bluuebunny/crossref_metadata_2025_split huggingface dataset:
Estimating seismic velocities below the sea‐bed using surface waves
Authors: Shtivelman V. 
Published: August 2009

ABSTRACTSeismic data acquired in shallow offshore surveys often display well‐defined dispersion patterns related to two types of surface wave, propagating in shallow subwater layers. The waves of the first type propagate as normal modes and are represented by a low‐velocity, low‐frequency wavetrain identified with Scholte waves. The phase velocities of Scholte waves are related to the shear‐wave velocities below the water‐bottom and can be inverted to estimate in the subwater layers. The waves of the second type propagate as leaking modes and are characterized by a number of distinctive features: their dispersion patterns have a resonant frequency‐tuned appearance, they have relatively high cut‐off frequencies and their phase velocities exceed the velocity in the water. When the subwater layers are composed of relatively soft saturated rocks with high Poisson’s ratio, the leaking modes can be closely approximated by acoustic waves. By inverting the approximating dispersion curves, the vertical distribution of the compressional‐wave velocity in the shallow subwater layers can be estimated.Estimation of and below the sea‐bottom using the two types of surface wave is illustrated by examples from shallow offshore surveys conducted at several sites in the eastern Mediterranean area.
Combination of 1D laterally constrained inversion and 2D smooth inversion of resistivity data with a priori data from boreholes
Authors: Wisén Roger , Auken Esben , Dahlin Torleif 
Published: August 2009

ABSTRACTResistivity imaging in combination with borehole information is a powerful tool for site investigation. We show that the combination of 1D laterally constrained inversion (1D‐LCI) with the use of a priori information from borehole data and 2D smooth inversion adds significant value to the interpretation of continuous vertical electrical sounding (CVES) data. The 1D‐LCI offers an analysis of the resolution of the model parameters. This is helpful when evaluating the integrity of the model. Furthermore, with the 1D‐LCI it is possible to constrain model parameters with a priori information, e.g. depth‐to‐layer interfaces, based on borehole information.We show that 2D smooth inversion resolves lateral changes well, while 1D‐LCI results in well‐defined horizontal layer interfaces. In geological environments where the lateral variations are not too pronounced, the 1D‐LCI contributes to a geological interpretation of the resistivity measurements. Depths to layers can be interpreted with greater certainty than if using results from 2D smooth inversion only. The inclusion of a priori information in the inversion reveals further details and enhances the geological interpretation significantly.
Characterisation of a NAPL‐contaminated former quarry site using electrical impedance tomography
Authors: Chambers J.E. , Meldrum P.I. , Ogilvy R.D. , Wilkinson P.B. 
Published: August 2009

ABSTRACTElectrical impedance tomography (EIT) was used to characterize two tar pits, containing a range of NAPL and aqueous contaminants, located within a former quarry site. The site, which is situated on Coal Measures strata near Belper, Derbyshire, UK, has been the subject of an extensive conventional site investigation. Consequently, the geometry and contents of the pits are relatively well defined. The aims of the survey were to test 3D EIT in field conditions against good ground‐truth data and, in combination with intrusive and historical site records, to improve the characterization of the waste pits.The EIT survey employed surface electrodes within a network of linear survey lines. Impedance magnitude and phase data were collected at frequencies of 0.6, 2, 10, 40 and 125 Hz, from which resistivity and phase models were calculated using a 3D smoothness‐constrained least‐squares inversion technique.Resistivity models displayed only weak frequency‐dependent variations. The resistivity images were proved to correlate well with the ground‐truth data, and were used to further constrain models of the distribution of waste and bedrock contamination. Furthermore, interrogation of the 0.6 Hz resistivity model permitted estimates of waste and contaminated bedrock volumes for the two pits. A reliable phase model could be produced only from the 0.6 Hz data, due to the increased noise levels affecting the data at higher frequencies. The 0.6 Hz phase model displayed phase variations consistent with the known distribution of the waste. The mechanisms causing the observed phase anomalies could not be conclusively determined; however, it is probable that they are related to the presence of metallic waste.EIT was shown to be a particularly effective tool for site characterization when used in conjunction with intrusive and historical data for the purposes of model calibration and interpretation.
Using trapped waves for mapping shallow fault zones
Authors: Shtivelman V. , Marco S. , Reshef M. , Agnon A. , Hamiel Y. 
Published: August 2009

