Emanuele Forte

2-D and 3-D processing and interpretation of multi-fold ground penetrating radar data: a case history from an archaeological site

A 2.5-D and 3-D multi-fold GPR survey was carried out in the Archaeological Park of Aquileia (northern Italy). The primary objective of the study was the identification of targets of potential archaeological interest in an area designated by local archaeological authorities. The second geophysical objective was to test 2-D and 3-D multi-fold methods and to study localised targets of unknown shape and dimensions in hostile soil conditions. Several portions of the acquisition grid were processed in common offset (CO), common shot (CSG) and common mid point (CMP) geometry. An 8×8 m area was studied with orthogonal CMPs thus achieving a 3-D subsurface coverage with azimuthal range limited to two normal components. Coherent noise components were identified in the pre-stack domain and removed by means of FK filtering of CMP records. Stack velocities were obtained from conventional velocity analysis and azimuthal velocity analysis of 3-D pre-stack gathers. Two major discontinuities were identified in the area of study. The deeper one most probably coincides with the paleosol at the base of the layer associated with activities of man in the area in the last 2500 years. This interpretation is in agreement with the results obtained from nearby cores and excavations. The shallow discontinuity is observed in a part of the investigated area and it shows local interruptions with a linear distribution on the grid. Such interruptions may correspond to buried targets of archaeological interest. The prominent enhancement of the subsurface images obtained by means of multi-fold techniques, compared with the relatively poor quality of the conventional single-fold georadar sections, indicates that multi-fold methods are well suited for the application to high resolution studies in archaeology.

Determination of Rayleigh Wave Dispersion Curves For Near Surface Applications In Unconsolidated Sediments

Multi-channel Analysis of Surface Waves (MASW) is a suitable technique for vertical shear-wave velocity profile determination whose efficiency and effectiveness depends on data acquisition parameters and processing procedures. In the present paper we compare the performances of three different methods to determine Rayleigh waves dispersion curves. Such curves are successively exploited for the inversion process, which eventually provides subsurface information of use in geological or geotechnical applications. We investigate the role of some processing procedures, with a special emphasis on the number of traces able to optimise the ratio between information content and acquisition and processing costs. We show that phase shift method is able to produce the best results in terms of accuracy and computation efficiency for the unconsolidated sediments considered in our work. Phase shift shows extremely stable results also when a reduced number of traces is considered and other methods fail due to spatial aliasing or severe noise content that prevents from unambiguous interpretation.

A multidisciplinary approach for rock spreading and block sliding investigation in the north-western coast of Malta

Landslides are widespread along the north-western coast of the Island of Malta and are strictly linked to the structural setting. Exemplary cases of rock spreading and block sliding phenomena characterise this stretch of coast. They are favoured by the overposition of two different geological units widely outcropping there, the Blue Clay Formation and the Upper Coralline Limestone Formation. The latter forms a wide plateau, bordered by vertical cliffs. At the foot of the cliffs, clayey terrains crop out and develop gentle slopes covered by large blocks detached and moved by rock spreading and block sliding phenomena. These mass movements are favoured by the fragile behaviour of limestones, which cap clays, otherwise characterised by visco-plastic properties. In order to investigate the kinematics and the evolution of these types of coastal landslides, a multidisciplinary and multitechnical approach was applied on a study site, named Il-Prajjet, which provides a spectacular case of rock spreading evolving into block sliding. This paper illustrates the results achieved by means of different engineering geological and geophysical techniques allied with traditional detailed geomorphological survey and mapping. In particular, the surface displacements of the landslides were determined using long-term GPS observations, acquired approximately every 6 months, over a 4.5-year period. A network of GPS benchmarks were distributed on the edge of a limestone plateau affected by rock spreading and on a series of displaced blocks making up a large block slide, finally enabling the definition of the state of activity and the rates of movement to be performed. In addition, the results deriving from two continuous fissurimeters more recently installed at the edge of two persistent joints over the block sliding area are outlined, with reference to the correlation between variations of crack apertures and precipitation input. In order to identify main structural discontinuities and to reconstruct variability of underground surface contact between clays and overlying limestones, Resistivity Tomography profiles and GPR investigations were carried out. Finally, the results obtained by combining the outputs of geophysical surveys and different field monitoring activities can be considered a first step on which numerical models can be developed and validated, in order to assess landslide hazard and risk of this stretch of Maltese coastline.

