bluuebunny/crossref_metadata_2025_split huggingface dataset:
A case study of two and three‐dimensional inversion of dipole‐dipole data: the Enfermarías Zn‐Pb (Ag, Sb, Au) Prospect (Moura, Portugal)
Authors: Represas P. , Monteiro Santo F. A. , Mateus A. , Figueiras J. , Barroso M. , Martins R. , Oliveira V. , Nolasco da Silva M. , Matos J.X. 
Published: August 2009

ABSTRACTThe Enfermarías Zn‐Pb(Ag, Sb, Au) prospect, located in the Moura‐Ficalho region (Alentejo, Portugal), was intensively explored during 1980s. A drilling programme was successful, intersecting massive and disseminated sulphide mineralizations. The geological and geophysical data set available for Enfermarias is quite vast and comprises 13 borehole geological logs, as well as gravity, magnetic and resistivity data. In this paper, the dipole‐dipole resistivity surveys are re‐examined and interpreted using both 2D and 3D inversion to provide an interesting comparison of the two different approaches. Results from gravity data modelling and data from borehole logs are used as independent validity tests on the quality of the results.The 2D and 3D models indicate a conductive overburden covering more resistive bedrock, which appears at a depth of 70–100 m in the western part of the profiles. Results obtained from 3D inversion, however, show a very clear separation of the most resistive domains in the two different bodies, whereas this separation does not appear in the 2D models. The 3D model also shows a higher degree of spatial continuity than the partial 2D models, which are, in general, richer in short‐wavelength anomalies, and show a higher variation from profile to profile.Comparison of the resistivity results with gravity in two coincident profiles reveals an excellent agreement between the main shallow features of both 2D and 3D models. This suggests that the resistivity model does indeed satisfy the main features of the available geological and geophysical data.
MRS contribution to hydrogeological system parametrization
Authors: Lubczynski M. , Roy J. 
Published: August 2009

ABSTRACTThe contribution of magnetic resonance sounding (MRS) to hydrogeological system parametrization is based on the hydrogeological interpretation of the two MRS output parameters, MRS water content () and decay‐time constant (). An overview of the hydrogeological parameters that can be defined with MRS is presented and is followed by a discussion on the nature, applicability and limitations of the relationships between MRS and hydrogeological parameters. The most important contributions to hydrogeological system parametrization are the estimations of effective porosity, specific yield and specific retention. The most important contributions are the estimations of hydraulic conductivity and aquifer transmissivity. The hydrogeological parameters obtained from parametrization are relatively well physically defined but insufficiently verified, where as the parameters obtained from have already largely been verified but they are based on an empirical relationship. All the hydrogeological parameters derived from and and obtained from a 1D MRS experiment, are integrated over a large subsurface volume of up to 106 m3.
Stability of MRS signal and estimation of data quality
Authors: Girard J.F. , Legchenko A. , Boucher M. 
Published: August 2009

ABSTRACTBased on the principle of nuclear magnetic resonance (NMR), the magnetic resonance sounding (MRS) method is used to measure the magnetic resonance signal from groundwater via a surface loop. The accuracy of the measurements depends on both the stability of the magnetic resonance signal under varying experimental conditions (geomagnetic field, electromagnetic noise, etc.) and the accuracy of the measuring device. The efficiency of the signal processing algorithms adopted by the NUMISPlus device, as well as sensitivity to the signal‐to‐noise (S/N) ratio (), are numerically investigated. The results are shown to be stable when .Numerical simulation of the magnetic resonance signal shows that with a fluctuating geomagnetic field and within a few Hertz of the frequency offset, amplitude remains stable whereas phase follows the geomagnetic field variations. An anti‐correlation is observed between phase and frequency shift.The numerical results were validated experimentally using the NumisPlus system at St‐Cyr‐en‐Val near the city of Orleans, France. For filtered data with and a frequency offset within ±0.5 Hz, the relative standard deviation of the measurements was found to lie within ±12% for amplitude and ±10° for phase. The standard deviation of the signal, which is strongly weighted by the S/N ratio, is higher for lower pulses than for higher pulses. Analysis of the stability of the current transmitted by NumisPlus reveals that for weak currents (<5 A), the standard deviation of the pulse amplitude is 3.7%, whereas for stronger currents (>100 A), it is only 1.3%. We thus conclude that for currents greater than 5 A, the instrumental instability of NumisPlus is significantly less than the electromagnetic noise, which can be considered as the major source of error. The S/N ratio can be significantly improved by stacking, which is efficient but time consuming. Note that the accuracy of measurements carried out over a long time (due to stacking) may decrease as measuring conditions can change during any one sounding.The accuracy of the field measurements for enables the complex MRS signal to be used in the inversion process instead of only amplitude. Nevertheless, any variation in the geomagnetic field with time must be taken into consideration during inversion of the MRS signal.
SNMR measurements in Thailand to investigate low‐porosity aquifers
Authors: Lange G. , Mohnke O. , Grissemann C. 
Published: August 2009

