Ground Penetrating Radar Sections Research Articles

Reservoir deltas and their role in pollutant distribution in valley-type dam reservoirs: Les Království Dam, Elbe River, Czech Republic

Interpretation of catchment pollution history from sediment cores is a common approach for dam reservoirs, but relatively little is known about the spatial distribution of pollutants in the lithologically diverse depositional subsystems. We studied the depositional system of a hundred-year-old valley-type reservoir of the Les Království Dam built in 1919 on the upper reaches of the Elbe River, Czechia. Based on the analysis of pre-dam river valley morphology using historical topography maps with 1 m vertical resolution, 30 ground-penetrating radar sections on the lake and 137Cs dating of four cores, we described a deltaic sediment body with maximum thickness of ~11 m, accumulation rates ranging from 3.6 to 9.7 cm/yr, and an annual loss in reservoir capacity of 0.8%. The sediment body has a distinct delta shape in the longitudinal section, with a delta platform dissected by a channel (topsets) and delta slope (foresets) and a ponded basin (bottomsets) located near the dam. The upstream sections of the channels are prone to erosion and sediment redistribution into downstream parts of the reservoir, as indicated by ground-penetrating radar sections and distribution of 137Cs in younger sediment strata.We retrieved four cores 1.0 to 1.6 m long from different parts of the delta and analysed the sediment grain size using laser granulometry, geochemical composition (X-ray fluorescence, ICP-MS lithogeochemistry, total organic carbon (TOC)) and concentrations of polycyclic aromatic hydrocarbons (PAH). The grain size variation between the delta platform (sandy silts and silty sands) and prodelta (clayey silts) sediments strongly controls the concentrations of lithogenic elements (Al, Fe, K and Ti) but it has a relatively little effect on distribution of toxic metals (Zn, Cu, Pb) and TOC concentrations. The toxic metals show strong affinity to TOC and PAH show a distinct sawtooth vertical distribution suggesting that they were deposited from depositional events during hyperpycnal flows induced by river floods. This study shows that a hyperpycnal delta may serve as a good predicable model for toxic metal- and organic pollutant distribution in dam reservoirs. The role of hyperpycnal flows should not be overlooked in contamination studies of dam reservoirs.

Reduction of conceptual model uncertainty using ground-penetrating radar profiles: Field-demonstration for a braided-river aquifer

Hydrogeological flow and transport strongly depend on the connectivity of subsurface properties. Uncertainty concerning the underlying geological setting, due to a lack of field data and prior knowledge, calls for an evaluation of alternative geological conceptual models. To reduce the computational costs associated with inversions (parameter estimation for a given conceptual model), it is beneficial to rank and discard unlikely conceptual models prior to inversion. Here, we demonstrate an approach based on a quantitative comparison of ground-penetrating radar (GPR) sections obtained from field data with corresponding simulation results arising from various geological scenarios. The comparison is based on three global distance measures related to wavelet decomposition, multiple-point histograms, and connectivity that capture geometrical characteristics of geophysical reflection images. Using field data from the Tagliamento braided river system, Italy, we demonstrate that seven out of nine considered geological scenarios can be discarded as they produce GPR sections that are incompatible with those observed in the field. The retained scenarios reproduce important features such as cross-stratified deposits and irregular property interfaces. The most convenient distance measure of those considered is the one based on wavelet-decomposition. Direct analysis of the distances is the most intuitive and fastest way to compare scenarios.

Determination of unstable tectonic zones in C–North deposit, Sangan, NE Iran using GPR method: importance of structural geology

Ground Penetrating Radar (GPR) is an effective and practical geophysical imaging tool, with a wide set of applications in geological mapping of subsurface information. This research study aims at determination of the geophysical parameter differences in the subsurface geological structures and construction of a 3D fracture model. GPR and resistivity methods were applied to detect the unstable tectonic zones in the C-North deposit. Structural geology investigations were, first, surveyed to detect the faults and fractures in the study area. Based on the structural features, the survey was conducted over an area of 1 km2 with a total of 30 profiles and low-resistivity zones in the C-North deposit which is a great help in reducing their impacts in slope stability studies. GPR sections were, then, obtained from low and high frequency antennas (10 and 50 MHz) to detect fractures and water content zones. The obtained data results demonstrated that the major structural trends in the study area were W–E, NE–SW, and NW–SE while fault zones that can create pathways for groundwater inflow into the deposit in the future. Information obtained from geological and GPR studies were also integrated with drill hole data. The geological information from structures are in good agreement with the actual geological situation. Method and results of this study could be useful in solving problems related to subsurface structures in mining engineering.