High resolution GPR imaging and joint characterization in limestone

We focus on the application of Ground Penetrating Radar (GPR) to evaluate limestone characteristics of interest in environmental and engineering studies, and in particular: a) to image joints, bedding planes and cavities; b) to improve accuracy and resolution of the method; c) to evaluate joint/bedding planes characteristics which affect the radar response with particular reference to thickness, sedimentary infilling, water/clay content and spatial frequency. The work is based on experiments carried out close to road cuts and cavities, where the exposed rock face allows calibration and validation of results. The test-sites are located in NE-Italy and are part of the Peri-Adriatic carbonate Platform. The rocks in the area of study date back to the Paleocene and are mainly peritidal regressive limestone sequences consisting of more than 90% carbonates. Joints and bedding planes surfaces, as results from direct inspection at the rock face, are sufficiently smooth to neglect roughness effects. We demonstrate that imaging can be improved by non-conventional data acquisition paradigms, and in particular by multi-offset (linear multi-fold, LMF) methods. Multi-offset/multi-azimuth (azimuthal multi-fold, AMF) techniques were exploited to select optimum grid orientation and offset range for LMF application. Multi-fold velocity analysis on CMP gathers shows rather constant wave propagation velocities. They slightly vary between different test-sites (between 10 cm/ns and 12 cm/ns) primarily due to macro/microscopic characteristics of limestone, such as spatial frequency of joints and porosity respectively, which affect water content in the rock mass. Constant velocities allowed application of post-stack time migration algorithms. Such algorithms attained imaging accuracy below 3% in the reconstruction of the detectable discontinuities at most test sites.A Kirchhoff algorithm proved to be the optimum solution as it effectively handled the steep dips (up to 70°) that characterize most of the examined sets of joints. Enhanced data quality further results from the application of original processing techniques, such as, in particular, Hough Transform based coherent noise/background removal and Wavelet Transform based instantaneous parameters computation and analysis. Maximum penetration depth at the examined test sites ranges between 15 m for 250 MHz (central frequency) bow-tie shielded antennas and 23 m for 50 MHz unshielded resistively loaded linear dipoles. As for resolution, antennas in the range of 200-250 MHz apparently provide effective discrimination of joints/bedding planes spaced not less than 40 cm, and seem therefore the adequate choice at the examined test sites. Low frequency antennas, in the range of 50 to 100 MHz (central frequency), provide a maximum 55% increment in penetration depth at the expenses of a substantially diminished resolution (around 20% of that provided by 200-250 MHz antennas).The comparison of modelling results to multi-fold data allows discrimination of radar response from joints filled by air and clayey deposits. Such results were validated by geological evidence at the exposed rock face.

Multifold ground-penetrating radar and resistivity to study the stratigraphy of shallow unconsolidated sediments

Reconstruction of shallow stratigraphy of unconsolidated sediments is a topic of primary interest in several environmental, hydrological, geotechnical, and engineering applications. The identification of porous layers and the assessment of their saturation, the characterization of sediments, the identification of bedrock and the analysis of shallow layering are some examples of topics of primary interest in near-surface applications. Recent ground-penetrating radar (GPR) research demonstrates the excellent results that can be attained in the study of shallow stratigraphy. Complex stratigraphic structures, involving cross-stratification, conflicting dips, and rapid lateral and vertical particle-size variations pose a challenge to the application of single-fold (constant offset) GPR methods. The objectives of the present work are imaging and resolution enhancement of GPR multifold records from shallow, unconsolidated sediments. The study is based, in particular, on prestack processing and imaging of data from alluvial plain sites in northern Italy, which are characterized by different stratigraphic and sedimentological conditions. Figure 1 shows the location map of the survey. We show the results obtained on a fluvial terrace of the Isonzo River that are characterized by a complete alluvial sequence including a range of sediments (gravel to clayey loam) and range of stratigraphic structures (depositional and erosional). The water table and vadose zone are in the GPR and resistivity depth range and affect the response of the geophysical techniques, particularly the lateral and vertical resistivity and GPR velocity variations. Figure 1. Map and aerial picture of the study area. The red rectangle shows the location of the 20 × 12 m study area. The site is close to the riverbank, where the different stratigraphic units identified by the geophysical survey were identified and sampled. A Mala Geoscience GPR system was equipped with shielded 250-MHz antennae for the study. Single-fold methods were used in reconnaissance surveys at all test sites. We successively performed …

Review of multi-offset GPR applications

GPR and reflection seismics share common physical and methodological bases but are sensitive to different subsurface physical properties. The peculiarities of the electromagnetic case impact data acquisition, processing and interpretation. We review multi-offset techniques in GPR applications focusing on similarities and differences through examples taken from different subsurface and target conditions.GPR multi-offset data acquisition methods basically involve common-offset and common midpoint geometries: accuracy and work load are the main factors that drive the choice, together with effectiveness of the solution for the objectives of the study.Multi-fold data processing algorithms can bring remarkable signal-to-noise ratio enhancement and offer the opportunity to extract additional information from field data. Velocity field and related dielectric constants distribution, attenuation and related conductivity variations, changes in the GPR response with offset are some of the examples. Coherent noise suppression and velocity analysis are key features in GPR multi-fold processing sequences and we review the relevant methods with examples of application in addition to technical aspects.Multi-channel acquisitions, full wave-form inversion, pre-stack depth migration, azimuthal and polarimetric analysis, are among the many topics in current and future research that are briefly reviewed to provide some highlights of the forthcoming developments in GPR methods. Multi-fold GPR allows to obtain enhanced subsurface imaging.EM velocity and other physical parameters can be extracted from Multi-Fold GPR data.Horizontal stacking is useful to identify and suppress random and coherent noises.Different Multi-fold GPR acquisition methods are analyzed.Recent advancements in GPR analysis and processing could be used in Multi-fold GPR.

