GPR Sections Research Articles

Integrated GPR analysis and numerical modelling for discovering archaeological mysteries at Kom Ombo temple

ABSTRACT Archaeological research is made accurate and detailed by non-invasive geophysical techniques. Hence, the thorough utilisation of GPR data sets alongside numerical modelling is imperative for reducing uncertainty surrounding archaeological sites. This study aims to assess the practicality and efficacy of GPR numerical modelling in analysing concealed archaeological elements within the renowned Kom Ombo temple in southern Egypt. Using 80 MHz and 270 MHz antennas to detect any buried structures on the temple grounds. From the processed 2D GPR sections, two distinct anomalies (deep and shallow) were identified, possibly indicating hidden archaeological structures. Given the likelihood of these features possessing complex geometries, four synthetic models were devised, maintaining consistent electrical properties while varying in buried body geometry, to evaluate the GPR response. Convolution perfectly matched layer (CPML) absorbing media was designed to absorb waves within the modelling grid, alongside a transverse magnetic (TM) mode formulation, were utilised. The findings suggest that the deep anomaly could potentially represent a tunnel or significant geological formation, while the shallow anomaly may correspond to an extension of the Turkish fort. Overall, numerical modelling emerges as an enhanced mapping tool for uncovering hidden archaeological features in areas worldwide where excavation via traditional methods is not feasible.

Should Anthropic Ridges Framed as Foredunes Be Considered Real Dunes?

Aeolian features framed as foredunes worldwide, whose classification faces challenges to separate wave-formed from aeolian ridges, are relevant to the context of anthropic control. An example of this kind of morphology, previously classified as foredunes, is arranged along Brazil’s northeastern coast, and the pertinence of its classification as such or even as truly developed dunes was examined. To contribute to the debate on this issue, detailed geomorphological mapping was carried out through UAV and LIDAR surveys, in addition to the multitemporal study of satellite images and aerial photographs. Ground penetration radar sections, trenches, and particle size analysis were also conducted. Although the obliquity of wind direction to the coastline, this study’s main bedform of focus shows coast-parallel positioning conditioned by the exogenous vegetation previously inserted about 70 m from the high tide line. This morphology showed practically no migration for over 15 years; however, the wind breaks through the vegetation barrier and develops depositional lobes and V-shaped low crests protruding into the mainland. GPR sections reveal cross-strata sets with a high dip angle in only two aeolian sequences, one preceding exogenous vegetation introduction and the other in a contemporary layer, amidst the dominance of horizontal to sub-horizontal strata. The sediments are characteristically aeolian, with cross-strata and morphology resembling incipient protodunes and a few stretches at the lee slope highlighting characteristics of retention or precipitation dunes. There was little similarity to the foredunes compared to the other occurrences along the equatorial northeast coast; there was also an inconsistency in the correlation between the cross-strata and the morphological stage. In addition, the disconnection between the aeolian dynamics and morphogenetic process also differentiates it from regional foredunes. This morphology, therefore, presents aspects that are not characteristic of the classification of foredunes or the naturally formed dunes in the region and thus demands a classification to emphasize the anthropogenic character.

A New Approach for Adaptive GPR Diffraction Focusing

Several researchers have utilized multipath summation to manage the common problem of scattered energy within GPR sections. Such energy results in degrading the lateral resolution and continuity of reflectors. If detailed velocity models are known, then it is fairly easy to focus the scattered energy by means of conventional migration methods. However, this is rarely the case in GPR sections, as the common-offset antenna array is mostly used, and therefore cannot provide velocity models. This gives an important advantage for the multipath summation method, which has proved to be successful in focusing such diffractions, without the need to build a detailed migration velocity field model. This multipath summation method is based on stacking (summation) of constant velocity migrated sections (weighted or not) over a predefined velocity range. The main drawback of this technique is the high computational cost and the need for user interference to select the appropriate stacking weights. We developed an improved implementation of the weighted multipath summation method that reduces both the computational cost, and the user interference in stacking weights selections. This data adaptive methodology can expedite the migration process, suppress the need for a detailed velocity model, and reduce the user subjectivity. Moreover, a data adaptive spectral scaling scheme was developed. This is applied on the output of the multipath summation process to reduce the expected blurriness in the resulting GPR sections.