Ground penetrating radar in the medieval oyster shell middens of Saint-Michel-en-l'Herm (Vendée, France)

This study presents the integrated results of GPR (Ground Penetrating Radar) and sedimentological analysis, performed on the giant shell middens of the medieval site of Saint-Michel-en-l'Herm (Vendée, France). Avoiding the use of destructive methods like large-scale digging, the purpose of this study was to determine the internal structure of the northern midden and the links with its substrate, and to obtain insight on the manner of its construction. Four GPR profiles were obtained along the main axis of the midden, on the shell mound, and at its foot, in an area leveled by the former extraction of shells. Sedimentological analysis is also presented from study of the extant parts of the midden and with hand auger cores taken along one of the GPR profiles. The integrated results show that each sedimentological facies is related to a specific radar facies, allowing the mapping of lateral and vertical facies variations along the GPR sections. Within these deposits, the major tilted reflectors correspond to thin soils and trampled surfaces. Their geometry and the stacking pattern highlight multiphase deposition triggered by successive inputs of large volumes of shells (in the range of 100m3), from west to east. Below the exploited parts of the midden, the data are interpreted to show the presence of undisturbed shells, although, an ancient dug area was also identified by GPR. Finally, sedimentological and GPR data both show evidence of a midden established on the west bank of a channel, progressively infilling the channel eastward. These results illustrate the relevance of GPR in answering major questions common to all large shell middens.

The research on the buried public monumental complexes of Lupiae (Lecce) by geophysical prospecting

Ongoing and extensive urbanisation may threaten important archaeological structures that are still buried in urban areas. The ground penetrating radar (GPR) method is the most promising alternative for resolving buried archaeological structures in urban territories. This paper presents a case study that involves a geophysical survey employing the surface three-dimensional (3D) GPR techniques, in order to archaeologically characterise the investigated areas. The site is located in the south-western sector of the historical centre of Lecce (Apulia, Italy), where the modern city overlaps the main public monuments of the Roman municipium of Lupiae, only partially preserved or excavated: the amphitheatre, the theatre, the baths and maybe also the Forum. GPR measurements, integrated with the results of archaeological excavations and the topographical surveys of the preserved remains, were carried out in several areas regarding sectors of the ancient roman city. The GPR data were collected along a dense network of parallel profiles. The GPR sections were processed applying specific filters to the data in order to enhance their information content. The GPR images significantly contributed in reconstructing the complex subsurface properties in these modern urban areas. Strong GPR reflections features were correlated with possible ancient structures and they were integrated in the digital archaeological map of the city.

3D architecture of cyclic-step and antidune deposits in glacigenic subaqueous fan and delta settings: Integrating outcrop and ground-penetrating radar data

