bluuebunny/crossref_metadata_2025_split huggingface dataset:
Forward modeling and inversion of MRS relaxation signals using multi‐exponential decomposition
Authors: Mohnke O. , Yaramanci U. 
Published: August 2009

ABSTRACTThe geophysical method of surface nuclear magnetic resonance (SNMR) or magnetic resonance sounding (MRS) allows a direct determination of the water‐content (amplitudes) distribution in the subsurface. In addition, the MRS signal also contains information about the distribution of pore sizes (decay times) and electrical conductivity (phases) in the ground.So far, the inversion of MRS data has mainly concentrated on the interpretation of the water‐content and decay‐time distributions. The inversion is performed by fitting the recorded relaxation curve for each excitation pulse with a single initial amplitude value and relaxation constant, i.e. decay time (mono‐exponential fit). However, MRS relaxation signals inherently exhibit a multi‐exponential behaviour that arises due to the superposition of signal contribution originating from layers or volume fractions that feature different decay‐time properties. Another geological situation that contributes to the multi‐exponential behaviour of MRS relaxation data is given by a possible multimodal decay‐time distribution within volume units or layers in the subsurface. This can be encountered in hard‐rock material, e.g. sandstone, and occasionally in sediments. As a further consequence of the signal contribution from sources with different decay‐time properties, the signal phase of MRS data is also affected.With the aim of a more quantitative and accurate aquifer characterization, in analogy with applications commonly used in borehole and laboratory NMR, we introduce a new approach to the inversion of MRS data taking into account the inherent multi‐exponential behaviour of MRS relaxation data due to layering and non‐uniform distribution of pores in the subsurface. Inversions carried out for synthetic data, i.e. the results of forward modelling, and MRS field data clearly show the advantages of such a comprehensive inversion (COIN) approach with respect to an improved determination of water content and decay times in the subsurface.
Characterization of crystalline basement aquifers with MRS: comparison with boreholes and pumping tests data in Burkina Faso
Authors: Vouillamoz J.M. , Descloitres M. , Toe G. , Legchenko A. 
Published: August 2009

ABSTRACTIn the Sahelian region of Burkina Faso (Western Africa), groundwater resources are scarce. The hydrogeological context is mainly crystalline basement aquifers that often present a challenge to hydrogeologists when investigating their exploration and management. A magnetic resonance sounding (MRS) survey was conducted to evaluate the ability of the method to answer the following main questions encountered by hydrogeologists in this hard‐rock context:

Where is the groundwater?
How deep and how thick are the water‐bearing formations?
What are the reserves of groundwater?
What is the productivity of the aquifer?
MRS measurements were implemented around recent boreholes drilled both in the weathered and in the fissured‐fractured units of the reservoirs. In order to evaluate the MRS method, MRS results are compared with borehole and pumping test data. The depths and thicknesses of the saturated aquifers encountered by the boreholes are compared with those estimated by MRS. The decay‐time constant of the magnetic resonance signal is used for calculating the storativity and transmissivity estimators from geophysical data. These MRS hydrogeological estimators are compared with the local transmissivity and storativity of the aquifer, estimated from pumping test results.The main conclusions of the comparison between the 13 MRS results and the borehole data are:

The depths and thicknesses of the saturated alterites are accurately described by the MRS results, and the mean differences with the borehole data are ±12% and ±17%, respectively.
The storativity estimated from MRS data is not reliable. The proposed estimators need to be confirmed with larger data sets, and further research needs to be conducted on this matter.
The transmissivity can be accurately estimated from MRS data after calibration with pumping test results. The mean difference between MRS and pumping test results is ±41%.
The main limiting factors of MRS applied in hard‐rock areas are the 1D approximation in a highly heterogeneous context, the screen effect that causes deep weathered‐fissured reservoirs to be poorly resolved when topped by shallow alterites reservoirs, and the suppression principle that causes deep narrow fractures to be undetectable.
MRS is a useful tool to characterize the saturated alterites and the weathered‐fissured zones of aquifers in a crystalline rock context. With knowledge of its limitations, its use within the framework of hydrogeological strategy is promising, both for borehole implementation and for groundwater reserve evaluation.
Seismic land‐marine acquisition survey on the Great Ancona Landslide
Authors: Stucchi E. , Zgur F. , Baradello L. 
Published: August 2009

