Luciana Orlando

Evaluation of tunnel stability using integrated geophysical methods

Abstract An integrated interpretation was made of data, from ground penetrating radar (GPR), seismic refraction and seismic transmission tomography, collected inside the catchment tunnels of a potable water source in central Italy. Rock fracturing and obsolescence of the concrete lining in a tunnel led a landslide that caused structural instability in the catchment work structures. To assess the stability of the rock close to the landslide, geophysical surveys were preferred to boreholes and geotechnical tests in order to avoid water pollution and the risk of further landslides. The interpretation of integrated data from seismic tomography and 200 MHz antenna GPR resulted in an evaluation of some of the elastic characteristics and the detection of discontinuities in the rock. Note also that an analysis of the back-scattered energy was required for the GPR data interpretation. The integration of seismic refraction data and 450 MHz antenna allowed us to identify the loosened zone around the tunnel and the extent of the mass involved in the cave-in, while GPR data from 225 MHz were used to evaluate the quality of contact between concrete lining and massive rock.

Lowstand terraces on Tyrrhenian Sea steep continental slopes

Abstract Wave-built terraces formed during sea-level Stillstands at depths lower than present are found along margins with fairly steep morphology and no- to ill-developed continental shelf, for instance the margin of volcanic islands. Terraces deposits are up to 20–30 m thick and are flat-topped with well-defined edges. They extend up to 1 km in dip section. The deposits are composed of intrabasinal sediments, mainly bioclastic sand and silt and are probably due to deposition below the storm wave-base during large-scale energy episodes. Two areas of the Tyrrhenian Sea have been analysed, one in a volcanic context (the Pontine archipelago) and the other in a highly active structural context (the Calabrian Arch). Similar wave-built terraces were found in both areas, with a main terrace placed at a depth of 100–150 m, which is thought to be Wurmian. Older terraces are also present, emplaced during the preceding lowstands in the Late Pleistocene. A possible present-day terrace is found at about 20 m depth in the Pontine archipelago. A mechanism of subsequent formation below wave base-level and removal because of sea level lowering has been hypothesised during long-lasting periods of Late Pleistocene sea level falls. Terraces are thus thought to preserve only at the minimum lowstand position and to remain relict on the slope during sea level rise and highstand. The analysis of the distribution and depths of these terraces allows some neotectonic interpretation. For example, in the volcanic archipelago, whose western sector has probably risen at an average rate of 2.5 mm/yr over the last 20,000 years.

Location of Archaeological Structures using GPR Method: Three-dimensional Data Acquisition and Radar Signal Processing

Generally, in order to detect shallow archaeological features, such as tombs, cavities, walls, etc., ground penetrating radar (GPR) data are acquired along parallel profiles. In some cases, the data collected using the GPR method are difficult to interpret owing to the presence of low signal-to-noise (S/N) ratio. These signals can be generated by several factors that significantly influence the radar profiles. To enhance the interpretation of radar sections, three-dimensional data acquisition, radar signal processing and time-slice representation are used. The archaeological investigated as a test site was the Sabine Necropolis (700–300 BC) at Colle del Forno (Montelibretti, Roma), believed to contain unexplored underground dromos chamber tombs. The measurements were carried out along parallel profiles in a test area, using Sir System 10 (GSSI) equipped with different antennas operating at 100, 300 and 500 MHz. The spatial interval used during the survey was 20 cm. To enhance the S/N ratio, a band-pass filter and subtraction of an average trace on the field data has been applied; furthermore, the two-dimensional migration technique on all profiles collected was used in order to move diffraction effects. A time-slice representation technique was adopted to obtain a planimetric correlation between anomalous bodies at different depths. The results indicate that the three-dimensional data acquisition, processing and the time slice representation can help determine the location, depth and shapes of buried features.

