A numerical simulation of GPR output data has a theoretical importance to advance the GPR technology. A GPR numerical modeling is also a practical way to ensure more effective and adequate GPR data processing and interpretation. For example, the complex radargrams can be interpreted in the simplest case by matching computer simulated and field measured GPR data by visually or computer correlation procedure for target's signatures.
The consideration of the corresponding pairs of mutually inverse algorithms for a 2-D impulse GPR data processing and simulation are presented in this study. The first algorithms implements a basic physical principles on the wave propagation and scattering phenomena, in other words, this mathematical model governs the transformation of a 2D picture of subsurface media with imbedded targets and layers to a radar return. The second algorithm makes possible to ascertain real geometry of subsurface media some its physical properties by computer processing of the GPR reflection profiles. The first algorithms (FDTD, ray-based model, FK-modeling etc.) are the forward-modeling direct algorithms as the second one (TD (time domain) focusing, inverse ray-based model, FK-migration etc.) are the inverse algorithms in commonly used notation.
The TD, frequency domain (FD) and others techniques with uniform (velocity constant) non-disperse media with scalar transmitted and scattered fields for simplicity reason are considered. These techniques are preferable for near and middle range sounding when hidden objects must be searched. The TD and FD techniques demonstrate various properties on numerical efficiency of computer implementation, ability to be useful for some kind of task geometry and character of signature for specific target's shape, possibility to take into consideration of antenna patterns, signal waveforms, monostatic /bistatic mode operation etc. There are some important common appointed features and differences between the direct/inverse TD /FD models. In parallel with the matching of the FD and TD techniques the other possible approaches for the GPR data processing and simulation are discussed.
All algorithms based on the simple mathematical formalism were effectively implemented with computer programs written by the author. The field measured and the simulated radar data of the actual GPR surveys are presented and compared. Some of the presented results can be achieved from the formal analyze of mathematical properties of diffraction integrals, Fourier transform etc. But proposed considerations enables to understand the TD and FD processing/modeling problems in the simpler manner under control of physical intuition.
The three electromagnetic properties appearing in Maxwell's equations are the electric permittivity, the electric conductivity and the magnetic permeability. The study of point diffractors in an homogeneous, isotropic, linear medium (Saintenoy, 1998, Saintenoy and Tarantola, 1998) suggests the use of logarithms to describe the variations of electromagnetic parameters in the earth. A small anomaly in electric parameters (permittivity and conductivity) responds to an incidentel ectromagnetic field as an electric dipole whereas a small anomaly in magnetic parameter responds as a magnetic dipole. No parameter variation is negligible compared to the others. Furthermore, considering radiation patterns of the different diffraction points, differentiating electric and magnetic variations is not an easy task with surface Ground Penetrating Radar. But using an effective electromagnetic impedance and an effective electromagnetic velocity to describe a medium, their corresponding radiation patterns behave completely differently with the source-receiver offset. Zero-offset reflection data give a direct image of impedance variations with depth. Large offset reflection data contain information on velocity variations. On the other hand speaking about VRP (Vertical Radar Profile) data, full wave form inversion should best resolve the electromagnetic effective velocity contrasts.
Another interesting feature is that an anomaly in electricconductivity alone diffracts an electric field that is the first time derivative of the incident electric field. But the field diffracted by an anomaly in electric permittivity alone is the second time derivative of the incident electric field. This difference gives a way of intrinsic separation of those two properties in an inversion program. VRP data were acquired at the Boise State University (Idaho) site for hydro-geophysical researches with Mala borehole antenna 250 MHz and Sensor and Software borehole antenna 100 MHz.
Saintenoy A, Radar Geologique: acquisition de donnees multi-deport pour une mesure multi-parametres, pHd thesis paris 7, (1998).
Saintenoy A & Tarantola A, Geophysics, submitted, (1998).