ABSTRACTThe paper deals with a reflection seismic acquisition survey carried out on the Great Ancona Landslide, a huge landslide body on the Italian Adriatic coast involving part of the city of Ancona. To investigate the area we carried out a joint land‐marine seismic acquisition that gave us a continuous image of the subsurface in the land‐sea transition zone where the landslide foot is supposed to be located. A specifically designed interface unit connected the analog ocean‐bottom cable (OBC) lying along the sea‐floor to the telemetry system used on land. The effectiveness of different source types for both marine and land shooting was tested in the field, by computing and analysing the frequency‐time (FT) response of the recorded seismograms. The various sub‐datasets obtained (land‐land, land‐marine, marine‐land and marine‐marine) were kept separate because of the different signal properties and the kinds of noise present; a short description of such noise is given. Finally, the acquisition statistics are discussed, and a preliminary image of the subsurface in time has been achieved from the marine‐marine data.
Application of anisotropy borehole radar tomography in Korea
Authors: Kim Jung‐Ho , Cho Seong‐Jun , Yi Myeong‐Jong , Sato Motoyuki 
Published: August 2009

ABSTRACTAlthough the main geology of Korea consists of granite and gneiss, it is not uncommon to encounter anisotropy phenomena in cross‐hole radar tomography even when the basement is crystalline rock. To solve the anisotropy problem, we have developed an anisotropic inversion algorithm, assuming a heterogeneous elliptic anisotropy, to reconstruct three kinds of tomogram: tomograms of maximum and minimum velocities, and of the direction of the symmetry axis. We introduce some case histories of the application of anisotropic radar tomography in Korea. The first two case histories were conducted to construct social infrastructure, and their main objective was to locate cavities in limestone. The last two were performed in a granite and gneiss area. The anisotropy in the granite area was caused by fine fissures aligned in the same direction, while that in the gneiss and limestone areas was caused by the alignment of the constituent minerals. Through these case histories we aim to show that the anisotropic characteristic itself provides additional important information for understanding the internal structure of basement rock.
Realistic FDTD modelling of borehole georadar antenna radiation: methodolgy and application
Authors: Ernst Jacques R. , Holliger Klaus , Maurer Hansruedi , Green Alan G. 
Published: August 2009

ABSTRACTHigh‐frequency electromagnetic‐wave propagation phenomena associated with borehole georadar experiments are complex. To improve our understanding of the governing physical processes and radiative properties of borehole georadar antenna systems, we have developed a modelling tool based on a finite‐difference time‐domain (FDTD) solution of Maxwell's equations in cylindrical coordinates. The computational domain is bounded by cylindrical symmetry conditions along the left edge of the model and uniaxial perfectly matched layer (UPML) absorbing boundary conditions along the top, bottom and right model edges. An accurate and efficient grid‐refinement technique allows us to account for detailed aspects of borehole georadar antenna systems, slim boreholes and materials with very high dielectric permittivities, such as water. Numerical experiments reveal that the radiation patterns of finite‐size Wu–King‐type antennae and infinitesimal electric dipoles in dry boreholes differ only slightly from the analytic solution of an infinitesimal electric dipole in a homogeneous full‐space. In contrast, there are substantial differences between the radiation patterns of antennae placed in water‐filled boreholes and their analytic full‐space equivalents without boreholes. The effects of placing the antennae in air‐ and water‐filled boreholes are explored using data acquired in crystalline rock and alluvial sediments, respectively. In both cases, simulations based on realistic transmitter antennae located in boreholes and spatially corrected receiver radiation patterns provide better agreement between the observed and modelled data than simulations based on infinitesimal transmitter and receiver dipoles.
Radar ‘lensing’ by a small river: Can a layer of surface water improve the signal?
Authors: Nobes David C. , Rother Henrik , van der Kruk Jan , Jol Harry M. 
Published: August 2009

ABSTRACTA radar survey was carried out on a gravel bar between the Hope and Boyle Rivers near the Hanmer Basin in the central South Island of New Zealand. One of the profiles crossed a shallow stream, where the depth of penetration and resolution both appeared to improve. The water's high dielectric permittivity, and consequent slower velocity, causes the signal to be directed more vertically into the subsurface. The superior resolution is also due in part to the slower average velocity; the water table is at or above the ground surface in the stream channel. Spectral analysis shows that there is a small increase in the amplitude of the higher frequencies in the stream portion of the profile, so that the improved resolution appears to be real. Multiple reflections from the stream may also contribute to the spectral broadening. The apparent increased depth of penetration is partly due to the increased time required for echoes to return at the slower velocity, but the larger two‐way traveltime does yield a marginally greater depth of penetration.
Semblance‐based topographic migration (SBTM): a method for identifying fracture zones in 3D georadar data
Authors: Heincke Bjoern , Green Alan G. , van der Kruk Jan , Willenberg Heike 
Published: August 2009