Semiquantitative evaluation of massive rock quality using ground penetrating radar

Abstract This work studies a methodology starting from georadar data that allows a semiquantitative evaluation of massive rock quality. The method is based on the concept that in good quality rock, most of the energy is transmitted, while in low quality rock, the energy is backscattered from fractures, strata joints, cavities, etc. When the energy loss due to spherical divergence and attenuation can be recovered by applying a constant spherical/exponential gain, the resulting energy function observed in the georadar section depends only on the backscattered energy. In such cases, it can be assumed that the amount of energy is an index of rock quality. Radar section interpretation is usually based on the reconstruction of reflected high-energy organized events. Thus, no consideration is given to backscattered not-organized energy produced by microfractures that greatly influences the geotechnical characteristics of the rock mass. In order to take into consideration all the backscattered energy, we propose a method based on the calculation of the average energy relative to a portion of predefined rock. The method allows a synthetic representation of the energy distributed throughout the section. The energy is computed as the sum of the square of amplitude of samplings contained inside cells of appropriate dimensions. The resultant section gives a synthetic and immediate mapping of rock quality. The consistency of the method has been tested by comparing georadar data acquired in travertine and limestone quarries, with seismic tomography and images of actual geological sections. The comparison highlights how effectively the energy calculated inside the cells give synthetic representation of the quality of rocks; this can result in maps where the high-energy values correspond to rock of poor quality and the low energy values correspond to a good quality region. The results obtained in this way can, in this case, be partly superimposed onto those of seismic tomography.

Georadar as a tool to identify and characterise solid waste dump deposits

Abstract This paper reports the ability of a ground-penetrating radar survey to detect and characterise uncontrolled waste dumps. The dumps in our study are located just outside Rome, date back to the 1960s, and have long been abandoned. It is probable that stored there are heterogeneous refuse, i.e. household refuse, building rubble and/or industrial refuse. The Ponte Malnome dump overlies Quaternary alluvial sediments and the Regione Lazio dump, located in an abandoned quarry of gravel and coarse-sand extraction, lies on Plio-Quaternary fine sediments. The sites were surveyed with georadar equipped with 50-, 100- and 200-MHz antennas. The results of the first site were compared and integrated with VES, multielectrode and boreholes and the second with VES and seismic refraction results obtained prior to the present study by Cardarelli et al. (1997). The study confirmed that georadar can be used to detect such dumps and define the geometry of the waste dump body. On the georadar records, the in situ sediments are stratified while the dump deposits produce diffraction and noncontinuous reflections. In the Ponte Malnome site, the georadar allowed us to define the actual eastern limit of the waste dump deposits, finding it wider than was presumed from geological survey, and define the type of refuse—mainly building rubble. The geophysical results were confirmed by boreholes. In the Regione Lazio, the georadar allowed us to define the limits of the waste dump and to discriminate between surficial household refuse and underlying quarrying waste. The study demonstrates that even in unfavourable conditions, georadar can provide useful information for locating and characterising abandoned urban dumps.

Some considerations on shallow seismic reflection surveys

Abstract High-resolution shallow seismic reflection surveys require more attention to the choice of source and configuration, receivers and recording geometry for optimizing data acquisition than conventional oil exploration surveys. Moreover, some standard processing techniques to increase signal/noise (S/N) ratio need special accuracy (for example, surgically precise removal of early-time coherent noise and iterative, small time shift static corrections). This paper compares results obtained using different sources at two test sites: explosive, cap, shotgun, hammer and weight drop. Data from experiments using geophones with different natural frequencies and using various acquisition geometries are also compared. In data processing, it is demonstrated how increasing the S/N ratio for high-resolution results requires special consideration in some common processing steps (F–K filter, first arrivals muting, elimination of air wave and static corrections). The comparison, based on shot gathers and stack sections, shows that attenuation of high frequencies by the earth is the most significant influence on the spectral properties of the data, as expected the source itself also does have some influence on frequency content, depending to some extent on surface conditions. The high-velocity explosive sources produced the highest frequency reflections and best S/N ratio, because they have higher energy related to higher burn/blast velocity and source containment then the other sources and they are used in hole (i.e. below ground surface where the air wave energy is more attenuated) but the shotgun also an explosive source was reasonably comparable to high explosive when used in hole. Special care must be taken during processing to insure artifacts are distinguished from real reflection events.

