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Near Surface Geoscience 2015 - 21st European Meeting of Environmental and Engineering Geophysics
- Conference date: September 6-10, 2015
- Location: Turin, Italy
- Published: 06 September 2015
41 - 60 of 164 results
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Electrical Geophysical Monitoring of Organic Waste Contamination
Authors D. Ntarlagiannis, J. Robinson, P. Kirmizakis, P. Soupios and L.D. SlaterSummaryOlive oil production in the Mediterranean area is a major industry, with significant impact in the economy of producer countries. During the production of extra virgin olive oil, a substantial amount of waste is produced and is improperly disposed. since no regulation for the environmental deposition of olive oil mill waste exists. Thus, there is an urgent need for the development of methods that will allow efficient monitor waste management processes, and enable rapid recognition of environmental degradation incidents.
The most common disposal practice involves unrestricted damping of the olive oil mill waste in open evaporation ponds, leading to significant degradation of the environment including groundwater and surface waters. Geophysical methods could be used to monitor olive oil waste management processes, and identify olive oil waste plumes right at their inception times. We applied a series of electrical geophysical measurements at an olive oil mill waste (OOMW) site in western Crete to test their applicability as monitoring aids. Our results suggest that electrical methods are very efficient in accurately delineating young OOMW plumes due to their conductive signature.
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Modeling of Electromagnetic Soundings to Detect Groundwater Contamination by Intensive Livestock Farming
Authors C.M. Sainato, R.A. Espada, B.N. Losinno and J.J. Márquez MolinaSummaryThe feedlots, farms for feeding animals in corrals, are spreading at Buenos Aires Province, Argentina. It is important to evaluate the contamination by mineralization of organic matter. Geophysical exploratory methods give an important tool for monitoring groundwater and soil quality. The objective of this work was to perform electromagnetic soundings and to do a modeling of these data by an inversion procedure for different frequencies at a corral of a feedlot and the surroundings. The survey was repeated the following year. Bellow the corral, the unsaturated zone has lower resistivity (between 8 and 17 ohm m, more conductive) than the test zone. The second year, the test zone showed greater range of values for resistivity (up to 54 ohm m) but the layers bellow the corral showed an important decrease of resistivity. Groundwater samples were taken along four years. An increase of water conductivity was found at the lower well along the time.
The EMI modelling allowed characterizing the distribution of resistivity bellow the corral and the background, reaching the groundwater, detecting an increase of salinity bellow the corral which may be mainly due to leaching of nitrates.
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TEM Profile Using Fixed-loop Array in the Paraná Basin, Brazil: Preliminary Results
Authors J.D.R. Campana, J.L. Porsani and C.A. BortolozoSummaryIn this work the authors shows preliminary results of TEM profile by using fixed transmitter loop and a 3D coil receiver mobile in the Paraná sedimentary basin, Brazil. The objectives were to map the sedimentary and crystalline aquifers. The sedimentary aquifer is characterized by Adamantina Formation and the crystalline aquifer is characterized by fractures zones inside of the basalt layers of the Serra Geral Formation. 1D inversion results for component-Z for each sounding are presented as an interpolated geoelectric profile of 1000m long. Two low resistivity zones were observed: the first between 40 – 60m depth, being related probably to the Adamantina Formation (sedimentary aquifer), and the second between 150m and 300m depth, being interpreted as a probable fractured zone filled with water inside of the basalt layer of the Serra Geral Formation (crystalline aquifer). These results are in accordance with the local geological information.
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Electric and Electromagnetic Investigation of a Karst System
Authors M.P. Miensopust, J. Igel, T. Günther, R. Dlugosch and S. HupferSummaryThe limestone of the western flank of the Innerste Mulde in Lower Saxony, Germany, host a 27 km long karst water system which is surface-mapped by sinkhole and collapse structures. The karst water emerges at springs — one of them is a spring called Kirschensoog near the village of Altwallmoden which only temporarily yields water. From early March to mid March 2015 the water level fell visibly. During that time electric and electromagnetic methods were applied in the field and soil and rock samples were taken for laboratory studies. Field measurements comprise electrical resistivity tomography (ERT), ground penetrating radar (GPR) and 2D nuclear magnetic resonance (NMR) surveys. The laboratory measurements (e.g. NMR and spectral induced polarisation measurements, petrophysical studies, dielectric spectroscopy) are applied to support the interpretation of the field data. The preliminary results show that the covering, clay-rich soil has a strong attenuation restricting the investigation depth of GPR. The ERT models indicate a 3D subsurface structure and — as well as GPR — suggest dipping structures possibly related to the bedding of the limestone or fissures/ fractures. The rather low resistivity of the limestone might be an indication of disruption and presence of fluids.