4-D quantitative GPR analyses to study the summer mass balance of a glacier: a case history

In order to assess the seasonal changes of the topography, the inner structure and the physical properties of a small glacier in the Eastern Alps, we performed a 4-D multi frequency GPR survey by repeating the same data acquisition in four different periods of the year 2013. The usual glacier mass balance estimation encompasses only topographic variations, but the real evolution is much more complex and includes surface melting and refreezing, snow metamorphism, and basal melting. We analyzed changes in both the imaged geometrical-morphological structures and the densities, estimated from GPR data inversion. The inversion algorithm uses reflection amplitudes and traveltimes to extract the electromagnetic velocities in the interpreted layers and the densities of the frozen materials through empirical relations. The obtained results have been compared and validated with direct measures like snow thickness surveys, density logs within snow pits and ablation stakes. This study demonstrates that GPR techniques are a fast and effective tool not only for glacial qualitative studies, but also for detailed glacier monitoring and accurate quantitative analyses of crucial glaciological parameters like density distribution and water runoff.

2D and 3D imaging of a buried prehistoric canoe using GPR attributes: a case study

We apply Ground Penetrating Radar (GPR) to detect a prehistoric canoe at the Maoshan site, Zhejiang Province, China. A complex attribute analysis of the GPR data allows enhancing the precision in target detection and provides more details about the canoe and the burial environment. The burial depth of the bottom interface of the prehistoric canoe is detected and the integrity of the whole canoe is assessed through a GPR survey. Difficulties in the application of dense sampling of 2D and pseudo 3D GPR data originate from micro-topographical disturbance that specifically affects the pseudo 3D investigation results obtained from high-frequency antennas. Data processing and advanced imaging techniques can only remove part of such effects. The research demonstrates that GPR can successfully image wooden cultural relics buried in the shallow subsurface with ultra, high-density trace spacing and high-frequency antennas even in totally saturated clay-rich soils based on 2D profiles and pseudo 3D methodologies, characterized by tight (cm) cross-line/in-line spacing.

Velocity spectra and seismic-signal identification for surface-wave analysis

Rayleigh-wave dispersion is observed every time acoustic-impedance stratification occurs, and its analysis is suitable for vertical shear-wave profile reconstruction. Accurate dispersion-curve identification is essential in order to properly determine the shear-wave velocity distribution of a medium. Data sets characterized by several events generate complex velocity spectra that can lead to possible misinterpretations. We analyse a real data set by taking into account theoretical dispersion curves and synthetic data obtained from numerical simulations in order to avoid possible pitfalls that could arise from the complex trends exhibited in the f–k (frequency–wavenumber) and v–f (velocity–frequency) domains. In the v–f domain in particular, we show that reflection events and their multiples generate coherences that could be misinterpreted because of their similarity to typical higher-mode dispersion curves. Another observed signal is interpreted in terms of guided waves and related phenomena. The results of the fundamental-mode dispersion curve inversion performed via genetic algorithms indicate a sedimentary cover stratification that simple reflection analysis cannot reveal. The present case study highlights the importance of a synergic approach, based on integrated synthetic and field data analysis, for correct interpretation of all the wavefield components in the velocity spectrum.

3D GPR imaging for paleoseismology in Central Appennines (Italy)

The knowledge of spatial position and geologic characteristics of active faults is fundamental to locate these elements on geologic maps and better define the potential seismological hazard of an area. A Quaternary fault in the “Piano di Castelluccio” basin, in Central Italy, has been already studied through paleoseismological analysis, provided stratigraphic data on sedimentary units and highlighted evidences of “recent” faulting related to past strong earthquakes. This fault has been defined “silent”, because events were not included in the current seismic catalogues, retaining uncertainties in the definition of the seismic hazard of that area. 2D/3D GPR surveys were done to image the fault zone and to provide new complementary data on the shallow sediments. The 3D data provides continuity of information over the investigation site, characterizing the structure in a total non-invasive way, whilst long 2D profiles were used to extend the study on a wider area. Final images show a characteristic GPR signature of a tectonic structure and faulted units: data reveal its position and continuity in space and highlight clear geometric features, providing useful qualitative and quantitative complementary data.