Ground-penetrating Radar Studies Research Articles

Evaluating LNAPL contamination using GPR signal attenuation analysis and dielectric property measurements: Practical implications for hydrological studies

Groundwater and sub-surface contamination by Light Non-Aqueous Phase Liquids (LNAPLs) is one of the industrial world's most pressing environmental issues and a thorough understanding of the hydrological, physical and bio-chemical properties of the sub-surface is key to determining the spatial and temporal development of any particular contamination event. Non-invasive geophysical techniques (such as electrical resistivity, electromagnetic conductivity, Ground-Penetrating Radar, etc.) have proved to be successful sub-surface investigation and characterisation tools with Ground-Penetrating Radar (GPR) being particularly popular. Recent studies have shown that the spatial/temporal variation in GPR signal attenuation can provide important information on the electrical properties of the sub-surface materials that, in turn, can be used to assess the physical and hydrological nature of the pore fluids and associated contaminants. Unfortunately, a high percentage of current LNAPL-related GPR studies focus on contaminant mapping only, with little emphasis being placed on characterising the hydrological properties (e.g., determining contaminant saturation index, etc.). By comparing laboratory-based, dielectric measurements of LNAPL contaminated materials with the GPR signal attenuation observed in both contaminated and ‘clean’ areas of an LNAPL contaminated site, new insights have been gained into the nature of contaminant distribution/saturation and the likely signal attenuation mechanisms. The results show that, despite some practical limitations of the analysis technique, meaningful hydrological interpretations can be obtained on the contaminant properties, saturation index and bio-degradation processes. A generalised attenuation/saturation model has been developed that describes the physical and attenuation enhancement characteristics of the contaminated areas and reveals that the most significant attenuation is related to smeared zone surrounding the seasonally changing water table interface. It is envisaged that the model will provide a basis for the interpretation of GPR data from analogous LNAPL contaminated sites and provide investigators with an appreciation of the merits and limitations of GPR-based, attenuation analysis techniques for hydrological applications.

Imagerie combinée géoélectrique–radar géologique des cavités souterraines de la ville de Zaouit Ech Cheikh (Maroc)

The basement of the Zaouit Ech Cheikh city shelters a dense network of underground cavities. These sub-surface voids make many buildings or infrastructures unstable or cause them to collapse. To test the efficiency of geophysical methods in detecting cavities, 2D electrical tomography and ground-penetrating radar (GPR) studies were carried out along the main streets of the city. The obtained results reveal the existence of a large number of underground cavities throughout the investigated area. They also show the interest of such combined geophysical methods as tools for mapping underground holes in an urban area.

Ground penetrating radar survey of a prehistoric site in southern Italy

In this paper, we present and discuss the results of a ground penetrating radar (GPR) study carried out in an area of southern Italy where a karst cave with prehistoric remains was found. The aim of the study involved the imaging of the subsurface stratigraphy, both inside and outside the cave, in order to assist archaeologists in an excavation programme. The survey grid comprised 18 intersecting GPR profiles. Problems associated with the use of the GPR within a cave environment are highlighted and the data processing sequence designed to extract useful geo-archaeological signatures is described. Interpretation of the radar sections was controlled by the excavation of two trenches (inside and outside the cave), allowing detailed outlining the complex stratigraphy and the reconstruction of Palaeolithic and Neolithic subsurface horizons.

Investigating small-scale variations of the recent accumulation rate in coastal Dronning Maud Land, East Antarctica

AbstractThe accumulation rate on Potsdam Glacier, East Antarctica, and its spatial and temporal variations are examined using ground-penetrating radar, snow samples and firn-core studies. Physical properties in snow samples and along firn cores provide distributions of density with depth, showing only small spatial variation. Counting of peaks in δ18O along the firn cores yields an age–depth distribution that is transferred to the stratigraphy of isochronal internal layers observed with radar. From two radar horizons we determine the spatial accumulation pattern, averaged over the periods 1970–80 and 1980–2004. The shape of internal layers indicates an ablation area at the eastern margin of the investigation area. Accumulation rates show a very high spatial variability, with a mean value of 141 kgm–2 a–1 for the period 1970–2004 and a standard deviation of almost 50%. Mean temporal variation of only a few per cent throughout the investigated area for the observed time interval is much less than the spatial variations. The mean accumulation values are somewhat less than values reported before from this region. Accumulation pattern and surface topography are linked in a way indicating that wind-borne redistribution of snow significantly contributes to the observed spatial variations of accumulation rates. The accumulation data and their variability complement and validate present and future satellite studies of Antarctica’s mass balance.