Bedforms related to supercritical flows are increasingly recognised as important constituents of many depositional environments, but outcrop studies are commonly hampered by long bedform wavelengths and complex three-dimensional geometries. We combined outcrop-based facies analysis with ground-penetrating radar (GPR) surveys to analyse the 3D facies architecture of subaqueous ice-contact fan and glacifluvial delta deposits. The studied sedimentary systems were deposited at the margins of the Middle Pleistocene Scandinavian ice sheets in Northern Germany.Glacifluvial Gilbert-type deltas are characterised by steeply dipping foreset beds, comprising cyclic-step deposits, which alternate with antidune deposits. Deposits of cyclic steps consist of lenticular scours infilled by backset cross-stratified pebbly sand and gravel. The GPR sections show that the scour fills form trains along the delta foresets, which can locally be traced for up to 15m. Perpendicular and oblique to palaeoflow direction, these deposits appear as troughs with concentric or low-angle cross-stratified infills. Downflow transitions from scour fills into sheet-like low-angle cross-stratified or sinusoidally stratified pebbly sand, deposited by antidunes, are common. Cyclic steps and antidunes were deposited by sustained and surge-type supercritical density flows, which were related to hyperpycnal flows, triggered by major meltwater discharge or slope-failure events.Subaqueous ice-contact fan deposits include deposits of progradational scour fills, isolated hydraulic jumps, antidunes and (humpback) dunes. The gravel-rich fan succession consists of vertical stacks of laterally amalgamated pseudo-sheets, indicating deposition by pulses of waning supercritical flows under high aggradation rates. The GPR sections reveal the large-scale architecture of the sand-rich fan succession, which is characterised by lobe elements with basal erosional surfaces associated with scours filled with backsets related to hydraulic jumps, passing upwards and downflow into deposits of antidunes and (humpback) dunes. The recurrent facies architecture of the lobe elements and their prograding and retrograding stacking pattern are interpreted as related to autogenic flow morphodynamics.

Relations of the groundwater quality and disorderly occupation in an Amazon low-income neighborhood developed over a former dump area, Santarém/PA, Brazil

Groundwaters are better protected than surface waters. However, in unconfined conditions, groundwater can be easily contaminated when the land use and occupation are considered as determining factor. In the city of Santarem/PA, Brazil, the district of Santo Andre has experienced a population growth until reaching areas close to a former dump, and this may have caused an impact on the quality of the Alter do Chao Aquifer. In order to analyze these impacts generated, seven ground-penetrating radar sections to set the dump boundaries, physicochemical and bacteriological analyses in six pumping wells to set the drinkability values of the water, and potentiometric surface map in order to understand groundwater flow directions were carried out. Geophysical data indicated the presence of lateritic crusts, buried pipelines, as well as faults and fractures, which may serve as preferred pathways for contaminant infiltrations up to the aquifer. The geophysical data also indicated the dump boundaries are larger than expected, where the population has built their residences over the compacted garbage. Consequently, every pumping well presented total amounts of coliforms higher than the allowed values, and three of these wells have above 10 mg/L values of nitrate, indicating the region has waters in disagreement to the drinkability standards set, directly influenced by the contamination of the former dump under the Alter do Chao Aquifer.

Imaging fracture distributions of the Al-Khuff Formation outcrops using GPR and ERT geophysical techniques, Al-Qassim area, Saudi Arabia

Khuff Formation is of utmost importance in Saudi Arabia for oil and gas reservoir although it is composed mainly of limestone. This reason refers to the existence of intensive fractures which play a vital role in the increase in porosity and permeability of this formation. The fracture pattern in the study area was verified through 2D and 3D ground penetrating radar (GPR)-defined and electrical resistivity tomography (ERT)-defined surveys. In this respect, ten of 2D GPR surveys were collected along an intersected grid of profiles covering the study area while ERT data were collected along three profiles of the GPR grid. The results were interpreted in light of the field-based structural and stratigraphic assessment of the outcropping rocks. The analysis of the inverted ERT and filtered GPR sections revealed the presence of fractures. Several resistivity and electromagnetic reflection anomalies were laterally and vertically identified across the measured sections clarifying fractures that extend to a depth of 24 m in the limestone. Most fractures are oriented vertical to sub-vertical dipping both east-west and north-south.

Attenuation analysis of real GPR wavelets: The equivalent amplitude spectrum (EAS)

