bluuebunny/crossref_metadata_2025_split huggingface dataset:
Subsurface imaging using measured near‐field antenna footprints
Authors: van Dongen Koen W.A. , van den Berg Peter M. , Nicolaescu Ioan 
Published: August 2009

ABSTRACTImages of the subsurface are made for the detection of land‐mines using a bistatic stepped‐frequency continuous‐wave spiral‐antenna system. While the system moves along the surface, the emitted electromagnetic wavefields are scattered by objects in the subsurface and cause changes in the voltages measured at the receiver. These changes are formulated as a convolution of a sensitivity function and a complex contrast function. Within the Born approximation, this sensitivity function is equal to the inner product of the wavefield emitted by the transmitter and the field from the receiver operating in transmitting mode. For true amplitude imaging purposes, knowledge of the wavefields in the subsurface is needed. Since it is difficult to obtain a model which describes the radiation characteristics accurately, we measure the footprints of the antennae at one level in the near‐field region and propagate the emitted wavefields using Huygens’ principle. We use both synthetic and experimental data to localize objects in a homogeneous space. First, we apply time‐domain synthetic‐aperture‐radar (SAR) imaging in its most basic appearance. Next, we apply a single‐step inversion algorithm to the data, where we use the measured radiation characteristics of the antenna system. This results in an increase in resolution. We refer to this method as ‘minimized back‐propagation’.
Finite‐difference modelling for tunnel seismology
Authors: Kneib Guido , Leykam Alice 
Published: August 2009

ABSTRACTThe diagonal differencing algorithm, which takes spatial derivatives along the diagonals of the finite‐difference grid, is well suited to solve the elastodynamic equations for tunnel seismic modelling because it is accurate and stable at strong low‐order discontinuities of the elastic medium. Spatial high‐order finite‐difference operators increase the accuracy and efficiency of the algorithm except at steps in the elastic properties, where these operators fail. A solution is to use high‐order finite differences in most of the grid but to employ two‐point second‐order operators at low‐order discontinuities. This keeps memory requirements and computational effort on a level that allows 3D simulations on moderately priced PC hardware. Measurements with the commercial tunnel seismic prediction method yield complex seismic common‐receiver gathers with long direct‐wave pulses. Finite‐difference simulations in two and three dimensions reveal that these long pulses are due to P‐wave reverberations in and around the water‐filled shothole. Direct S‐waves originate close to the detonation point by conversion at the shothole–rock boundary. A dispersive, low‐frequency guided wave reverberates with the average S‐wave velocity inside the excavation damaged zone around the tunnel. The guided wave creates multiple body waves inside the undisturbed rock as it reverberates. It carries useful information about the degree and depth of the rock excavation damage but the guided wave often overlaps tunnel surface‐wave modes and may be difficult to extract from seismograms. Receivers should be buried deeper than the extent of the excavation damaged zone so that the guided waves and tunnel surface waves, which circulate around the tunnel, do not dominate seismic records and hide direct S‐waves, reflections or other late arrivals.
Propagation of errors due to incorrect positions of sources and detectors in wave‐field tomography
Authors: Cosentino P.L. , Sanfratello V. 
Published: August 2009

ABSTRACTTomographic data processed by 2D inversion programs can produce fairly large distortions due to incorrect source and/or detector positions. This problem is very serious in high‐frequency electromagnetic tomography (GPR), due to the dimensions of the transmitter and receiver antennae. The errors can even be larger when coupled antennae are used (receiver and transmitter inside the same box) whose positions are not clearly known. Similar errors can be involved in seismic tomography, for instance when the mechanical connection between transducers and sample is defective.In this paper the problem has been studied using synthetic data which were calculated for different acquisition geometries. Synthetic data have been distorted assuming a position error in the whole set of sensors, transmitters or receivers. Tests have been carried out using a homogeneous, isotropic medium and various simple inhomogeneous models, characterized by different geometric boundaries.The responses for all the models have been calculated (both forward and inverse problems) assuming straight raypaths, also in the case of velocity variations. The calculated traveltimes were processed using an LSQR algorithm implemented in an iterative inversion program, containing two damping factors to reduce total and local velocity contrasts. The inversion results have been reproduced graphically and the analysis of residuals has been carried out. Finally, propagation of these residuals, both in time and velocity, is discussed.
An investigation into the use of geophysical methods in the study of aquifer contamination by graveyards
Authors: Senos Matias M.J. , Marques da Silva M. , Goncalves L. , Peralta C. , Grangeia C. , Martinho E. 
Published: August 2009