Small-scale Seismic Stratigraphy and Paleogeographical Evolution of the Continental Shelf Facing the Se Elba Island (Northern Tyrrhenian Sea, Italy)

ABSTRACT Approximately 200 naut. mi. of high-resolution, single-channel seismic reflection profiles from the SE Elba continental shelf were analyzed. The analysis included a detailed three-dimensional reconstruction of the upper 100 m of shelf sediments. Six depositional sequences were identified, with an average thickness of less than 10 m. These sequences were bounded by unconformities which were mostly erosional and which extended over the entire area. Commonly used exploration seismology methods were applied; they included an analysis of the configurations of reflectors inside each sequence and their terminations against the basal unconformity. The lower five sequences experienced a similar history. They appear to represent coastal environments which developed at more than 100 m below the present sea level during Pleistocene sea level changes. The uppermost sequence was correlated with the muddy shelf sedimentation of the present highstand. The recurrent distribution in the five lower seismic sequences can be explained by the fact that, during rises of sea level, sedimentation was largely controlled by local paleogeography and paleotopography. During such periods, the investigated area was located at the apex of a wide gulf connecting Elba Island to the mainland. In this area, progradations were mostly controlled by the slope of the paleocoast, that is, by the structural constraint represented by the Monte Calamita headland. /P> The paleoenvironmental interpretation for the upper sequences was largely substantiated by seafloor cores. Cores were collected from relict transgressive littoral deposits below a mud interval related to the present eustatic environment.

Using GPR to Monitor Cracks in a Historical Building

The paper treats the application of GPR to detect and monitor cracks induced on a historical building by a landslide. The data were acquired on a floor inside the building with profiles 10 cm apart recorded at three different times. A 2GHz bipolar antenna was used, which allows the acquisition of multi-component data. The data were processed to produce vertical profiles and a 3D cube for each component and survey. The interpretation of the profiles is in good correlation with the structure of the floor. The profiles and time slices have detected two different kinds of anomalies, only a few of which can be due to utilities and the metallic mesh. The others were associated with cracks induced by the landslide. The time slice obtained from the last survey shows anomalies not detected before.

Electrical permittivity and resistivity time lapses of multiphase DNAPLs in a lab test

Dense Non-Aqueous Phase Liquids (DNAPLs) induce variation in electromagnetic characteristics of the ground, e.g., electric permittivity and resistivity. The most used indirect methods in the mapping of these physical characteristics are electrical resistivity and ground penetrating radar. To better understand the effect of DNAPL release on electrical permittivity and resistivity in a water saturated medium, we carried out a controlled laboratory experiment where the host material was simulated by glass beads and the DNAPL by HFE-7100 (hydrofluoroether). The experiment measured the electric resistivity and permittivity of each fluid, the multiphase fluid system, and the host material, along with time-lapse electrical resistivity and GPR measurements in a controlled cell. We found that the different phases of DNAPL within a saturated medium (free, dissolved, and gaseous phase) affect the physical characteristics differently. The reflection pull-up behind contaminated sediments, which is normally detected by GPR, was mainly inferred from the HFE free phase. The dissolved phase causes small variations in electric permittivity not usually readily detected by GPR measurements. Both the dissolved and free HFE phases induce variation in resistivity. The study showed that GPR and electrical resistivity differ in sensitivity to the different HFE phases, and can be complementary in the characterization of DNAPL contaminated sites.

Using polarimetric GPR antenna to characterize hidden cylinders

The recent appearing of multi-component GPR antennas in the research and market fields is attracting interest towards the possibility to exploit polarization diversity in order to gain more information on the probed scenario. In this framework, based on a forward model, we investigate the potential of discerning the electromagnetic nature of cylindrical targets staring from B-scan measurements. The study is carried out via a FDTD numerical analysis and is supported by analytical results referred to the 2D case. The preliminary obtained results suggest that is actually possible to discriminate conducting and dielectric circular cylinders from multi-component GPR B-scans. Experimental results are also provided.