Rock fractures analysis using Structure from Motion technology: new insight from Digital Outcrop Models

Fractures are one of the most important features of the rocks of the upper crust since they strongly influence their physical and chemical behavior and reflect their tectonic history. For this reason, fracture study plays a key role in different branches of the geosciences. Notwithstanding, the quantification of the features and parameters describing fractures could be unsatisfactory using the standard field techniques because they are mainly based on direct-contact methodologies that are affected by errors, as orientation bias and trace censoring, and scarce representativeness, due to the limited possibility of acquiring information of outcrops partially or totally inaccessible. Recently new remote sensing technologies, such as Terrestrial Laser Scanner (TLS) and Digital Photogrammetry (DP), can help to overcome these limitations. Whereas TLS could be very expensive and difficult to use in geological study, DP permits to obtain similar results in an easier way due to cheaper and lighter equipment and more straightforward procedures. Moreover, DP becomes even more useful when combined with Unmanned Aerial Vehicle (UAV) because permits to acquire digital images from positions inaccessible to humans, allowing to analyze geological objects from points of view previously unimaginable. The images acquired from the ground and/or by the UAV can be then processed using different digital algorithms, such as Structure from Motion (SfM), that permit to create 3D model of the studied outcrop. In geosciences, the 3D model representing the surface of the outcrop is often called Digital Outcrop Model (DOM). Despite DOMs can be really useful in different branches of geosciences, their applications are quite well limited because the procedures of their development and sampling/analysis are scarcely analyzed in literature. It is important to highlight that whereas the UAV-based SfM approach is fairly discussed in literature for simple flat areas, is scarcely treated for application to near vertical and not-planar slopes. Moreover, the validity of some procedures of fracture sampling on 3D model, with special regards to the automatic ones, that have been recently presented in literature, is not well treated for real cases of study. The scarce knowledge about these approaches could cause different troubles to the scientific-users: from the application of avoidable time-consuming routine, to the acquisition and interpretation of erroneous data. This research aims to contribute to the scientific knowledge of the use of digital photogrammetry for fractured rock mass analysis, creating and defining new approaches and procedures for the development, analysis and application of DOMs. Here, a workflow for the fracture analysis of steep rocky outcrops and slopes using the 3D DOM is presented. In particular, the following steps are discussed: (i) image acquisition; (ii) development of 3D model; (iii) sampling of DOM; (iv) quantification and parametrization of the 3D measures; (v) application of the 3D quantitative data and parameters to different case of study. Four different cases of study were selected to validate the proposed method: the upper Staffora Valley and Ponte Organasco (Northern Apennines, Italy), Ormea (Ligurian Alps, Italy), and Gallivaggio (Western Alps, Italy) cases of study. However, this methodology could not completely replace the 'direct-contact' field activity, because some information as roughness, infilling and aperture of fractures cannot be measured satisfactory, and because, where possible, field control measures to validate the 3D data are necessary. However, this methodology could be considered as a new necessary procedure for rock-fracture studies because it allows to overcome the inevitable errors of the ground-based traditional methodology and because the DOMs are always available for the analysis, promoting data sharing and comparison, two fundamental principles on which science have and will have to be basedFractures are one of the most important features of the rocks of the upper crust since they strongly influence their physical and chemical behavior and reflect their tectonic history. For this reason, fracture study plays a key role in different branches of the geosciences. Notwithstanding, the quantification of the features and parameters describing fractures could be unsatisfactory using the standard field techniques because they are mainly based on direct-contact methodologies that are affected by errors, as orientation bias and trace censoring, and scarce representativeness, due to the limited possibility of acquiring information of outcrops partially or totally inaccessible. Recently new remote sensing technologies, such as Terrestrial Laser Scanner (TLS) and Digital Photogrammetry (DP), can help to overcome these limitations. Whereas TLS could be very expensive and difficult to use in geological study, DP permits to obtain similar results in an easier way due to cheaper and lighter equipment and more straightforward procedures. Moreover, DP becomes even more useful when combined with Unmanned Aerial Vehicle (UAV) because permits to acquire digital images from positions inaccessible to humans, allowing to analyze geological objects from points of view previously unimaginable. The images acquired from the ground and/or by the UAV can be then processed using different digital algorithms, such as Structure from Motion (SfM), that permit to create 3D model of the studied outcrop. In geosciences, the 3D model representing the surface of the outcrop is often called Digital Outcrop Model (DOM). Despite DOMs can be really useful in different branches of geosciences, their applications are quite well limited because the procedures of their development and sampling/analysis are scarcely analyzed in literature. It is important to highlight that whereas the UAV-based SfM approach is fairly discussed in literature for simple flat areas, is scarcely treated for application to near vertical and not-planar slopes. Moreover, the validity of some procedures of fracture sampling on 3D model, with special regards to the automatic ones, that have been recently presented in literature, is not well treated for real cases of study. The scarce knowledge about these approaches could cause different troubles to the scientific-users: from the application of avoidable time-consuming routine, to the acquisition and interpretation of erroneous data. This research aims to contribute to the scientific knowledge of the use of digital photogrammetry for fractured rock mass analysis, creating and defining new approaches and procedures for the development, analysis and application of DOMs. Here, a workflow for the fracture analysis of steep rocky outcrops and slopes using the 3D DOM is presented. In particular, the following steps are discussed: (i) image acquisition; (ii) development of 3D model; (iii) sampling of DOM; (iv) quantification and parametrization of the 3D measures; (v) application of the 3D quantitative data and parameters to different case of study. Four different cases of study were selected to validate the proposed method: the upper Staffora Valley and Ponte Organasco (Northern Apennines, Italy), Ormea (Ligurian Alps, Italy), and Gallivaggio (Western Alps, Italy) cases of study. However, this methodology could not completely replace the 'direct-contact' field activity, because some information as roughness, infilling and aperture of fractures cannot be measured satisfactory, and because, where possible, field control measures to validate the 3D data are necessary. However, this methodology could be considered as a new necessary procedure for rock-fracture studies because it allows to overcome the inevitable errors of the ground-based traditional methodology and because the DOMs are always available for the analysis, promoting data sharing and comparison, two fundamental principles on which science have and will have to be base