Geophysics applied to the optimization of blasting design at the S11D mine, Carajás, Brazil

Carajás mineral province, Pará State, Brazil, is one of the world's largest producers of iron ore. The study area is located inside this province at the S11D Mine in its southern portion. The iron ore is composed by banded iron formations (BIFs), consisting basically of hematite and jaspilite bodies. The weathering processes promoted formation of blocks of jaspilite immersed in friable hematite. The current methodology applied in the production benches, using just drillhole investigations, is limited to detect boulders of compact rock, that depending on their sizes, can compromise the Truckless System operation making it costly and unproductive. Electrical and electromagnetic dataset were able to differentiate these two types of materials, one, more conductive (<8000 Ω-m), with attenuated signal in the GPR section associated with jaspilites and another, most resistive (> 8000 Ω-m) with the coinciding contrasted signal from GPR sections associated with the friable hematite. The near surface geophysical dataset interpretation constrained by a detailed bench geological mapping, was capable to solve the referred problem by indicating the location of the compact bodies through the interpretation of the 3D resistive / reflective models, enhancing geological model and rescaling blast design. The final Geophysical model guided the optimization of the blasting design with geometric and energetic improvements.

Delineation of Fractures Using a 2D GPR Processing Strategy for 3D Imaging: Weak Zones within Carbonates at the Archaeological Site of Xochicalco in Mexico

The use of GPR data multipath summation on data acquired over parallel study lines is presented here within the framework of a study on the effects of natural hazards on cultural heritage areas in order to image weak zones within carbonates, such as fractures and caverns. This study was realized at the archeological site of Xochicalco in Mexico, where fractures and caverns are potential sources of the degradation of the archeological remains. Dense parallel GPR study lines spaced every 0.25 m were surveyed using a 400 MHz monostatic antenna with the aim to image possible weak zones in three dimensions. We used a 2D imaging approach, namely, the method of multipath summation, which efficiently focused the scattered energy within the GPR sections. The study revealed, at depths of 1.6m and 1.8m, several linear events attributed to fractures, leading to the preliminary conclusion of this on-going project that cracks on the walls of the Quetzalcoatl Temple after a large earthquake in 2017 are prone to instability of carbonates rocks.

Stratigraphic analysis and geomorphological reconstruction of Grumari coastal plain, Rio de Janeiro, Brazil

Between Marambaia spit and Jacarepagua barrier island system, two major coastal plains in Rio de Janeiro State, Grumari's type of evolution remains unknown, since it was anthropologically modified from the 1960s, where all superficial sedimentary deposits were devastated, intensively changing its natural landscape, and hiding how it has been formed through the Holocene. Aiming to fill these blanks in Grumari's geological history and complementing Rio de Janeiro's coastal studies, the Ground Penetrating Radar method (GPR) was applied to enable the 2D subsurface imaging. The reflection patterns recognition and their correlation with geological features allow a radarfacies interpretation, in a radar stratigraphy context. For coastal plain studies, geological interpretation takes into consideration the sedimentary processes involved in their formation, driven by the balance between sedimentary supply and sea level variations. In the adjacent areas' researches, geological and geophysical data show, from bottom to top, continental deposits formed during the last regional marine regression, in the Late Pleistocene, covered by sand deposits and marine fossils deposited during the last marine trangression, already in the Holocene, followed by sand barriers and lagoons formed and exposed during the last sea falling until the present shoreline. For Grumari coastal plain, this work presents geophysical data interpretation of five GPR sections, collected in about 15 m thick sedimentary deposit, with a sequence of three main units, separated by regional stratigraphic surfaces: a transgressive unit followed by a regressive one, being this one covered by the most recent deposits composing a third unit. The Unit 1 is related to the Holocene sea level rise, from about 7,000 yr BP, and is characterized by truncation relationships between seawards downlapping reflectors and flat-parallel ones, showing a constant change in deposition direction, as the accommodation space was still small at the beginning of this process. At the Unit 1 top, as the accommodation space rate increased, the deposition became more regular, seen by a flat-parallel reflection pattern. While mean sea level reached its maximum range, from about 5,500 yr BP, its rising rate decreased, while the sedimentary supply rate raised due to the continental shelf and the mountain system erosion. This variation in radarfacies sequence is registered by a regional surface, marking the regressive Unit 2 beginning. The change in the rates led to the continental progradation through sand ridges formation, represented by seawards sigmoidal radarfacies. The mean sea level continued to drop gradually to the current level, exposing the sand ridges to erosion, and newer aeolian, fluvial, lagoonal and storm deposits could be formed above them, composing the Unit 3, limited at the bottom by a second regional high amplitude surface. Storm deposits from 4,450 ± 180 yr BP and 3,780 ± 200 yr BP, as washover fans, have been already identified in Jacaperagua plain, therefore, there were episodical events in the region, in the last 4,450 years, seen also in Grumari's data. This primary radar stratigraphic analysis and geomorphological reconstruction of Grumari most recent deposits show that the most recent evolution occurred through the prograding sand ridges formation, mostly attached to each other, with some inter-ridge areas development, and then is likely to be considered as a strandplain.

