Seismic evidence for Messinian salt deformation and fluid circulation on the South Balearic margin (Western Mediterranean)

European Geosciences Union General Assembly 2014 (EGU2014), 27 april - 2 may 2014, Vienna, Austria.-- 1 pageThe south Balearic margin is characterized by an abrupt tectonically-controlled transition between a steep continental slope (Emile Baudot escarpment) and the Algero-Balearic abyssal plain, in which Messinain salt-induced deformation affects the seafloor morphology. Multichannel seismic profiles, multibeam bathymetry, and shallow seismic data demonstrate that the extent of salt deformation does not coincide with the bathymetric plain-slope transition. Instead, deformation occurs south of linear structure in the abyssal plain located some tens of kilometres from the base of the slope. The quality of the multi-channel seismic record in the deep water deformed area is severely decreased by the three dimensional character of the salt structures. However, the abyssal plain near the base of the slope reveals details on the Messinian sequence, its structure, post-Messinan deformation, and relation with subsurface fluids. The analysis of part of the EUROFLEETS SALTFLU multichannel seismic data set has included detailed RMS velocity analysis, post-stack and pre-stack time migration. An anomalously thick (up to 800 ms twt) acoustically laminated unit comprising the Messinian Upper Unit (UU) is present near the base of the slope and is characterized by syn-sedimentary gentle symmetric folding. The crests of such folds are affected by small-offset, layer-bound fractures and faults propagating from the upper part to the UU to the Plio-Quaternary sequence. Amplitude anomalies, polarity inversion and at times acoustic blanking reveal the presence of fluids (presumably gas) within the Messinian sequence. A clear seismic evidence for the Mobile Unit (MU, or salt layer) is missing in this area. Seismic evidence for the MU exists south of the linear structural boundary, where salt induced deformation has created vertical displacements of several hundreds of metres, diapiric growth, and at least two salt/mud piercement structures at the seafloor. In the highly deformed area, the UU and the Lower Unit (LU) appear to amalgamate as a consequence of complete salt withdrawal around diapirs. The seismic analysis is focussed on determining whether the boundary between low and high degree of deformation in the abyssal plain is determined by the limit of the salt distribution. In this case the northern limit of the Messinian pure salt basin would not coincide with the present day continental slope, thus requiring either a strong control of Messinian tectonic structures an salt deposition and/or a contamination of salt with clasticsPeer Reviewe

Regional Mapping of the Average Shear Wave Velocity for Geotechnical Applications - An Example in North-eastern Italy

We utilised the Refraction Microtremors (ReMi) seismic technique to conduct a regional survey targeting the average shear wave velocity in the uppermost 30 m in the Province of Treviso in north-eastern Italy. The ReMi technique was chosen because of its cost effectiveness as the field operations can be easily conducted by single person The data acquisition grid was optimised using a multidisciplinary approach. Each VS recording station was precisely located based on a high-resolution depositional model of the entire venetian plain. A total of 250 recording stations, scattered over an area of 2000 square km, were collected in a time window of a month in the early 2006. Data processing faced some problems as in some cases spectral quality was not satisfying and the dispersion curve could not be identified with an adequate degree of confidence. A percentage of 10% of the collected were discarded. The final VS map showed a surprisingly correlation with the alluvial and fluvio-glacial depositional model acting during late Pleistocene and Holocene. The ReMi technique and more generally the surface wave based techniques proved to be a valuable tool for the regional mapping of average values of VS in the near surface strata

Algero-Balearic Basin

4 pagesPeer Reviewe

Passive seismic exploration in an urban environment along the projected route of an underground railway

We utilised the Refraction Microtremors (ReMi) seismic technique to conduct a survey in the city of Malaga in Spain to support a project for the construction of a new subway line in the vicinity of the coast. The ReMi technique was chosen to carry out the survey because of the expected difficulties of conventional seismic operations in a densely populated urban area with its logistical and administrative restrictions and with a very high level of background noise. A total of about 1400 m of profiles were collected in two days of field work. The correlation of shear wave maps with the available borehole stratigraphy was generally good. In a few areas, probably because of structural complexity, ReMi interpretation of the data was not definitive. Poor ReMi performance was seen as an indicator of zones of geological complexity requiring either further geophysical investigation or special attention during tunnel excavation. The ReMi technique proved to be adequate for characterising the shallow subsurface in the study site in terms of geometry and shear wave velocity down to a depth of 5070 meters. The bandwidth andamplitude of the spectrum of the traffic and urban noise were sufficient to generate usable surface waves

Seismic Imaging of Fractured Reservoirs in the Scansano-Amiata area, Southern Tuscany

