bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultra‐shallow seismic reflection imaging in a region characterized by high source‐generated noise
Authors: Schmelzbach C. , Green A. G. , Horstmeyer H.
Published: August 2009

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

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

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

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

This paper aims to investigate the magnetic resonance signal generated by shallow aquifers and its influence on the interpretation of magnetic resonance sounding (MRS) data. To improve the accuracy of MRS modelling in the presence of shallow aquifers, an enhanced mathematical model, which takes into account the higher harmonics of the transmitted pulse and a non‐zero frequency offset between the Larmor frequency and the pulse frequency, is used. It is shown that in the presence of shallow aquifers, the magnetic resonance signal is complex even when the subsurface has a low electrical conductivity. At large pulse moments, water in shallow aquifers may generate a signal comparable in amplitude to a signal generated by water in deeper aquifers. Two types of error related to a shallow aquifer may occur. For a one‐aquifer system, interpretation using the simplified model may reveal an artefact that appears to be a deeper aquifer, although in reality it is non‐existent (a phantom aquifer). For a multi‐aquifer system, the signal from a shallow aquifer may offset the signal from a deeper aquifer, thus reducing the depth of investigation in comparison with a simple one‐aquifer formation (offset of a deep aquifer).

It is shown numerically that the depth resolution of the MRS method could be significantly improved by considering not only the amplitude but also the phase of the MRS signal. However, taking into account the non‐linearity of the spin system for a large flip angle, the mathematical model for the inversion of complex signals needs to be further improved or at least checked for consistency before inversion.

For verification of the numerical results, a re‐interpretation of MRS field data reported in the literature is presented.
Magnetic resonance soundings with separated transmitter and receiver loops
Authors: Hertrich Marian , Braun Martina , Yaramanci Ugur
Published: August 2009

Triggered by an extended mathematical formulation of the response signal for magnetic resonance soundings (MRS), which allows the treatment of individual transmitter and receiver loops, we make a comprehensive evaluation and assessment of the emerging new possibilities of the technique. Based on a reformulation of the basic equation, we indicate and interpret the evolving effects. The influence of loop separation on MRS sounding curves in terms of offset and direction is also assessed as is the corresponding sensitivity to depth and lateral spin variation. Interpretation of field data measured with separated loops using the extended formulation is found to fit the predicted response extremely well. From the encouraging results, we derive new aspects of two‐dimensional investigation of groundwater resources. Furthermore, new perspectives of future developments of the MRS technique, with individual loops for optimized field measurements and hydrological applications, are discussed.
Forward modeling and inversion of MRS relaxation signals using multi‐exponential decomposition
Authors: Mohnke O. , Yaramanci U.
Published: August 2009

The 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

In 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

The 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

Although 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

High‐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

A 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

Steep‐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

We 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

In 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

The 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

The 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.
Geophysical investigation of the June 6, 1944 D‐Day invasion site at Pointe du Hoc, Normandy, France
Authors: Everett M.E. , Pierce C.J. , Save N. , Warden R.R. , Dickson D.B. , Burt R.A. , Bradford J.C.
Published: August 2009

A near‐surface geophysical survey at the D‐Day invasion site on top of the cliffs at Pointe du Hoc in Normandy, France, was carried out using ground‐penetrating radar, electromagnetic induction and magnetic gradiometry equipment. The subsurface targets of investigation are predominantly buried concrete and steel structures and earthworks associated with the German coastal fortifications at this strongpoint of Adolf Hitler’s Atlantic Wall. The targets are readily detectable, embedded within the vadose zone of a weakly magnetic, electrically resistive, loess soil cover. The radar and electromagnetic induction responses lend themselves to plan‐view imaging of the subsurface, while the magnetics data reveal the presence of buried magnetic bodies in a more subtle fashion. Several intriguing geophysical signatures were discovered, including what may be the buried remains of a railway turntable, ordnance fragments in the bomb craters, a buried steel‐reinforced concrete trench, and a linear chain of machine‐gun firing positions. Geophysical prospecting is shown to be a very powerful tool for historical battlefield site characterization.
Resistivity tomography as an aid to planning gas‐pipeline construction, Risha area, north‐east Jordan
Authors: Batayneh Awni T.
Published: August 2009