Multivariate taphonomic analysis of mollusk shell concentrations in Holocene deposits of southern Brazil: An integrated approach

Shell concentrations are one of the most distinctive features of the Holocene coastal stratigraphy in the southern portion of Santa Catarina State, southern Brazil. In this paper, we integrated stratigraphic, geochronological, geophysical, and taphonomic data to discuss the processes involved in the genesis of shell concentrations in a siliciclastic and subtropical setting, a useful analogy for ancient shell concentrations in semi-closed environments affected by storms. The data indicated the succession of three facies associations (FA): bay, lagoon, and back-barrier. This succession implies a grain size fining-upward trend that is representative of a gradual shallowing. Interpretation of GPR sections revealed that the passage from the bay to lagoonal deposits occurred by forming a laterally continuous, high-amplitude surface overlain by sigmoidal reflections, which characterized the progressive basinward backstepping of the lagoonal margin. Shell concentrations in bay deposits are an allochthonous product of storm flows in shallow waters. In contrast, accumulations of the bivalve Anomalocardia brasiliana had their valves reworked in the lagoonal margins and transported to the bottom, characterizing a parautochthonous accumulation. In this study, the correlation of taphonomy and grain size indicates that in bay FA, the loss of periostracum is correlated with increased gradients of fine sand, and part of the shell samples correlates to corrosion and abrasion in increased gradients of medium sand and decreased ones of fine sand. Additionally, in the lagoon FA, the loss of periostracum depends on the decrease in mud-enriched sediments and the increase in fine and medium sand, whereas brightness loss and color alteration are related to the increase in fine and medium sand, as well as margin modification. Corrosion was positively correlated to the mud-dominated fraction.

Automatic estimation of inversion parameters for Microwave Tomography in GPR data using cooperative targets

Knowing the dielectric constant of a geologic medium is an important task in GPR studies. Accurate estimation allow realistic results in migration process, whilst inaccurate values lead to incorrect time-to-depth conversion of GPR sections. Dielectric constant is also needed for target imaging through Microwave Tomography, which is an inverse scattering problem. In this paper an approach for dielectric permittivity evaluation through the tomographic imaging process in order to retrieve the best image possible for a given target is proposed. Input parameters required for the tomographic imaging process were evaluated through Ant Colony Optimization, an efficient global search algorithm. An experiment was carried out using a 2600 MHz antenna for surveys over metallic and Styrofoam targets of different cross sections embedded in a sand box, in five different scenarios. Accurate results were achieved for estimating dielectric permittivity, with slight variations depending on the scenario complexity. True target positions were well retrieved in most cases; however, depth and shape estimation presented higher errors related to known limitations of the tomographic imaging. In addition, automatic estimation of the regularization factor avoids eventual errors related to subjective analysis. Low computational times required to retrieve the models from real data makes this approach suitable for application in realistic GPR surveys.