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Seasonal Monitoring of Saline Intrusion in an Unconfined Coastal Aquifer in New Zealand Using DC Resistivity Traversing
More LessSummaryLong term time-lapse DC resistivity measurements along six 155 to 275 meter long traverses at the Kapiti Coast in New Zealand have been made to image a seasonal cycle in the saline interface movement. Two-monthly repetition of the measurements showed a continuous decrease of the bulk resistivity at depth and over an extended horizontal area when coming into the drier summer season. This observation is somewhat contradictory to the expectations and has its origin most possibly in a decrease of the fluid resistivity, likely due to a lower fresh water flow coupled with saltwater intrusion. Although, the changes from time step to time step are not always major on all profiles, ERT time-lapse monitoring proved itself a suitable tool to image the saline interface shape and movement along with changes in the subsurface resistivity distribution in an unconfined coastal sand aquifer and in a seasonal time frame.
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Prestack Depth Migration of Ultrashallow Seismic Data. A Critical Analysis
Authors G.P. Deidda, G. Satta, E. Battaglia, C. Strobbia and Z. HeilmannSummaryPSDM is increasingly becoming rule rather than exception in near surface seismic imaging when the goal is to get detailed sub-surface images and accurate velocity field. PSDM produces the seismic image of an earth model in depth converting the seismic reflection data into a depth section by means of a velocity field. Although different implementations of velocity model building tools exist, they usually start with an estimation of an initial velocity model using moveouts in the time domain and proceed with an iterative model refinement using residual moveouts in the depth domain and seismic reflection tomography. In addition, if available, a priori velocity/depth information, is also considered to improve the accuracy of the velocity model. In shallow seismic reflection imaging, instead, refraction tomography has been used not only to improve the velocity model built with PSDM but also as the exclusive tool to build the velocity model. In our tests, building a velocity model by means of a priori information unrelated with the recorded data or by means of the exclusive use of refraction tomography, PSDM produces depth images that have an imprint of the velocity models and not a result coming only from seismic reflection data.
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Groundwater Storage in a Karst Vadose Zone Evidenced Using Gravimetric and Surface-to-borehole ERT Monitoring Systems
Authors A. Watlet, O. Kaufmann, O. Francis and M. Van CampSummaryHydrogeophysical tools allow characterizing the heterogeneity of karsts and evidencing groundwater storage in their subsurface. A permanent gravimetric monitoring highlights seasonal variations in gravity that can be related to vadose zone effects after the discrimination of the saturated zone signature with direct measurements. A surface ERT monitoring improved with daily borehole-to-surface measurements through the vadose zone allows identifying preferential flow paths. Groundwater storage variation is then monitored by time-lapse inversion of the ERT datasets.
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Assessing Electrical Resistivity Tomography for Hydrofacies Detection Using a Sensitivity Dependent Probabilistic Method
Authors T. Hermans and J. IrvingSummaryAlluvial aquifers are generally composed of several facies with complex architectures and interconnections depending on the fluvial system. In this context, electrical resistivity tomography (ERT) may provide important information on the spatial distribution of hydrogeological parameters. However, ERT inversion introduces some bias in the resulting resistivity distribution due to regularization and resolution issues. In this study, we refine ERT inversions by incorporating prior information in order to improve the identification of facies through a probabilistic relationship derived from collocated measurements. We then analyze with synthetic cases the effect of spatially varying sensitivity on the probabilistic relationship. As expected, when sensitivity decreases, the distributions of resistivity for the different facies tend to be superimposed. A mean distribution thus overestimates the ability of surface ERT to discriminate hydrofacies in depth.