Investigation of the Luco dei Marsi DSGSD revealing the first evidence of a basal shear zone in the central Apennine belt (Italy)

Deep-seated gravitational slope deformations (DSGSDs) show a wide range of geomorphological characteristics and kinematic behaviours. In many cases, deforming rock masses move on a continuous surface or a thick basal shear zone (BSZ) overlying the stable bedrock. The nature of this boundary is a significant issue in scientific debates since examples of BSZs have been observed or inferred in some DSGSDs worldwide. In the central Apennines, although several cases of DSGSDs have been described in recent decades, no evidence of BSZs has been documented thus far. This work presents the first case of a BSZ found in the region at the bottom of a large-scale gravitational deformation that affects the Mesozoic-Cenozoic carbonate ridge overhanging the Luco dei Marsi village (Abruzzi region). The BSZ consists of several metres-thick, cataclastic breccia developed within middle-Upper Cretaceous biodetritic limestone. The breccia level is exposed for approximately 200 m with a subhorizontal geometry and shows severe rock damage and weathering. The DSGSD hosting the BSZ affects an NNW-SSE-oriented and wide Miocene anticline whose eastern limb is dismembered by Pliocene-Quaternary normal faults delimiting the edge of a large Quaternary intermontane basin (the Fucino Basin). Field survey, aerial photointerpretation, and remote sensing (DInSAR technique) analyses outline an active gravity-driven process. This is characterized by several kinds of geomorphological features, including downhill- and uphill-facing scarps, ridge-top depressions, gravitational grabens and trenches in the upper and middle parts of the ridge, and bulging at the toe of the slope. These features, which can be distinguished from tectonic elements due to their shape and extension, are an indication of a high degree of internal deformation and a compound sagging geometry for the Luco dei Marsi DSGSD. The short-term activity of the process was revealed by DInSAR time series covering almost thirty years of satellite datasets, including ERS1/2, ENVISAT, COSMO-SkyMed, and SENTINEL 1 constellations. Strain rates on the order of a few mm/yr were inferred, with a marked difference between different sectors of the DSGSD area. The long-term (y > 102) lifespan of the DSGSD was framed into a multiple-step conceptual model summarizing the Early Pleistocene-Holocene geological evolution of the area. The model results outline the control exercised by extensional tectonics on DSGSD development, as progressive displacements along normal faults in the latest Pleistocene were the cause of lateral unconfinement at the toe of the slope. This work further contributes to the increasing knowledge on DSGSDs in the central Apennines and the understanding of the relationship between deformation features induced by slope morphogenesis, such as the BSZ, and Quaternary tectonics within the mountain belt

