillaceous limestone. This paper compares the imaging obtained from different array sequences. Calibration of the 3D resistivity block with well logs indicates that: the Wenner-Schlumberger (WS) array shows the tendency to enhance layering, to locate bodies at a shallower depth and to laterally extend them; the Pole-Dipole (PD) array shows larger lateral heterogeneities, more compact and vertically extended bodies and poor data fitting; the hybrid array sequence, obtained by the combination of WS and PD array sequences, despite a poor data fitting, similar to PD, shows a better correlation with respect to well log results. In this setting, the hybrid array sequence shows better imaging, due to the combination of the large vertical resolution of WS, large lateral resolution and penetration depth of PD. It allows passing through the thick, low resistivity shallow layer. Indeed, the results are affected by the occurrence of the shallow, 30 m thick, low resistivity argillaceous limestone that reduced the investigation depth as revealed by synthetic datasets modelling and sensitivity analysis. Modelling also revealed that the occurrence of the argillaceous limestone led to a severe underestimate of the Dogger Limestone resistivity values with respect to well resistivity logs; it also allowed verifying the detectability limits when investigating shallow karst limestone intervals located at depths of up to 50 m.

SEISMIC IMAGING OF THE MAIN CRATER OF THE PROPOSED SIRENTE METEORITE CRATER FIELD (CENTRAL ITALY)

We present seismic imaging of the subsurface structure of the main crater of the proposed Sirente meteorite crater field (Abruzzo, Central Italy). The crater field has been suggested to have formed by a small meteorite impact and consists of a main, dominant crater (~120 m in diameter) and a group of much smaller craters (~10 m in diameter on average). The main crater has a prominent elevated rim. The crater field is found within lacustrine sediments overlying limestone. Two shallow reflection profiles with P waves were acquired across the structure when the small lake, which occupies the main crater, was ice-covered. Profile RFL 2 is 130 m long and crosses the main structure rim to rim. Profile RFL 1 is 78 m long and roughly transversal to profile RFL 2. Two CMP processing sequences were applied on raw data. A short processing sequence allowed recognition of the main features of the subsurface structure of the crater: a deep (53 m on average), rootless, bowl-shaped geometry, a deepseated central uplift structure and three different seismic facies representing the infilling of the bowlshaped basin. These include lateral onlap reflectors may be interpreted as an analogue to the “breccia lens” in craters formed on rocky targets, indicating the occurrence of collapse events (slumping) during the crater modification stage. A long processing sequence allowed a more detailed imaging of the bowlshaped basin and the structures underlying and surrounding the crater, such as compaction of strata below the rim. The structural features interpreted from our survey are consistent with the impact hypothesis. Apparently, they do not support other proposed mechanisms of formation as the structure seems both rootless and deep.

Working Strategies for Addressing Microzoning Studies in Urban Areas: Lessons from the 2009 L’Aquila Earthquake

The Italian territory is affected by frequent seismic events producing high intensity in the near-field areas. Hence the seismic effects in Italy “can be considered as historical variables of the degree of intensity: historical building techniques, economic levels and population size”. In recent years, the Department of Civil Protection (DPC) evidenced that the high economic losses due to the strong earthquakes occurred in Italy up to 2003 rapidly increase over time due to the rapid urbanization of several portions of the territory. According to these concerns, the present study focuses on microzoning studies in the Aterno Valley after the 2009 L’Aquila earthquake. The authors show the role played by geo-lithological conditions in the amplification effects occurred at different sections of the Aterno Valley and discuss the efficiency of some geophysical techniques to characterize the local seismic response at the site.

3D POROSITY BLOCK OF A FRACTURED-KARST AQUIFER: COMPARISON BETWEEN THE POROSITY MODEL ACHIEVED BY 3D SEISMIC AND ERT IMAGING IN THE EXPERIMENTAL SITE OF POITIERS (FRANCE)

Three-dimensional total porosity blocks of a Dogger Limestone fractured-karst aquifer were obtained at the Hydrogeological Experimental Site of Poitiers, France. Dogger Limestone occurs at a depth ranging between 30 and 120 m and is overlain by argillaceous limestone. Three karst intervals at depth of 35-40 m, 85-87 m and 110-115 m have been revealed from vertical flowmeter data and highresolution imagery of borehole walls. This paper compares the total porosity blocks obtained from 3D seismic and resistivity imaging. Reflection seismic survey allowed generating a 3D seismic pseudo velocity block. The seismic interval velocities have been converted into resistivity. The empirical relationship between seismic velocity and true formation resistivity proposed by Faust (1953) has been used. Resistivity values were then converted into porosity values, by using the Archie’s law (1942). The 3D seismic pseudo porosity block allowed identifying three different large porosity layers that are consistent with flowmeter and borehole imagery data. Three-dimensional resistivity imaging was obtained from full inversion of combined 2D ERT data collected using both Wenner-Schlumberger (WS) and Pole-Dipole (PD) array sequences. 3D resistivity block obtained from the hybrid (combination of WS and PD) sequence seemed to apparently identify the first two main karst intervals. It was converted into porosity by using the Archie law (1942). The total porosity blocks obtained from seismic and resistivity imaging show very different resolutions (above all at the karst layers depth), but agree in identifying the karst layers depth. The resistivity block shows an overestimation of the total porosity values: this is due to the occurrence of the shallow, thick, low resistivity layer (drift deposits and argillaceous limestone) that led to a severe underestimate of the limestone bulk resistivity values. Both the seismic and the resistivity blocks agree in suggesting that not karst carbonates would be affected by low permeability but significant storability.