Absorption of a Ground Penetrating Radar (GPR) pulse is a frequency dependent attenuation mechanism which causes a spectral shift on the dominant frequency of GPR data. Both energy variation of GPR amplitude spectrum and spectral shift were used for the estimation of Quality Factor (Q*) and subsequently the characterization of the subsurface material properties. The variation of the amplitude spectrum energy has been studied by Spectral Ratio (SR) method and the frequency shift by the estimation of the Frequency Centroid Shift (FCS) or the Frequency Peak Shift (FPS) methods. The FPS method is more automatic, less robust. This work aims to increase the robustness of the FPS method by fitting a part of the amplitude spectrum of GPR data with Ricker, Gaussian, Sigmoid–Gaussian or Ricker–Gaussian functions. These functions fit different parts of the spectrum of a GPR reference wavelet and the Equivalent Amplitude Spectrum (EAS) is selected, reproducing Q* values used in forward Q* modeling analysis. Then, only the peak frequencies and the time differences between the reference wavelet and the subsequent reflected wavelets are used to estimate Q*. As long as the EAS is estimated, it is used for Q* evaluation in all the GPR section, under the assumption that the selected reference wavelet is representative. De-phasing and constant phase shift, for obtaining symmetrical wavelets, proved useful in the sufficiency of the horizons picking. Synthetic, experimental and real GPR data were examined in order to demonstrate the effectiveness of the proposed methodology.

GPR background removal using a directional total variation minimisation approach

Ground penetrating radar (GPR) is a leading geophysical subsurface imaging tool for various purposes. This efficiency, however, is compromised by the interference of different types of noise. Background noise (clutter) is one of the nagging types of noise that undermines the high-resolution imaging capabilities of GPR. This study presents the experience of applying a directional total variation minimisation (DTVM) filter to attenuate clutter in GPR data. The application of DTVM to both synthetic and field GPR data proves its great capability to attenuate clutter without affecting the features of interest of a GPR section. The results also show that the proposed DTVM method affords superior image quality than both the most commonly used and the most recently published background removal techniques.

Integrated Geophysical Mapping of Near-surface Structures and Utilities in Ile-Ife, Nigeria

Magnetic, self-potential (SP) and Ground-penetrating radar (GPR) methods were employed to investigate the near-surface conditions beneath a road in Obafemi Awolowo University Campus, Ile-Ife with the aim of mapping previously suspected dyke, fractures, buried utilities in the area. Fifty stations were established for the magnetic and SP methods along five and four traverses respectively while GPR data was acquired along three traverses, all parallel to the general strike of the rocks in the area (NW-SE). The magnetic and SP readings obtained were interpreted qualitatively and quantitatively using least-squares to identify the location/trends and determine source parameters of features of interest. Also, GPR sections were analyzed using basic radar processing tools. The magnetic field readings ranged from -310nT to 220nT while the SP readings with range -126mV to 55mV. The anomaly source was delineated on low closures on the iso-magnetic map while the inflexion points on the SP profiles lie above the target source. Results of quantitative interpretation of magnetic and SP anomalies revealed a spherical, sub-vertical source with depth to top values of 1.6m to 3.85m. The radargrams revealed the positions and depths of utilities such as pipes and cables, shallow-dipping fractures, a road culvert and partially weathered bedrock layers overlying fresh basement in the area. The penetration depth of the GPR was 14 m and the depth to top of the basement ranged from 4 m to 6 m. The techniques employed mapped the subsurface structural/cultural features in the study area and determined their positions and depth extents. crust than the other deeper layers. As a result increased understanding of the dynamics of this part of the Earth is paramount (1). In this paper, the primary aim was to map the shallow portions of the earth within a defined area in Ile-Ife; up to a depth of about 15 m, and confirm the presence of previously suspected fractures, fault, dyke and man-made utilities. Three geophysical methods namely Magnetic, Self-potential (SP) and Ground Penetrating Radar (GPR) were employed in the present study. The magnetic method involves the measurement of variations in total magnetic field of the earth, caused by local differences in the magnetization of the subsurface rocks and soils. It has a wide range of applications and has been used in locating buried metalliferous man-made objects such as cables and pipelines, location of boundaries between rocks, faults, and dykes (2). The SP method is a passive geophysical technique that responds to naturally-occurring potential differences generated mainly by electrochemical, electrokinetic, and thermoelectric sources in the subsurface and has many applications ranging from mineral exploration, hydrogeological, environmental, geotechnical and engineering investigations. There is recorded success in the use of the SP method in shallow subsurface mapping of simple geometrical bodies (3) and sheet-like bodies (4). The GPR technique (which is a novel technique in Nigeria) detects electrical discontinuities due to features such as bedding planes, fault planes etc. in the shallow subsurface (usually less than 50m) by the generation, transmission, propagation, reflection and reception of discrete pulses of high-frequency electromagnetic energy with high-resolution imagery (5). GPR is effective for subsurface fracture and fracture zone imaging (6), soil and rock stratigraphy mapping (7) and underground utility mapping (8). Consequently, this integrated study was done to verify the presence of the geologic structures and cultural utilities in the area, and also to demonstrate the efficacy of the GPR technique in shallow subsurface mapping in a basement complex terrain of southwestern Nigeria.