Fresh water availability has recently become a serious concern in the Italian Apennines as various activities rely on a predictable supply. In southern Tuscany along the Scansano-Amiata ridge the situation is further complicated because of the contamination of the nearby alluvial aquifers. Locally aquifers consist of thin fractured reservoirs, generally within low-permeability formations, and planning exploitation of resources, based on conventional techniques, could be complicated. An integrated investigation program, based on remote sensing, structural geology and geophysical exploration, was initiated in 2002 to associate tectonics and groundwater circulation and better define the hydrological model. After the regional identification of fault and fracture patterns a major structure has been investigated in detail to accurately map the spatial position, and understand the associated aquifer geometry and properties to assess exploitation potentials. The subsurface around the fault zone was clearly imaged with Ground Probing Radar, 2D and 3D resistivity tomography and more recently with 3D shallow seismic. The vertical and the horizontal contacts between the different geological units of the Ligurian and of the Tuscan Series were resolved with a high degree of spatial accuracy. 3D high-resolution geophysical imaging proved to be very effective in small-scale fractured reservoir characterisation

3D Seismic Imaging of a Deep Landslide in the Eastern Alps (Northern Italy)

The Lamosano Town in the Eastern Alps, was built on the top of a ridge, involved in a slow landslide movement that caused several damages to various buildings. Former studies, conducted in the early eighties, suggested of classify the town as a centre to be relocated. The Italian Government recently funded a new investigation program aimed to better understand the landslide dynamics and assess the possibility of mitigating the surface effects avoiding the need of transferring part of the local population to a different site. In this context was undertaken an integrated 3D seismic refraction tomography / reflection survey targeting the geological structure of the ridge. The subsurface below the ridge was clearly imaged by the seismic and the major contacts between the different units were resolved with an adequate degree of horizontal and vertical resolution. The high-resolution acoustic image of the subsurface led to the formulation of new hypotheses about the major cause of the soil movements and the possible relationship between groundwater circulation and surface displacements. 3D high-resolution seismic imaging proved to be very effective in the characterisation of areas affected by deep landslide phenomena

3D-imaging of shallow fractured aquifers along the Scansano-Amiata ridge (Tuscany, Italy)

A series of high-resolution geophysical surveys including 3D seismic refraction and resistivity tomography, 2D resistivity tomography and Ground Probing Radar, were recently conducted in three sites of the Scansano-Amiata ridge in southern Tuscany (Italy). The goal of the survey was to clearly image the geometry and the properties of some small scale aquifers hosted in high-angle faulted and fractured zones. In the recent years fresh water availability has become a major issue in the Italian peninsula as various agricultural and industrial activities as well as urban settlements depend on a constant and predictable supply. In the Appennine area, aquifers very often consist of fractured reservoirs within low-permeability formations, where established alluvial aquifer modeling techniques appear inadequate to protect the resource from pollutants or plan new extracting plants. The existing well distribution was compared with the fault network revealed by satellite image interpretation. The most productive aquifers resulted invariably located along major faults, thus pointing out the importance of secondary (tectonic) permeability. A prominent sub-vertical fault system ("Patrignone" fault), oriented north-south was investigated with high-resolution geophysical methods and direct excavation at three different sites: "Aione", "Ripacci" and "Poderino". We report here some of the results obtained at the "Poderino" site where 3D-geophysical data have been also collected (Figure 1). The different geophysical images clearly outlined the structural framework of the subsurface down to a depth greater than 100 m and the final synthesis provided a comprehensive insight into the architecture of faulted and fractured zones and in the overall permeability structure

THE MEDITERRANEAN SEA: A NATURAL LABORATORY TO STUDY GAS HYDRATE DYNAMICS?

International audienceGas hydrates have been proven by coring at one site in the (eastern) Mediterranean Sea, but their wider extent remains uncertain. Here we present results from investigations of the potential Mediterranean gas hydrate system, suggesting that clathrates occur more widely and have been strongly impacted by glacial-interglacial climate forcing. Modeling of the methane hydrate stability zone (MHSZ) shows it to be present in most of the Mediterranean Sea, albeit in deep waters (>1000 m) due to warm bottom waters (12.5-14°C) and in greater thicknesses (200-500 m) in the geothermally cooler eastern basin. Comparison of the MHSZ with known or possible zones of gas flux to seabed suggests prospective areas for hydrate occurrence, mainly in the eastern basin. One is the Nile fan, where evidence of the first BSR in the Mediterranean Sea (presented sseparately, Praeg et al. this volume) confirms the potential for additional hydrate discoveries. During glacial stages, gas hydrate stability in the Mediterranean increased due to bottom waters up to 4°C cooler; even allowing for sea levels 125 m lower, the modeled glacial-stage MHSZ was up to 25% thicker and 300 m shallower on basin margins. Glacial-to-interglacial transitions thus corresponded to a marked reduction in hydrate stability, with downslope migration of the upper limit of the MHSZ across depths of c. 700-1000 m. A compilation of submarine landslides in the Mediterranean Sea indicates a peak in the age of slide deposits during the last deglaciation and includes abundant headwalls in mid-to upper slope depths (<1200 m), including on the Nile fan. Together these results suggest that the Mediterranean Sea, in particular its gas-rich eastern basin, offers natural laboratory conditions to test the hypothetical linkages between climate-driven changes in gas hydrate stability and slope instabilities over glacial-interglacial timescales