Two‐dimensional (2D) electrical resistivity tomography measurements were conducted with the Iris Syscal R2 resistivity instrument to map the ground resistivity in advance of gas‐pipeline construction in the Risha area, north‐east Jordan. The cost of pipeline construction is strongly dependent on the ground conditions encountered. Accurate prediction of these conditions can reduce the planning risk considerably. The determination of ground conditions reduced the time and cost associated with gaining land access and permission for trenching. It is also served to detect areas of high ground conductivity due to clays or saline soils, which can create conditions in which pipeline corrosion is accelerated. Resistivity measurements were carried out utilizing a Wenner‐array with 1‐m electrode spacing along approximately 52 km of the proposed gas‐pipeline lane. The survey took six days to complete, providing a map of the resistivity of the surficial deposits and the bedrock. The results obtained, based on 2D inversion of field data and borehole information, were interpreted to determine the extent of shallow bedrock, which would require blasting, and deeper overburden, which could be trenched down to the depth necessary for the pipeline. The thickness of the overburden layer along the gas‐pipeline lane in the north‐eastern Jordanian plateau was found to be relatively thin: in the range 0.5– 8 m. The largest values are observed over topographic lows because of the tectonic uplift of the area which leads to fast erosion. Correlation between the field results obtained shows that the overburden depth is the main factor controlling the electrical resistivity of the near‐surface material.
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.
Integrated study of the sinkhole development site on the Western shores of the Dead Sea using geophysical methods
Authors: Ezersky Michael , Bruner Ilan , Keydar Shemer , Trachtman Pini , Rybakov Michael
Published: August 2009

This paper presents a combination of high‐resolution seismic diffraction, reflection, refraction, continuous vertical electric sounding (CVES), ground‐penetrating radar (GPR) and microgravity methods to study the structure and properties of the shallow subsurface at the sinkhole development site in the Nahal Hever southern area (on the shore of the Dead Sea) in Israel. These methods have different resolutions and penetration depths. The methods are complementary to each other and reveal useful information about the subsurface. Each one investigates certain characteristics, but when combined they provide us with new information about the subsurface. The seismic refraction method is utilized to map salt layers and to detect dissolved zones. The analysis of all of the seismic refraction data obtained at the Dead Sea area, verified by the information gained from a borehole, has shown that the P‐wave velocity of the salt layer varies from 2900 to 4250 m/s. The velocity of 2900 m/s was used as the minimum velocity criterion for salt‐layer identification at this site.

A low‐velocity zone (VP < 2800 m/s) revealed within a high‐velocity area ( m/s) is interpreted as a dissolved salt or non‐saline unit. The site is characterized by two significant geological features. The first feature is a water‐filled cave, detected recently by a borehole in the depth interval 23–28.5 m, and the second feature is a depression at the surface. Geophysical results show that the depression site is presently characterized as a heavily perturbed zone in both lateral and vertical directions. Soil decompaction in the subsurface is clearly identified by the CVES method as a high‐resistivity anomaly. This decompaction is assumed to be associated with the voids and fractures occurring down to a depth of 100–120 m (by diffraction data) and in the uppermost subsurface (by GPR data). The zone containing the cave is characterized by a lateral (funnel) and vertical (faults) distortion of the subsurface structure (by seismic reflection data) and has a faint diffraction anomaly in the shallow subsurface. A slightly increased resistivity anomaly appears in the depth interval 6–12 m (from CVES data). Drilling data reveal this interval as a decompaction zone. No GPR anomalies were detected at the buried cave site. The microgravity survey at the buried void site in 1999 detected an anomaly that is very similar to the funnel‐shaped one.
A study of the groundwater seepage at Hibis Temple using geoelectrical data, Kharga Oasis, Egypt
Authors: Sultan S.A. , Santos F.A.M. , Helal A.
Published: August 2009

Geoelectric techniques have been used to detect and define the subsurface stratigraphy and structures around Hibis Temple in Kharga Oasis, Egypt. We used 2D and 3D inversion approaches to interpret the data set obtained from 20 dipole‐dipole resistivity profiles with electrode spacings of 3 and 5 m and lengths between 39 and 85 m.

Five vertical electrical soundings, with a maximum array length of 200 m, along a profile crossing the study area were also carried out. A preliminary quantitative interpretation of the vertical electrical sounding curves was achieved using two‐layer standard curves and generalized Cagniard graphs. The final models were obtained by 1D inversion using the results of the manual interpretation as initial models. Model results were used to construct a geoelectric cross‐section that correlated very well with the stratigraphic units.