ABSTRACTGraveyards are the traditional way of dealing with the deceased and often their location is unsuitable so that there is a potential contamination danger for local aquifers and soils. Such a contamination is complex as organic, inorganic and biological pollutants will be present. This problem has been studied by direct methods, i.e. boreholes. However borehole location can be difficult and so it is proposed here to use geophysical multiparameter surveys, as well as hydrochemistry studies, to investigate the contamination in the vicinity of two graveyards located in very porous and permeable formations. Based on the geophysical interpretation, two shallow boreholes were drilled and groundwater sampling confirmed local aquifer contamination. The geophysical response is shown to depend on the water table depth, and when this is close to the ground surface, it is demonstrated that geophysical methods can be used to map contamination plumes originated in the graveyards and, therefore, assist borehole location.
Surface waves in inversely dispersive media
Authors: Ryden N. , Park C.B. 
Published: August 2009

ABSTRACTThe dispersion of surface waves in inversely dispersive media where the shear‐wave velocity decreases with depth is studied. Theoretical dispersion curves are calculated in the complex wavenumber domain. Excitability and attenuation due to leakage are calculated for each point on the dispersion curves. These additional parameters are critical for a correct understanding of the dispersion properties of surface waves. Mode shapes are included in the study to visualize displacements inside the medium. The results of the study show that, for inversely dispersive media, the Rayleigh‐wave assumption is not valid, and other types of interface waves and leaky Lamb waves contribute to the surface wavefield. They also show that, in this case, true theoretical dispersion curves can be approximated by Lamb‐wave dispersion curves for a free plate in a vacuum, provided that the stiffness contrast between the top layer and the underlying half‐space is large, and also that the shear‐wave velocity of the stiff layer is greater than the compressional‐wave velocity in the underlying media. The error in the phase velocity resulting from this approximation is investigated and it is shown that the error does not exceed 5% for the fundamental antisymmetric Lamb‐wave dispersion curve. Because of the numerical simplicity of calculating its theoretical dispersion curves, the Lamb‐wave approximation can provide an effective evaluation method to resolve the thickness and elastic parameters of the stiff top layer. This is exemplified using a set of field data.
Surface‐wave inversion using a direct search algorithm and its application to ambient vibration measurements
Authors: Wathelet M. , Jongmans D. , Ohrnberger M. 
Published: August 2009

ABSTRACTPassive recordings of seismic noise are increasingly used in earthquake engineering to measure in situ the shear‐wave velocity profile at a given site. Ambient vibrations, which are assumed to be mainly composed of surface waves, can be used to determine the Rayleigh‐wave dispersion curve, with the advantage of not requiring artificial sources. Due to the data uncertainties and the non‐linearity of the problem itself, the solution of the dispersion‐curve inversion is generally non‐unique. Stochastic search methods such as the neighbourhood algorithm allow searches for minima of the misfit function by investigating the whole parameter space. Due to the limited number of parameters in surface‐wave inversion, they constitute an attractive alternative to linearized methods. An efficient tool using the neighbourhood algorithm was developed to invert the one‐dimensional profile from passive or active source experiments. As the number of generated models is usually high in stochastic techniques, special attention was paid to the optimization of the forward computations. Also, the possibility of inserting a priori information into the parametrization was introduced in the code.This new numerical tool was successfully tested on synthetic data, with and without a priori information. We also present an application to real‐array data measured at a site in Brussels (Belgium), the geology of which consists of about 115 m of sand and clay layers overlying a Palaeozoic basement. On this site, active and passive source data proved to be complementary and the method allowed the retrieval of a profile consistent with borehole data available at the same location.
Using transfer function for estimating dissipative properties of soils from surface‐wave data
Authors: Foti S. 
Published: August 2009

ABSTRACTThe quality factor (or damping ratio) can be estimated by analysing the spatial attenuation of surface‐wave data. However, because of the link between geometrical and material dispersion, a coupled analysis of dispersion and attenuation curves is preferable. Using a transfer‐function approach, it is possible to estimate the dispersion and attenuation curves simultaneously, provided the seismic source is known. A formulation based on the deconvolution of seismic traces is used to extend the transfer‐function approach to ordinary seismic gathers in which the source wavelet is not known. The measured transfer function is used in a regression analysis to obtain estimates of the complex wavenumbers, which, in the framework of viscoelasticity, contain all the information relating to phase velocity and attenuation of surface waves for a layered medium. Application of this procedure to experimental data leads to results consistent with those obtained using conventional techniques (e.g. analysis and amplitude regression).
Mechanical characterization of heterogeneous soils with surface waves: experimental validation on reduced‐scale physical models
Authors: Abraham O. , Chammas R. , Cote Ph. , Pedersen H.A. , Semblat J.‐F. 
Published: August 2009