ABSTRACTIntegrated geophysical surveys have been carried out at two different locations in Thailand in the context of the Thai‐German Research Project “Investigation of Waste Disposal Sites ‐ WADIS”. Surface nuclear magnetic resonance (SNMR, MRS) was used in addition to conventional geophysics. Although a high total intensity of the earth’s magnetic field (TMI) offers advantageous physical conditions for SNMR, only very low signal amplitudes could be measured. Due to the long acquisition times caused by high stacking rates, diurnal variations in TMI may have had a considerable influence on the SNMR signal.The two investigated sites are characterized by different geological and physical ground conditions. An unusual feature is an extremely high local magnetic anomaly caused by magnetic landfill material at the Nong Harn dump site in northern Thailand. Because of strong field inhomogeneities, SNMR measurements were impractical in the sphere of influence of this anomaly. The Dan Khun Thot site in central Thailand is characterized by very high ground conductivities, obviously due to saline pore fluids. Despite the evidence of water occurring in several narrow sand layers observed in shallow holes, the water content deduced from the SNMR measurements turns out to be very low. An explanation for the apparent discrepancy is the irregular distribution of the water in lens‐shaped confined aquifers. Forward modelling of an elementary 3D situation and its comparison with 1D inversion illustrate that this hydrogeological situation cannot be treated correctly using the usual 1D data collection and 1D interpretation algorithms.
3D inversion of VES data from the Saqqara archaeological area, Egypt: a case study
Authors: El‐Qady G.M. , Monteiro Santos F.A. , Hassaneen A. Gh. , Trindade L. 
Published: August 2009

ABSTRACTSaqqara is one of the most important archaeological sites in Egypt. The monuments in this area are suffering from the effects of urbanization and temporal and spatial variations in the groundwater level. In an attempt to understand better the problems associated with the groundwater, a geoelectrical DC resistivity survey, comprising 47 vertical electrical soundings using a Schlumberger array, has been conducted. These data have been inverted using a 3D code based on a smoothness‐constrained least‐squares method. To determine the reliability of the inversion results, a range of tests has been performed. The final 3D resistivity model obtained for the study area provides useful new subsurface geological and hydrogeological information. The new model allows a more integrated image and interpretation of the complex hydrogeological conditions. It also depicts the mutual relationship between the two water‐bearing units in the area that was not clear in the previous 1D and 2D interpretations.
Seismic prospecting in archaeology: a 3D shear‐wave study of the ancient harbour of Miletus (Turkey)
Authors: Woelz Susanne , Rabbel Wolfgang 
Published: August 2009