Remote sensing-GIS and GPR studies of two active faults, Western Gangetic Plains, India

Northern parts of the Ganga–Yamuna Interfluve in the Gangetic Plains, India have been investigated by remote sensing and Ground Penetrating Radar (GPR) techniques. Digital analysis of remote sensing data and Geographical Information System (GIS) techniques were used to locate a new active transverse Muzaffarnagar Fault and confirmed an earlier described Solani-II Fault in almost flat or gently sloping terrain. The Solani-II and Muzaffarnagar faults are members of two major systems of surficial faults i.e. longitudinal and transverse faults, respectively. Longitudinal faults are curvilinear in nature, trending N–S in the northern regions and veering to E–W in the southern regions of the plains and transverse faults are normal to the longitudinal faults occurring in the Upper Gangetic plains. GPR survey was carried out by common offset method across the Muzaffarnagar and the Solani-II faults, using a 100 MHz antenna. Our GPR data indicate that both regions around the Solani-II Fault and Muzaffarnagar Fault are characterized by 2–3 major steeply dipping normal faults at shallow depth (< 20 m). One of the major faults is always associated with cliffs on the surface. In addition, a number of smaller synthetic and antithetic faults are present, thus forming members of an array covering wide regions. Also, several small folds observed in all the three radargrams which are probably seismites. Both the Solani-II and Muzaffarnagar faults probably developed at about 2.5 ka and almost at the same time fans were deposited on the downthrown block of the Muzaffarnagar Fault.

Late Cenozoic geomorphic evolution in response to inversion: Evidence from field and GPR studies in Kim drainage basin, western India

This study presents field and ground penetrating radar (GPR) data for understanding geomorphic evolution of the Kim-drainage basin in response to tectonic inversion of the Narmada–Ankleshwar block of the Cambay rift basin located on the western-continental margin of India. The Narmada–Ankleshwar block underwent continuous sedimentation up to Mio–Pliocene and subsequently was subjected to compressional inversion. Four geomorphic surfaces are recognised that formed in response to the varying degrees of tectonic activity related to phases of inversion. These are the early Pleistocene erosional surface (EPES), late Pleistocene depositional surface (LPDS), early Holocene erosional surface (EHES) and the late Holocene depositional surface (LHDS). The phases of increased tectonic activity relate to two erosional episodes, while two depositional periods relate to waning tectonic activity or quiescent periods. We show that the Narmada–Ankleshwar block suffered inversion earlier than other parts of the Cambay basin to the north of the NSF. Localisation of early inversion movements occurred in an area characterised by thinner crust and involved the development of new reverse faults and transformation of the NSF from a normal to a reverse fault. GPR studies indicate the reverse nature of the various faults in the Kim drainage basin and provide evidence of their late reactivation under compression. The study demonstrates that the parts of a large sedimentary basin can undergo inversion at different times depending on the orientation of pre-existing crustal discontinuities with respect to the controlling stress trajectories.

Ground-penetrating radar tomography for soil-moisture heterogeneity

Many ground-penetrating radar (GPR) studies incorporate tomographic methods that use straight raypaths for direct model reconstruction, which is unrealistic for media with gradually changing petrophysics. Ray-bending algorithms can sometimes lead to unreliable resolution, especially at interfaces of abrupt dielectric changes. We present an improved GPR tomography technique based on a combination of seismic tomographic methods and a finite-difference solution of the eikonal equation. Our inversion algorithm uses velocity gradient zones and bending rays that represent realistic geology in the subsurface. We tested the technique on theoretical and experimental models with anomalous bodies of varying saturations and velocity and applied it to data from a GPR field experiment that analyzed the root zones of trees. Synthetic results showed that the resolution of our technique is better than that of published methods, especially for local anomalies with sharp velocity contacts. Our laboratory experiments consisted of four objects buried in sand with various water saturations. The GPR tomogram could map the objects and determine their degree of saturation. The velocities are compatible with those of the complex refraction index method; their relationship to the water content fits a previously published empirical equation. Our original field experiment around a poplar tree could map the heterogeneous subsurface and distinguish a central low velocity beneath the tree from the peripheral negative anomaly of a refill. This zone reflects the whole root zone and is caused by its bulk water content of both the organic root network and its surrounding soils.