ABSTRACTThe characterization of heterogeneous soils using common geotechnical techniques often proves impossible when the size of the heterogeneity is larger than a few tens of centimetres. Geophysical investigation techniques based on seismic wave propagation can help engineers to characterize the mechanical properties of such materials. In this paper, both refracted and surface waves are used to estimate the mechanical properties of an equivalent homogeneous medium.A summary of the main results obtained numerically using finite‐element computations and homogenization theory is presented. It is shown that, for first‐mode surface‐wave wavelengths larger than 7.5 times the nominal size of the heterogeneity and within certain heterogeneity concentration ranges (up to 50% for matrix dominant soils), surface waves homogenize the soil in accordance with classical homogenization theory.To validate these numerical results, a reduced‐scale model was built and seismograms, generated with a falling weight, were recorded. The phase‐velocity dispersion curve of the generated surface waves is inverted in order to obtain the shear‐wave velocity of the heterogeneous layer. The compressional‐wave velocity is calculated by means of seismic refraction analysis. Velocities obtained on the reduced‐scale model correspond to those predicted by homogenization theory from individual measurements of matrix and inclusion velocities.
Modelling azimuthal resistivity sounding over a laterally changing resistivity subsurface
Authors: Watson Kathryn A. , Barker Ron D. 
Published: August 2009

ABSTRACTRotational or azimuthal resistivity sounding is frequently employed for determining the electrical anisotropy of the subsurface. From this, the orientation of fracturing which might give rise to the anisotropy is interpreted. However, 4‐electrode arrays, such as the Wenner or Schlumberger, are strongly ambiguous and will produce an anisotropy‐style signature also over dipping strata or a gra‐dational lateral change in rock resistivity. However, this problem may be overcome by use of a 5‐electrode offset Wenner array. Simple computer modelling of rotational soundings is sufficient to demonstrate that azimuthal Wenner surveys, conducted on ground where there is a lateral variation in resistivity, will produce results similar to those recorded on truly anisotropic ground, whereas alternative techniques, such as offset Wenner azimuthal sounding, will successfully differentiate the two models.
Magnetic, GPR and geoelectrical measurements for studying the archaeological site of ‘Masseria Nigro’ (Viggiano, southern Italy)
Authors: Rizzo E. , Chianese D. , Lapenna V. 
Published: August 2009

ABSTRACTThe results of a geophysical survey carried out at the archaeological site of Masseria Nigro, located on a fluvial terrace on the north‐eastern flank of the Agri valley (Viggiano, Basilicata Region), are presented and discussed. The site includes an important ancient structure, developed in successive phases between the 4th and 3rd centuries BC and destroyed after the Roman conquest. Magnetic mapping, GPR profiling and electrical resistivity tomography were applied in the course of geophysical exploration in the archaeological area. The application of these methods revealed information about buried ancient walls, helping to define the boundaries of the whole building. During the first phase of the excavation, archaeologists targeted only the western part of the ancient structure (about 1000 m2), but the geophysical survey performed in a second phase was able to cover the entire ancient structure (1400 m2), enabling the archaeologists to define the extent of the buried structures. Our findings confirmed the hypothesis of the larger extent of the archaeological area in the eastern part and revealed the complete plan of the ancient building.
Ultra‐shallow seismic reflection imaging in a region characterized by high source‐generated noise
Authors: Schmelzbach C. , Green A. G. , Horstmeyer H. 
Published: August 2009

ABSTRACTCoincident 30 Hz and 100 Hz ultra‐shallow seismic data sets acquired across a sedimentary sequence in northern Switzerland are characterized by unusually high levels of source‐generated noise (i.e. guided and surface waves). To image reflections in these noisy data it is necessary to apply an extensive processing sequence that includes semi‐automatic first‐break picking, amplitude scaling, multichannel spiking deconvolution, bandpass‐frequency filtering, careful selection and application of top mute functions, iterative velocity and residual static correction analyses, application of NMO corrections and appropriate stretch mutes, linear τ–p filtering and stacking. After processing, nearly identical reflection patterns are observed on the resultant 30 Hz and 100 Hz stacked seismic sections. In particular, an ultra‐shallow reflector is mapped beneath the entire length of the approximately 50.3 m long profiles at 1.4–2.3 m depth; differences in depth between the two stacked sections are less than 0.15 m. Since this reflector is practically coincident with an ultra‐shallow refractor across which the velocity increases from about 275 m/s to over 2100 m/s, it is interpreted as the groundwater table. Such an interpretation is consistent with the groundwater level observed in nearby boreholes. Underlying the ultra‐shallow reflection is a quasi‐continuous event that originates from 1.9–10.0 m depth. This deeper reflection probably represents the boundary between glaciofluvial and underlying glaciolacustrine sedimentary units. We conclude that state‐of‐the‐art processing techniques may allow ultra‐shallow reflections to be extracted from seismic data dominated by source‐generated noise. As shown in earlier studies, the ultra‐shallow seismic reflection method is capable of supplying high‐resolution subsurface images at locations where the geo‐radar technique is limited by lack of depth penetration.
Use of surface waves for geotechnical characterization of soft alluvial deposits ‐ the Póvoa de Santa Iria case study
Authors: Lopes I. , Santos J. A. , Almeida I. 
Published: August 2009

