bluuebunny/crossref_metadata_2025 huggingface dataset:
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

Electrical 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

Recent 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

Among 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

Magnetic 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

The 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

(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

The 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

In 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

Ground‐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

Multicomponent 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

We 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

Potential 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.
Multiple‐gradient array measurements for multichannel 2D resistivity imaging
Authors: Dahlin Torleif , Zhou Bing
Published: August 2009

Two‐dimensional resistivity imaging using multiple gradient, Wenner and dipole‐dipole electrode arrays was carried out at two field sites in Sweden and one in Nicaragua, with the objective of confirming the practical applicability of results obtained with numerical modelling. The results support earlier numerical modelling studies that concluded that the gradient array, using multiple current electrode combinations, has resolution as good as or better than the commonly used Wenner array. The array behaved well in terms of sensitivity to noise at the test sites, and the results obtained generally agree with dipole‐dipole array results, although the latter at two of the sites gave resistivities that differed significantly from the other arrays in the deeper parts of the inverted models. A formula proposed for pseudosection plotting works well for data quality assessment, where it can be an advantage to make separate plots for each m‐factor (the distance between the midpoints of the current and potential electrode pairs) or for each a‐spacing. The gradient array is well suited for multichannel data acquisition, and can significantly increase the speed of data acquisition in the field and at the same time give higher data density, but it is also an attractive option for single‐channel data acquisition. The Wenner array, on the other hand, is not suitable for measuring in more than one channel. Compared to the dipole‐dipole array, it offers lower sensitivity to noise which may be a major advantage in real data acquisition, and the remote electrode needed for the pole‐dipole array is avoided, which is often a significant advantage for field logistics.
Identification of buried valleys using the BGR helicopter‐borne geophysical system
Authors: Eberle D.G. , Siemon B.
Published: August 2009

A study of four airborne electromagnetic data sets collected during the past decade by the BGR helicopter‐borne geophysical system over various geological settings shows that such data sets indicate not only layered horizontal geological features but also vertically and laterally confined structures, such as buried valleys. Indicative geological markers comprise conductive cover layers, conductive fills in resistive bedrock, and incisions in a conductive host filled with resistive materials. In cases where the valleys are incised into a magnetic basement and refilled with non‐magnetic material, the magnetic data records can also be used for identification and thickness estimates of the infill.

From a hydrogeological point of view, buried valleys are becoming increasingly important as they host groundwater reserves, which are in many cases big enough to satisfy the future demand for fresh water. This paper describes two examples from the North‐West German lowlands and another two from the Namib coastal plains, where the Kuiseb and the Omaruru ephemeral rivers reach the shore of the South Atlantic Ocean.
Subsurface water‐content identification in a crypt using GPR and comparison with microclimatic conditions
Authors: Leucci Giovanni , Cataldo Rosella , De Nunzio Giorgio
Published: August 2009

The effects of climate, pollution and human negligence cause severe and sometimes irreversible damage to buildings and monuments of cultural interest. It is well known that the presence of water and/or moisture content in a porous material is the initial cause of deterioration. In a previous paper, the authors reported an integrated study on a building of cultural importance, namely the crypt of the Cattedrale di Otranto in Apulia, Italy, based on non‐destructive integrated biological and physical surveys. The method described was able to identify the ‘internal’ factors responsible for deterioration. It was discovered that the distribution of moisture in the stone depended mainly on adverse environmental conditions, and the presence of wet buried structures in the ground.

The first aim of the present study was to identify subsurface water‐content in this same crypt using a ground‐penetrating radar (GPR) technique, and to compare these results with those of the previous microclimatic survey. In particular, the existence of underground discontinuities was verified; we located them and analysed their influence. Moreover, by means of velocity analysis, we obtained a quantitative estimate of the volumetric water‐content under the pavement of the crypt. This finding completes the results of the previous research, as it indicates the causes of the deterioration in the crypt and provides significant information, on the basis of which, effective decisions can be made for safeguarding the historic building.
Detecting brine zones in salt deposits with the ground penetrating radar (GPR) for safety assessments of underground waste disposals
Authors: Igel Jan , Kurz Gerhard , Schulz Rüdiger
Published: August 2009

The presence of brine is one of the principal hazards of underground waste disposal. A clear reflection at a brine reservoir would be expected when carrying out ground‐penetrating radar (GPR) measurements because of the high contrast in the electrical conductivity between dry rock‐salt and brine. However, this is not the case as many previous measurements have shown, and as is also demonstrated by some of our GPR measurements.