Depositional and erosional architectures of gravelly braid bar formed by a flood in the Abe River, central Japan, inferred from a three-dimensional ground-penetrating radar analysis

We conducted a ground-penetrating radar (GPR) survey of gravelly braid bars in the Abe River, central Japan, to clarify the three-dimensional (3D) variations in their depositional facies under various geomorphologic conditions. In September 2011, a ten-year return-period flood in the study area reworked and deposited braid bars. After the flood, we surveyed three bars with different geomorphologies using a GPR system with a 250-MHz antenna and identified seven fundamental radar depositional facies: Inclined reflections (facies Ia and Ib), horizontal to subhorizontal reflections (facies IIa and IIb), discontinuous reflections (facies IIIa and IIIb), and facies assemblage with a large-scale channel-shaped lower boundary (facies IV). Combinations of these facies in\\dicate bar formation processes: channel filling, lateral aggradation, and lateral and downstream accretion. In the Abe River, aerial photographs and airborne laser scanning data were obtained before and after the flood. The observed changes of the surface topography are consistent with the subsurface results seen in the GPR sections. This study demonstrated that the erosional and depositional architecture observed among bars with different channel styles was related to river width and represented depositional processes for high-sediment discharge. The quantitative characterizations of the sedimentary architecture will be useful for interpreting gravelly fluvial deposits in the rock record.

Image-guided inversion in steady-state hydraulic tomography

In steady-state hydraulic tomography, the head data recorded during a series of pumping or/and injection tests can be inverted to determine the transmissivity distributions of an aquifer. This inverse problem is usually under-determined and ill-posed. We propose to use structural information inferred from a guiding image to constrain the inversion process. The guiding image can be drawn from soft data sets such as seismic and ground penetrating radar sections or from geological cross-sections inferred from the wells and some geological expertise. The structural information is extracted from the guiding image through some digital image analysis techniques. Then, it is introduced into the inversion process of the head data as a weighted four direction smoothing matrix used in the regularizer. Such smoothing matrix allows applying the smoothing along the structural features. This helps preserving eventual drops in the hydraulic properties. In addition, we apply a procedure called image-guided interpolation. This technique starts with the tomogram obtained from the image-guided inversion and focus this tomogram. These new approaches are applied on four synthetic toy problems. The hydraulic distributions estimated from the image-guided inversion are closer to the true transmissivity model and have higher resolution than those computed from a classical Gauss–Newton method with uniform isotropic smoothing.

Identifying damaged areas inside a masonry monument using a combined interpretation of resistivity and ground-penetrating radar data

The Bayon Complex in the Angkor heritage site, Cambodia, has been damaged by weathering. To plan its long-term preservation, it is essential to investigate its internal structure and the degree of damage within the masonry monument. This study shows results of ground-penetrating radar (GPR) and electrical exploration surveys, and an interpreted section of the internal structure and moisture distribution in the masonry monument. The GPR can detect boundaries between stone blocks and between stone blocks and compacted soil. Electrical resistivity can indicate moisture distribution with high reliability in combination with GPR sections. The top surface zone of the terrace structure of this monument is composed of three layers of stone blocks, and the zone below a depth of 55–60 cm is composed of compacted soil. Rainwater penetrates into the terrace through gaps between the stone blocks and drains from vertical walls through cavities in the top part of the compacted soil. Damaged areas are limited to a part of the terrace, and a large area has remained in good condition. This study shows that a combination of electrical resistivity and GPR data is useful for investigating the internal structures and classifying the degree of damage to old stone structures.

