I. Finetti

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.

Structure and Evolution of the Central Mediterranean (Pelagian and Ionian Seas)

Four main extensional phases affected the Central Mediterranean from the Permo-Triassic to the Quaternary. The first, active during the Middle-Upper Triassic, produced continental rifting with prominent effects in the Streppenosa Trough (Sicily-Malta platform area) and in the Ionian Sea. A second extensional phase occurred in the Middle Jurassic and opened the Ionian Sea and Eastern Mediterranean. The Pelagian Sea and Sirte Rise (excluding the upper Sirte Slope, where sedimentation commenced in the Upper Cretaceous), comprise a thick to very thick sedimentary sequence that includes deposition from the Triassic to the Quaternary. The Sirte Rise and other geological provinces in the studied area were stretched considerably during the third extensional phase of the Middle-Upper Cretaceous.

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 …

Messinian Evaporites in the Mediterranean: A Model of Continuous Inflow and Outflow

A model of brine circulation and evaporite precipitation is proposed that presumes a steady inflow/outflow regime across a severely restricted entrance strait and intermittent precipitation of sulfates and chlorides. Frequent minor freshenings of the brine, owing to climatic oscillations, cause interruptions in precipitation.

Seismic modeling study of the Earth's deep crust

We use seismic modeling methods to validate the interpretation of deep‐crust seismic exploration. An approximation of the stacked section is obtained with the nonreflecting wave equation and the exploding‐reflector approach. Using this technique and ray‐tracing algorithms, we obtain a geological model by comparing the synthetic section with the real stacked section. An isotropic constitutive equation is assumed in this phase. The exact synthetic stacked section is then obtained by applying the standard processing sequence to a set of synthetic common‐shot profiles computed with the variable‐density acoustic wave equation. We introduce elliptical P‐wave anisotropy and the effects of small‐scale inhomogeneities by using a von Karman autocovariance probability function that simulates scattering Q effects. Verification of the geological model by poststack migration constitutes an additional test. The methodology, which is suitable for areas of complex geology, is applied to a seismic line acquired in the nort...

Shear‐wave profiling via SH reflection analysis and rayleigh wave inversion

We analyse and compare the shear-wave velocity distribution obtained from multi-fold SH reflection data and from Rayleigh wave dispersion curve inversion. The analysis focuses on two seismic datasets from a waste disposal site. We use an optimisation scheme based on a Genetic Algorithm and on the evaluation of the Marginal Posterior Probability Density to invert the dispersion curves and handle the strong multi-modality of the inversion problem. The number of reflectors in the inversion process is constrained by the SH-wave reflection survey to allow better comparison of the results. The final velocity models, separately obtained by the two techniques, are in excellent agreement and are validated by the available borehole data. The results of the study demonstrate the benefits of joint shear and Rayleigh wave surveys with particular reference to media characterized by strong attenuation.

Interpretive Migration Velocity Analysis with Applications to GPR

Velocity model refinement by a combination of Deregowsky and focusing techniques is an effective solution to image complex shallow subsurface conditions by means of GPR. Including geological knowledge in the velocity analysis of CRP gather considerably reduces costs and uncertainties of the final velocity model and allows rapid convergence to the correct solution. Applications to high resolution GPR study of alluvial sediments confirm the accuracy of the results attainable by PSDM in imaging complex sedimentological patterns.

Pre-stack Processing of Multi-fold GPR Data to Study the Stratigraphy of Shallow Unconsolidated Sediments

We carried out multi-fold GPR surveys in alluvial plains located in northern Italy to develop and test acquisition and processing procedures suited for the 3-D reconstruction of radar stratigraphy in unconsolidated sediments. The study was integrated by resistivity measurements and by particlesize distribution analysis for a physical and sedimentological characterization of materials, which range from sandy to silty loams interbedded with sparse gravel and clay lenses. Data acquisition was performed at frequencies ranging from 800 MHz to 250 MHz to achieve maximum resolution at different depth levels. The average maximum penetration is around 7 metres at most of the test sites. Data processing was basically performed in the prestack domain to obtain a complete amplitude and velocity characterization of the radar facies. Primary objective of the processing sequence was resolution enhancement to image thin layers and lenses characterized by rapid lateral velocity variations and conflicting dips. A deconvolution algorithm based on Burg spectral estimation proved to be very efficient in wavelet compression. Pre-stack depth migration performed by means of a Kirchhoff algorithm was the optimum imaging solution . The results of multifold data processing allow the interpretation of sedimentary layers and lenses as thin as 30 cm and a characterization of the radar response from different materials based on the analysis of amplitude variations with offset. The velocity models obtained from pre-stack data analysis allow discrimination of zones of saturation and aeration and the evaluation of leakage from streams into unconsolidated sediments.

Hough transform based multiple removal in the XT domain

The proposed method for multiple reflection attenuation is based on a two-step procedure for the construction of optimized model traces. Statistical optimization of the model traces at each offset (step 1) is followed by a Hough transform based scaling (step 2) which further reduces the non-multiple energy in the model traces. The major advantage is the substantial enhancement of the nonmultiple/multiple energy ratio in the near offset, obtained through the Hough transform scaling. We apply the method to CMP gathers corrected for the NMO of the multiples. The multiple reject zone is defined by the analyst in the velocity analysis phase. At the present state of implementation the proposed method is effective in the removal of horizontally aligned events. The first step is separately carried out for each multiple reflection, thus resulting in a time-variant and offset-variant number of traces which contribute to the model-trace construction. A measure of the coherence obtained in the multiple reject window, as a function of the number of adjacent traces considered, is used to determine the optimum number of traces to be summed for the construction of the individual model traces at each offset. A scaling function is calculated from the input data which is applied to the model traces in order to minimize the non-multiple energy. A single scaling function for all the offsets is calculated from the NMO corrected CMP gather by applying a shape detection algorithm based on the Hough transform. Maximum values of the scaling function are obtained where the algorithm detects the presence of horizontal alignments, corresponding to multiple reflections in the input data. The scaled model traces are subtracted from the data. The method was tested on real data from the Ross Sea (Antarctica) and from the Rockall Trough (offshore U.K., courtesy of BIRPS). The results indicate that the method provides superior multiple attenuation and preservation of the primary signal in most cases when compared with conventional techniques, such as filtering in the FK and τp domains. The present implementation of the algorithm carries out in parallel model trace and scaling function computation. The CPU time requested for a complete run of the proposed algorithm is approximately one tenth of that requested by conventional FK filtering.