The lack of a distinct reflection at higher frequencies can be explained by the presence of a gradient zone leading to a frequency‐dependent reflection coefficient. Therefore, measurements at two frequencies (25 and 100 MHz) were carried out in a potash salt‐mine with a massive brine reservoir. When comparing the energy distributions of these measurements, the area of the brine reservoir shows a frequency‐dependent reflection coefficient as opposed to areas without brine. Several finite‐difference forward calculations using different frequencies and models deduced from drill‐cores from the area of the brine reservoir are presented. They demonstrate that brine zones are detectable by comparing different frequencies if they are not enclosed by a sharp interface but by a gradual increase in the brine content.
Use of borehole radar tomography to monitor steam injection in fractured limestone
Authors: Grégoire C. , Joesten P.K.
Published: August 2009

Borehole radar tomography was used as part of a pilot study to monitor steam‐enhanced remediation of a fractured limestone contaminated with volatile organic compounds at the former Loring Air Force Base, Maine, USA. Radar tomography data were collected using 100‐MHz electric‐dipole antennae before and during steam injection to evaluate whether cross‐hole radar methods could detect changes in medium properties resulting from the steam injection. Cross‐hole levelrun profiles, in which transmitting and receiving antennae are positioned at a common depth, were made before and after the collection of each full tomography data set to check the stability of the radar instruments. Before tomographic inversion, the levelrun profiles were used to calibrate the radar tomography data to compensate for changes in traveltime and antenna power caused by instrument drift. Observed changes in cross‐hole radar traveltime and attenuation before and during steam injection were small. Slowness‐ and attenuation‐difference tomograms indicate small increases in radar slowness and attenuation at depths greater than about 22 m below the surface, consistent with increases in water temperature observed in the boreholes used for the tomography. Based on theoretical modelling results, increases in slowness and attenuation are interpreted as delineating zones where steam injection heating increased the electrical conductivity of the limestone matrix and fluid. The results of this study show the potential of cross‐hole radar tomography methods to monitor the effects of steam‐induced heating in fractured rock environments.
Electrical resistivity tomography to detect buried cavities in Rome: a case study
Authors: Cardarelli E. , Di Filippo G. , Tuccinardi E.
Published: August 2009

Cavities located in shallow layers of the subsoil represent a major hazard especially in sites located in town centres. For this reason, it is necessary to determine their location and size in order to evaluate the risk of subsidence and to draw up plans for restoration and safety.

In this study, electrical resistivity tomography was carried out, both to detect cavities and to test a new inversion algorithm that we developed. The cavities are located in a volcanic formation in a public recreation park where there are municipal buildings. The survey was performed using two different arrays: dipole‐dipole and Wenner. A new approach to adopting an initial model and inequality constraints was used and the results from different algorithms were compared. The effective range of the dimensions of cavities was determined in order to make an approximate evaluation of ground stability and any potential subsidence hazard.
Mine action: status of sensor technology for close‐in and remote detection of anti‐personnel mines
Authors: Acheroy Marc
Published: August 2009

After a broad introductory discussion on the mine action problem, this paper presents the status of close‐in mine detection technologies, including operational characteristics, without aiming at being exhaustive. Signal processing aspects and important lessons on data fusion are also discussed briefly. The detection is considered as a global process in which the outputs of the sensors, considered as skilled specialists, are integrated in a fusion operation. Next, the paper briefly addresses the problem of area reduction using remote sensing. In this case, information, collected with appropriate sensors and associated with context information from the field, is integrated in a geographical information system. This part of the paper is intentionally limited to a very short description of the SMART project funded by the European Commission, which uses multispectral and full polarimetric radar data in order to assist image analysts in their interpretation of mined scenes during an area reduction process.
Mutually and laterally constrained inversion of CVES and TEM data: a case study
Authors: Christiansen Anders V. , Auken Esben , Foged Nikolai , Sørensen Kurt I.
Published: August 2009

Mutually constrained inversion in combination with laterally constrained inversion (MCI‐LCI) between transient electromagnetic (TEM) and direct current (DC) resistivity methods was successfully used to characterize a buried valley structure. Although both methods measure, in some sense, the electrical resistivity, or conductivity, of the subsurface, they sample different volumes and have different sensitivities, which are exploited with mutually and laterally constrained inversion of combined, coincident profile data sets. The output models incorporate the information from both data sets to obtain optimum layered 1D models, fitting both data sets and constraints.