Remote sensing and geophysical investigations of Moghra Lake in the Qattara Depression, Western Desert, Egypt

The Western Desert covers two-thirds of the land area of Egypt and occupies one of the driest regions of the Sahara. Seven depressions within the desert – Siwa, Qattara, Fayum, Bahariya, Farafra, Dakhla, and Kharga – may represent parts of old drainage systems with deflation, extensive erosion, and possibly, some tectonic activity. Oases with freshwater exist in these depressions. Geological and geophysical investigations in the Qattara Depression indicate the presence of buried fluvial channels with southeast to northwest flow directions from the highland areas. The origin of these fluvial systems, as well as the origin of the depressions themselves, is still unresolved, and many ideas have been suggested. Moghra Lake at the northeastern tip of the Qattara basin may be a remnant of a larger paleolake, including the mouth of a paleo-river.We present here the results of our recent work in this area using ALOS PALSAR radar remote sensing data, which indicated the presence of buried channels that may have fed the larger Moghra paleolake. Ground penetrating radar (GPR) data along 2D profiles were acquired; the migrated GPR sections identified a major paleochannel with numerous minor channels at its margins. GPR interpretations are verified by field observations, trenching, and stratigraphic information from outcrop data. Potential field analyses identify possible aquifers that are controlled by regional structures. Density contrasts within the sedimentary units, physical boundaries of uplifted basement blocks and depths to causative sources were also identified. This work contributes to the reconstruction of paleodrainage of this region and helps in understanding processes involved in the formation of the Qattara Depression.

Geologia e litogeoquímica de migmatitos, charnockitos e granulitos do Complexo Guaxupé na região de São João da Boa Vista (SP)

Nesse trabalho são apresentados os procedimentos metodológicos envolvidos no imageamento digital de paleocavernas colapsadas com ground penetrating radar (GPR), que ocorrem em tufas carbonáticas, situadas na borda oeste da Bacia Potiguar. Foi mapeada uma área em Quixeré (CE) e selecionado um afloramento na rodovia CE-377, denominado de Afloramento Quixeré. Em uma porção desta área foi confeccionado um fotomosaico e levantada uma seção-teste, com o GPR, para a comparação e parametrização das geometrias cársticas (aflorantes e as imageadas na linha geofísica). Os resultados foram satisfatórios e permitiram a adoção de critérios para a interpretação das paleocavernas colapsadas nas outras seções GPR, levantadas na região do afloramento. Foram adquiridas duas malhas de linhas GPR, a primeira mais ampla e espaçada norteou a locação da segunda, mais adensada, na porção sul do Afloramento Quixeré. Nos radargramas foram interpretadas e parametrizadas as geometrias das principais paleocavernas colapsadas. Para cada malha foi elaborado um modelo de sólido digital do Afloramento Quixeré, sendo que o primeiro modelo permitiu o reconhecimento da distribuição geral das tufas e a localização das paleocavernas colapsadas, enquanto o segundo modelo digital, por ser mais detalhado, possibilitou não apenas a individualização 3D das principais paleocavernas mas também o cálculo dos seus respectivos volumes. Os modelos de sólidos digitais são aqui apresentados como uma nova fronteira no estudo de afloramentos análogos a reservatórios (hídricos ou petrolíferos), na qual a parametrização e a caracterização volumétrica dos corpos geológicos passam a ser fundamental para compor bancos de dados, que conjuntamente com informações de propriedades petrofísicas serão utilizados em simulações computacionais mais realísticas de reservatórios.

Ground-penetrating radar attribute analysis for visualization of subsurface archaeological structures

Attribute analysis was originally developed for enhanced data visualization in seismic data interpretation. Recently attribute analysis has been applied to ground-penetrating radar (GPR) data to improve visualization and associated interpretation for archaeological investigations. In this study, we evaluate the application of energy, instantaneous-amplitude, and steepness event attributes, which are traditionally used to highlight changes in the subsurface, to the GPR data acquired at Pan Sao Temple in Chiang Mai City, Thailand. The survey area is located on the eastern side of an old pagoda, the suspected location of a buried Buddha hall. After routine data processing, the energy, instantaneous amplitude, and steepness event attributes were calculated for the GPR sections. Visualization of the GPR attributes provides clearer images of the edge of buried archaeological structures than GPR data with traditional processing. The instantaneous amplitude provides images with the most reflection continuity while the steepness event highlights high-contrast features. GPR attribute analysis of the type used here is applicable to other areas undergoing archaeological investigations.