ABSTRACTA 3D multicomponent shear‐wave survey was performed in the area of the ancient city of Miletus (western Turkey). To achieve high spatial resolution of near‐surface targets, data were acquired with a geophone array of 1 m grid spacing. The survey served: to investigate archaeologically important details of the basement of one of the silted harbours of Miletus, and to evaluate the potential of close‐meshed shear‐wave seismics for shallow high‐resolution prospecting in a more general sense. Methodically, we could show that isotropic radiation patterns of compressional shear‐ and surface waves can be reconstructed with simple vector arithmetic even if hand‐driven hammer blows are applied as a horizontal seismic source. Presenting time‐slices of S‐wave propagation, we demonstrate that side‐swipe may significantly affect the propagation of refracted waves. It is shown that the wavefronts of surface waves may be far from circular in the case of typical near‐surface lateral heterogeneity. As a further phenomenon of wave propagation, we could visualize the coda of a ground‐coupled air wave probably caused by mud cracks. Regarding the archaeological background of our study, we could show that the basement of the Milesian Lions’ Harbour is shallow enough to provide appropriate ground for ancient construction work. However, by ‘true 3D’ refraction mapping, we could image small‐scale topographic details of the harbour basement, proving that no large ancient stone walls are hidden under the ground. Magnetic anomalies were found to coincide with a lateral increase in Rayleigh‐wave velocity. The rectangular shape of this velocity contrast may indicate the remnants of an artificially shaped rock platform or an embankment. Spectral analysis of the surface wavefield shows that this seismic anomaly will be found at a depth of 3–6 m.
Nuclear magnetic resonance (NMR) properties of unconsolidated sediments in field and laboratory
Authors: Müller Martin , Kooman Stefan , Yaramanci Ugur 
Published: August 2009

ABSTRACTSurface nuclear magnetic resonance (SNMR) is a geophysical technique that has proved to be a useful tool for the investigation of hydrological properties of aquifers (porosity , saturated hydraulic conductivity ) in the past 10–15 years. Until recently, laboratory NMR has focused on consolidated sediments. In this research, an attempt is made to investigate unconsolidated sediments. To enable an enhanced understanding and interpretation of SNMR data, laboratory nuclear magnetic resonance (NMR) properties of synthetic and natural unconsolidated samples (glass beads, sand mixtures, bore‐core samples from a Quaternary environment west of Berlin (Germany) and coarse sand samples with a varying clay content) were analysed. To verify the NMR measurements, pore‐space properties (specific surface, porosity, pore‐size distribution) were analysed. Finally, hydraulic conductivity measurements were conducted to verify both the hydraulic conductivity derived from NMR relaxation times and the hydraulic conductivity estimates based on grain size. The laboratory data were compared to SNMR field data to assess scaling effects due to the dispersion of relaxation.The results show that the relationship used to obtain hydraulic conductivity from NMR relaxation is suitable for predicting the saturated hydraulic conductivity of samples composed of clay, silt and coarse sand. Furthermore, it is found that the predictions of hydraulic conductivity of intermediate grain sizes with differing clay content vary over a wide range. A more individual approach with regard to the paramagnetic properties of the material might be needed to achieve successful estimations.Clay (because of its high specific surface and large surface‐to‐pore‐volume (S/V) ratio) has a strong influence on NMR relaxation and the associated saturated hydraulic conductivity. The NMR relaxation of a series of coarse sand samples (d=1.0–0.5 mm) with a clay content in the range 3–20% shows an exponential decay for the hydraulic conductivity.
Multi‐electrode measurements at Thai Binh dikes (Vietnam)
Authors: Weller Andreas , Canh Tran , Breede Katrin , Trong Vu Nguyen 
Published: August 2009

ABSTRACTThe province of Thai Binh is located in the delta of the Red River in Vietnam where an extensive system of river and sea dikes protects the population and infrastructure from flooding in the rainy season. The integrity of the river dikes is affected by a diversity of problems. Importantly, various termite species dig their nests in the dikes causing water leakage. Leakage can also occur near sluices and passages.Geoelectrical multi‐electrode profiling has been applied to locate defects in the dikes. Surveys carried out on several dikes demonstrated that a combined half‐Wenner configuration yields high‐resolution images in which termite nests are indicated by resistive anomalies. Petrophysical investigations of soil samples and density logs yield additional information on the dike material. Elsewhere, imaging surveys confirmed the subsurface extent of a buried sluice structure in the dike body.
GPR studies in the Piano di Pezza area of the Ovindoli‐Pezza fault, central Apennines, Italy: Extending palaeoseismic trench investigations with high‐resolution GPR profiling
Authors: Jewell Chris J. , Bristow Charles 
Published: August 2009