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Integration of Geoelectrical Imaging and Hydrochemistry Data in a Coastal Aquifer in India
Authors P.R. Pujari, R. Sanam, C. Padmakar and P. LabhasetwarSummaryGroundwater is a vital source of drinking water on earth. The quality of groundwater is under increasing stress due to various anthropogenic stresses. The impact of ash ponds has been a major concern and is a subject of research. The present study seeks to assess the impact of an ash pond on the near sub-surface through Resistivity and IP imaging survey. The Resistivity and IP imaging data has been acquired on profiles close to the ash pond and at some far off locations. Groundwater and surface water samples have been collected from the ash pond and also from the wells in and around the ash pond. The study indicates very low resistivity formations in the near sub surface at location close to the ash pond as compared to the profile approximately 1 km away from the pond. The water chemistry data from the monitoring wells indicates very high Total Dissolved Solids (8658 mg/L) and chloride (1402 mg/L).
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Ground Penetrating Radar Applications in Seismic Microzonation
Authors S. Santos-Assunçao, V. Perez-Gracia, R. González-Drigo, V. Salinas and O. CasellesSummaryThe work presents a methodology that combines a first GPR survey and a subsequent measurement of seismic ambient noise vibration. The GPR signal characteristics used in this analysis are two: the amplitude of the background noise in the A-scans, and the frequency content of the received signal. The background noise could be consequence of three main sources: clutter as consequence of external reflections, electronic noise and energy randomly scattered in the medium. The first source could be identify in the GPR B-scans and usually produce anomalies similar to those caused by reflections in the targets inside the medium. The second source produce a continuous noise in the A-scans characterized because the average value is approximately constant. The third source introduces noise in the A-scans but its amplitude depends on the randomly backscattered energy. As consequence, the amplitude of this noise could be used in order to identify sudden changes in the shallow geology, always depending on the grain size distribution. The frequency content depends strongly on the water content. The analysis of the spectrum, combined with the analysis of the B-scans and the backscattering noise in the A-scans could be used in order to locate active subterranean streams.
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Efficient Acquisition and Holistic Analysis of Rayleigh Waves
Authors G. Dal Moro, S.S.R. Moustafa and N. Al-ArifiSummarySurface-wave analysis is nowadays widely considered for a number of goals ranging from geotechnical investigations to seismic-hazard studies. The most common approach is surely represented by the analysis of active or passive multi-channel data used to depict the phase velocity spectrum from which dispersion curves are then depicted and inverted.
The presented methodology attempts to obtain a robust sub-surface model in spite of the light equipment (a single [at least] bi-axial geophone) and the straightforward field procedures.
The active method here presented can be considered as an improvement of the classical MFA (Multiple Filter Analysis) method and is based on the joint analysis of the vertical and radial group-velocity spectra, together with the Radial-to-Vertical Spectral Ratio (RVSR).
After introducing the fundamental points charactering its implementation, a case study is presented also comparing the obtained sub-surface model with the one obtained considering a standard approach based on multi-channel active (MASW — Multichannel Analysis of Surface Waves) and passive (ESAC — Extended Spatial Autocorrelation) data jointly considered with the Horizontal-to-Vertical Spectral Ratio (HVSR).
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Sensitivity Analysis of Rayleigh-wave Ellipticity with Application to Near Surface Characterization
By M. CercatoSummaryThe joint inversion of surface-wave measurements and Rayleigh-wave ellipticity has gained popularity in recent years for near-surface soil characterization. The common approach is to use low-frequency, single-station ellipticity data in conjunction with high-frequency dispersion measurements obtained employing small aperture arrays. A complete understanding of the diagnostic potential of ellipticity in such conditions can be assessed only with a complete sensitivity analysis.
To this end, a new analytical method is presented for computing the sensitivity of Rayleigh-wave ellipticity with respect to the structural parameters of a layered elastic halfspace. This method employs a layer stacking procedure based on the subdeterminant formulation of the surface-wave forward problem and is numerically stable at high frequencies.
The sensitivity of the ellipticity curve is then evaluated quantitatively with specific focus on near-surface examples and compared to the dispersion patterns and sensitivity of modal phase velocity.
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Approximate Direct Estimate of S-wave Velocity Model from Surface Wave Dispersion Curves
Authors L.V. Socco and C. CominaSummarySurface wave dispersion curves are inverted to obtain 1D local VS model. The inversion is based on a layered model and the result is non-unique. A linear relationship can be found between weighted average S-wave velocity at different depths and surface-wave phase velocity at different wavelengths. This relationship, if known for a category of models, can be used to directly estimate an approximated velocity profile from the dispersion curve without the need for inverting. We show on a synthetic case that the error between estimated and true velocity is of the same order of that obtained by inversion.
