bluuebunny/crossref_metadata_2025_split huggingface dataset:
Geophysical surveying an active well catchment in the presence of significant topography and anthropogenic disturbances
Authors: Paasche Hendrik , Tronicke Jens , Maurer Hansruedi , Green Alan G. , Auken Esben , Stauffer Fritz 
Published: August 2009

ABSTRACTTraditional approaches to surveying well catchments usually involve investigations at two quite different spatial scales: very high resolution borehole‐based studies and low resolution tracer and pump tests. In recent years, various researchers have attempted to fill the wide gap between the two scales of acquired data by using a variety of geophysical tools that have high‐to‐medium resolution capabilities. Although geophysical surveying may be relatively straightforward at many locations, there are numerous regions where topography, the presence of ubiquitous metal objects and/or a lack of suitably placed boreholes cause the acquisition, processing, inversion and interpretation of certain types of geophysical data to be extremely challenging. It was necessary for us to address such problems in an investigation of an active well catchment near Zurich, Switzerland. The principal goals of our project were to determine the geometries and physical properties of the shallow sedimentary layers, with emphasis on a key water‐bearing gravel unit. To achieve these goals, four different geophysical methods were employed. In a first step, frequency‐domain electromagnetic measurements allowed the locations of buried metal pipes to be established. A 2D tomographic seismic refraction survey then provided information on the interface between the low‐velocity surface loamy topsoil and underlying high‐velocity morainal material. The high electrical resistivity of the important gravel unit allowed its upper boundary to be outlined in 2D models derived from geoelectric data and its lower boundary to be delineated in models based on 1D linked inversions of geoelectric and transient electromagnetic data.
Plus�minus method to solve large�amplitude near�surface static corrections
Authors: Bridle Ralph 
Published: August 2009

ABSTRACTThe surface conditions in Saudi Arabia demonstrate significant variability ranging from flat gravel plains to sand�dunes,sabkhas(salt flats) and desertwadis(washouts). Saudi Aramco has used a single�layer velocity model from surface to datum for most of the prospective areas for both 2D and 3D seismic data. Source gathers have been interpreted by picking the first arrivals to obtain a linear fit for the direct and refractor arrivals using the intercept�time method. This creates depth and velocity control input for a multiple‐layer model of the near‐surface, from which static corrections to datum are calculated. On 2D lines with very large amplitude near‐surface anomalies, the plusminus technique has been applied to solve local areas. The implementation of the plus‐minus technique utilizes a variable near‐surface velocity derived from either upholes or the direct‐arrival velocity determined from source gathers. With this method, multiple source‐pairs build up effective spreads. Then by compiling high fold of delay‐times at a location, a statistical average is taken where the high fold reduces the significance of picking errors. The model is built utilizing multiple refraction layers and is referenced to the datum, thus tying it to the regional model.The example 2D seismic line has a static model calculated from the single‐layer velocity model. The intercept‐time method was applied locally over a near‐surface anomaly caused by unconsoli‐dated material at the foot of a cliff. Further analysis of the near‐surface involved the plus‐minus method to solve the high amplitude time shifts.
Efficient ERT measuring and inversion strategies for 3D imaging of buried antiquities
Authors: Papadopoulos N.G. , Tsourlos P. , Tsokas G.N. , Sarris A. 
Published: August 2009