Krzywa airbase is a former soviet military base, which is highly polluted due to soil contamination during the dismantling of fuel tanks. In the extreme cases, the pollution is so intense that the spilled fuel can be exploited in the system of drainage wells. In general, the contamination can be correlated with the geological structure. The geometrical features of the floating hydrocarbons on the water table can be identified and mapped with the ground penetrating radar (GPR) technique. Since aviation gasoline has a relative permittivity of 2 and water has a permittivity of 80, the detection is mainly based on the permittivity contrast between the hydrocarbon and ground-water saturated layers. Differences in conductivity constitutes an additional effect to aid in the interpretation of the radargrams. In this work, we evaluate the degree of hydrocarbon saturation that can be detected with the GPR technique and, in addition, determine the antenna frequency required to resolve the contaminated layer. The radar simulation is based on a pseudospectral forward modeling technique, and the model for computing the effective permittivities and conductivities of sand/clay mixtures is based on a self-similar theory for the sandy component and a transversely isotropic constitutive equation for the shaly component.
The integrated analysis of radar wave propagation velocity and instantaneous attributes (Amplitude, Phase and Frequency) of the radar trace was exploited in the high resolution study of several sites of archaeological and environmental interest. Radar wave velocities obtained from multi-fold 2-D gathers provided indications to correlate the imaged reflectors across 2-D grids. This is of use in mapping planar interfaces which are the characteristic target of geological and pedological studies. The capability of correlating reflectors across localized discontinuities is of major interest in areas of archaeological interest, where excavations or boreholes may not be available and not easily allowed, whereas ditches, brick-works and other features related to human activity may frequently interrupt pedological discontinuities. The tests performed indicate that velocities obtained from CMP gather analysis can be used to detect lateral variations of facies and water content, to identify buried targets which exhibit a low contrast of electrical properties in environments characterized by high energy background noise, to correlate reflectors across faults, major fractures or man made interruptions. Further information is obtained by means of 3-D acquisition and velocity analysis techniques. We use a CMP acquisition scheme iterated along four different azimuths thus achieving a multi-fold subsurface coverage with 45° azimuthal spacing. The records thus obtained allow to perform azimuthal velocity analysis and identify azimuthal variations which are of use in the identification of localized targets and volumes characterized by peculiar anisotropic properties. Complex trace analysis was used to study lateral changes of the radar response due to localised targets and time-varying discontinuities due to the diffusion of liquid contaminants in the subsurface. The analysis was performed on pre-stack gathers with minimum processing. Envelope, instantaneous frequency and phase were calculated on stack traces obtained summing different offset intervals at near, median and far offset. Envelope analysis was exploited to study the inner structure of funerary platforms and burial sites and provided good results at several locations where the features of interest exhibited a hardly detectable response on conventional records. Further tests where carried out to study the stratigraphy of waste disposal sites and the diffusion of liquid contaminants in the subsurface. In such cases, the response of the envelope at different offset ranges brings additional information to that provided by conventional and stack images and is comparatively more reliable than the instantaneous phase and frequency. Examples include detection of targets of archaeological interest, mapping of paleosols in archaeological sites, study of fractured rocks, study of waste disposal and contaminated sites.
Depending on the geological setting, ground-penetrating radar (GPR) surveys are nonintrusive, rapid, reliable, and cost effective for mapping shallow subsurface sediments, hydrogeologic conditions, and potential geologic hazards. In particular, GPR data gathered at sites in central Utah and northern Arizona have been very helpful in mapping soil and groundwater contamination produced by leaking undergouund fuel tanks. Analysis of monitoring well data shows that assessments of the contamination inferred from the GPR data are in good agreement with the depths and contamination levels found in the wells. GPR data collected along the Wasatch fault zone in central Utah also demonstrate the reliable detection of shallow concealed faulting in appropriate geologic settings. Interpretations of the GPR data sets coincide with many features observed in recently-opened, nearby trenches. The net offsets and dips of the faults correlate well between the GPR data, other geophysical data, and existing geotechnical data. GPR can also be successful in detecting near-surface cavities. GPR data collected in central Utah have provided useful information about the location and approximate depths of apparent cavities and associated areas of subsidence beneath highways and other structures. Procedures can then be implemented to help protect huyman life and produce minimal damage to building structures and to the environment.