ABSTRACTSteep‐dipping fracture zones are generally difficult to delineate using traditional ground‐penetrating radar (georadar) techniques. Evidence for their presence in standard georadar images may be either completely absent or limited to diffractions and/or chaotic reflection patterns. To address this issue, we present a novel three‐dimensional (3D) migration scheme based on computations of semblance. This new approach, which accounts for undulating surface topography, emphasizes diffractors while markedly reducing the effects of specular reflectors. After demonstrating the efficiency of the technique on 3D synthetic data, we apply it to a 3D georadar data set acquired across an unstable mountain slope in the Swiss Alps. This region is characterized by rugged topography and numerous shallow‐ to steep‐dipping fracture zones and faults. Only the shallow‐ to moderate‐dipping structures are imaged as reflectors in conventionally migrated versions of the georadar data. Our semblance‐based topographic migration (SBTM) scheme produces a 3D volume containing clouds of high‐semblance values. Application of morphology image processing to these clouds reveals the presence of geologically meaningful structures, most of which are very steeply dipping (>75°). Several of the steep‐dipping structures project to open fracture zones and associated lineaments at the surface, thus demonstrating the capability of the combined SBTM and morphology procedure for mapping near‐vertical fracture zones.
Experimental studies for monitoring water‐level by dipole‐antenna array radar fixed in the subsurface
Authors: Ebihara Satoshi , Nagoya Kaoru , Abe Noriyasu , Toida Masaru 
Published: August 2009

ABSTRACTWe propose a method for monitoring the movement of moving targets, such as water‐level, at considerable depth, using dipole array‐type radar with the dipoles fixed at various positions in the subsurface. We report on monitoring the water‐level variation in a sand tank over several hours with the dipole‐antenna array. A dipole‐antenna with an optical modulator made of non‐metallic material is used as the receiving antenna element. The antenna has little effect on the electromagnetic field being measured, and the optical modulator does not need electrical power. The basic idea of a radar system fixed in a subsurface is to distribute the dipole‐antennae and to monitor the movement of a target, such as water distribution, over long periods of time, using scattered waves from the target. In order to demonstrate the validity of the radar system, we carried out laboratory experiments. We positioned several receiving dipole‐antennae with the optical modulators, in a small package inside the sand. We raised the height of the water‐level in the sand and measured waves scattered from the boundary between the saturated and unsaturated layers. We analysed the radar signals with the MUSIC algorithm utilizing Method of Moment analysis and monitored the 2D location of a moving scattering centre over several hours. By comparing the water‐level in a vertical borehole with the estimated positions, we found that the estimation is reasonable.
Detailed shallow geometry and vertical displacement estimates of the Maleme Fault Zone, New Zealand, using 2D and 3D georadar
Authors: Tronicke Jens , Villamor Pilar , Green Alan G. 
Published: August 2009

ABSTRACTIn an attempt to map the shallow geometry of the Maleme Fault Zone (North Island, New Zealand) and estimate vertical displacements of selected fault strands, we have collected 2D and 3D georadar data using 100 MHz antennae. The 2D data consisted of three parallel georadar lines recorded perpendicular to the axis of the well‐defined graben of the Maleme Fault Zone. These ~160 m long lines, which were 7.5 m apart, crossed several fault strands on either side of the graben axis. The processed georadar sections revealed two prominent parallel reflections that originated from the boundaries of Late Pleistocene lacustrine and tephra deposits. Distinct vertical offsets of these reflections allowed us to estimate displacements at individual fault strands across the entire inner graben. The total displacements represented by these offsets was ~10–20 % greater than that inferred from geomorphological studies, thus demonstrating the limitations of surface observations for determining cumulative fault movements. The 3D georadar data set, recorded across an area of ~70×~20 m to one side of the graben axis, provided key details on individual fault strands. For the 3D visualization of fault‐related structures, various spatial attribute analyses based on the cosine of the instantaneous phase proved to be useful.
A stochastic interpretation of GPR data concerning the location of hydrocarbon plumes
Authors: Marcak Henryk , Golebiowski Tomislaw 
Published: August 2009

ABSTRACTThe flow of hydrocarbon contamination in near‐surface geological structures is correlated with changes in the electrical properties of the rock, such as permittivity and resistivity. The flow of large amounts of contamination over long periods is unstable and it can be only described with stochastic models, in which hydrocarbon contamination takes the form of plumes of variable size. The Ground Penetrating Radar (GPR) wavefield in a stochastic medium also has a stochastic character, and the properties of the scattered waves from the plumes, when observed, can be used for locating the contaminated areas. Field measurements carried out in former Soviet bases in Poland support this opinion.
The LSDARC method of seismic refraction analysis: principles, practical considerations and advantages
Authors: Wright C. 
Published: August 2009