ABSTRACTThe optimum processing technique (2D vs. 3D inversion) to interpret and visualize parallel and/or orthogonal two‐dimensional surface Electrical Resistivity Tomography data collected from archaeological sites is examined in this work. A simple modification of a standard resistance‐meter geophysical instrument was implemented in order to collect parallel two‐dimensional sections along the X‐, Y‐ or XY‐direction in a relatively short time, employing a pole–pole array.The sensitivity analysis showed that the distance between the parallel 2D lines must be smaller or, at the most, equal to the basic inter‐electrode spacing in order to produce reliable 3D resistivity images of the subsurface. This was confirmed by modelling and inversion of both synthetic and real data.Direct comparisons of the quasi‐3D images, resulting from a posteriori combination of the inverted 2D sections, with the full 3D inverted resistivity models indicated the superiority of the 3D inversion algorithm in the reconstruction of buried archaeological structures, even in complex archaeological sites. Due to the inherent three‐dimensionality of many archaeological targets, quasi‐3D images suffer from artefacts. The combination of a single survey‐direction with a full 3D processing and interpretation scheme is adequate to image the 3D subsurface resistivity variation in detail. Furthermore, the implementation of a quasi‐Newton Jacobian matrix update technique reduced the processing time by one‐half without any significant loss of accuracy and resolution.
GPR surveying over burial mounds: correcting for topography and the tilt of the GPR antenna
Authors: Goodman Dean , Hongo Hiromichi , Higashi Noriaki , Inaoka Hiromichi , Nishimura Yasushi 
Published: August 2009

ABSTRACTA new method for static corrections of radargrams, which accounts for the tilt that the GPR antenna encounters on sites with topography, has been developed. The method uses the average electromagnetic wave velocity and assumed antenna tilt according to the first derivative of the topography profile, in order to adjust the normal ray projecting from the GPR antenna into its correct subsurface location. Static tilt corrections for radargrams with under sampled scan locations can be effectively adjusted using scan sweeping across small angles. Radargrams that are corrected for topography and antenna tilt show that changes in the imaged locations of subsurface structures can be significant, resulting in improved focusing and more accurate positioning of reflections.
Extended comparison of different GPR systems and antenna configurations at the Roman site of Carnuntum
Authors: Seren Sirri , Eder‐Hinterleitner Alois , Neubauer Wolfgang , Löcker Klaus , Melichar Peter 
Published: August 2009

ABSTRACTTo evaluate the suitability of ground‐penetrating radar (GPR) systems for archaeological prospecting, three different GPR systems with several antenna configurations were tested on one test area at the Roman site of Carnuntum, under identical environmental conditions, in 2004. Due to developments in hardware and software within the last year, these measurements where repeated and compared with the previous results. The data processing considers the needs of the different systems and antennas and aims to produce depth slices for best interpretation from an archaeological point of view. The results show advantages and disadvantages of the different systems and antennas, the need to process the data differently and the necessity to increase the power of the antennas.
Verification and topographic correction of GPR data in three dimensions
Authors: Leckebusch Jürg , Rychener Jürg 
Published: August 2009

ABSTRACTTwo sites in a Roman town allowed a direct comparison between GPR interpretation and excavation results in three dimensions. All the reflection features mapped by the geophysical survey were identified in the subsequent archaeological excavation. Some of these anomalies were shown to be walls preserved as a single layer of stone or completely robbed out. However, a number of walls that were later excavated were either not detected at all by the GPR or not imaged to their full depth. The reason for this is the limited penetration depth of the radar energy due to attenuation, which can be visually estimated from the data. In addition, the presentation of the GPR results was enhanced when the depth information was coded into the map of the interpretation. Finally a new processing step called topographic correction was developed to compensate for the antenna tilt and elevation in three dimensions. The differences and advantages of this method compared to 2D tilt corrections are discussed. Apart from migration, this topographic correction significantly improves the geometrical 3D accuracy and interpretation of the GPR data.
Estimating subsurface porosity and salt loads using airborne geophysical data
Authors: Ley‐Cooper Yusen , Macnae James , Tweed Sarah 
Published: August 2009