This paper is devoted to a microwave sounding technique for Non Destructive Testing (NDT) in building engineering. This technique uses advanced GPR technology for investigation of invisible internal structure for some complex reconstruction building projects in the Ukraine during the past 1997 and the current 1998. There were the reconstruction projects of the Odessa's State Opera Theater and the Kiev's Olympics National Stadium, the rebuilding projects of the Kiev's two main church cathedrals and others. The GPR survey allowed to overcome the lack of exact plans and maps caused by the 1917 revolution and the World War II destructive events.
The GPR sounding gives following features: the depth, span and size of fundaments; the thickness of wall, ceiling and floor; the presence of wire mesh and reinforcement in concrete, walls, floors etc.; the presence and location of various underground and internal hidden objects etc. Several novel engineering and data processing approaches were developed and implemented for internal structure imaging of walls, floors, ceilings, columns, pillars, foundations, underground cavities, tunnels and other buried construction structures and their elements. Detecting of humidity areas and leakage from underground water-transport systems under buildings was important solved task.
Complexity associated with multiple subsurface objects on the place of old churches, monasteries and others places with long historical period of occupation requires more sounding with various parameters and additional data processing for accurate identification of radar returns. The presence of superimposed archeological objects was also important problem for most of the building and reconstruction projects in the oldest city like Kiev. The frequency band of GPR sounding used in this investigation was from 100 MHz up to 4 GHz.
Also many interference signals and artifacts must be remove from GPR data which are connecting with air and surface wave propagation under conditions of limited area and volume of survey sites. Some of artifacts caused by the effect of surface wave excitation and propagation were used to delineate building's foundation performances. Presented studies indicate that GPR has potential in solving the problem of building engineering as a powerful non-invasive tool.
Pulse UHF range ground penetrating radar was used during tunneling in Moscow for advance detection of water-filled cavities, quicksands and other hydrogeological objects in heavily absorbing moist clayey ground. Regular measurements (2-5 times a week) were carried out on the surface of tunnel front wall (4 m in diameter) during service stops of tunneling machine. Each time 250-300 scans were recorded in the nodes of 2-dimensional grid with 0.1 m step. Subsequent data processing using original software allowed us to obtain 3-dimensional distribution of objects reflecting radiowaves up to depths of 2-3 m. The processing included correlation analysis of signals and aperture synthesis.
The radar survey was done in the immediate vicinity (1-2 m) of the tunneling machine. Radar slot antennas (resonator size 0.4 m x 0.4 m x 0.1 m) were developed in Moscow Institue of Physics and Technology, their characteristics are given in (Druchinin, 1998) and (Chubinsky et al, 1998). These antennas have very low level of radiation into the air. In spite of this fact, reflections from metallic parts of mechanisms caused serious interference to the radar operation. These reflections were especially strong when the spacing between the antenna apertures and the ground was not small enough due to roughness of the ground surface. Therefore additional shielding of antennas was required.
A series of experiments with radio absorbing material sheets covering antennas partially or completely in several combinations was carried out. A thin layer (3 cm) of the material was placed between the antenna apertures and ground surface. Antenna resonator tops were covered with 80 cm x 60 cm x 5 cm sheets of the material, and blocks of the material were attached to side walls of resonators in E-plane. Conductivity of the material was about 0.015 Sm/m.
The analysis of the results obtained allowed us to define optimal designof the resistive coating of the slot antenna. It included combination of sheet under antenna apertures and covers for resonator tops. Improvements due to use of side blocks were not big enough to compensate for considerable increase of antenna dimension (doubled) and difficulties in antenna handling in tunnel. As a result, the amplitude of interference signals was decreased by a factor of 2-4.