The set‐up of constraints contains three parts. First, we constrain the individual data sets along their profile using lateral constraints producing a chain of TEM data and a chain of DC data. Next, we merge the information from these two chains by setting up mutual constraints between the TEM and the DC models. Finally, we adjust the mutual constraints to resemble the increasing sampling volumes with depth, i.e. wide constraints at large depths and short constraints at shallow depths. All data sets are inverted simultaneously; a common objective function is minimized, and the number of output models is equal to the number of 1D soundings. The lateral and mutual constraints are part of the inversion, and consequently the output models are balanced between the constraints and the data‐model fit. Information from one model will spread to the neighbouring models through the constraints, helping to resolve parameters that are poorly resolved by any of the individual data sets.

A field example illustrates that MCI‐LCI allows the governing information from each method to dominate the inversion process. Thus, the model resolution in both the shallow and the deeper parts of the model is significantly enhanced. This could not be obtained by inverting the two data sets separately with a subsequent comparison of the output models. Our results are confirmed by drill‐hole data.
An experiment of non‐invasive characterization of the vadose zone via water injection and cross‐hole time‐lapse geophysical monitoring
Authors: Deiana Rita , Cassiani Giorgio , Kemna Andreas , Villa Alberto , Bruno Vittorio , Bagliani Andrea
Published: August 2009

The characterization of the vadose zone, i.e. the part of the subsurface above the water table, is a challenging task. This zone is difficult to access with direct methods without causing major disturbance to the natural in‐situ conditions. Hence the increasing use of geophysical methods capable of imaging the water presence in the vadose zone, such as ground‐penetrating radar (GPR) and electrical resistivity tomography (ERT). This type of monitoring can be applied both to processes of natural infiltration and to artificial injection (tracer) tests, by collecting multiple data sets through time (time‐lapse mode). We present the results of a water‐injection experiment conducted at a test site in Gorgonzola, east of Milan (Italy). The site is characterized by Quaternary sand and gravel sediments that house an extensive unconfined aquifer, potentially subject to pollution from industrial and agricultural sources. ERT and GPR profiles were acquired in 2D cross‐hole configuration and time‐lapse mode over a period of several days preceding and following the injection of 3.5 m3 of fresh water in a purpose‐excavated trench. A 3D model of the water‐infiltration experiment was calibrated against the time‐lapse cross‐hole data, particularly focusing on the ability of the model to reproduce the vertical motion of the centre of mass of the injected water as imaged by GPR and ERT. This model calibration provided an estimate of the isotropic hydraulic conductivity of the sediments in the range of 5–10 m/d. However, all isotropic models overpredict the measured excess of moisture content, caused by water injection, as imaged by GPR. The calibration of anisotropic models for the vertical hydraulic conductivity, with the horizontal hydraulic conductivity determined by direct measurement, also leads to a good fit of the sinking of the centre of mass, with a better mass balance in comparison with field data. The information derived from the experiment is key to a quantitative assessment of aquifer vulnerability to pollutants infiltrating from the surface.
HELP ACEH – A post‐tsunami helicopter‐borne groundwater project along the coasts of Aceh, northern Sumatra
Authors: Siemon Bernhard , Steuer Annika , Meyer Uwe , Rehli Hans‐Joachim
Published: August 2009

After the earthquake and the tsunami event on December 26, 2004, the Indonesian and German governments set up a project dedicated to re‐installing the public life of the people in the coastal region of Aceh Province in northern Sumatra, Indonesia. As the tsunami waves caused large‐scale coastal salt‐water intrusions and destroyed thousands of shallow drinking‐waterwells, the focal point was water assessment along the shorelines of Aceh about nine months after the tsunami. The target areas were the city of Banda Aceh and the district of Aceh Besar on the north coast and the area on the west coast between the towns of Calang (district of Aceh Jaya) and Meulaboh (district of Aceh Barat). A helicopter‐borne survey including electromagnetics (HEM), magnetics and gamma‐ray spectrometry was conducted by the airborne group of the German Federal Institute for Geosciences and Natural Resources from August to October 2005. The HEM surveys revealed shallow coastal salt‐water occurrences caused by the tsunami, deep salt‐water occurrences up to several kilometres inland, and a number of potential freshwater resources. Numerous requests for information on the geological and hydrogeological situation at sites for planned water‐wells were successfully evaluated.
Characterization of small local noise sources with array seismology
Authors: Ritter Joachim R.R. , Sudhaus Henriette
Published: August 2009