AbstractRecent studies have shown that shallow fault zones can be identified by anomalous behaviour of the wavefield recorded by high‐resolution seismic surveys. Analysis of seismic records acquired using off‐line shooting geometry, where sources and receivers are located along two separate parallel lines crossing a suspected location of a fault zone, reveals prominent anomalies which may be identified with the waves trapped within the zone. On seismic records, trapped waves usually appear as regular low‐frequency high‐amplitude wavetrains. In order to facilitate the identification of the trapped‐wave‐related anomalies, we propose two procedures utilizing specific properties of the trapped waves. The first procedure is based on stacking time‐scaled seismic traces in the common‐shot and common‐receiver domains with the subsequent application of eigenimage analysis. In the second procedure, band‐limited spectral energy of time‐scaled traces is represented as a map in the source–receiver coordinates. By detecting and mapping trapped‐wave anomalies in the source and receiver domains, the spatial location of a fault zone can be estimated by linear interpolation between the locations of the corresponding anomalies on the source and receiver lines.Application of the above procedures is illustrated by a number of synthetic and real data examples.
Use of antenna arrays for GPR surveying in archaeology
Authors: Leckebusch Jürg 
Published: August 2009

ABSTRACTAmong the different geophysical prospecting methods applied in archaeology, ground‐penetrating radar is one that still requires a lot of time to carry out field recordings. A duo‐antenna configuration is proposed that will halve the time needed in the field. Tests have shown that this can be done without any interference between the two antennae. The signals from different antennae do not have the same characteristics and the spectrum and amplitude of the traces are not the same. The use of spectral whitening and energy matching for adjacent traces or samples makes the data sets as similar as possible to allow for unperturbed time‐ or depth‐slices. As a side effect, spectral whitening enhances the vertical resolution and penetration depth of the method.
The links between MRS parameters and the hydrogeological parameters
Authors: Lachassagne Patrick , Baltassat Jean‐Michel , Legchenko Anatoly , de Gramont Hubert Machard 
Published: August 2009

ABSTRACTMagnetic resonance sounding (MRS) is a non‐invasive geophysical method that can be used to discriminate between the geophysical signals of pore water of the surrounding rock. This method is thus particularly attractive for hydrogeological applications.In order to investigate MRS for the benefit of applied hydrogeology, a comprehensive interdisciplinary study was carried out. The aim of this project was to develop an optimal methodology which could be easily adopted by hydrogeologists. This paper summarizes the main results obtained, with the objective of translating the MRS parameters into parameters that can be directly used by hydrogeologists.It was found that MRS is well adapted to the working scales of the hydrogeologist (i.e. field scale, well scale). It is able to provide significant information for the hydrogeologist, including:

Reliable detection of the presence of water in the subsurface. This is the basic and most significant advantage of the method, which proved to be useful, particularly in arid to semi‐arid environments.
Locating water‐saturated formations (top and bottom), situated at depths between 0 and 100 m approximately.
Estimation of the hydrodynamic parameters of detected aquifers:

– Through a rigorous inversion and calibration process, the MRS method enables the quantification of the specific yield (effective porosity) of aquifers and of the storage coefficient, the latter in unconfined aquifers only.