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Measurements of Surface Waves Phase Velocity with a Large Ring Laser Gyroscope and a Seismometer
Authors A Simonelli, J. Belfi, N. Beverini, G. Carelli, A. Di Virgilio, E. Maccioni, R. Santagata, G. De Luca and G. SaccorottiSummaryIn recent years the development of large ring laser gyroscopes gave birth to a new discipline: the rotational seismology. This field of study is rapidly expanding and many areas of geophysics are or can be involved, from geodesy to seismology to the monitoring of critical structures. New rotational sensors based on optical interferometry and Sagnac effect has been developed in the last two decades. These sensors are called ring laser gyroscopes (RLG) and are now able to cover the range of rotations of seismological interest i.e. from 10–2 to 10–9 rad/sec. A measure of rotations in seismology is of fundamental interest for several reasons:
Quantitative seismology requires a measure of all the six degrees of freedom that characterize a rigid body’s motion.
Standard seismological observations are contaminated by rotations that produce a bias in the estimate of ground translation.
Rotational observables provide important informations about the elastic properties of the subsoil. The extension of rotational observations to exploration seismology can improve resolution and accuracy of subsurface imaging.
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SH-wave Seismic Reflection at the Patigno Landslide and Integration with P-wave Reflection Data
Authors A. Tognarelli, E. Stucchi, A. Ribolini, E. Lauriti and L. MeiniSummaryIn this work we describe the acquisition and processing, up to the depth migrated image, of an SH-wave reflection seismic survey carried out on a complex deep seated landslide located in the Northern Apennines in Italy. We also show a comparison with a recently acquired P-wave seismic reflection profile that investigates the same landslide body. The P-wave survey was able to delineate the deep sliding discontinuity, but was unable to give a detailed description of the small reactivation slip surfaces delineating minor landslides at shallow depths, that are responsible of the major observed damages. Our experience shows that the combined use of both P-waves and SH-waves offers the possibility to obtain detailed insights of the whole landslide body from the deepest discontinuity up to the very shallow portion of the landslide, overcoming the limitations due to the low resolution of P-wave method for imaging shallow horizons and the low investigation depth of SH-wave method. The deeper knowledge of the landslide internal setting that can be gained by a joint application of both methodologies is of primary importance to plan adequate and effective defence strategies.
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Time-lapse Monitoring of the Slopes of a Heritage Earthwork by Means of Near-surface Seismic Techniques
Authors P. Bergamo, S. Donohue, D.A. Gunn, B. Dashwood, S. Uhlemann, J.E. Chambers and D. WardSummaryA significant portion of UK’s infrastructures earthworks was built more than 100 years ago, without modern construction standards: poor maintenance and the change of precipitations pattern experienced in the past decades are currently compromising their stability, leading to an increasing number of failures. To address the need for a reliable and time-efficient monitoring of earthworks at risk of failure we propose here the use of two established seismic techniques for the characterization of the near surface, MASW and P-wave refraction. We have regularly collected MASW and P-wave refraction data, from March 2014 to February 2015, along 4 reduced-scale seismic lines located on the flanks of a heritage railway embankment located in Broadway, SW of England. We have observed a definite temporal variability in terms of phase velocities of SW dispersion curves and of P-wave travel times. The accurate choice of ad-hoc inversion strategies has allowed to reconstruct reliable VP and VS models through which it is potentially possible to track the temporal variations of geo-mechanical properties of the embankment slopes. The variability over time of seismic data and seismic velocities seems to correlate well with rainfall data recorded in the days immediately preceding the date of acquisition.
