Patrizio Torrese

Shallow Electrical and Seismic Imaging of The Pineto Mud Volcano (Central Italy)

Shallow underground electrical and seismic imaging was undertaken at the Pineto (Central Italy) Mud Volcano site using 2D-3D electrical resistivity tomography (ERT) and 2D reflection seismic surveys. This mud volcano or mud lump (known by the local population as “Cenerone Mud Volcano”) appears as a dome of about 15x10 m in size. The height is approximately 2 m with a crater 2.5 m of diameter where emission of fluids and solids (cold brine, mud, gas) occurs. Upper Pliocene-Lower Pleistocene foredeep pelitic deposits (clays with silty-sandy levels) overlain by clayey-silty deposits outcrop in the area. Below the Middle Pliocene deposits, a NNW-SSE anticline which runs approximately 2 kilometers to the WSW of the site represents the main element of the compressional tecnonic setting. Ditchs and streams in the area run along fault systems that are oriented NW-SE and NE-SW. These could be related to the upward migration of deep mud fluids. 2D-ERT results were used to determine the geometry of the high conductivity body related to the uprising of mud fluids and detecting changes in deposits. Detailed geometry reconstruction of the shallow upward migration of mud fluids was obtained by using 3D-ERT. A shear wave reflection seismic survey was undertaken to determine stratigraphic limits and to assess the occurrence of fractured zones along which mud fluids could migrate towards the surface. The survey results revealed that the uprising of deep fluids doesn’t occur exactly below the mud volcano at present. Instead, a high conductivity body occurs at approximately 60 m to the ENE, within a fractured zone in the undercompacted clays. The probable occurrence of a high permeability layer approximately between 20 m and 30 m below ground level, confined by clay layers, gives rise to an overpressured mud reservoir. This is the source of mud fluids which flow to the surface through a mud conduit that appears nearly horizontal at depth and then inclined towards the surface up to the mud volcano crater.

The Uprising of Deep Saline Paleo-Waters into the Oltrepò Pavese Aquifer (Northern Italy): Application of Hydro-Chemical and Shallow Geophysical Surveys

The alluvial aquifer of the Oltrepo Pavese plain sector (Po Valley, Northern Italy) is naturally contaminated by Na–Cl paleo-waters, rising from the tertiary marine substratum and mixing with the shallow groundwater. This phenomenon is localised along a buried tectonic discontinuity known as Vogherese Fault. A hydro-chemical (sampling for chemical analysis, log of electrical conductivity and temperature, continuous monitoring system) and a geophysical (VLF-EM, 2D and 3D ERT surveys, resistivity profiling, resistivity depth soundings) study was carried out to investigate the phenomenon. The research highlighted a spot spatial distribution of the saline contamination, suggesting the existence of isolated plumes of saltwater within the aquifer and a temporal variability of groundwater’s salinity. These results allowed at formulating a conceptual model based on the regional hydraulic continuity between the fracture network of the aquifer in the Apennines mountain range and the porous aquifer in the Oltrepo Pavese plain. Severe recharge events in the Apennines could be able to activate a mass or pressure transfer from the fractured to the porous aquifer and to cause the uprising of deep paleo-waters into the alluvial aquifer.

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.

COMPARISON BETWEEN VLF-EM AND RESISTIVITY ANOMALIES ASSOCIATED WITH SALT PALEO-WATERS CONTAMINATIONS IN AN ALLUVIAL AQUIFER: THE SAN RE TEST SITE (NORTHERN ITALY)

The alluvial aquifer of the Oltrepo 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 km 2 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.

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.