– In the present state‐of‐the‐art, MRS is able to estimate the hydraulic conductivity (or trans‐missivity) of aquifers in localized favourable configurations, when calibration is available; otherwise, aquifers can be compared qualitatively.
Thus the MRS method provides data that cannot be obtained with other non‐invasive geophysical tools, and is already a valuable tool for applied hydrogeological projects.
Magnetic resonance sounding (MRS) and resistivity characterisation of a mountain hard rock aquifer: the Ringelbach Catchment, Vosges Massif, France
Authors: Baltassat J.M. , Legchenko A. , Ambroise B. , Mathieu F. , Lachassagne P. , Wyns R. , Mercier J.L. , Schott J.J. 
Published: August 2009

ABSTRACTThe Ringelbach catchment, which has been studied since 1975, is highly representative of the crystalline Vosges massif, where the water supply is mainly derived from small aquifers in heterogeneous superficial formations and in the weathered and fissured bedrock. Vertical electrical soundings (VES), resistivity imaging profiles and magnetic resonance soundings (MRS) were applied in order to investigate the 3D structure and estimate the hydrodynamic characteristics of the subsurface, which is composed of granites partly covered by Triassic sandstone.Despite poor MRS signal‐to‐noise conditions, the general structure of the catchment is defined using geophysics (electrical methods and MRS combined) and geology. MRS also makes it possible to map the water volume per unit surface within the different geological formations and structural blocks. The corresponding maps are proposed as a basis for evaluating the water storage within the catchment and improving the understanding of its hydrological functioning. The initially proposed geological model is refined and a four‐block structure intersected by faults is defined. The schema of a gradual stratified weathering of the granite from surface to depth is not clearly reflected in the resistivity images, partly because of insufficient investigation depth. It is completed with highly weathered, deeply rooted fractured zones associated with faults in order to explain deeply sited low‐resistivity zones.The MRS water content in the weathered granite of Ringelbach appears abnormally low in comparison with the higher values observed in similar resistivity settings in other granite regions. Because of the rough catchment topography, these low values may be attributed to unsaturated weathered formations on hillsides or saturated, fine material (colluvial deposits, highly weathered granite) in the valley bottom. The control‐wells and laboratory measurements on core samples proposed in the next step of this research will attempt to calibrate the MRS results and validate the proposed structural and weathering schema. Re‐interpreted and validated geophysical results will then be used as effective input for improved hydrological modelling.
MRS multi‐exponential decay analysis: aquifer pore‐size distribution and vadose zone characterization
Authors: Roy J. , Lubczynski M.W. 
Published: August 2009

ABSTRACT (the NMR signal decay‐time constant) spectra are analysed for two purposes: (1) to determine the pore‐size distribution below the water table for lithological and flow parameter purposes and (2) to determine the moisture content as a function of water‐film thickness/water‐drop size to characterize water storage and fluxes in the unsaturated zone. Unsaturated‐zone hydraulic conductivity versus moisture‐content relationships are non‐linear and normally have a memory effect (hysteresis). Such relationships are dependent on the partitioning of moisture into pores of different sizes, which are wetted and drained in an order that depends on the soil composition and texture. Magnetic resonance sounding (MRS) multi‐exponential decay analysis may supply a unique insight in this component of water‐resources quantification through the differentiation of the moisture content into different film/droplet sizes. Field examples from two sites, one in The Netherlands and the other in Botswana, are shown, in which the free induction decay NMR signal is analysed in terms of three spectral components.
Improved karst exploration by VLF‐EM‐gradient survey: comparison with other geophysical methods
Authors: Bosch F.P. , Müller I. 
Published: August 2009

ABSTRACTThe knowledge of size, density and both orientation and vertical distribution of fractures as well as their opening and filling material or overburden thickness is a valuable contribution to estimating hydraulic conductivity and to evaluating the vulnerability and protection strategy of karst aquifers. To obtain some of these parameters and to ascertain high‐permeability zones, the Very Low Frequency‐Electromagnetic Gradient (VLF‐EM GRAD) method was applied, together with Radio Frequency‐Electromagnetics (RF‐EM), Radiomagnetotellurics (RMT), Geoelectrical Tomography and refraction seismics, over a karstic terrain in the Swiss Jura Mountains. In this area, karst springs infiltrate a porous aquifer. This survey investigated the highly permeable karst structures, which provide fast water‐infiltration pathways into the karstic flow network. A dye tracer test validated the geophysical results. The results show the efficiency and reliability, particularly of the VLF‐EM GRAD method, for high‐resolution investigation at shallow depths and for its potential for fast data acquisition over large surfaces at catchment area scale without ground contact.
Electromagnetic propagation features of ground‐penetrating radars for the exploration of Martian subsurface
Authors: Pettinelli E. , annaroni G. , Mattei E. , Di Matteo A. , Paolucci F. , Pisani A.R. , Cereti A. , Del Vento D. , Burghignoli P. , Galli A. , De Santis A. , Bella F. 
Published: August 2009