Urban Deformation Monitoring using Persistent Scatterer Interferometry and SAR tomography

This book focuses on remote sensing for urban deformation monitoring. In particular, it highlights how deformation monitoring in urban areas can be carried out using Persistent Scatterer Interferometry (PSI) and Synthetic Aperture Radar (SAR) Tomography (TomoSAR). Several contributions show the capabilities of Interferometric SAR (InSAR) and PSI techniques for urban deformation monitoring. Some of them show the advantages of TomoSAR in un-mixing multiple scatterers for urban mapping and monitoring. This book is dedicated to the technical and scientific community interested in urban applications. It is useful for choosing the appropriate technique and gaining an assessment of the expected performance. The book will also be useful to researchers, as it provides information on the state-of-the-art and new trends in this fiel

Book of Abstracts

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Multi-component and multi-source approach to model subsidence in deltas. Application to Po Delta Area

This thesis focused on the definition of a study approach able to deal with the complexity of the land subsidence phenomenon in deltas. In the framework of the most up- to-date multi-methodological and multi-disciplinary studies concerning land subsidence and targeting to predict and prevent flooding risk, the thesis introduces a procedure based on two main innovations: the multi-component study and the multi-source analysis. The proposed approach is a “multi-component” procedure as it investigates, in the available geodetic datasets, the permanent component apart from the periodic one, and, at the same time, it is a “multi-source” approach because it attempts to identify the relevant processes causing subsidence (sources) by a modelling based on multi-source data analysis. The latter task is accomplished first through multi-disciplinary and multi-methodological comparative analyses, then through modelling of the selected processes. With respect to past and current approaches for studying subsidence phenomena, the developed procedure allows one to: i. overcome the one-component investigation, improving the accuracy in the geodetic velocity estimate; ii. fix the “analyses to modelling” procedure, enhancing qualitative or semi-quantitative procedures that often characterize the “data to source” and the “residual to source” approaches; iii. quicken the source validation phase, accrediting the relevance of the source on the basis of the analysis results and before the modelling phase, differently from the “peering approach”, which validates the source on the basis of the model findings. The proposed procedure has been tested on the Po Delta (northern Italy), an area historically affected by land subsidence and recently interested by accurate continuous geodetic monitoring through GNSS stations. Daily-CGPS time series (three stations), weekly- CGPS time series (two stations) and seven sites of DInSAR-derived time series spanning over the time interval 2009 – 2017 constituted the used geodetic datasets. Several meteo/hydro parameters collected from fifty-seven stations and wide stratigraphic-geological information formed the base for the performed comparative analyses. From the application of the proposed procedure, it turns out that the periodic annual component highlighted in the continuous GPS stations is explained by two water mass-dependent processes: soil moisture mass change, which seems to control the ground level up-or-down lift in the southern part of the Delta, and the river water mass change, which influences the ground displacement in the central part of the Delta. As it concerns the permanent component, the lower rate found over 2012 - 2016 period in the central part of the Delta with respect to the eastern part is interpreted as due to the sediment compaction process of the Holocene prograding sequences and to the increase of rich-clay deposits