ABSTRACTThe LSDARC (least‐squares difference and reciprocal combination) method of seismic refraction analysis is an adaptation of the reciprocal method that optimizes resolution of refractor depths and lateral variations in refractor wavespeed, and provides direct estimates of errors. Both time depths and differences between time depths are calculated separately to allow determination of more complete and accurate adjusted time depths, and therefore optimal estimates of refractor depth. The most efficient way of estimating refractor wavespeed is not to use the ‘velocity function’, but to subtract the time‐depth terms from the measured times to project the sources and receivers on to the refractor surface. A least‐squares procedure is then used to calculate corrections to the times for each shot gather and apply them to make all times appear to be produced by a single shot placed on the refractor surface at one end of the profile. Refractor wavespeeds are estimated from these corrected times using variable distance windows that can be adjusted to get a preferred trade‐off between variance in wavespeed and window length. An alternative and simpler method of wavespeed determination involves using first‐break times to prepare slowness profiles in opposite directions, with a model for refraction across a dipping interface, to compute true refractor wavespeeds. However, although the alternative method will generally resolve significant variations in wavespeed, the estimated errors are larger than when time depths are used.The methodology is valuable for engineering‐scale refraction surveys in which economic considerations prevent dense spatial sampling of the ground. The main features of the method are illustrated using short refraction profiles undertaken during student field schools.
Mapping fly‐ash water pond leakage with TEM and IP data at Loy Yang coal‐mine (Australia)
Authors: Viezzoli A. , Cull J.P. , Massie D. 
Published: August 2009

The fly‐ash water pond at the Loy Yang power station (Latrobe Valley, Victoria, Australia) seeps and therefore presents a hazard to the local environment from its high total dissolved solids content. A transient electromagnetic survey (TEM) was completed to investigate the contaminant plume in more detail.The contaminate plume was successfully delineated; however, multiple contiguous soundings featured late‐time negative decays not typically recorded in coincident‐loop TEM surveys. This indicates that conductivity is frequency‐dependent. These anomalous soundings suggested the presence of a shallower contaminate plume than that mapped by the boreholes and therefore further analysis was required. An induced polarization survey revealed that the area where the late‐time negative responses were recorded coincided with a shallow conductive and extremely chargeable anomaly. A simple power‐law relationship holds between the negative transients decay rates and normalized apparent chargeability. Analysis on soil samples obtained from auger holes related the polarization anomaly to clay‐, organic‐ and metal‐rich layers of fill material.
Gravity investigation of buried Pleistocene subglacial valleys
Authors: Gabriel Gerald 
Published: August 2009

Buried Pleistocene subglacial valleys are important geological structures in northern Germany and are used extensively for groundwater production. Details of their occurrence, their structure, and the hydraulic properties of their sediments are not well known. In recent years, seismic and geoelectric methods, including airborne‐electromagnetics, have been applied in reconnaissance surveys. This paper summarizes the results of gravity surveys conducted across four Pleistocene valleys in northern Germany. Each buried valley causes a significant gravity anomaly. The observed anomalies in the residual gravity field range from −0.5 mGal to 0.7 mGal. The Ellerbek valley in Schleswig‐Holstein is the only buried valley causing a negative anomaly. Its infill is dominated by sandy sediments. Two other Pleistocene valleys in Schleswig‐Holstein, the Trave valley and the Curau valley, are filled with boulder clay. Both cause positive anomalies in the residual gravity field. In the area of the Bremerhaven‐Cuxhaven valley in Lower Saxony, a 4‐km2 survey area shows a quite different situation. Strong negative anomalies caused by shallow silt/clay layers are superimposed on positive anomalies caused by the valley sediments, resulting in a complicated pattern of gravity anomalies. The horizontal gravity gradient proved to be appropriate for locating the edges of the valleys.The quality of the gravity results was confirmed by the seismic data. Quantitative interpretations of the residual gravity anomalies, which give deeper insight into the structure of the valley fill, often suffer from a lack of information about the rock density. This lack of information can be partly compensated for by converting seismic velocities into densities, although in some cases the normally used velocity‐density formulae yield unrealistic density values. Nevertheless, the additional information obtained from gravity surveys can support the seismic interpretation or provide information in areas where other geophysical methods fail.