ABSTRACTIn this work, the effects of magnetic inclusions in a Mars‐like soil are considered with reference to the electromagnetic propagation features of ground‐penetrating radars (GPRs). Low‐frequency and time‐domain techniques, using L‐C‐R meters and TDR instruments, respectively, are implemented in laboratory experimental set‐ups in order to evaluate complex permittivity and permeability and wave velocity for different scenarios of a dielectric background medium (silica) with magnetic inclusions (magnetite). Attenuation and maximum detection ranges have also been evaluated by taking into account a realistic GPR environment, which includes the transmitting/receiving antenna performance and the complex structure of the subsurface. The analysis and the interpretation of these results shed new light on the significant influence of magnetic inclusions on the performance of Martian orbiting and rover‐driven GPRs.
Buried target signature extraction from ground‐penetrating radar signal: measurements and simulations
Authors: van den Bosch Idesbald , Lambot Sébastien , Druyts Pascal , Huynen Isabelle , Acheroy Marc 
Published: August 2009

ABSTRACTGround‐penetrating radar (GPR) proves to be a very valuable tool in the field of humanitarian de‐mining, especially for the detection of plastic land‐mines. Recently, a monostatic stepped‐frequen‐cy continuous‐wave (SFCW) GPR, together with a conceptual model of the radar‐antenna‐soil system, has been developed for the characterization of the electromagnetic parameters of soil, i.e. dielectric permittivity (), magnetic permeability () and electric conductivity (). This approach is extended here to the extraction of the GPR signal and to modelling the signatures of buried targets. The equivalence principle is used to decompose the GPR signal into its soil and target‐in‐soil components, as well as to model the radar‐soil‐target system. It permits the soil contribution to be subtracted from the total GPR signal to provide the signature of the buried target. This signature is compared to simulations. For a proof of the concept, the GPR return signal from a buried metal sphere has been simulated using the Method of Moments and it shows good agreement with its measured counterpart. We also have extracted clean frequency‐ and time‐domain signatures of a PMN‐2 plastic mine embedded in a multilayered medium, subject to various water contents. The method is also applied to a B‐scan above a buried conducting cylinder. Finally, a study of the main sources of errors in the extraction of the signature of a buried target shows that mistakes in antenna height measurement lead to errors more important than those due to misestimating the relative dielectric permittivity of the soil.
Three‐dimensional multicomponent georadar imaging of sedimentary structures
Authors: Streich Rita , van der Kruk Jan , Green Alan G. 
Published: August 2009

ABSTRACTMulticomponent georadar methods that involve recording data using several different antenna configurations and orientations should provide better constrained information about the subsurface than conventional single‐component techniques. We test this hypothesis using three‐dimensional (3D) multicomponent data recently acquired across braided river sediments deposited adjacent to the Alpine Fault in New Zealand. Strong reflections were observed in the two pairs of co‐ and cross‐polarized data sets. Relatively high reflection amplitudes in the cross‐polarized data were a consequence of the dip and oblique orientation of most sedimentary structures relative to the antennae. Many differences in structural information content and reflection amplitude observed in the different components could be explained on the basis of rather simple wavefield models. To obtain reliable subsurface information, a novel 3D multicomponent imaging (migration) algorithm was applied to the data. This algorithm accounted for the effects of antenna orientation, antenna offset and far‐field electromagnetic radiation patterns. In addition to correcting the dips, positions and lengths of reflections, the multicomponent imaging also adjusted their relative amplitudes according to the far‐field wavefield model. The contributions of the cross‐polarized data to the images were particularly important in our data set, because they selectively enhanced dipping reflections oriented at oblique angles to the antenna axes. Phase characteristics of reflections with certain orientations and dips were not recovered consistently, indicating a need for further investigation.
Delineating the near‐surface geometry of the fracture system affecting the Valley of Querétaro, Mexico: Correlation of GPR signatures and physical properties of sediments
Authors: Carreon‐Freyre Dora C. , Cerca Mariano 
Published: August 2009