GPR measurements to detect major discontinuities at Cheshmeh-Shirdoosh limestone quarry, Iran

In recent years, the Iranian rich quarry industry has been looking for efficient scientific investigations to improve the extraction operations in different dimension stone quarries. Kerman Province is one of the most potential zones with a variety of dimension stone quarries near the city of Kerman. In this research, GPR measurements were carried out to detect major discontinuities at Cheshmeh-Shirdoosh limestone quarry, northeast of Kerman city. This quarry is being extracted by the diamond wire sawing method. As the first GPR study in Iranian quarries, a total length of about 1200 m was surveyed with 50 MHz and 250 MHz GPR antennas collecting data on the surface of the three extraction benches of the quarry. A 800 MHz antenna was also used to map the main defects of a block, which was extracted from a fractured section of the quarry. Six parallel profiles at 10 cm intervals were measured along one side of the block. The results obtained from the 250 MHz dataset were very encouraging and could detect all the major discontinuities. Interpreted profiles were also used to prepare depth slices of the density of joints for two main survey areas. As expected, GPR sections obtained from the 50 MHz antenna had a lower resolution but could clearly detect fault zones. The 800 MHz antenna could map the main defects of the extracted block. However, a higher frequency antenna (e.g., 2GHz or more) is recommended for mapping thin fractures.

Caracterização geológica, geomorfológica e evolutiva holocênica do litoral sul de Jaguaruna, SC, Brasil

Compreender os processos evolutivos que ocorreram para a conformação atual do ambiente costeiro é de suma importância para estudos relacionados ao planejamento e gestão do litoral. O presente trabalho tem como objetivo caracterizar a barreira costeira holocênica do setor sul de Jaguaruna, Santa Catarina, do ponto de vista geológico, geomorfológico e evolutivo, através de dados de superfície e de subsuperfície, a fim de subsidiar a gestão costeira da região. O estudo foi realizado com base na interpretação de fotografias aéreas e na análise de dados geofísicos obtidos com um GPR. Em superfície foram identificados quatro depósitos sedimentares compondo a barreira costeira: Depósito Lagunar Praial, Depósito Paludial, Depósito Eólico e Depósito Marinho Praial. Do ponto de vista geomorfológico, esses depósitos se apresentam na forma de planície de cordões litorâneos, banhados, dunas e praia, respectivamente. Além destes, cristas de precipitação e paleocanais se destacam na morfologia da barreira. Quanto às informações de subsuperfície, a principal observação está relacionada à influência do Rio Urussanga sobre o ambiente marinho raso e praial durante a evolução da barreira. A presença de paleocanais em subsuperfície, juntamente com outras informações identificadas em superfície, permitiram caracterizar o comportamento evolutivo da barreira como regressiva. As seções de GPR possibilitaram identificar os paleocanais devido à presença de uma superfície erosiva em forma côncava, com alta amplitude e continuidade lateral. Acima dessa superfície ocorre o preenchimento dos paleocanais em fases distintas. Estas informa- ções de caráter geológico e geomorfológico têm implicações importantes para o planejamento e gestão territorial.

Background matrix subtraction (BMS): A novel background removal algorithm for GPR data

Abstract Background noise is a common type of coherent noise that severely compromises the integrity of the high-resolution images provided by ground penetrating radar survey. Several existing techniques employ different approaches to attenuate background noise. In this study, we present the background matrix subtraction (BMS) as an alternative technique to remove horizontal background noise and we compare its efficiency to that of the conventional background removal technique. Instead of calculating an average trace that is subtracted from the GPR data in the conventional background removal methods, the BMS technique is based on calculating a complete background matrix of the same size of the GPR section. The background matrix is created through a series of windowing, sample exclusion, weighting, and iteration. This series of processes guarantees that the background matrix is least affected by target response and is composed purely of horizontal background noise. The computed background matrix is then subtracted from the GPR data to remove horizontal events. Results of experiments conducted on both synthetic and real GPR data show that the BMS technique yields better results than the commonly used background removal technique.