Additional shielding increased damping coefficient of own oscillations of the antennas and shifted down operation frequencies. Use of absorbing sheets between antenna apertures and ground surface smoothed signal variations due to surface roughness and helped to get rid of false objects near the surface revealed during processing.
Druchinin SV, 7th International Conference on Ground Penetrating Radar, Lawrence, KS, USA, 2, 643-648, (1998).
Chubinsky NP, Chernokalov AG, Druchinin SV, Krampuls AY, 4th Meeting of EEGS, European Section, Barcelona, Spain, 789-792, (1998).
This paper presents preliminary results of investigation in a non-invasive manner with GPR conducted by authors during the spring-summer period of the 1998 in the famous and oldest monastery of east Christian church, the Kiev's Petherskaja Lavra, Kiev, Ukraine. The investigation place with long prehistoric and historic occupations contains abundant variety of subsurface non-documented archeological galleries, tunnels, subway, caves which were made and used during the near 1000 years period from AD 800-900 till 1800-1900. There are also many graves, crypts, ossuaries etc. Any exact and complete plans, schemas, cartography and maps of this complex subsurface region are absent due to numerous wars and other destructive historical events. Only about of 10% or less of total old subsurface infrastructure are known and used partially at present.
Other non-destructive techniques and geophysical methods for underground investigation tested in this place were not useful here due to the site's tremendous complexity. But this territory is not simple site for the GPR survey also. Variety approaches, data collected strategies and processing procedures were tested to obtain valuable sounding information on subsurface objects, including using of well-mapped objects to get reference signatures. GPR return profiles were widely modeled and integrated in GPR data processing procedures to better reveal information on subsurface phenomena and identification of buried man-made structures. The 3-D computer processing and interpretation techniques were used for reconstructing the complex shape of underground objects.
Many buried structures were located and 3-D mapped as results of this investigation. The presented data has preliminary character due to large total area of investigated site and author's common work with historians and archeologists is continuing at present. The radar data at survey site indicated the presence of variety of subsurface objects would be presented in this paper. The presented studies have clearly demonstrated that GPR survey has potential in solving the complex problem of this complex site.
Rushen Abbey, Isle of Man, is the site of a Cistercian monastery that was originally founded as a Savignian community in 1134. The abbey was in use until 1540 and some elements of the physical structures still remain. Parts of the site have been excavated during the past century and have confirmed some of the supposed layout of the monastery. Manx National Heritage is currently overseeing a programme of archaeological excavation and restoration of the site which will open to the public in the year 2000. As part of this project geophysical investigations have been undertaken over large parts of the site by GSB Prospection.
Conventional fluxgate gradiometry and twin probe resistance have been successfully carried out over the lawned areas. However, large portions of the site are under tarmac or paved and in these areas GPR has been employed. A GSSI SIR 2 system was used with 300 MHz and 500 MHz antennae. Many of the features identified by the GPR survey were excavated in the summer of 1998. This paper will discuss the usefulness of the technique in such conditions, and its limitations, based on the excavation results.
Ground penetrating radar is a powerful tool to perform studies on cultural heritage buildings and historical remains because of its non destructive applications and its high resolution. We performed two GPR surveys on a roman theater restored by using modern tecniques and materials and under an old stone gothic bridge where a main sewer was designed. In the first case, we used 900 MHz and 500 MHz center frequency antennas; the purpose of the study was to investigate the contact between the remains and the new used materials in order to detect damages in the old stones caused by the restoration actions in the theater. We were able to determine the dielectric parameters of the involved materials (old and new) because in some areas of the theater the size and the surface between the old and new construction were visible. In this way we were able to characterize the location and dimensions of the investigated anomalies. In the second case, we used 100 MHz and 500 MHz centre frecuency antennas; the main purpose was to determine the quality and geometry of the bridge foundations in order to avoid that the construction of the main sewer caused damage in the bridge. We were able to detect wood into the foundations, several trunks had been inserted between ground and the first stones used in its foundation.