ABSTRACTWe present a combined ground‐penetrating radar (GPR) and geotechnical investigation of a fault‐fracture system that affects the Valley of Querétaro. The main fracture, known as Falla Central (FC), is aligned with N–S‐orientated regional faults suggesting that the buried fault scarps influence the geometry and propagation of fractures. Although the origin of fracturing is closely related to geological factors, in the urbanized area of Querétaro the mechanical and hydraulic equilibrium in the subsoil is also perturbed by anthropogenic activities, such as overexploitation of groundwater and overloading of compressible ground. In order to delineate the fracturing geometry, we obtained several vertical GPR profiles, perpendicular and oblique to the main trace of the fracture, with two different prospecting frequencies, 900 and 300 MHz. Processing of radar profiles consisted of background removal, topographic correction and AGC amplitude correction to enhance stratigraphic‐related records. The near‐surface stratigraphy consists of partially saturated fluvio‐lacustrine granular and pyroclastic deposits. Detailed measurements of specific gravity, grain‐size, plasticity, water content, and electrical conductivity were performed on samples collected from two shallow trenches in order to relate physical changes in the sedimentary sequence to the recorded electrical contrasts. The analysis of the GPR profiles, plus the stratigraphic record, enabled us to identify variations in the deformation of layers, and changes in direction, width and vertical displacement of fractures. Our results suggest that the propagation of the fracture is influenced by the conditions of deposition of the geological materials and by the anthropogenic activity.
Linking regional‐scale lineaments to local‐scale fracturing and groundwater inflow into the Päijänne water‐conveyance tunnel, Finland
Authors: Lipponen Annukka , Airo Meri‐Liisa 
Published: August 2009

ABSTRACTPotential structural or lithological controls of groundwater flow into the mostly rock‐surfaced Päijänne water‐conveyance tunnel in southern Finland were investigated by integrating geological, hydrogeological and aerogeophysical data. Fracture‐zone geometry and distribution, interpreted from processed aeromagnetic images, and topographic and geological information are compared for three different sections of the Päijänne Tunnel. Documented observations on fracturing and ground‐water inflow from inside the tunnel provide subsurface information on the fractures and their hydraulic properties. Topographic and aeromagnetic lineaments both exhibit general NE–SW and NW–SE orientations at a regional scale. Based on the aeromagnetic interpretation, bedrock fractures intersecting the tunnel are part of a regional fracture network and their evolution can be connected to the main tectonic stages in southern Finland. Their distribution is lithologically controlled and reflects the rock type and its structure. For more detailed scales, a superficial deposit relief map is a powerful tool for obtaining a more accurate location of fracture zones and for evaluating their connection to superficial deposits as possible groundwater reservoirs. Depending on the scale, integrated interpretation using both topographic and aeromagnetic data gives the best results, particularly when supported by independent verification such as the observed fracturing inside the tunnel.NW–SE appears to be the most common strike trend among the measured fractures linked with water inflow. In the magnetic data, this orientation is displayed (1) as swarms of short, faint signatures indicating brittle, shallow features, and (2) as extended, broad, linear magnetic gradients denoting block boundaries. Their lengths, of tens to hundreds of kilometres, suggest that they may also be deep‐reaching. The coincidence and the parallel orientation of these two magnetic features of different scales suggest their genetic relationship: the faulted block boundaries were reactivated at later tectonic stages, resulting in brittle fracturing along the same earlier structural weakness zones. Many of the locations where water‐conducting fracturing occurs, or where groundwater inflow has been measured at a larger scale, are associated with intersecting or individual topographically interpreted fracture zones, especially NW–SE trending ones.