Michele Pipan

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.

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.

Synthetic and field examples of ground‐penetrating radar (GPR) profile improvement using two‐phase detection techniques

Ground-penetrating radar (GPR) has been used extensively in a wide range of subsurface imaging applications, including underground utilities, hazardous waste and liquid contaminants, buried archeologic sites, geological interfaces (fractures, bedding planes), and pavement–concrete structure assessment (Hugenschmidt, 2000; Olhoeft and Smith, 2000). In the latter case, industrial applications such as damage assessment before road rehabilitation require processing and analysis of data sets that may be orders of magnitude larger than normal GPR ones. In such conditions, the reliable identification of radar echoes in a large range of S/N ratio values can be exploited to apply automatic tracking algorithms for a cost-effective interpretation. A number of studies deal with applying GPR to pavement assessment (Hugenschmidt et al., 1998; Saarenketo and Scullion, 2000), analyzing water content in the pavement or in the subgrade (Benedetto and Benedetto, 2002; Emilsson et al., 2002), and implementing or exploiting automatic tracking algorithms to interpret GPR data sets from pavement surveys Spagnolini and Rampa, 1999). Anomalous lateral variations in water content of the pavement and subgrade are recognized as a major source of pavement damage. At the same time, they affect the radar response by causing an overall decrement of the S/N ratio, most probably because of the defects (primarily delamination and cracks) produced by the anomalous saturation. No examples exist in the available literature of dedicated processing techniques to improve the detection of weak transients in low S/N ratio conditions and to enhance the performance of automatic tracking methods. In this work we exploit wavelet transform-based instantaneous parameter analysis for this purpose and compare the results with those obtained by Hilbert transforms on synthetic and real GPR data.

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.

Instantaneous parameters calculation and analysis of impulse ground penetrating radar (GPR) data

We use the wavelet transform (WT) method to analyse impulse GPR signals received from anti-personnel (AP) plastic mines. The mine radar echoes examined in this work are characterized by an average low amplitude and low signal-to-noise ratio (SNR). We focus on the use of WT to extract such weak transients from the noisy background. In particular, we calculate the instantaneous parameters of the radar signals by means of the WT. Instantaneous wavelet power and energy spectra allow enhanced identification of plastic mines echoes in low SNR. We further propose a novel non-linear transform method to process the results after WT in order to increase the energy difference between the mine signal and the clutter.

Early Roman military fortifications and the origin of Trieste, Italy

Significance Archaeological evidence from the Trieste area (Italy), revealed by airborne remote sensing and geophysical surveys, provides one of the earliest examples of Roman military fortifications. They are the only ones identified in Italy so far. Their origin is most likely related to the first year of the second Roman war against the Histri in 178 B.C., reported by Livy, but the sites were in use, perhaps not continuously, at least until the mid first century B.C. The main identified San Rocco military camp is the best candidate for the site of the first Trieste. An interdisciplinary study of the archaeological landscape of the Trieste area (northeastern Italy), mainly based on airborne light detection and ranging (LiDAR), ground penetrating radar (GPR), and archaeological surveys, has led to the discovery of an early Roman fortification system, composed of a big central camp (San Rocco) flanked by two minor forts. The most ancient archaeological findings, including a Greco–Italic amphora rim produced in Latium or Campania, provide a relative chronology for the first installation of the structures between the end of the third century B.C. and the first decades of the second century B.C. whereas other materials, such as Lamboglia 2 amphorae and a military footwear hobnail (type D of Alesia), indicate that they maintained a strategic role at least up to the mid first century B.C. According to archaeological data and literary sources, the sites were probably established in connection with the Roman conquest of the Istria peninsula in 178–177 B.C. They were in use, perhaps not continuously, at least until the foundation of Tergeste, the ancestor of Trieste, in the mid first century B.C. The San Rocco site, with its exceptional size and imposing fortifications, is the main known Roman evidence of the Trieste area during this phase and could correspond to the location of the first settlement of Tergeste preceding the colony foundation. This hypothesis would also be supported by literary sources that describe it as a phrourion (Strabo, V, 1, 9, C 215), a term used by ancient writers to designate the fortifications of the Roman army.