Some dificulties found during these two surveys and in the interpretation of the obtained records will be discussed. The main goals achieved in this work will be presented.
The island of Thera has a present-day landscape that results directly from the Late-Bronze-Age (LBA) eruption of Santorini volcano. The cataclysmic Plinian eruption created a 70 - 80 km2 caldera and blanketed the ancient Minoan living surface with 10's of metres of pyroclastic fall and flow deposits. Previous workers have established a stratigraphy of 4 mappable pyroclastic units (Phases 1-4). These deposits drape and obscure most of the LBA living surface but, except along the crater wall or along the coast, are relatively poorly exposed. We have used GPR to map these volcanic deposits in the subsurface. Individual volcanic deposits can show strong facies variations and have distributions that are heavily influenced by paleotopography, however they are amenable to study by GPR, because they tend to be relatively thin (<50 m) and they are electrically resistive. Both attributes allow radar to probe the entire deposit.
We present data for GPR traverses across the Akrotiri peninsula collected via a pulse EKKO 100 radar system (Sensors & Software Inc.) using a 50 MHz antennae and 1000 V transmitter. All surveys used a 1 m station spacing and a fixed, 3 m antennae separation. Data were collected using time windows of 512 - 900 ns and were stacked 64 to 256 times. Our survey of the Akrotiri peninsula includes a 900 m traverse run parallel to the southern caldera wall, and 2 traverses which were oriented radially to the caldera. The latter are: i) a 600 m traverse on the eastern edge of the Akrotiri excavation site, and ii) a parallel, 1500 m traverse situated approximately 800 m further east. Our radargrams define structures as deep as 18 - 20 m (0.1 m/ns) and can accurately trace the thickness of the LBA deposits from the caldera wall to Thera's southern coast. Furthermore, our best datasets suggest that the Phase 1 fall deposits can be differentiated from the Phase 2-4 deposits. These results allow for refined estimates of eruption volumes, accurate tracking of facies variations in the LBA deposits and better tracing of individual deposits. In several instances, we clearly image the interface between the volcanic deposits and the underlying LBA living surface. Consequently, our radargrams delineate lows and valleys associated with paleo-streams or drainages. The shape and size of the LBA Thera landmass also remains a contentious and important issue; our data suggest that GPR could serve to refine the position and nature of the paleo-shore line on the Akrotiri peninsula or help define the ancient Minoan harbour.
Surface Penetrating Radar (SPR) is a high frequency (from 10 MHz to 1 GHz) electromagnetic (EM) method that provides high-resolution images of near-surface structure. The depth of investigation depends on the frequency used and on the medium crossed by EM waves, and for low-loss geological materials it does not exceed 50 m. SPR has been used for a variety of tasks like finding ice or permafrost thicknesses, detecting subsurface cavities, buried channels and tunnels, mapping contamination plumes, detecting buried objects in archaeological surveys, determining the thicknesses of soil horizons and depth to water table, etc. In some cases, SPR data show strong diffraction hyperbolae due to objects on the ground as well as some clutter noise (continuous flat reflections) caused by breakthrough between the antennas and by multiple reflections between the antenna and the ground surface. We try to image by SPR the geometry of Quaternary sedimentary deposits and identify possible recently active faults hidden under a vegetal cover. The purpose of this presentation is to show some case histories of the SPR surveys acquired over Quaternary sedimentary deposits. The first example, from the Rhine valley, shows how SPR data can be perturbated by diffractions from surface objects. A method of modeling and filtering of these diffractions gives a clear image of a paleo-scarp as well as some progradation structures. The second example, from New Zealand, shows the importance of the clutter filtering in order to produce an image of the SPR that can readily be interpreted by the geologists.