Image-guided inversion of electrical resistivity data

Electrical resistivity tomography (ERT) is based on solving a Poisson equation for the electrical potential and is characterized by a good sensitivity only in the vicinity of the electrodes used to gather the data. To provide more information to ERT, we propose an image-guided or structure-constrained inversion of the apparent resistivity data. This approach uses structural information obtained directly from a guiding image. This guiding image can be drawn from a high resolution geophysical method based on the propagation equation (e.g. migrated seismic or ground penetrating radar images) or possibly from a geological cross-section of the subsurface based on some prior geological expertise. The locations and orientations of the structural features can be extracted by image processing methods to determine the structure tensor and the semblances of the guiding image at a set of pixel. Then, we introduce these structural constraints into the inversion of the apparent resistivity data by weighting the four-direction smoothing matrix to smooth along, but not across, structural features. This approach allows preserving both discontinuities and coherences in the inversion of the resistivity data. The image-guided inversion is also combined with an image-guided interpolation approach used to focus a smooth resistivity image. This yields structurally-appealing resistivity tomograms, while the whole process remains computationally efficient. Such a procedure generates a more realistic resistivity distribution (closer to the true ones), which can be, in turn, used quantitatively using appropriate petrophysical transforms, to obtain parameters of interest such as porosity and saturation. We check the validity of this approach using two synthetic case studies as well as two real datasets. For the field data, the image used to guide the inversion of the electrical resistivity data is a GPR section in the first case and a combination of seismic and structural information in the second case, which corresponds to a geothermal site at Pagosa Springs, in Colorado.

Application of alternative seismic‐stacking techniques to ringing noise removal from GPR data

ABSTRACTRinging is a common type of coherent noise that degrades the quality of ground‐penetrating radar data. Conventional ringing attenuation or a background removal algorithm works by generating an average ensemble that is then subtracted from all traces in a GPR section. This study presents four alternative algorithms, usually used in stacking‐seismic reflection data, for generating such an ensemble. These algorithms include median stack, diversity stack, alpha‐trimmed stack and smart stack. The traditional algorithm and the four alternative algorithms are tested using two real GPR data sets. The outcomes of the five background removal methods are compared to each other and to the original data using statistical analyses and visual inspection. The results show that all the four alternative techniques are more efficient in background removal than the conventional technique, with the smart ensemble yielding the best results followed by the alpha‐trimmed ensemble.

Deterministic deconvolution for GPR data in the t‐f domain

ABSTRACTDeconvolution methods encounter difficulties in increasing the temporal resolution of GPR data due mainly to non‐stationarity of the records. GPR wavelets are typically mixed phase, which is additionally a major failure of standard deconvolution methods. Here, we propose a deterministic deconvolution method for GPR data, implemented in the t‐f domain, which utilizes narrow time windows and sets spectral balancing as a precondition. A reference wavelet is estimated experimentally for the calculation of a time varying deconvolution operator. Its phase variation is extracted from the spectrally balanced deconvolved GPR trace. The algorithm, tested on synthetic and real data, produces very promising results. In particular the deconvolved GPR section acquired over sands exhibits better temporal resolution and reveals reflected waves travelling through high loss media.

Fracture imaging within a granitic rock aquifer using multiple-offset single-hole and cross-hole GPR reflection data

The sparsely spaced highly permeable fractures of the granitic rock aquifer at Stang-er-Brune (Brittany, France) form a well-connected fracture network of high permeability but unknown geometry. Previous work based on optical and acoustic logging together with single-hole and cross-hole flowmeter data acquired in 3 neighboring boreholes (70-100 m deep) have identified the most important permeable fractures crossing the boreholes and their hydraulic connections. To constrain possible flow paths by estimating the geometries of known and previously unknown fractures, we have acquired, processed and interpreted multifold, single- and cross-hole GPR data using 100 and 250 MHz antennas. The GPR data processing scheme consisting of time-zero corrections, scaling, bandpass filtering and F-X deconvolution, eigenvector filtering, muting, pre-stack Kirchhoff depth migration and stacking was used to differentiate fluid-filled fracture reflections from source-generated noise. The final stacked and pre-stack depth-migrated GPR sections provide high-resolution images of individual fractures (dipping 30-90{\deg}) in the surroundings (2-20 m for the 100 MHz antennas; 2-12 m for the 250 MHz antennas) of each borehole in a 2D plane projection that are of superior quality to those obtained from single-offset sections. Most fractures previously identified from hydraulic testing can be correlated to reflections in the single-hole data. Several previously unknown major near vertical fractures have also been identified away from the boreholes.