Seismic noise is often regarded as a stochastic, incoherent and unwanted signal. However, at a closer look, noise also contains coherent signals which can be used to characterize its sources and/ or the subsurface. Here, we propose the use of array seismological methods for engineering purposes to locate and identify small, local noise sources. A detailed knowledge of noise sources enables appropriate measures to be taken to mitigate unwanted noise effects. As a case study, we report results from a temporary array of ten seismic broadband stations which were installed at the building site of a high‐precision physics laboratory. Standard analyses of power spectra and amplitude measurements indicate that the highest noise amplitudes are caused by heavy trucks passing by. Burst‐like coherent waves, excited by vehicles, appear to have a recurring pattern. Using array techniques, we were able to measure the slowness and backazimuth of the coherent arrivals. These could be identified as Rayleigh waves generated at small pot‐holes and bumps of a near‐by road and were found to propagate with a speed of 130–200 m/s.
Non‐destructive DC resistivity surveying using flat‐base electrodes
Authors: Athanasiou E.N. , Tsourlos P.I. , Vargemezis G.N. , Papazachos C.B. , Tsokas G.N.
Published: August 2009

The application of flat‐base electrodes to geoelectrical measurements is examined. This study is motivated by the inability to apply geoelectrical techniques in environments where conventional spike electrodes cannot be inserted into the ground. The performance of flat‐base electrodes is examined in various environments, using different measuring modes. It is shown that flat‐base electrodes can be satisfactorily used in most cases, producing data that are almost identical to the measurements obtained using standard electrodes. Several case studies in various urban locations in Greece, where flat‐base electrodes have been successfully employed, are also presented. The results indicate that flat‐base electrodes provide the advantage of a fully non‐destructive application and, therefore, the extension of geoelectrical methods to environments that, otherwise, would not have been considered suitable.
Seismic tomography on the castle hill in Quedlinburg
Authors: Schicht Th. , Lindner U. , Heckner J. , Strobel G. , Rappsilber I.
Published: August 2009

The collegiate church of Saint Servatius (part of the UNESCO world heritage list) and other buildings on the castle hill in Quedlinburg show evidence of frequent damage caused by foundation subsidence. The geological structure consists of sandstone of differing hardness from the Cretaceous period. Geotechnical investigations to date have provided information about the underground structure at various points, but no comprehensive 3D spatial model of the entire area was heretofore available. Therefore, the non‐destructive seismic tomography method was used. On different locations around and on top of the castle hill, elastic waves were generated with a weight drop. On the other hillside the arrivals of the seismic waves were recorded by geophones. The result was a tomographic model of the 3D distribution of P‐wave velocities. The results showed a zonally differentiated seismic velocity scheme. Considerable differences in the rock strength were shown. Areas with slower seismic velocities indicate lower rock quality and therefore geotechnically weak zones. The areas of our investigations, which were identified as deconsolidated rock, correspond to damage to the buildings above.
TDEM magnetic viscosity prospecting using a Slingram coil configuration
Authors: Thiesson Julien , Tabbagh Alain , Flageul Sébastien
Published: August 2009

Soil magnetic properties are significantly modified by pedogenesis and human activity; consequently their study is an important source of information about soil structure and history. In addition to measurements of magnetic susceptibility, now commonly practised, magnetic viscosity measurements can be undertaken as a routine survey method if well‐matched instruments that are easy to use in the field and that offer a sufficient depth of investigation, exist.

A theoretical study, supported by field tests of a prototype, demonstrates that a Slingram perpendicular coil configuration with a one‐metre inter‐coil spacing allows the required one‐metre depth of investigation to be reached for both 1D and 3D models. The sensitivity of the instrument response to the viscosity is also very high over the whole measurement range (10μs to 1ms), while the sensitivity to electrical conductivity is reduced to a minimum.

A first experiment at the Roman site of Vieil‐Évreux illustrates how viscosity measurements can complement magnetic field and susceptibility measurements for a more accurate identification of an archaeological feature.
Time‐lapse electrical resistivity tomography to delineate mud structures in archaeological prospections
Authors: Ranieri G. , Sharpe L. , Trogu A. , Piga C.
Published: August 2009

Archaeological sites in rural areas are often characterized by structural remains that are made of mud or raw brick, and that produce a very small contrast in physical characteristics, as the surrounding materials of both anthropogenic and natural origins have similar lithological and mineralogical properties. The main measurable differences are due to changes in compaction and porosity and, as a consequence, in permeability, which influence the hydrological behaviour of the subsurface materials.