ABSTRACTA high‐resolution 200 MHz frequency GPR profile with 0.1 m station spacing was collected using a Pulse EKKO 100 system, adjacent to a trench site that had been previously excavated and interpreted. The profile intersected the fault scarp on one of the alluvial fans on the northern slope of the Piano di Pezza; this fault scarp was thought to be associated with repeated earthquake activity along the Ovindoli‐Pezza fault. The GPR profile was conducted to evaluate the effectiveness of the technique in radar mapping and in extending the trenching information laterally and in depth in this environment. The work was carried out as part of a wider research programme of GPR profiling along the Ovindoli‐Pezza fault system. The GPR data were processed using Win EKKO radar data processing software and time–depth conversion was facilitated using field‐derived CMP velocity analysis. The results provided a detailed image of the GPR data, which shows a generally high spatial correlation with the mapped fault and 14C‐dated lithologic/pedologic units. Furthermore, the GPR data has provided a framework for estimating the extent of extensional and episodic catastrophic activity along the fault, and has elucidated the geometric relationship between the faulting in response to recent tectonic events.
Electrical resistivity imaging of tree trunks
Authors: al Hagrey S.A. 
Published: August 2009

ABSTRACTAn electrical resistivity technique using a ring array of needle electrodes is applied to image the internal electrical structures of the trunks of living standing trees and trunk disks. Measured electrical resistance data are inverted using a 2D iterative finite‐element algorithm which incorporates the cylindrical geometry of the trunk. The technique is successfully tested using synthetic models showing that the resolution obtained when mapping anomalous zones inside the trunk is higher for the dipole‐dipole configuration than for the Wenner configuration. Measurements on a trunk column show a well‐established inverse relationship between the imaged resistivity and the moisture content determined from wood cores.Healthy tree trunks always show a concentric ring structure with a central maximum of resistivity, which decreases towards the outside, corresponding to an increase in moisture from the inner dry heartwood to the outer wet ring of sapwood. Asymmetry in the ring structure is interpreted as the influence of branching or the direction of sunshine and wind. Strong local resistivity anomalies are related to infections causing wet, rotting or dry cavities. The resistivity of the heartwood is a maximum in olive and oak trunks, intermediate in young fruit trees and a minimum in cork oak trunks that are considered to be wet. The resistivity imaging technique can also monitor the dynamic process of sapflow if adequate tracers are used.
Karst investigations using microgravity and MASW; Application to Orléans, France
Authors: Debeglia Nicole , Bitri Adnand , Thierry Pierre 
Published: August 2009

ABSTRACTAn integrated geophysical approach to detecting and characterizing karst structures in an urban environment was applied experimentally to partially explored karst conduits located in Orléans, France. Microgravity was performed to detect voids, in conjunction with multichannel analysis of surface waves (MASW) for the purpose of identifying areas of mechanical weakness.Microgravity detected negative anomalies corresponding to known conduits and succeeded in identifying the probable extensions of this network in unexplored areas. Control boreholes located on these extensions encountered several levels of water‐saturated voids, probably belonging to the shallowest part of the karst system, overlying the main conduits. Buried urban networks, accurately located by ground‐penetrating radar (GPR), were shown to have no significant gravity effect. Simulations using the compact inversion approach to characterize the size and density of environmental disturbances confirmed this conclusion. In this context, the gravity method has been shown to be suitable for detecting near‐surface (<25 m deep) karst features.The MASW method, which analyses Rayleigh‐wave propagation, can determine the mechanical behaviour of superficial formations and serve as an indicator for subsurface heterogeneities such as voids or fractures. At the Orléans site, MASW provided evidence of perturbed zones superimposed on gravity anomalies, characterized by the appearance of several dispersion modes, velocity inversions and the attenuation of seismic markers. One of these features was characterized by low velocities and was interpreted as an area of mechanical weakness, confirmed by pressure measurements in the boreholes.Repeated gravity measurements, or time‐lapse microgravity, were conducted on the anomalous areas to ascertain gravity reproducibility and detect possible temporal variations due to subsurface mass redistribution that may indicate site instability. A two‐year experiment revealed low‐amplitude gravity changes that were recorded in the two sensitive zones. However, their significance is still unclear and these changes need to be validated by further repeat experiments.
Analysis of multi‐offset GPR data: a case study in a coarse‐grained gravel aquifer
Authors: Becht A. , Appel E. , Dietrich P. 
Published: August 2009