A planície costeira holocênica de Campos Verdes (Laguna, SC): evolução sedimentar inferida a partir de georradar (GPR), granulometria e minerais pesados

Campos Verdes region, in Laguna, mid-south coast of Santa Catarina, is characterized by sets of beach ridges alignments, oriented transversal to the present coastline and separated from each other by truncations with a concave-convex pattern. This strandplain is limited towards SW by a higher area (1m) without beach ridges, where four shell mounds are located. The alignments are superposed by several groups of inactive parabolic dunes, attributed to the aeolian generation 3 (post maximum Holocene flooding). The sedimentary growing began in the southwest (higher area), where the former beach ridges developed anchored on a sandy spit. This spit was formed according to the Zenkovitch’s models of lagoonal circulation; moreover, it would have worked as a “trap” for the accumulation of sediments that formed the strandplain. This hypothesis is highlighted by the GPR sections, with reflectors dipping towards N, indication of progradation towards this azimuth. The McLaren’s granulometric variation pattern of “fining, better sorting and more negative skewness” towards N can be interpreted as resulting from a progressive sedimentary reworking during progradation. In a longitudinal profile, the pattern of “coarsening, better sorting and more positive skewness” towards W, suggests transport by longshore currents towards this azimuth. The variation in the heavy minerals assemblage shows a higher concentration of unstable components in opposite to the decrease of ultrastable components towards N and W, with an increase in the relation tourmaline/zircon towards NW. This decreasing mineralogical maturity towards the sector formed in the end of the progradation can be attributed to three factors: 1. progressive concentration by hydraulic sorting; 2. increase in the sedimentary input of newer sediments, because of the stronger influence of Tubarao river deltaic front; and 3. least time for the action of pos-depositional solution in the newer beach ridges.

A planície costeira holocênica de Campos Verdes (Laguna, SC): evolução sedimentar inferida a partir de georradar (GPR), granulometria e minerais pesados

Campos Verdes region, in Laguna, mid-south coast of Santa Catarina, is characterized by sets of beach ridges alignments, oriented transversal to the present coastline and separated from each other by truncations with a concave-convex pattern. This strandplain is limited towards SW by a higher area (1m) without beach ridges, where four shell mounds are located. The alignments are superposed by several groups of inactive parabolic dunes, attributed to the aeolian generation 3 (post maximum Holocene flooding). The sedimentary growing began in the southwest (higher area), where the former beach ridges developed anchored on a sandy spit. This spit was formed according to the Zenkovitch’s models of lagoonal circulation; moreover, it would have worked as a “trap” for the accumulation of sediments that formed the strandplain. This hypothesis is highlighted by the GPR sections, with reflectors dipping towards N, indication of progradation towards this azimuth. The McLaren’s granulometric variation pattern of “fining, better sorting and more negative skewness” towards N can be interpreted as resulting from a progressive sedimentary reworking during progradation. In a longitudinal profile, the pattern of “coarsening, better sorting and more positive skewness” towards W, suggests transport by longshore currents towards this azimuth. The variation in the heavy minerals assemblage shows a higher concentration of unstable components in opposite to the decrease of ultrastable components towards N and W, with an increase in the relation tourmaline/zircon towards NW. This decreasing mineralogical maturity towards the sector formed in the end of the progradation can be attributed to three factors: 1. progressive concentration by hydraulic sorting; 2. increase in the sedimentary input of newer sediments, because of the stronger influence of Tubarão river deltaic front; and 3. least time for the action of pos-depositional solution in the newer beach ridges.