Five geoelectric units were identified: the first (the uppermost) is a high‐resistivity layer consisting of fill deposits (rubble); the second is a muddy clay with moderate resistivity values; the third is also a muddy clay but with decreased resistivity due to the increase in salt content originated by the evaporation of the groundwater seepage; the fourth unit, at a depth of 7–13 m, is a muddy clay saturated with water seepage from the agricultural areas surrounding the temple; the final unit is a more resistive layer corresponding to dry muddy clay. The differences in the groundwater level, and its salt content, correlated with the irrigation activities around the temple. We concluded that the high corrosion potential of the seepage water might be connected with its salt content.
2M‐SASW: Multifold multichannel seismic inversion of local dispersion of Rayleigh waves in laterally heterogeneous subsurfaces: application to the Super‐Sauze earthflow, France
Authors: Grandjean G. , Bitri A.
Published: August 2009

The approach presented here deals with the inversion of Rayleigh wave dispersion (SASW: Spectral Analysis of Surface Waves) in laterally heterogeneous media. The traditional method consists of computing the frequency versus phase‐velocity curve, known as the dispersion curve, from seismic records, then inverting to obtain a 1D shear‐wave velocity model. When media are laterally heterogeneous, S‐wave velocity changes lead to phase variations in Rayleigh waves in the offset dimension. This phenomenon can drastically alter the dispersion image, since it affects the local slopes of Rayleigh waves in the shot gather, and thus their phase‐velocity dispersion properties. In the case of multifold acquisitions (2M‐SASW: Multifold and Multichannel SASW), we redefine the manner in which the dispersion‐image calculation is formulated in order to compute the local dispersion stack (LDS). In principle, this consists of gathering receivers within a restricted window for a series of shots and computing the local dispersion images that are then stacked in order to improve the signal‐to‐noise ratio. Even though local dispersion diagrams are noisy because they are computed from a limited number of traces, the summation over multiple dispersion images enables the quality of the final LDS to be improved. Because it is related to the dispersion properties of the windowed wavefield, the LDS is an efficient input for the 1D inversion process. The method is tested on synthetic data to demonstrate its contribution compared to that of the traditional SASW technique. An application of the 2M‐SASW method to the Super‐Sauze earthflow confirms that is well‐suited to inverting the shear‐wave velocity from Rayleigh‐wave dispersion when high‐contrast media are considered.
A review of practical numerical modelling methods for the advanced interpretation of ground‐penetrating radar in near‐surface environments
Authors: Cassidy Nigel J.
Published: August 2009

As ground‐penetrating radar (GPR) technology improves, there is an increasing demand for more sophisticated and detailed interpretational tools. Numerical modelling has become one of the most popular advanced analysis methods and there is a wide range of electromagnetic modelling approaches available to the GPR user. The finite‐difference time‐domain (FDTD) technique is one of the most common, as it provides modellers with a robust, flexible, yet accurate, modelling scheme that is capable of simulating GPR wave propagation in complex, three‐dimensional, heterogeneous, lossy, subsurface environments. Through the use of a well‐constrained example, the realistic modelling of near‐surface GPR is evaluated including the practical limitations, computational procedures and application constraints of the FDTD method. The results show that, despite some obvious deficiencies, even a relatively basic FDTD model can provide important additional information for the advanced interpretation of observed GPR data.
Thin‐wire antenna design for GPR applications using a multi‐objective GA
Authors: Moreno de Jong van Coevorden C. , Rubio Bretones A. , Fernández Pantoja M. , García Ruiz F. , Garcia Salvador G. , Monorchio A. , Gómez Martín R.
Published: August 2009

A Pareto multi‐objective genetic algorithm (MOGA) has been applied to the design of non‐dispersive, ultra‐wideband (UWB), resistively loaded, thin‐wire antennas for ground‐penetrating radar (GPR) applications. The radiation characteristics of the antennas are optimized, seeking to maximize the bandwidth and the front‐to‐back ratio. The variables in the design are the value and position of the resistive loads located along the antenna wires, the number of wires and the angular distances between those wires. The radiation characteristics of the optimized antennas are explored and an example of an application to detect cracks in marble blocks is presented.
A linear inverse scattering algorithm for realistic GPR applications
Authors: Soldovieri Francesco , Hugenschmidt Johannes , Persico Raffaele , Leone Giovanni
Published: August 2009

The paper deals with a microwave‐tomography‐based solution algorithm tailored for use with GPR data‐processing applications. The algorithm tackles an inverse scattering problem in the frequency domain through the use of a linear model of the electromagnetic scattering, based on the Born approximation. In particular, we evaluate the reconstruction capabilities of the linear inversion algorithm in terms of the retrievable spatial variations of the unknown contrast function, whilst considering the problem of choosing an optimal frequency measurement step, theoretically, using diffraction tomography arguments. A numerical analysis of the technique is performed by means of the singular‐value decomposition tool, which allows us to extend the theoretical results to more realistic cases involving lossy soils. Finally, we present a series of reconstructions, obtained using synthetic and experimental data, which show the performance of the method under realistic conditions.
Clutter reduction in GPR measurements for detecting shallow buried landmines: a Colombian case study
Authors: Lopera Olga , Milisavljević Nada , Lambot Sébastien
Published: August 2009