Land Streamer Aided Geophysical Studies at Saqqara, Egypt

During December 2002 and January 2003, Montana Tech in collaboration with Ain Shams University, Cairo, collected Ground Penetrating Radar (GPR) and seismic data at Saqqara, Egypt. The purpose of this study was to see if GPR and seismic methods could detect manmade structures in the subsurface at Saqqara. In particular, land streamer aided, seismic diving-wave tomography was tested as a method to detect archaeological features. Saqqara was one of the principal necropolises of Memphis, an ancient capital of Egypt. The research site was near the 3rd Dynasty pharaoh Djoser’s Step Pyramid—the first monumental structure built entirely of stone. A preliminary GPR study of our site yielded numerous, possibly manmade features in the subsurface with a [Formula: see text] depth of penetration using [Formula: see text] antennas. A follow-up three-dimensional (3-D) GPR survey over one of the more interesting features showed a broad trench underneath the flat-lying sand that is seen at the surface. This feature is most likely manmade because the horizontally layered limestone rocks of Saqqara are inconsistent with the shape of this feature. Seismic diving-wave tomograms show that this probable manmade feature extends to a depth of [Formula: see text] into the subsurface. Moreover, we were able to complete the seismic survey faster using a land streamer consisting of gimbaled geophones than could be done using conventional planted geophones. This site has potential for further investigation and possible excavation.

3-D sedimentary architecture of a Quaternary gravel delta (SW-Germany): Implications for hydrostratigraphy

This paper investigates a Quaternary Gilbert-type gravel delta that was formed in an ice-marginal environment at the end of the last glacial period. Outcrop, sediment core and ground-penetrating radar (GPR) studies reveal the sedimentary facies and depositional architecture of the delta that comprises three major units: (1) a 2–5 m thick, gravelly topset with an erosional base, formed by accretion of bedload sheets in a braided river; (2) an up to 40 m thick, steeply inclined (13–35°) foreset, dominated by gravelly lithofacies being the product of cohesionless debris flows and debris falls as well as gravity slides while sandy lithofacies was deposited by traction currents; and (3) a 10–20 m thick, sandy bottomset comprising low-density turbidites. Syn- to postdepositional deformation of parts of the bottomset deposits largely resulted from rapid deposition of overlying gravels and differential loading of the prograding foreset beds. The development of the delta was most likely controlled by a high sediment supply and lake level fluctuations. The overall coarsening-upward succession reflects delta progradation and aggradation into a glaciolacustrine environment. Outcrop sedimentology served as a direct analogue in order to characterise the three-dimensional sedimentary and hydraulic architecture of the nearby gravel-delta aquifer. Applying a multidisciplinary approach, sedimentological, geophysical, and hydrogeological data were integrated within the 3-D modelling package Gocad (Earth Decision Sciences) to develop high-resolution 3-D aquifer models.

Use of geophysical methods in urban archaeological prospection: The Basilica de Nuestra Señora de La Salud, Patzcuaro, Mexico

AbstractThe purpose of this investigation was to locate the foundations of an ancient wing of the Basilica de Nuestra Señora de la Salud, which, according to historical documents, was part of this 16th‐century structure. The site was located within the southern vestibule of the Basilica. Ground‐penetrating radar (GPR) and vertical magnetic gradient (VMG) field methods were employed. The geophysical survey consisted of 20 GPR profiles measured in the study area. A series of 16 VMG profiles was also surveyed. The GPR studies located a linear reflector going in a NE‐SW direction, with an angle of almost 45°. The VMG made it possible to identify a rectangular pattern and other features that are probably associated with either collapsed portions of the foundations or Pre‐Hispanic remains. These results confirmed the presence of the reflector found with GPR, as well as other features of interest. © 2005 Wiley Periodicals, Inc.