In this paper, we present some applications based on Electrical Resistivity Tomography (ERT) surveys that are routinely applied in archaeological prospection, carried out over two different weather conditions. Measurement of the changing resistivity, corresponding to different moisture conditions in the subsoil, allowed us to identify archaeological strata and structures surrounded by natural, undisturbed soil.

A number of tests carried out on a physical model permitted a further definition of the acquisition parameters and methodologies to be used to secure the best results. The field surveys were carried out in the necropolis of “Pill’e Matta” (IV BC ‐ V AD), located in the suburbs of the metropolitan area of Cagliari, in the southern part of Sardinia, and in a Punic and Roman site near the village of Terralba, located in the Campidano plane, in the western part of Sardinia. The results show that time‐lapse prospecting can increase the resistivity contrast between archaeological structures and soil.
Aquifer characterization in the Ouarzazate Basin (Morocco): A contribution by TEM and RMT data
Authors: Steuer A. , Helwig S.L. , Tezkan B.
Published: August 2009

In this case study we present the first application of the radio magnetotelluric (RMT) technique in Africa. We applied the RMT and transient electromagnetic (TEM) methods to study an aquifer of the Ouarzazate Basin between the High Atlas and the Anti Atlas in Morocco. The main objective of the survey was to constrain the geometry of the near surface sediment layer by mapping the aquitard, an underlying clay layer.

We collected RMT data in the frequency range of 16 kHz to 207 kHz and TEM data in the time range of 4 μs to 1 ms along an 8 km long profile in a desert area crossing an oasis region. After 2D inversion of the RMT data and 1D inversion of the TEM data, we carried out 1D joint inversion to enhance the resolution of the subsurface resistivity structure.

From the inversions we derived the resistivity distribution down to a depth of about 100 m and identified a relatively resistive layer as the aquifer sediments, overlying a more conductive clay layer. Inversion statistics indicate that the depth of the clay layer is well resolved. Compared with the lithological logs of drillings located at distances of 150 m to 3 km from the profile, the depth of the clay layer beneath the aquifer in the south of the geophysical section agrees to ± 3 m. Furthermore, we observed a lateral resistivity contrast close to the surface separating the desert area and the oasis, due to differences in humidity.
Semi‐automatic feature extraction from GPR data for archaeology
Authors: Leckebusch Jürg , Weibel Andreas , Bühler Flurin
Published: August 2009

A new method and working tool to interpret 3D ground‐penetrating radar data has been developed. Algorithms for feature extraction based on three‐dimensional regression techniques are developed to generate 3D models of the archaeological reflection features in the subsurface.

After several attempts to reasonably visualize the information of a 3D ground‐penetrating radar (GPR) data cube by manual and automatic techniques, we propose a new approach: the visualization and interpretation steps are combined in an efficient and objective process in order to extract the geometrical information of the mapped archaeological features. Each object is modelled by six independent planes that form the structure that should be extracted. This is an appropriate approximation for walls, floors, stones as well as for pipes. To speed up calculation and because routines operating on the complete data set were not successful so far, the system utilizes a start line which is defined by the user. After 3D pre‐filtering with a sequence of median – dilation – erosion, followed by a 3D gradient filter, planes are matched to the borders of the anomaly by using a least‐median‐of‐squares algorithm. Finally, the found planes are refined by an analytical regression and the final box displayed in a 3D GIS software. The small computing time for a single object makes this method feasible for routine application of feature extraction from a 3D GPR data cube. The presented technique allows visualizing and extracting the imaged archaeological features rapidly and into an easy understandable representation.
High‐resolution magnetic survey in a quasi‐urban environment
Authors: Ciminale Marcello , Gallo Danilo
Published: August 2009

A high‐resolution magnetic survey was performed on a large cultivated field located inside a heavily built‐up town in Southern Italy. Measurements with a vertical gradiometer were even carried out relatively close (down to 35 m) to sources of strong magnetic noise. Despite these particularly unfavourable surroundings, the geophysical investigation made it possible to detect and recognize buried archaeological remains from the Neolithic age.