Analysis and calibration of Ground Penetrating Radar shielded antennas

Ground-Penetrating Radar (GPR) is a non-destructive geophysical method applied in many civil engineering applications. The knowledge of the GPR antennas behavior is crucial to obtain accurate results and comprehensive data interpretations. However, the age of the antennas and the use can modify the values provided by manufacturers. Consequently, calibration of antennas is recommended. Calibration involves determining several parameters. In this paper, the evaluation of two parameters (stability of the signal and zero-time position) and the analysis of the effects of stacking are presented. Three antennas characterized by nominal center frequencies of 500 MHz, 800 MHz and 1.6 GHz are analyzed. The direct wave was used for the stability analysis in two propagation media: air and concrete. The results of the stability evaluation seems to indicate that the emitted signal and the reconstruction of A-scans are quite stable and noise as consequence of electronic noise is low. Zero time position was measured in air, calculating the propagation time for the reflection on a metallic surface. © 2018 IEEE.Postprint (published version

Lab non destructive test to analyze the effect of corrosion on ground penetrating radar scans

Corrosion is a significant damage in many reinforced concrete structures, mainly in coastal areas. The oxidation of embedded iron or steel elements degrades rebar, producing a porous layer not adhered to the metallic surface. This process could completely destroy rebar. In addition, the concrete around the metallic targets is also damaged, and a dense grid of fissures appears around the oxidized elements. The evaluation of corrosion is difficult in early stages, because damage is usually hidden. Non-destructive testing measurements, based on non-destructive testing (NDT) electric and magnetic surveys, could detect damage as consequence of corrosion. The work presented in this paper is based in several laboratory tests, which are centered in defining the effect of different corrosion stage on ground penetrating radar (GPR) signals. The analysis focuses on the evaluation of the reflected wave amplitude and its behavior. The results indicated that an accurate analysis of amplitude decay and intensity could most likely reveal an approach to the state of degradation of the embedded metallic targets because GPR images exhibit characteristics that depend on the effects of the oxidized rebar and the damaged concrete. These characteristics could be detected and measured in some cases. One important feature is referred to as the reflected wave amplitude. In the case of corroded targets, this amplitude is lower than in the case of reflection on non-oxidized surfaces. Additionally, in some cases, a blurred image appears related to high corrosion. The results of the tests highlight the higher amplitude decay of the cases of specimens with corroded elements.Peer ReviewedPostprint (published version

Development of new GPR methodologies for soil and cement concrete pavement assessment