ABSTRACTMulti‐offset ground‐penetrating radar (GPR) data were collected in a coarse‐grained gravel aquifer located in a glacial delta environment within dipping foresets (at the Tettnang aquifer test site, SW Germany). We apply prestack processing techniques and normal‐moveout (NMO) velocity analysis in preparation for stacking. The estimation of propagation velocities is of considerable importance for converting time‐domain radargrams into depth‐sections and for an interpretation in terms of petrophysical properties. In our case, interval velocity determination is difficult because reflector dip angles are variable, and stacking and NMO velocities differ significantly since the antenna offset is comparable to the reflector depth. Using a synthetic two‐layer model, we systematically study possible errors in interval velocity determination. We compare the cases of decreasing velocity with depth (typical for ground‐penetrating (GPR) surveys of aquifers) and increasing velocity with depth (typical for seismic surveys). In the case of decreasing velocity with depth, the discrepancy between stacking and NMO velocity is considerable and, consequently, interval velocities calculated with the Dix equation or the more accurate 2D NMO approximation for dipping layers show unacceptably high errors. The errors are much smaller in comparable models with increasing velocity with depth. Thus, the small‐offset approximation inherent in the NMO concept is not appropriate for the first case. Consequently, it is necessary to model the true common‐midpoint (CMP) raypaths to determine realistic interval velocities from our GPR field data.Stacked GPR sections of multi‐offset data show significant improvements in imaging of reflectors and depth of investigation compared with a standard constant‐offset section. This allows a more reliable interpretation of geological structures. Simultaneous acquisition of four channels, which is possible with some commercial GPR systems, yields significantly better results than constant‐offset standard acquisition without increasing the efforts for data acquisition. Major reflectors in the GPR section can be correlated with distinct contrasts of porosity, represented by the base of open‐framework gravels and sand beds within poorly sorted gravels.
Velocity spectra and seismic‐signal identification for surface‐wave analysis
Authors: Dal Moro G. , Forte E. , Pipan M. , Sugan M. 
Published: August 2009

ABSTRACTRayleigh‐wave dispersion is observed every time acoustic‐impedance stratification occurs, and its analysis is suitable for vertical shear‐wave profile reconstruction. Accurate dispersion‐curve identification is essential in order to properly determine the shear‐wave velocity distribution of a medium. Data sets characterized by several events generate complex velocity spectra that can lead to possible misinterpretations. We analyse a real data set by taking into account theoretical dispersion curves and synthetic data obtained from numerical simulations in order to avoid possible pitfalls that could arise from the complex trends exhibited in the (frequency–wavenumber) and (velocity–frequency) domains. In the domain in particular, we show that reflection events and their multiples generate coherences that could be misinterpreted because of their similarity to typical higher‐mode dispersion curves. Another observed signal is interpreted in terms of guided waves and related phenomena. The results of the fundamental‐mode dispersion curve inversion performed via genetic algorithms indicate a sedimentary cover stratification that simple reflection analysis cannot reveal.The present case study highlights the importance of a synergic approach, based on integrated synthetic and field data analysis, for correct interpretation of all the wavefield components in the velocity spectrum.