A multidisciplinary study of Benetutti thermal waters (Tirso Valley, Sardinia)

We report a multidisciplinary study of Benetutti thermal waters based on hydrochemistry, isotopic techniques, structural geology and geophysical surveys. Seven thermal waters and two cold spring waters have been studied. Thermal waters show temperature values ranging between 28 °C and 42 °C, a medium mineralization and a sodium-chloride chemical hydrofacies. Isotopes of the water molecule allowed verifying that these waters have not been affected by isotopic exchange phenomena with the Sardinian granite complex aquifer. VLF-EM (Very Low Frequency-Electromagnetics) surveys were undertaken for a rapid assessment of the distribution of thermal waters over vast areas. A northeast-southwest trend of high conductivity anomalies was revealed. This trend can be correlated to the occurrence of sub-parallel discontinuities along which thermal waters uprise.

SITE RESONANCE FREQUENCY ASSESSMENT THROUGH DYNAMIC LOAD TESTING (DLT): EARLY RESULTS OF A TEST CARRIED OUT IN THE EXPERIMENTAL SITE OF FIVIZZANO (MS, ITALY)

The assessment of the local seismic response is one of the fundamental phases when it comes to defining the levels of seismic hazard at a detailed scale. these are typically known as seismic zonation studies. the estimation of the site specific resonance frequency is a parameter of relevant interest within such studies as it is associated with the maximum seismic amplification which has to be related to the frequency of resonance of buildings. the ambient noise based methods and other experimental methods for measuring this parameter are not always applicable because they are strongly influenced by contrasts of impedence and by lateral subsurface heterogeneities. Furthermore, they sometimes present limits that are not always clearly known. On the other hand, the classic analytical approach that makes use of integrated and multidisciplinary Investigation methods, which estimate the local amplification parameters, mainly in terms of amplification factors and response spectrums, currently presents uncertainties and limitations. these are essentially associated with the difficulty of modelling the mechanisms of propagation of the seismic waves within a 3D system. the authors of this paper propose the experimental measurement of the site resonance frequency by means of Dynamic Load Testing (DLT). This is done by presenting the operative procedures, the data processing and the results obtained from a test undertaken at the experimental site of Fivizzano (Massa-Carrara, Italy) where a detailed study of the local seismic response was also undertaken. the test involved the delivery of sinusoidal forces with maximum amplitude of 20 kN and a range of frequencies between 0 and 15 Hz by means of an electro-mechanical generator. Simultaneous seismic monitoring was undertaken through a multichannel acquisition system. Velocimeters were placed on the ground and in a nearby strategic building. the measured frequency of resonance was compared to the predicted value obtained from empirical-analytical solutions and application of horizontal to vertical spectral ratios (HVSR) method and to the value measured by a seismic station equiped with two coupled accelerometers, one at the surface and one at depth; these sensors allowed an accurate analysis of the effects of the drift deposits overlying the bedrock, that is the transfer function of the stratigraphic column at the site.

An Application of Seismic Tomography Method for Studying an Urban Geology Problem

Geophysical methods have been used more recurrently within urban geology in the past years. These techniques, when correctly applied, offer a more or less complete frame on the physical and mechanical properties, on discontinuities and on subsurface geometries of investigated mediums. Geophysical prospecting is also used because of its low cost/advantages ratio. It s also capable of overcoming the limits of more classical geognostical investigations (i.e. borehole drilling) that, being localised, can often give results, which are not univocally interpretable, especially for reconstruction of subsurface geometries in complex scenarios. This work presents an example of the use of high resolution, P wave seismic tomography that has been applied within a study intended to securing an anthropic cavity so as to not represent a threat for human lives and possessions. Instability conditions are due to the poor mechanical conditions of existing lithotypes and the high grade of weathering of the lithologies. Proof of poor structural conditions have been the numerous falls of debris during seismic events (i.e. after the seismic event of 1997). Firstly, applicability and intrinsic limitations of the methodology are discussed on the bases of experimental tests conducted in laboratory with the use of ultrasound on materials of different complexity. Eventually, the acquisition techniques and the data processing are discussed, coadiuvated by the use of direct investigations like geological and structural survey, exploration trial pitting and rotary drilling.