Caracterização geofísica e hidrogeológica do cemitério Bom Jardim, Fortaleza - CE

A multidisciplinary study, involving geophysical, geological, hydrogeological and hydrochemical data, was developed to the geoenvironmental characterization of the Bom Jardim cemetery in Fortaleza, Ceara State - Brazil. The characterization of the subsurface environment has been carried out based on several vertical electric sounding, GPR and EM-34 sections, compositional, granulometric and hydraulic conductivity analyses of soil samples collected in boreholes. The hydro-geological evaluation of the cemetery site was based on the subsurface geophysical signature, topographical data, hydrogeological parameters and physical-chemical analyses of the groundwater, obtained in monitoring wells. The local groundwater quality was evaluated through physical-chemical and microbiological analyses to investigate the groundwater contamination by the leachate from the corpse decomposition. Periodic samplings allowed evaluating the seasonal influences in the hydrogeological parameters of the aquifer along a complete water cycle. This study also attempted to identify the possible geophysical signatures of contamination plumes into the cemetery site. However, high concentration of dissolved elements and organic compounds are present in the unconfined aquifer of the whole studied area, originate from residential septic tanks. This pollution source probably masks possible contrasts of electric or electromagnetic properties between the supposed contamination plume into the necropolis and the surrounding vicinity.

Application of 1-D Convolutional Modeling to Interpretation of Ground Penetrating Radar Profiles-Turbidite Channel Sandstone 1, Lewis Shale, Wyoming

Complex stratigraphic relationships, distinct lithologic facies, and variable bed thickness characterize channel-levee elements of turbidite sections examined in outcrop. The 3-D configuration of these features in the subsurface cannot be determined accurately using only logs in sparsely distributed and unevenly spaced boreholes. The missing interborehole information can be filled in by three-dimensional GPR surveys so long as the geological elements can be recognized in the data. This recognition can be aided by 1-D convolutional waveform modeling, which is able to separate multiples and refracted events from primary reflections, to predict severe amplitude loss due to attenuation in shales, to constrain reflection polarities and traveltimes, and to elucidate the role of interference within thin sandstone beds. Two-dimensional GPR profiles and a 3-D GPR survey image the lowermost and largest of a series of laterally offset, vertically stacked channel sandstones of the Lewis Shale, near Dad, WY. We combine measured sections, borehole logs, and cuttings to form a one-dimensional, blocked framework into which are embedded the laboratory-measured electrical properties of four facies. Zero-offset, non-dispersive impulse responses are computed and convolved with the waveform used in the surveys to produce synthetic radargrams. Modeling applies a frequency-independent attenuation and includes multiples up to second order. These synthetics are compared to nearby GPR sections that have been datum static-corrected using differential GPS elevations. The present modeling makes possible the interpretation of four features of major geological significance—the base of channel, the top of a shale-clast conglomerate, a featureless sandstone, and eroded channel∕levee mounds (erosional remnants). This case history shows that the use of 1-D models can reveal internal consistency among geological mapping, borehole information, laboratory measurements, and radar stratigraphy.

GPR utilizado na deteção da geometria de cavas usadas para disposição de resíduos de óleos lubrificantes

Uma variedade de problemas ambientais ocorre devido à disposição inadequada de resíduos. Metodologias geofísicas aplicadas a estudos do meio físico mostram ótimos resultados na investigação de contaminantes. O georadar é um método geofísico prático e rápido nestas investigações, pois a resposta do sinal eletromagnético auxilia na delimitação do contaminante em subsuperfície, direcionando técnicas diretas de investigação (furos e poços de monitoramento). Este estudo foi desenvolvido numa antiga área de disposição de resíduos de óleos lubrificantes (dense nonaqueous phase liquids - DNAPL) no município de Ribeirão Preto - SP, situada nos domínios geológicos dos derrames basálticos da Formação Serra Geral e arenitos da Formação Botucatu. A aquisição de seções GPR foi realizada dentro e próximo de 4 cavas de disposição de rejeitos, com antenas não-blindadas de 100 MHz, possibilitando uma profundidade de investigação máxima de 7 metros. Os limites laterais e as bases das cavas puderam ser identificados nas seções GPR, com exceção da cava 1, cuja base não é bem caracterizada pelo sinal GPR. Os resultados mostram que o sinal eletromagnético do GPR é bastante atenuado abaixo das cavas. Este fato pode ser resultado de uma atividade bacteriológica no resíduo ou aumento de sais dissolvidos no meio causado pelo pH ácido do resíduo.