Detection of land‐mines from ground‐penetrating radar data is a challenging task demanding accurate and useful filtering techniques to reduce soil‐surface and antenna reflections, which obscure the landmine response. In this paper, we apply and adapt a recently proposed filtering approach to enhance the detection of shallow buried anti‐personnel land‐mines from data acquired in typical mine‐affected soils in Colombia. The methodology combines a radar‐antenna‐subsurface model with phase‐shift migration to filter out antenna and soil‐surface effects from off‐ground monostatic radar two‐dimensional data. Firstly, antenna multiple reflections are removed using linear transfer functions. Secondly, simulated Green's functions accounting for the surface reflection are subtracted. These functions are derived using the relative dielectric permittivity of the surface, which is estimated by full‐wave inversion of the radar signal for measurements taken in local land‐mine‐free areas. Finally, the antenna radiation pattern effect is filtered out by performing phase‐shift migration, and information about size and shape is extracted. Data are acquired using a hand‐held vector network analyser connected to an off‐ground monostatic horn antenna. Typical Colombian targets such as low‐metallic anti‐personnel land‐mines and low‐ and non‐metallic improvised explosive devices are used. Results prove that the proposed technique effectively reduces clutter under non‐controlled conditions and yields target features that are useful for detection of these land‐mines.
GPR, VES and refraction seismic surveys in the karstic area “Spedicaturo” near Nociglia (Lecce, Italy)
Authors: Nuzzo Luigia , Leucci Giovanni , Negri Sergio
Published: August 2009

The present work shows some results of an early ground‐penetrating radar (GPR) study performed in a hazardous karstic area near the village of Nociglia in the central part of the Salento peninsula (Apulia, southern Italy), after a sudden subsidence event occurred on March 13th, 1996. The geophysical study, also involving vertical electric sounding (VES) and refraction seismics, is part of a multidisciplinary research project, aimed at determining the climatic, geological, morphological and hydrogeological conditions of the area in order to assess geological hazards and to prevent groundwater pollution due to agricultural activities. GPR transects led to the identification of some shallow subsurface structures including a known cave and probable karstified bedding joints, weakened shallow layers, fractures, fault zones, and burrows. The VES and seismic surveys resulted in a better understanding of the geology of the area and supported the interpretation of the GPR data.
Effective frequency and attenuation measurements of glass beads/magnetite mixtures by time‐domain reflectometry
Authors: Mattei E. , De Santis A. , Pettinelli E. , Vannaroni G.
Published: August 2009

Samples with different percentages (5–30%) of magnetite and different ranges of grain size are analysed by time‐domain reflectometry (TDR). The passband, the attenuation factor and the effective frequency are derived from the TDR measurements. The passband was obtained from the second edge of the sample by eliminating the DC component and applying a Fourier transform to the residual signal. The bandwidth of the resulting spectral shape decreases as the magnetite percentage increases. The second edge was also used to evaluate the effective attenuation factor , using multiple‐reflection theory, terminated at the second reflection. The attenuation values can be estimated independently by means of the electromagnetic parameters obtained using an LCR meter. To compare and , an effective TDR frequency is introduced. The agreement between the values obtained from TDR and LCR‐meter techniques supports the reliability of the attenuation derived from the second TDR reflection, and that computed from the effective frequency. The shapes of the passbands and the and values for magnetite/glass‐beads mixtures, which simulate dry soils with increasing iron oxides content, are reported and discussed.
On hydrogeological noise in superconducting gravimeter data
Authors: Hokkanen Tero , Virtanen Heikki , Pirttivaara Mika
Published: August 2009

The purpose of this research was to gain a better knowledge of the signals contained in superconducting gravimeter data at the Metsähovi station in southern Finland. Because superconducting gravimeter data are very sensitive to various environmental effects, we must first identify and understand the characteristics of these effects and only after that are we able to correct for them if necessary. In this study we used the continuous wavelet transform to analyse correlations between the gravity residuals, precipitation, and groundwater level. As a result, we determined and isolated the source of a long‐term effect, i.e. the varying flow rate of groundwater in an inhomogeneous ground. The causes of this flow‐rate variability are changes in the porosity of the ground due to changes in the type of medium, and the amount of fracturing in the bedrock. Topography is also a cause of the flow‐rate variation. Therefore, a single observation well to determine the groundwater level at the study site is not sufficient to provide coefficients that are precise enough to reduce the gravity effect due to variations in the groundwater level, and several access tubes will be needed to assess the correction coefficients accurately.