The study of surface geology by means of Ground Penetrating Radar (GPR) can provide information about abrupt lateral changes in the terrain. The location ofthese changes is very useful in seismic nanozonation studies, since it allows determining a priori the sectors in which the meas ures should be intensified . The thesis analyzes the effect of changes in granulometry and com paction in the subsurface radar records . lt is observed that a greater heterogeneity of the terrain produces an increase in the background noise ofthe signa! (clutter) dueto the backscattering ofthe waves. The analysis of the amplitude ofthe incoherent signa!, determined in each A-sean by means ofthe average value obtained in a certain time window, makes it possible to obtain graphs ofthe amplitude ofthe background noise versus the position ofthe trace in the profile . The results clearlydefine the sectors in which the amplitude value increases, associating them with the existence of heterogeneous materiaIs in the most superficial zone of the subsoil. On the other hand, the analysis of the frequency content ofthe received radar signa! shows abrupt changes that are sometimes related to the increase in the clutter amplitude. However, at other times they do not appear , even though there is an increase in noise. To analyze the effect, a seasonal study was carried out in an area eros sed by underground streams , comparing the results in the dry season with the results in the rainy season . In temporal space, the underground streams could be detected thanks to the increase in clutter. The results were compared with those obtained by passive seismic . In the frequency space, greater changes in frequencywere observed in the rainy season, while in the dry season these changes only appeared in the records obtained in the area where there was one of the streams. lt was concluded, therefore, that the study of the amplitude s pectrum could be used to determine the presence ofwater and, in the case ofstreams, to differentiate between active streams and drychannels. These results are of great interest for the planning of trans port infrastructures. In the second part ofthe thesis, damage to transport infrastructure is analyzed. Specifically, on rigid pavements. Often these pavements built by using reinforced concrete slabs, are covered by an asphalt layer, either by design or dueto periodical restorations. The interna! cracks that are generated are not observed on the surface until very advanced stages. Severa! laboratory experiments and field case studies were proposed to determine the ability of the GPR to locate cracks, studying the effect ofthickness, depth and fill material. lt was observed that cracks of a size that allow them to be considered as incipient can be detected, although the length ofthe vertical crack could only be detected in the case of cracks already in an advanced state of damage (of great width) or filled with saturated sand or water. The decrease in the speed ofthe signal due to the presence ofwater allowed a higher resolution. The experimental results were compared and verified with field case studies, on real roads. Similar effects were observed although with a more limited resolution. Us ing com putational methods, the anomalies recorded in the radargrams were related and verified in most ofthe cases, both experimentally and in field studies, with different types of cracks.El estudio de la geologia superficial mediante georradar puede proporcionar información acerca de cambios laterales abruptos en el terreno. La localización de estos cambios resulta especialmente útil en los estudios de nanozonificación sísmica, ya que permite determinar a priori los sectores en los que se deben intensificar las medidas. En la tesis se analiza el efecto de los cambios de granulometría yde compactación en los registros de radar de subsuelo. Se observa que una mayor heterogeneidad del terreno produce un incremento del ruido de fondo de la señal ("clutter") debida a la retrodispersión de las ondas. El análisis de la amplitud de la señal incoherente, determinada en cada A-sean mediante el valor promedio obtenido en una cierta ventana temporal, permite obtener gráficas de amplitud de ruido de fondo frente a la posición de la traza en el perfil. Los resultados definen claramente los sectores en los que se incrementa el valor de la amplitud, asociándolos con existencia de materiales heterogéneos en la zona más superficial del subsuelo. Por otro lado, el análisis del contenido frecuencial de la señal de radar recibida presenta cambios bruscos que en algunas ocasiones están relacionados con el aumento de la amplitud de "clutter". Sin embargo, en otras ocasiones no aparecen aunque se registre un aumento del ruido. Para analizar el efecto se realizó un estudio estacional en una zona atravesada por rieras subterráneas, comparando los resultados en la época seca con los resultados en la época de lluvias. En el espacio temporal, las rieras subterráneas se podían detectar gracias al aumento del "clutter". Los resultados se compararon con los obtenidos mediante sis mica pasiva. En el espacio frecuencial se observaron mayores cambios en la frecuencia en la época de lluvia, mientras que en la época seca únicamente aparecían estos cambios en los registros obtenidos en la zona en la que había una de las rieras. Se concluyó, por lo tanto, que el estudio del espectro de amplitudes se podía utilizar para determinar la presencia de agua y, en el caso de las rieras, para diferenciar entre rieras activas y cauces secos. Estos resultados son de gran interés para la planificación de infraestructuras de transporte. En la segunda parte de la tesis se analizan los daños en infraestructuras de transporte. En concreto en pavimentos rígidos. A menudo estos pavimentos, construidos mediante losas de hormigón armado, están cubiertos por una capa asfáltica, ya sea por diseño o debido a restauraciones. Las grietas internas que se generan no se observan en superficie hasta fases muy avanzadas. Se propusieron ensayos para determinar la capacidad del georradar para localizar grietas, estudiando el efecto del grosor, la profundidad y del material de relleno. Se observó que se pueden detectar grietas de un tamaño que permite considerarlas como incipientes, aunque la longitud de la grieta vertical sólo se podía detectar en el caso de grietas ya en un estado de daño avanzado (de gran anchura) o bien rellenas de arena saturada o de agua. La disminución en la velocidad de la señal debido a la presencia de agua permitía una mayor resolución. Los resultados experimentales se comprobaron en estudios de campo, en carreteras reales. Se observaron efectos similares aunque con una resolución más limitada. Mediante métodos computacionales se relacionaron las anomalías registradas en los radargramas, tanto experimentalmente como en estudios de campo, con distintos tipos de grietas.Postprint (published version

Background clutter amplitude and frequency of GPR signals to analyse water content in sedimentary deposits: Urban infrastructure environment

Soil site studies are crucial in the analysis of seismic hazard in populated areas. This study focusses on the use of the Ground Penetrating Radar as a non-destructive geophysical method to analysis the water content of a sedimentary basin of a local urban area in Cervantes Parks in Barcelona city. Main objective of this work is to understand further the local seismic soil site, and analysis the background clutter amplitude and frequency of the GPR signal applied on the seasonal changes of the underground water content. Changes produces in the GPR signal could be associated to the changes of the ground material due to the existence of water content and water flow, which influence the grain size distribution of the soil characterisation. Previously, several methodologies have been applied to determine the changes of the ground material due to the existence of water content and most importantly effect of the seasonal changes on the soil characterisation. GPR has been applied as a new technology and non-invasive to further analysis water content in shallow geology. Methodology applied on this study compromised in four different phases; Two seasonal data collection using 100 MHz centre frequency shielded antenna in both dry and wet season, radar images are proceeded with common post-processing approaches, following more innovative processing approach based on the Fast Fourier Transform (FFT) method in order to analysis background clutter amplitude and frequency spectrum to achieve an accurate interpretation of underground water content activities, in conclusion a comparative study of the results driven from radar images are elaborated considering historical knowledge. Results show that GPR is an efficient technique to analysis water content invasively considering low frequency antennas to achieve deeper penetration depth in sedimentary deposits, and predict shifting and increasing of the underground streams due to changes in weather conditions. Results could contribute to the enhancing local seismic site behaviour in populated areas considering continuous monitoring of underground water activities in sedimentary basin sites.Peer ReviewedPostprint (author's final draft