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Structurally Constrained 4D ERT Monitoring to Image Hydrological Processes Leading to Landslide Reactivation
Authors S.S. Uhlemann, J.E. Chambers, S. Hagedorn, H. Maurer, P. B. Wilkinson, T.A. Dijkstra, B. Dashwood, A. Merritt and D.A. GunnSummaryFuture environmental and climate change will affect the frequency and magnitude of landslide occurrences. A major focus of research is therefore to gain an improved understanding of the processes contributing to unstable slopes and the associated triggering mechanisms. This study considers the use of a combination of one-off seismic and geoelectrical monitoring measurements (from a 34 month period leading to landslide reactivation) to gain detailed understanding of the hydrological conditions leading to landslide reactivation. The study site is the Hollin Hill landslide field observatory that comprises a suite of geophysical, geotechnical and environmental sensors, thus offering the opportunity to compare and inform interpretation of the different data streams. The 4D electrical resistivity tomography (ERT) inversion was structurally constrained employing results of a combined P- and S-wave seismic refraction tomography (SRT). The ERT results were temperature corrected and translated into values of gravimetric moisture content (GMC) using laboratory derived GMC-resistivity relationships. The results show seasonality effects for the first year of monitoring, followed by imaging of crack built up and deep moisture penetration leading to failure of the back scarp. Elevated moisture contents, as an effect of prolonged rainfall, were imaged throughout the landslide prior to its reactivation.
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Comparison of Geotechnical and Geophysical Prospecting Approaches in the Study of a Landslide in Northern Iran
Authors M. Kazemnia Kakhki, H. Rezaiy and A. SadidkhouySummaryOne of the most challenges in landslides is deciphering of slide surface. As we know, there are different landslide types which make us the first problem to create accurate observations to distinguish the type and the stage of landslide. The following step is to define where to focus our monitoring. Electrical resistivity measurement is one of the most common methods which can be applied in landslide studies, although it is not accurate in most cases. On the other hand, its lower expense makes it applicable. For this, 2D electrical resistivity observations and Geotechnical methods such as standard penetration tests were combined to distinguish the subsurface conditions on a big landslide surface in NargesChal, a village in north of Iran in Golestan province.
The results were controlled by geotechnical logs and the extracted slide surfaces were confirmed by direct subsurface information.
The depth of 11.5 to 12.5m in all profiles has confirmed as sliding surface in SPT and soil moisture variation diagrams which is somehow consistent to Geoelectric method.
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High-resolution Seismic Imaging of the Pian di Neve Glacier on the Adamello Massif (Italy)
Authors M. Giorgi, S. Picotti, R. Francese, F. Pettenati, A. Bondesan, C. Baroni and M.C. SalvatoreSummaryActive seismic investigations, generally employed to image the earth interior, are also widely applied in glaciology to map the thickness of glaciers and to study the basal sediment properties. In this work we present the results of a 1-km multichannel seismic survey carried out on the Pian di Neve, a high altitude glacier located in the Adamello massif (Italy), with the purpose to image the glacier bottom and obtain the ice thickness profile. The Pian di Neve occupies a 18-km2 plateau at an altitude of between 3100 and 3400 m, and together with the adjacent Mandrone glacier forms the largest glacier mass in the Italian Alps. From the maximum Little Ice Age extension to present, these two glaciers lost about 36% of their total area.
The seismic survey evidenced an ice thickness ranging from 170 m to 270 m and the presence of a relatively thick layer of sediments at the bottom. Comparison with previous investigations seems to indicate that, in the last 50 years, the ice thickness in the survey area has not significantly changed. This evidence is contrary to the general retreat trend and could be due to a sort of thermal inertia in this glacial accumulation area.
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Seasonal Monitoring of Snow Properties by WCR and up-GPR
Authors A. Godio, D. Franco, B. Chiaia, B. Frigo, L. Dublanc, M. Freppaz, M. Maggioni, E. Ceaglio and P. DellavedovaSummaryThe upward Ground Penetrating Radar (upGPR) permits to monitor in time lapse the snow properties using a single antenna or an array of several antennas, disposed on the soil and radiating upward (on the snow). Water Content Reflectometer (WCR) is a special TDR (Time Domain Reflectometer) system to monitor the electrical permittivity of soil and snow. We discuss the effectiveness of an integrated monitoring using an up-GPR with an antenna of 1500 MHz and WCR sensor on the snow and into the ground.
The data acquisition refers to the winter season 2014–15, in a test site of interest for monitoring and prediction of avalanche phenonema in the Italian alps, at elevation of 2.100 m a.s.l.
GPR data permit to monitor the different phases of snow accumulation and layering of the snowpack during the season, while the analisys of WCR data allow us to estimate with good accuracy density and water content of the snow. Particularly, the uppermost layers are characterized by density in the range between 250–300 kg/m3, while at deeper level, density values are around 400–450 kg/m3.
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