Constraints on the Structure and Evolution of the Malawi Rift from Active- and Passive-Source Seismic Imaging

Located at the southernmost sector of the Western Branch of the East African Rift System, the Malawi Rift exemplifies an active, magma-poor, weakly extended continental rift. This work focuses on the northern portion of the Malawi Rift, which is flanked by long (>100 km) basin-bounding border faults and crosses several significant remnant structures. This combination of characteristics makes the Malawi Rift the ideal location to investigate the controlling processes governing present-day extension throughout the lithosphere. To investigate these processes I image shallow basin- to uppermost-mantle structure beneath the region using a combination of passive- and active-source seismic datasets. I conduct passive-source imaging of the crust and upper mantle using ambient-noise and teleseismic Rayleigh-wave phase velocities between 9 and 100 s period. This study includes six lake-bottom seismometers located in Lake Malawi (Nyasa), the first time seismometers have been deployed in any of the African rift lakes. I utilize the resulting phase-velocity maps to invert for a shear velocity model of the Malawi Rift discussed below. I utilize active-source tomographic imaging to obtain new constraints on rift basin structure in the Malawi Rift from a 3-D compressional velocity (Vp) model. The velocity model uses observations from the first wide-angle refraction study conducted using lake-bottom seismometers in one of the great lakes of East Africa. The 3-D velocity model reveals up to ~5 km of synrift sediments, which smoothly transition from eastward thickening against the Livingstone Border Fault in the North Basin to westward thickening against the Usisya Border Fault in the Central Basin. I use new constraints on synrift sediment thickness to construct displacement profiles for both faults. Both faults accommodate large throws (> 7 km) but the Livingstone Fault is ~30 km longer. The dimensions of these faults suggest they are nearing their maximum size. The presence of >4 km of sediment within the accommodation zone suggests fault length was established early pointing the "constant length" model of fault growth. The presence of an intermediate velocity unit with velocities of 3.75-4.5 km/s is interpreted to represent prior rifting (Permo-Triassic and/or Cretaceous) sedimentary deposits beneath Lake Malawi. These thick (up to 4.6 km) packages of preexisting sedimentary strata improve the understanding of the Tanganyika-Rukwa-Malawi rift system and the role of earlier stretching phases on synrift basin development. I use the previously obtained local-scale measurements of Rayleigh wave phase velocities between 9 and 100 s combined with constraints on basin structure and crustal thickness to robustly invert for shear velocity from the surface to 135 km for the Malawi Rift. We compare our resulting 3-D model to a 3-D model of shear velocity obtained for the mature Main Ethiopian Rift and Afar Depression using commensurate datasets and identical methodologies. Comparing the Vs models for the two regions reveals markedly different seismic velocities particularly pronounced in the upper mantle (average velocities in the Malawi Rift are ~9% faster than the Main Ethiopian Rift). Our 3-D Vs model of the Malawi Rift reveals a strong, localized low velocity anomaly associated with the Rungwe Volcanic Province within the crust and upper mantle that can be explained without requiring the presence of partial melt. Away from the Rungwe Volcanic Province, velocities within the plateau regions are fast (> 4.6 km/s) and representative of depleted lithospheric mantle to depths of 100 and >135 km to the west and east of the rift, respectively. Thinned lithosphere, represented by the absence of similarly high velocities, is centered directly beneath the rift axis and footwall escarpments of the rift basins. The correlation between the localization of lithospheric thinning, the boundaries between abutting Proterozoic mobile belts, and the positions of the basin-bounding border faults may point to the controlling role of preexisting large-scale structures in localizing strain and allowing extension to occur here