Implementation of Reliability-Based Backcalculation Analysis

The reliability concept provides a means of incorporating some degree of certainty into the pavement design process to ensure that the outcomes of the process will provide acceptable levels of service until the end of the intended design life. Pavement structural performance and rehabilitation design are highly dependent on the in situ layer properties. Pavement layer thickness is an essential input in backcalculation analysis performed with measured surface deflections to evaluate the in situ structural capacity of a pavement. Inaccurate thickness information may lead to significant errors in the backcalculated layer moduli and, hence, in the rehabilitation design. Because pavement layer thickness has some degree of variability (normal variability), it is important to consider this variability in the backcalculation analysis and rehabilitation design. A procedure was developed to implement the reliability concept in backcalculation analysis to account for the normal variability in layer thickness within structurally homogeneous sections. This procedure was developed on the basis of in situ layer information obtained from a ground-penetrating radar study performed for the New Jersey Department of Transportation. This paper provides an overview of the procedure, along with the results of the pilot implementation of the procedure. This reliability procedure complements the reliability factor of the 1993 AASHTO pavement design guide, as the latter reliability factor does not account for the in situ layer thickness.

Implementation of Reliability-Based Backcalculation Analysis

The reliability concept provides a means of incorporating some degree of certainty into the pavement design process to ensure that the outcomes of the process will provide acceptable levels of service until the end of the intended design life. Pavement structural performance and rehabilitation design are highly dependent on the in situ layer properties. Pavement layer thickness is an essential input in backcalculation analysis performed with measured surface deflections to evaluate the in situ structural capacity of a pavement. Inaccurate thickness information may lead to significant errors in the backcalculated layer moduli and, hence, in the rehabilitation design. Because pavement layer thickness has some degree of variability (normal variability), it is important to consider this variability in the backcalculation analysis and rehabilitation design. A procedure was developed to implement the reliability concept in backcalculation analysis to account for the normal variability in layer thickness within structurally homogeneous sections. This procedure was developed on the basis of in situ layer information obtained from a ground-penetrating radar study performed for the New Jersey Department of Transportation. This paper provides an overview of the procedure, along with the results of the pilot implementation of the procedure. This reliability procedure complements the reliability factor of the 1993 AASHTO pavement design guide, as the latter reliability factor does not account for the in situ layer thickness.

Stratigraphy of Back-Barrier Coastal Dunes, Northern North Carolina and Southern Virginia

Ground penetrating radar studies of four representative active back-barrier dunes, combined with radiocarbon and photon-stimulated-luminescence dating techniques and soils analysis, reveal phases of alternating dune activity and stabilization along the North Carolina–Virginia coast. Two smaller dunes represent only the current phase of dune activity. Two larger dunes preserve evidence of three phases of dune development (ca. 740, 1260 and 1810 AD) and intervening phases of soil development. Climate, particularly moisture conditions, played a part in the timing of dune activity and stabilization events. All three dune phases are associated with drier conditions whereas soils formation is associated with humid conditions. Modern (phase 3) dunes are more widespread along the coast and their formation is attributed to a combination of dry conditions, increased storminess associated with the Little Ice Age, and rising sea level. Tidal inlet closing and storm overwash processes likely provided sediment point sources for individual dune masses. The longer history and much greater volume of dune sand in the area of the two larger dunes suggests a greater sediment supply in this locality.

Sedimentological and GPR Studies of Subglacial Deposits in the Joux Valley (Vaud, Switzerland): Backset Accretion in an Esker Followed by an Erosive Jökulhlaup

During the Würmian glaciation, the Jura ice sheet covered the Joux Valley (Vaud, Switzerland). A geomorphological study reveals many drumlins in this valley. Some are composed of gravels and sand, others of till. Outcrops show that the surface of the sandy-gravel drumlins is a major and sharp erosion surface. Given the lack of shearing structures in sediments below this erosion level, its origin cannot be linked to ice action of the glacier. Very high-energy subglacial meltwater floods (jökulhlaups), probably due to the drainage of subglacial or supra-glacial lakes, are the more likely cause of the erosion. Results of a ground penetrating radar (GPR) survey show the internal structure of one of these sandy-gravel drumlins to depth of 15 m. These GPR data, together with sedimentological observations, indicate that prior to erosion, subglacial sedimentation occurred in closed conduits (eskers) with strong and rapid flow variations. The sediments contain large chute-and-pool structures (high flow energy backset accretion) with dimensions comparable to the conduit width. Therefore, we interpret these sandy-gravel drumlins as portions of eskers, their present drumlin shape being the result of erosion by one or many jökulhlaups.The good preservation of the subglacial meltwater deposits is the result of the closed-basin geometry of the Joux Valley, which limited movement at the base of the glacier. This new contribution to the interpretation of the Joux Valley glacial features underlines the importance of meltwater in sedimentological processes under the Jura ice sheet.