ABSTRACTThe surface‐wave method using a multistation approach is a powerful field test as it is fast, robust and cost‐effective for geotechnical zonation of large areas. A case study is presented to illustrate the application of the method for the geotechnical characterization of an area located at Póvoa de Santa Iria, where Miocene bedrock is covered by thick Quaternary terraces and soft alluvial deposits. The results are analysed along with other relevant data: boreholes with standard penetration test data, resonant‐column tests on undisturbed samples and other available geological and geotechnical data.
Evaluation of electrical methods, seismic refraction and ground‐penetrating radar to identify clays below sands ‐ Two case studies in SW Sweden
Authors: Turesson Anita , Lind Gustaf 
Published: August 2009

ABSTRACTBecause of their geophysical properties, making a detailed interpretation of clay layers beneath surface layers of sand or gravel is sometimes a challenge. We chose two sites in south‐western Sweden where sand over clay has been documented by drilling, to test the effectiveness of four different geophysical methods in mapping the buried clay. We used geoelectrical methods (resistivity and induced polarization), seismic refraction and ground‐penetrating radar. Resistivity was found to be the best method for identifying the presence of the clay layer. Induced polarization (IP), in combination with resistivity, provides additional information but can give erroneous results if IP responses are weak. The seismic refraction method was not effective in detecting the clay layer due to the low contrast in seismic velocities between saturated sand and clay, although the method did give useful constraints on resistivity modelling. With ground‐penetrating radar, it was possible to map the upper boundary of clay with good accuracy.
Weathering zone determination in the province of Rio Negro, Argentina, with high resolution gravimetry
Authors: Gimenez M. , Martínez P. , Ruíz F. , Introcaso A. , La Vecchia J. 
Published: August 2009

ABSTRACTThis investigation demonstrates the advantages of microgravity results obtained with a digital gravimeter. On this basis, we determined the physical parameters of the weathering zone used in the calculation of static corrections in hydrocarbon exploration. A significant result presented here is, that by combining information from upholes with the application of microgravity results, the accuracy of the determination of the weathering thickness was improved by 5‐6 metres. It is also shown that application of this technique is not only time‐saving and economical, but it is also a more efficient method of determining anomalous bodies, which are overlooked by the conventional techniques currently used in prospecting.
Improved modelling of the magnetic resonance signal in the presence of shallow aquifers
Authors: Legchenko Anatoly 
Published: August 2009

ABSTRACTThis paper aims to investigate the magnetic resonance signal generated by shallow aquifers and its influence on the interpretation of magnetic resonance sounding (MRS) data. To improve the accuracy of MRS modelling in the presence of shallow aquifers, an enhanced mathematical model, which takes into account the higher harmonics of the transmitted pulse and a non‐zero frequency offset between the Larmor frequency and the pulse frequency, is used. It is shown that in the presence of shallow aquifers, the magnetic resonance signal is complex even when the subsurface has a low electrical conductivity. At large pulse moments, water in shallow aquifers may generate a signal comparable in amplitude to a signal generated by water in deeper aquifers. Two types of error related to a shallow aquifer may occur. For a one‐aquifer system, interpretation using the simplified model may reveal an artefact that appears to be a deeper aquifer, although in reality it is non‐existent (a phantom aquifer). For a multi‐aquifer system, the signal from a shallow aquifer may offset the signal from a deeper aquifer, thus reducing the depth of investigation in comparison with a simple one‐aquifer formation (offset of a deep aquifer).It is shown numerically that the depth resolution of the MRS method could be significantly improved by considering not only the amplitude but also the phase of the MRS signal. However, taking into account the non‐linearity of the spin system for a large flip angle, the mathematical model for the inversion of complex signals needs to be further improved or at least checked for consistency before inversion.For verification of the numerical results, a re‐interpretation of MRS field data reported in the literature is presented.
Magnetic resonance soundings with separated transmitter and receiver loops
Authors: Hertrich Marian , Braun Martina , Yaramanci Ugur 
Published: August 2009

ABSTRACTTriggered by an extended mathematical formulation of the response signal for magnetic resonance soundings (MRS), which allows the treatment of individual transmitter and receiver loops, we make a comprehensive evaluation and assessment of the emerging new possibilities of the technique. Based on a reformulation of the basic equation, we indicate and interpret the evolving effects. The influence of loop separation on MRS sounding curves in terms of offset and direction is also assessed as is the corresponding sensitivity to depth and lateral spin variation. Interpretation of field data measured with separated loops using the extended formulation is found to fit the predicted response extremely well. From the encouraging results, we derive new aspects of two‐dimensional investigation of groundwater resources. Furthermore, new perspectives of future developments of the MRS technique, with individual loops for optimized field measurements and hydrological applications, are discussed.