Joint application of ground penetrating radar and electrical resistivity measurements for characterization of subsurface stratigraphy in Southwestern Nigeria

The frequent building collapses in Nigeria have been attributed to a lack of pre-construction investigations, which assist engineers in obtaining in situ geotechnical information. Further, the structural subsurface settings are often ignored or investigation is haphazardly carried out. To address this issue and demonstrate the importance of such a survey, a combination of ground penetrating radar (GPR) and vertical electrical sounding (VES) data were acquired in a part of Southwestern Nigeria. A 200 MHz antenna was used for the data acquisition along four traverses. The data were subjected to standard GPR processing techniques, and attribute analysis such as instantaneous frequency, amplitude and phase. Also, for comparative and engineering characterization purposes, longitudinal conductance and coefficient of anisotropy were computed from the VES results and used for determining the competency of the bedrocks. From the GPR results, it was observed that the mapped subsurface is characterized as erosional truncated at a low angle, which is southerly dipping and includes tangential reflections. Further, stratified rocks dipping at an angle of 32° occur between 1.0 and 4.5 m depth in all of the GPR sections; these strata were truncated by topsoil at shallow depths. Also, some of the sections depict ancient channel structures that have a dimension of 70 m × 40 m. The resistivity data suggest that the study area is characterized by four distinct geoelectric sequences. These comprise topsoil which is composed of clay-like sand to lateritic clay whose thickness ranges between 0.25 and 8.12 m, weathered bedrock with a thickness between 3.84 and 12.61 m, stratified bedrock with a thickness between 0.33 and 7.51 m, and fresh bedrock. These results reveal a complex subsurface geology and this characterizes the study area. The area has low to moderate longitudinal conductance and coefficient of anisotropy values, which suggest that incompetent to semi-competent bedrock exists beneath the subsurface. A good correlation between the GPR and resistivity derived thicknesses was established.

Geophysical investigation using resistivity and GPR: A case study of an oil spill site at Seberang Prai, Penang

Subsurface contamination by light non-aqueous phase liquids (LNAPLs) is one of the most pressing environmental issues in the industrialized and developing world. Non-invasive geophysical techniques have proven to be effective in identifying contaminated areas. In this study, a 2D electrical resistivity imaging and ground penetrating radar (GPR) survey has been carried out at a site to investigate the nature and extent of an oil spill into sandy soil and groundwater from old transformersat an abandoned power station. Hand-augered boreholes revealed that the water level was at 1.5 m and shallower towards the coastline, which is about 100 m from the site. The presence of several oil plumes were detected at the top of the water table in 2D resistivity sections as a zone of high resistivity with values ranging from 450 to 1000 Ωm. The low resistivity zones below the water table are interpreted as saline water. In the GPR section, the oil contaminated layer exhibits discontinuous, shadow and chaotic high amplitude patterns.

Single-hole GPR reflection imaging of solute transport in a granitic aquifer

Identifying transport pathways in fractured rock is extremely challenging as flow is often organized in a few fractures that occupy a very small portion of the rock volume. We demonstrate that saline tracer experiments combined with single-hole ground penetrating radar (GPR) reflection imaging can be used to monitor saline tracer movement within mm-aperture fractures. A dipole tracer test was performed in a granitic aquifer by injecting a saline solution in a known fracture, while repeatedly acquiring single-hole GPR sections in the pumping borehole located 6 m away. The final depth-migrated difference sections make it possible to identify consistent temporal changes over a 30 m depth interval at locations corresponding to fractures previously imaged in GPR sections acquired under natural flow and tracer-free conditions. The experiment allows determining the dominant flow paths of the injected tracer and the velocity (0.4-0.7 m/min) of the tracer front.