Potential fields data modeling: new frontiers in forward and inverse problems

Since the '50, potential fields data modeling has played an important role in analyzing the density and magnetization distribution in Earth's subsurface for a wide variety of applications. Examples are the characterization of ore deposits and the assessment of geothermal and petroleum potential, which turned out to be key contributors for the economic and industrial development after World War II. Current modeling methods mainly rely on two popular parameterization approaches, either involving a discretization of target geological bodies by means of 2D to 2.75D horizontal prisms with polygonal vertical cross-section (polygon-based approach) or prismatic cells (prism-based approach). Despite the great endeavour made by scientists in recent decades, inversion methods based on these parameterization approaches still suffers from a limited ability to (i) realistically characterize the variability of density and magnetization expected in a study area and (ii) take into account the strong non-uniqueness affecting potential fields theory. The prism-based approach is used in linear deterministic inverse methods, which provide just one single solution, preventing uncertainty estimation and statistical analysis on the parameters we would like to characterize (i.e, density or magnetization). On the contrary, the polygon-based approach is almost exclusively exploited in a trial-and-error modeling strategy, leaving the potential to develop innovative inverse methods untapped. The reason is two-fold, namely (i) its strongly non-linear forward problem requires an efficient probabilistic inverse modeling methodology to solve the related inverse problem, and (ii) unpredictable cross-intersections between polygonal bodies during inversion represent a challenging task to be tackled in order to achieve geologically plausible model solutions. The goal of this thesis is then to contribute to solving the critical issues outlined above, developing probabilistic inversion methodologies based on the polygon- and prism-based parameterization approaches aiming to help improving our capability to unravel the structure of the subsurface. Regarding the polygon-based parameterization strategy, at first a deep review of its mathematical framework has been performed, allowing us (i) to restore the validity of a recently criticized mathematical formulation for the 2D magnetic case, and (ii) to find an error sign in the derivation for the 2.75D magnetic case causing potentially wrong numerical results. Such preliminary phase allowed us to develop a methodology to independently or jointly invert gravity and magnetic data exploiting the Hamilton Monte Carlo approach, thanks to which collection of models allow researchers to appraise different geological scenarios and fully characterize uncertainties on the model parameters. Geological plausibility of results is ensured by automatic checks on the geometries of modelled bodies, which avoid unrealistic cross-intersections among them. Regarding the prism-based parameterization approach, the linear inversion method based on the probabilistic approach considers a discretization of target geological scenarios by prismatic bodies, arranged horizontally to cover it and finitely extended in the vertical direction, particularly suitable to model density and magnetization variability inside strata. Its strengths have been proven, for the magnetic case, in the characterization of the magnetization variability expected for the shallower volcanic unit of the Mt. Melbourne Volcanic Field (Northern Victoria Land, Antarctica), helping significantly us to unravel its poorly known inner geophysical architecture

Multidimensional Interpretation of Near Surface Electromagnetic Data Measured in Volvi Basin Northern Greece