ABSTRACTWe aim to map dissolved salts in aquifers which, with rising water tables, are a threat to fresh water resources and ecosystem health. Interpolated groundwater electrical conductivity (EC) maps from borehole measurements are combined with spatially detailed bulk electrical conductivities estimated from airborne electromagnetics to provide, through Archie’s law, a detailed prediction of subsurface porosity and total dissolved salt load of any near‐surface aquifer. This technique quantitatively integrates airborne electromagnetic and groundwater measurements of EC to provide the subsurface detail necessary for hydraulic properties. The process has been applied in Australia’s Murray River Basin, where the presence of salt is a threat to agriculture and fresh water supplies. Porosity predictions range between 1% and 50% and are spatially consistent with known geology, dissolved salt loads are estimated to be between 100 and 4000 tonnes per hectare. The predictions are yet to be compared to in‐situ measurements but if proven, will be a useful tool when in‐situ porosity is under‐sampled or is not available.
Application of tensorial electrical resistivity mapping to archaeological prospection
Authors: Varga Mihály , Novák Attila , Szarka László 
Published: August 2009

ABSTRACTAt an archaeological site (Pilisszentkereszt Cistercian Monastery, Hungary) we carried out 3D tensorial geoelectric mapping measurements. We applied the well‐known tensorial form of Ohm’s differential law, where a 2 × 2 resistivity tensor relates to the horizontal current density vector and the corresponding electric field vector. In the DC apparent resistivity tensor there are three independent rotational invariants, and we defined two alternative sets. In the field, the current electrode distance was fixed to 15 m; two perpendicular AB directions were used, and 16×15=240 potential electrodes (with an equidistant space of ) were put in the central (nearly square, 7.5 m × 7 m) area between the current electrodes. Due to a four‐channel measuring system, it was possible to determine both components of a horizontal electric vector at the same time. The time needed to measure all potential differences between the neighbouring potential electrodes (thus to obtain 15×14=210 resistivity tensors), was about 40 minutes. So far, about 239 such maps have been completed, thus we have more than fifty thousand resistivity tensors around the monastery. The tensorial results are shown together with the results of traditional measurements. In field conditions, any resistivity estimation provides reliable information about the subsurface (both the tensor invariants and the traditional mean values). At the same time, the multidimensional (2D and 3D) indicators proved to be informative only in case of significant subsurface inhomogeneities.
Electrical strike imaging and anisotropy diagnosis from surface resistivity measurements
Authors: Senos Matias M.J. 
Published: August 2009

ABSTRACTField resistivity measurements are often affected by the inhomogeneity and anisotropy of subsurface formations. The crossed square array is an electrode array particularly suited to investigating these problems. Herein, its use to obtain concealed formations electrical strike pseudosections will be shown. Field and 2D data modelling will be discussed and images of computed strike in depth and space will be presented and interpreted. Data modelling was carried out by using theoretical solutions based on the application of image theory to simple 2D outcropping models. More complex 2D concealed model solutions were obtained by physical analogue modelling. Furthermore, it has also been difficult to distinguish ground inhomogeneity and lateral resistivity variations from anisotropy in field data. Hence, it will also be shown that with crossed square array data anisotropy analysis it is possible to distinguish fracture anisotropy and structural anisotropy from media that conform closely to real anisotropy situations.
GPR multi‐component data analysis
Authors: Capizzi P. , Cosentino P.L. 
Published: August 2009

ABSTRACTIn order to test the capabilities of ground‐penetrating radar (GPR) detection, models were reproduced in a laboratory using both plastic and metal cylinders as well as sheets to represent anomalies buried in a plastic tank containing sand. These models, which represent pipes and dipping layers in a homogeneous medium (i.e., sand), were used to detect the object’s position and study its polarization properties based on the different dielectric contrasts between the object and the surrounding medium. We paid particular attention to the choice of medium through which the wavefield propagated, to be sure of its isotropy and homogeneity, in order for the velocity to be the same for every path travelled.Experimental tests were carried out by changing the dielectric contrasts from the target to the surrounding medium. In particular, the dielectric cylinders (which have a lower permittivity than that of the surrounding sand) were filled with different substances to represent targets such as PVC pipes, either empty or filled with hydrocarbons or water. The resulting scattered field depended on the electrical properties of both the cylinder and the surrounding material, the distance from the cylinder and the scattering angle.Further experimental tests were carried out by using multi‐component GPR data to study the possibility of calculating the angle between the direction of the profile and that of the anomalies. The experimental data obtained as responses to the physical models proved that this method can be successfully applied to detect the orientation of dipping layers, cylinders and vertical fractures.
Two‐dimensional inverse filtering for the rectification of the magnetic gradiometry signal
Authors: Tassis G.A. , Hansen R.O. , Tsokas G.N. , Papazachos C.B. , Tsourlos P.I. 
Published: August 2009