Modelling of GPR waves for lossy media obeying a complex power law of frequency for dielectric permittivity

ABSTRACTThe attenuation of ground‐penetrating radar (GPR) energy in the subsurface decreases and shifts the amplitude spectrum of the radar pulse to lower frequencies (absorption) with increasing traveltime and causes also a distortion of wavelet phase (dispersion). The attenuation is often expressed by the quality factor Q. For GPR studies, Q can be estimated from the ratio of the real part to the imaginary part of the dielectric permittivity.We consider a complex power function of frequency for the dielectric permittivity, and show that this dielectric response corresponds to a frequency‐independent‐Q or simply a constant‐Q model. The phase velocity (dispersion relationship) and the absorption coefficient of electromagnetic waves also obey a frequency power law. This approach is easy to use in the frequency domain and the wave propagation can be described by two parameters only, for example Q and the phase velocity at an arbitrary reference frequency. This simplicity makes it practical for any inversion technique. Furthermore, by using the Hilbert transform relating the velocity and the absorption coefficient (which obeys a frequency power law), we find the same dispersion relationship for the phase velocity. Both approaches are valid for a constant value of Q over a restricted frequency‐bandwidth, and are applicable in a material that is assumed to have no instantaneous dielectric response.Many GPR profiles acquired in a dry aeolian environment have shown a strong reflectivity inside dunes. Changes in water content are believed to be the origin of this reflectivity. We model the radar reflections from the bottom of a dry aeolian dune using the 1D wavelet modelling method. We discuss the choice of the reference wavelet in this modelling approach. A trial‐and‐error match of modelled and observed data was performed to estimate the optimum set of parameters characterizing the materials composing the site. Additionally, by combining the complex refractive index method (CRIM) and/or Topp equations for the bulk permittivity (dielectric constant) of moist sandy soils with a frequency power law for the dielectric response, we introduce them into the expression for the reflection coefficient. Using this method, we can estimate the water content and explain its effect on the reflection coefficient and on wavelet modelling.

Magnetic and Electromagnetic Induction Studies at Archaeological Sites in Southwestern Jordan

In support of ongoing archaeological excavations in southwestern Jordan, a number of geophysical studies have been performed in this region. The work reported was executed in three different years and, in the first year, electromagnetic induction, magnetometry, and ground penetrating radar studies were performed at three sites in the region known to have buried stone walls. While integrated geophysical methods were planned, this first field experience revealed that electromagnetic induction provided, by far, the most useful results and this became the mainstay of subsequent geophysical investigations. To date, five large sites and numerous smaller sites have been surveyed ranging in dates of occupation from the Pre-Pottery Neolithic to the Iron Age. Results from three of the larger sites are reported here. These sites are sufficiently large that they cannot be completely excavated in a single field season and the results of the geophysics are used to identify the most interesting areas at a particular site for subsequent excavation. At the oldest site, the Pre-Pottery Neolithic, the three-dimensional structure of buried stone walls are characterized. At an Iron Age site, walls and magnetic artifacts within a fortress are revealed and a possible tomb containing metal artifacts was discovered immediately outside of the fortress. A third site surveyed, a Biblical copper mine, was exploited by cultures spanning several millennia.