The Volvi basin is an alluvial valley located 45 km northeast of the city of Thessaloniki in Northern Greece. It is a neotectonic graben (6 km wide) structure with increasing seismic activity where the large 1978 Thessaloniki earthquake occurred. The seismic response at the site is strongly influenced by local geological conditions. Therefore, the European test site “EURO-SEISTEST” for studying site effects of seismically active areas is installed in the Volvi-Mygdonian Basin. The ambient noise measurements from the east area of EURO-SEISTEST give strong implication for a complex 3-D tectonic setting. Hence, near surface Electromagnetic (EM) measurements are carried out to understand the location of the local active fault and the top of the basement structure of the research area. The Radiomag- netotelluric (RMT) and Transientelectromagnetic (TEM) measurements are carried out along eight profiles, which include 446 RMT and 107 TEM soundings. The correlation between the borehole data and the interpreted TEM and RMT model generally shows six layers. The layers are identified as sedimentary and metamorphic rocks which in detail are: silty sand (10 - 30 Ωm), silty clay (10 - 30 Ωm), silty clay marly (30 – 50 Ωm), sandy clay (50 - 80 Ωm) and marly silty sand (> 80 Ωm) and basement (gneiss and schist) (> 80 Ωm) with varying thick- nesses. To analyze the structure of the research area, interpretation of multidimensional models (1-D, 2-D, 3-D) is carried out. The 1-D model and the 2-D model derived from RMT data show a clear indication of the fault structure distribution in the research area. From the analysis, there can be found that the fault structure is associated with marly silty sand with a resistivity of more than 80 Ωm. The correlation of the RMT 2-D model with the geological map provides a good fitting to the surface structure. Due to the high resistivity of the top layer, the skin depths of the RMT soundings are approximately 35 m. The TEM data gives a detailed description about the deeper structure down to the depth of 200 m. Joint and sequential inversions of RMT and TEM data can provide clear information from the surface to the deep structure. Single and joint inversions of RMT and TEM give a consistent result in which both identify the fault structure. Three dimensional modeling of RMT data is implemented to provide a representative model of all conductivity structures in the research area. The overall number of cells in the 3-D model is 2,317,000 cells (nx = 220 cells, ny = 220 cells and nz = 45 cells) modeling the research area with size of 2.4 km × 2.4 km. 3-D models provide a detailed description of the normal fault structure at depths of about 5 to 25 m and thicknesses of 20 m. According to the analyses, a normal fault is located next to the EURO-SEISTEST site, with a strike direction of N 70◦ E

Topography effects in the 1999 Athens earthquake : engineering issues in seismology

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2004.Includes bibliographical references.It is well known that irregular topography can substantially affect the amplitude and frequency characteristics of seismic motion. Macroseismic observations of destructive earthquakes often show higher damage intensity at the tops of hills, ridges and canyons than at lower elevations and on flat areas. Systematic seismic motion amplification over convex topographies has been confirmed by instrumental studies and also predicted by theoretical and numerical simulations of wave diffraction. Nonetheless, for the most part, the former have been limited to weak motion data and the later have treated topographic asperities as simple geometric irregularities on the surface of homogeneous, linearly elastic halfspaces. Despite the qualitative agreement between theory and observations on topography effects, there is still much uncertainty concerning the actual severity of amplification near topographic irregularities, inasmuch as predictive methods are still lacking on the quantitative aspects of seismic amplification near such features. Focusing of seismic rays by convex topographies does play a significant role as shown theoretically, yet it is not the only physical phenomenon involved. On the other hand, weak motion data may not be applicable to describe topography effects for strong shaking, and indeed there exist very few- if any- well documented case studies demonstrating the severity of topographic effects for strong ground motion. In this dissertation, we find that topography and local soil conditions need to be accounted for simultaneously for the prediction of site amplification factors, especially when earthquake motions are strong enough to elicit clear nonlinear soil behavior.(cont.) We examine how local stratigraphy, material heterogeneity and nonlinear soil response can alter the focusing mechanism at the vertex of cliff-type topographies, and how the free-field response is further modified on account of soil-structure interaction. By means of a case-study from the Athens 1999 earthquake, we validate the effects of local soil conditions by comparison with weak motion data, and illustrate the effects of nonlinear soil behavior and soil-structure interaction on strong motion amplification. Our finite-element, nonlinear simulations seem to explain the uneven distribution of severe damage in the community of Adàmes that borders the crest of the Kifissos river canyon at its deepest point. They also resolve in part previously unexplained discrepancies, often observed between strong amplification during actual earthquakes and moderate values predicted by simple theoretical models. Combining our findings with earlier published results, we propose a period- and space-dependent factor, referred to as Topographic Aggravation Factor (TAF), which can be used in engineering design to modify site-specific design spectra of seismic code provisions to account for topography effects.by Dominic Assimaki.Sc.D