ABSTRACTThe magnetic difference, the quantity measured by magnetic gradiometers, is considered to be the convolution between a function that controls the anomaly pattern and another that controls the strength signal. These are called shape and amplitude functions, respectively. They are distinct and analytically determined; thus, after the assessment of a suitable model, its shape function can be inverted to serve as a filter. If this filter is next convolved with the measured field, a series of amplitude functions is recovered, provided that the subsurface structures can be simulated by a combination of a number of models similar to the one whose anomaly was inverted. The recovered series is essentially the subsurface distribution of the amplitude function.Alternatively, the scheme can be viewed as a transformation of the original field of magnetic differences. The signals, transformed in this context, comprise rectified monopolar versions of the originals, positioned directly above the centre of the targets. Furthermore, their amplitude is a measure of the magnetization of the targets. Thus, the new anomalies can be viewed as a kind of magnetization or susceptibility mapping. Inversion filters are computed in the Wiener mode by inverting the shape function of the anomalies caused by vertical sided finite prisms. This particular model is appropriate for a wide variety of targets that are commonly met in archaeological prospecting. Converting the magnetic signal from the dual lobe pattern to single monopolar anomalies cause them to resemble the usual outcome of resistivity exploration. In this context, this work aims to form a scheme that makes the magnetic gradiometry outcome directly comparable to the outcome of resistivity mapping. Furthermore, it can be applied without contradicting or excluding any other processing operation.The method is implemented in a FORTRAN program that is reasonably user‐ friendly. The efficiency of the scheme is tested both on synthetic and real data sets.
Inversion stability analysis of multimode Rayleigh‐wave dispersion curves using low‐velocity‐layer models
Authors: Liang Qing , Chen Chao , Zeng Chong , Luo Yinhe , Xu Yixian 
Published: August 2009

ABSTRACTSurface wave inversion is increasingly applied to estimate near‐surface shear (S)‐wave velocities for geological structures. Our study addresses the sensitivity and stability of the Rayleigh‐wave multimode dispersion‐curve inversion for earth models containing a low‐velocity layer. Due to the dependence of Rayleigh‐waves on the S‐wave velocity over a range of depth, the inverted generally have uncertainties that vary with depth and structure. An evolutionary algorithm was used to provide a population of final models from inversion of multimode Rayleigh‐wave dispersion curves. We analysed the uncertainties of inversion results for three irregular structures, i.e., three models with a low S‐wave velocity layer.The results show that 1) because the low‐velocity layer traps the energy of Rayleigh‐waves and makes the wave travel within it, Rayleigh‐wave phase velocities are insensitive to variations in layers beneath a low‐velocity layer. This characteristic can influence the inversion stability for these layers’ parameters. 2) The high degrees of uncertainties of inverted for these layers still remain although the higher mode Rayleigh‐wave data are included in the inversion procedure. It can be concluded that estimates for layers beneath a low‐velocity layer are with a low degree of confidence and need to be treated with extra caution.
The very low‐frequency step‐frequency GPR system and its application to active fault detection
Authors: Zeng Zhaofa , Huang Ling , Liu Sixin , Liu Fengshan , Sun Jiguang 
Published: August 2009