Radar RAMAC2/GPR profiling with 50 MHz, 100 MHz and 200 MHz antenas are made in different geological conditions in open-cast mines, quarrys, unerground mines, on the valley terasses and on the embankments (dam, track-way).
During the first stage the mesurements are made in the bigest open-cast mines and quarrys in Poland: Brown-coal mine "Belchatów", Clay mine "Jaroszów", Limestons quarry, "Dzialoszyn", Limestons quarry "Tarnów Opolski", Limestons quarry "Góra~dze", Oranites quarry "Gniewków", Sands & grawels pit "Obora - KGHM Polska Miedz' S.A.". In this mines, after photographical documentations and geological mapping of the escapments, there are made radar proffies along this escarpments but about 20 m behind. The depth scale of this profiles are compared by next geological mapping of escarpments after mining excavation, too. On the radar profiles are collected the pictures of paleovalleys (sands infiling erosion form in till), bottom of the brown-coal deposits (contact brown-coal and underleying sands), layers of tills in the sands, infiled fossil sink-holes in limestones, caves, fissures and faults. Also there are mesured the waethering covers on the limestones and on the granites and glacitectonic deformations of tertiary and quaternary deposits, too.
During the second stage of the mesuring and collection of the geological structures on the radar profiles, the depth scale of the radar profiles are compared by bore-holes. On this same mines and in underground copper mines Polkowice-Sieroszowice there are made radar profiles for geological forecast of the exploatation. The continuous radar profiles to enable good recognizing of the sedimentological structures and its deformations. Radar profiling in the valley give good informations about structures od terrases. Radar monitoring of the dam in Odra River valley after flood 1997 enabled quickly informations about the places of structures destruction inside this dam. Radar monitoring of the track-way embankments enabled quickly informations about the thicknes of the breakstone below sleepers. Now there are made the experiments on the different archeological sites.
On the radar profiles the geological structures are identificated up to 80 m deep in salt-anhydrite deposits, up to 20 - 50 m in limestones and granites, and up to 10 m in sands, clays and silts deposits.
The Kobe (1995 Hyogoken-Nanbu) Earthquake with a magnitude of 7.2 hit big cities between Kobe and Osaka. After the earthquake, we examined subsurface structures in the Kobe urban areas, especially those of strong ground damages during the earthquake using the ground-penetrating radar (GPR) method. The SIR-2 system (GSSI) was employed using 200 MHz and 100 MHz antennas. The GPR data are processed by horizontal high pass filtering and by horizontal and vertical low pass filtering. Data were collected on 23 survey lines (whole line; about 4200 m). The geology of the Rokko Mountains is chiefly Cretaceous granites, while the urban area is composed mainly of Plio-Pleistocene and Holocene sediments (about 1,500 m thick). The Gosukebashi fault is one of obvious active faults in the Rokko Mountains. The following results were obtained: (1) Detected anomaly, showing discontinuity or flexure, was found in a subsurface sediments of 10 m wide: The discontinuity was found in the thin Late Pleistocene strata, overlying the granites, while the flexure was developed in the thick Holocene strata. (2) The vertical displacement, estimated from the flexure, in the Holocene strata is about 80 cm. (3) This detected anomaly is located along the southwestward extension of the pre-exiting Goshukebashi fault in the Rokko Mountains. (4) The site of the discontinuity and the flexure of subsurface sediments on the GPR profiles coincides with that of the concealed fault, which has already found by the seismic reflection survey, in the basement granites beneath the Kobe area. Judging from these results, we conclude that the Gosukebashi fault extends to the urban area. The discontinuity or flexure in the subsurface strata was probably caused by fault movement on the buried Gosukebashi fault beneath the Kobe area. The existence of the vertical displacement is in harmony with the distribution of small-scale slopes in the urban area. Furthermore, the distribution of the detected anomaly is also in harmony with that of strong ground damages during the earthquake. Therefore, the GPR method is thought to be useful for study of buried active fault in the urban area.
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