Ground-penetrating radar facies characterization of deepwater turbidite outcrops

High-resolution outcrop studies are becoming an increasingly important tool in building reservoir architectural models (Slatt, 2000). By providing attributes of strata in both the lateral and vertical directions, outcrop studies can help constrain reservoir characterization models (e.g., identify seals or barriers to fluid flow). Outcrop exposures, at best, are limited in their extent. Boreholes, even when they are spaced tightly together, may yield an inaccurate picture of sedimentary features. Because the type and scale of information important to hydrocarbon production can be beneath the resolution of 3D seismic data, there is a need for a geophysical method that yields a high-resolution, continuous image of the subsurface. Ground-penetrating radar (GPR) is one of the more recent geophysical technologies used in examining outcrops (Slatt, 2000;Young et al., 2001). GPR is an electromagnetic imaging procedure that generates reflections from boundaries between layers and can use modified seismic processing to create images (Young et al., 1995). The short wavelengths of GPR signal provide data, which is subseismic scale in resolution. The resulting profiles are good for interpreting small-scale sedimentary features of bed continuity, stratification patterns, and facies boundaries. This paper presents the results from a GPR study of the Lewis Shale turbidite outcrops in Wyoming. The objective of the project is to collect subseismic scale resolution data to aid in determining small-scale sedimentary features. The interpretations, in turn, could be used to provide a better understanding of fluid flow and reservoir production modeling. The Lewis Shale, a Cretaceous age turbidite deposit, is an active gas exploration target and analog for the lower Cretaceous sequences in South Africa and tertiary deposits in the Gulf of Mexico. The formation crops out along the western edge of the Rawlins-Sierra Madre uplift within southeast Wyoming. The geology of three areas along the Lewis Shale outcrop has been studied. …

Ground penetrating radar study for hydrogeological conditions related with mining activity

In the Bicholime, Goa, there is an iron-ore mine surrounded by villages and small towns. Exploitation of such an enormous amount of iron ore may disturb the local groundwater table, thereby causing groundwater depletion. However, these effects are dependent mainly on the geological formations in the region and the mining method. The effects of Bicholime mining on the surrounding groundwater resources and subsurface strata conditions are the main objective of this study. For this purpose, ground penetrating radar surveys were carried out to study the possible effects of mining on the hydrological regime of this open-pit mine, and the subsurface conditions in the Bicholime opencast iron-ore mine, Goa. The results of these surveys show that laterite layers exist from the surface to depths varying from 5 to 22 m at all study locations in the mining area, and in most places clay layers exist, except for the GPR section along the profiles DD1 and EE1 in the village at depths of 8–22 m. Due to the presence of impervious clay layers, mining and dewatering in the leasehold areas will most likely not affect the surrounding hydrological regime of this mining area. It is concluded that no hydrological connection exists between the open pits and the surrounding villages, so that sufficient groundwater is available in the village area, in spite of the enormous extraction of iron ore from the underground.

Sedimentary architecture and radar facies of a fan delta, Cypress Creek, West Vancouver, British Columbia

Abstract Fan deltas are common depositional features along the macro-tidal, fjorded coast of southern British Columbia. To determine the sedimentary architecture of the Cypress Creek fan delta, West Vancouver, British Columbia, a combined ground penetrating radar and geomorphological study was carried out. Five radar facies were identified and their associated geomorphic environments were established: oblique clinoform radar facies (delta front); mounded radar facies (debris-flow deposits); undulating subhorizontal radar facies (beach zone); concave radar facies (river channels) and horizontal radar facies (salt-water intrusion). The delta plain was formed primarily by debris flows, some of which reached the delta front to form prograding foreset beds. Radar facies analysis established the occurrence of arcuate scars and masses of slumped material, signifying mass movement on the delta front. Wave action, working over a 4.9 m tidal range, has reworked the distal delta-plain debris-flow sediments into an armoured gravel beach. A depositional model is proposed for a fan delta in a macro-tidal fjord setting.

A GPR study of sedimentary structures within a transgressive coastal barrier along the Danish North Sea coast

Abstract The Danish North Sea coast is characterized by the presence of coastal barrier systems. One of these systems, the Holmsland Barrier, is a transgressive wave-dominated barrier. For the purpose of studying large-scale architecture in a transgressive barrier, as well as small-scale sedimentary structures, a ground penetrating radar (GPR) field experiment has been carried out. The study focuses on the identification of high-amplitude reflections of the large-scale architecture of the barrier and the recognition of small-scale structures for interpretation of coastal processes. The observed radar facies fall into two groups, both interpreted as storm washover deposits. One group, dominated by parallel to subparallel reflection, is related to the seaward horizontal stratification in the washover fans. The other group, dominated by sigmoid and oblique clinoforms, is related to delta foreset stratification, indicating that the washover fans are terminated in standing water. The observations derived from the GPR study of the Holmsland Barrier suggest that this transgressive barrier is composed almost entirely of washover deposits with local small amounts of aeolian deposits. This study has shown that the GPR method is outstanding in mapping both large-scale architecture and small-scale internal structures in a coastal barrier.