ABSTRACTGround‐penetrating radar (GPR) in the high‐frequency range is subject to attenuation, which limits its application in many fields, such as underground water detection and other near‐surface geological studies. We built a very low‐frequency GPR system based on a vector network analyser (VNA) and developed the system controlling software using visual basic for application (VBA). We also built a wire‐line antenna with a central frequency of 10 MHz on the ground. Results from an experiment on a water reservoir dam showed that the system works well.Active fault evaluation is important for city planning and construction. The soil layer in Changchun City, Northeast China, has low‐resistivity and a depth of 10–20 metres. It is difficult for conventional GPR systems to detect bedrock in Changchun City. In this paper, we investigate the active fault in Changchun City using the very low‐frequency step‐frequency GPR system. The results from the step‐frequency GPR shows clearly the undulation of the bedrock. From this and the resistivity soundings we are able to interpret the location and the depth of the fault.
Vertical radar profiling for the assessment of landfill capping effectiveness
Authors: Cassiani Giorgio , Fusi Nicoletta , Susanni Daniele , Deiana Rita 
Published: August 2009

ABSTRACTIn this paper we present the results of the characterization of a large landfill cap by means of ground‐penetrating radar (GPR) measurements. The GPR data were collected in boreholes, using a vertical radar profile (VRP) configuration, where one antenna was kept at the ground surface while the other was progressively lowered into the borehole. This yields a vertical profile of GPR velocity from which a moisture content profile can be obtained. VRPs were conducted in 15 boreholes available on‐site, all having been drilled through the protective cap and the waste mass into the underlying native soil. The separation between the boreholes (many tens of metres) makes it infeasible to characterize the site via other forms of hole‐to‐hole GPR measurements, with the exception of a few pairs of holes drilled at the periphery of the waste mass and therefore of limited usefulness. The VRP data allowed for the characterization of the moisture content profile across the waste body down to the water table, very close to the natural land surface, providing evidence that the waste is generally fairly dry (moisture content less than 10% on average). In order to assess the effectiveness of the cap, two surveys where conducted in March and April 2005 using all the boreholes with the aim of identifying moisture content changes due to natural rainfall and especially artificial irrigation over a limited area surrounding one of the boreholes. The time‐lapse VRP results show that in the irrigated area a measurable amount of water seeps through the cap and changes the waste moisture content. Elsewhere, we basically observed no changes in moisture content due to natural infiltration during the same period: this fact does not confirm or exclude that some degree of leakage can occur. Direct in‐situ permeability measurements, albeit more localized than VRP data, confirm that the landfill cap is not totally impermeable, thus corroborating the results derived from geophysical measurements.
Geophysical characterization of derelict coalmine workings and mineshaft detection: a case study from Shrewsbury, United Kingdom
Authors: Pringle J.K. , Stimpson I.G. , Toon S.M. , Caunt S. , Lane V.S. , Husband C.R. , Jones G.M. , Cassidy N.J. , Styles P. 
Published: August 2009

ABSTRACTA study site of derelict coalmine workings near Shrewsbury, United Kingdom was the focus for multi‐phase, near‐surface geophysical investigations. Investigation objectives were: 1) site characterization for remaining relict infrastructure foundations, 2) locate an abandoned coalmine shaft, 3) determine if the shaft was open, filled or partially filled and 4) determine if the shaft was capped (and if possible characterize the capping material).Phase one included a desktop study and 3D microgravity modelling of the relict coalmine shaft thought to be on site. In phase two, electrical and electromagnetic surveys to determine site resistivity and conductivity were acquired together with fluxgate gradiometry and an initial microgravity survey. Phase three targeted the phase two geophysical anomalies and acquired high‐resolution self potential and ground penetrating radar datasets. The phased‐survey approach minimised site activity and survey costs.Geophysical results were compared and interpreted to characterize the site, the microgravity models were used to validate interpretations. Relict buildings, railway track remains with associated gravel and a partially filled coalmine shaft were located. Microgravity proved optimal to locate the mineshaft with radar profiles showing ‘side‐swipe’ effects from the mineshaft that did not directly underlie survey lines.Geophysical interpretations were then verified with subsequent geotechnical intrusive investigations. Comparisons of historical map records with intrusive geotechnical site investigations show care must be taken using map data alone, as the latter mineshaft locations was found to be inaccurate.