Georadar technology has proven to be highly effective in the diagnosis and analysis of soils and the most diverse applications thanks to its effectiveness and non-destructive characteristics. This article describes and analyzes the use of georadar technology supported by other complementary ones, such as the ground electromagnetic survey, infrared thermography, infrared photography and multispectral photography, all of them used together for a common objective in concrete examples related to the field of the construction. Subsequently, the characteristics of the construction systems and the environment that can condition the effectiveness of the results and reduce the advantages of the joint use of the different systems are analyzed, which will allow future actions to adjust the scope of the objectives, optimize the advantages the complementarity of the different systems and, therefore, will open a range of possibilities for future use.
La tecnología del georradar se ha probado de gran eficacia en la diagnosis y análisis de suelos y las más diversas aplicaciones gracias a su efectividad y sus características no destructivas. El presente artículo describe y analiza la utilización de la tecnología del georradar apoyada en otras complementarias, como son el perfilómetro, la termografía infrarroja, la fotografía infrarroja y la fotografía multiespectral, empleadas conjuntamente en ejemplos concretos de análisis de construcciones con objetivos relacionados con la detección y diagnóstico de deficiencias en los proyectos. Posteriormente, se analizan las características de los sistemas constructivos y del entorno que pueden condicionar la efectividad de los resultados y merma las ventajas de la utilización conjunta de los distintos sistemas, lo que permitirá en futuras actuaciones ajustar el alcance de los objetivos, optimizar las ventajas de la complementariedad de los distintos sistemas y, por tanto, permitirá abrir un abanico de posibilidades de utilización futura.
The georadar is included among the non-destructive techniques used in construction, among other sectors, which allows the analysis and identification of parameters with a greater degree of efficiency, and which is experiencing a moment of study due to its wide possibilities (
Also called Terrestrial Penetration Radar or Ground Penetration Radar (GPR), it is a system based on the measurement of the transmission of ultra-wide band electromagnetic waves (
The history of the georadar begins in 1904, when Christian Hülsmeyer developed the first patent on radar technology. Already in 1910 Gotthelf Leimbach and Heinrich Löwy applied this technology to locate buried objects in the subsoil, patenting a system that used surface antennas together with continuous wave radar. The system had a substantial advance when in 1926 Dr. Hülsenbeck developed and patented a pulse radar system, which allowed a significant improvement in the resolution of the results and in the depth of the range of the system, so we can say that it is the basis of current systems.
The use of the system had an example of relevance in 1929, when W. Stern measured the depth of a glacier in Austria (
Rodriguez Abad (
This paper aims to analyze projects carried out by the company Falcon High Tech, a company highly specialized in the use of various technologies that, together, complement their results by leveraging of the various advantages of each of the systems to improve the scope of the final results. The analysis of these results allows us to establish parameters that limit the scope and therefore possible uses in the construction sector
Before studying the different technologies, it is necessary to summarize some concepts that support the principles on which the operations of the different systems are based.
Conductivity in materials is the ability they have to allow electric current to pass through them. It is also defined as the natural property characteristic of each homogeneous environment that represents the ease with which electrons (and holes in the case of semiconductors) can pass through it.
The electrochemical activity caused by electrolytes, which circulate in materials, is the basis for electromagnetic methods, both self-potential and induced polarization (
At present, geophysical methods are the main procedures for prospecting and detecting anomalies in structures involving air or water. The chosen technique depends fundamentally on the environmental context and the materials to be used.
To know the conductivity of water you have to know what type of water we are talking about. Pure water, H2O, does not conduct electricity. However, practically all the water with which we are in contact (in the tap, mineral, rain, sea ...) is water with a solution of salts in different concentrations. Since the salts within the water have the ability to carry electrical energy (
Next, we analyze the technologies that will be used in the projects, both the main one, such as the georadar, and others complementary to the first, such as thermography, infrared aerial photography, multispectral and electromagnetic profiler.
Ground Penetrating Radar technology is based on the transmission and reflection of electromagnetic waves, that is, on their emission and subsequent reception of the different reflections that the different materials which make up the object studied.
This technology uses a balanced matrix for a pair of bistatic antennas, which allows combining the results on a grid at a fixed distance. The matrix results in a high resolution definition of the structures of different materials (
Once the general parameters have been established, such as the dielectric constants of the materials that we can expect the sample is made up, the size of the analysis area, the depth of the area that we intend to study and the resolution that we will need to achieve, we must proceed to choice of the most suitable antenna frequency for the objective we intend to obtain, since the lower frequency counts greater depth of penetration, but at the cost of a decrease in the resolution of the results.
When finding a zone with different electromagnetic characteristics, part of these waves are reflected and captured by the antenna. Given the cone shape of the pattern emitted by the transmitting antenna, the areas that extend with respect to the vertical will arrive with a greater delay, reflecting data that will appear to be of deeper than its reality (
The horizontal resolution of the GPR will depend fundamentally on the wavelength of the antenna used, in such a way that a 200Mhz antenna achieves a low resolution at depths of up to 10m, while a 2.4 Ghz antenna achieves a very high resolution, which reaches 20,000 points per square meter, although its penetration capacity decreases, being in this case below 0.5m (
Regarding the representation, the georadar uses electromagnetic pulses of very short duration, 1-60 nanoseconds (ns = 10-9 seconds), in the VHF / UHF band (20-1000 MHz), which are repeated with a frequency of 50 KHz. These impulses are grouped into wave packets made up of 1,000-15,000 of them. When these impulses are generated by means of the transmitting antenna, these impulses, in their trajectory through the subsoil, may encounter a change in the geological stratum, a construction, cavities, objects, humidity or water table levels, etc.; In short, although other factors such as density and relative humidity also influence , what they detect is mostly a change in the electrical properties (dielectric constant) of the media in which they propagate. This causes part of the energy to be reflected and collected by the receiving antenna, while the rest continues its way through the interior of the analyzed sample.
Residual moisture is also important, because it affects the speed of the wave (the higher the humidity, the lower the speed) which can distort the results. In metallic elements, density is also important, given that being high, the radar wave does not penetrate it. It also influences if there is salinity in the subsoil or clay, the worst enemies of the radar wave.
This energy is represented by color-amplitude windows that are usually chosen so that they are representative of the objectives of each job. The positive amplitudes correspond to the passage to a medium with a higher dielectric constant, the target being the maximum positive amplitude recorded. The negative amplitudes correspond to the passage to a medium with a lower dielectric constant, gray being the maximum negative amplitude. Since it can be considered that one works with a normal incidence when the reflecting surface is flat and in the case of working in non-magnetic media, the reflection coefficient of the georadar is given by the simplified expression (
Where ε2 corresponds to the dielectric constant of medium 2, while ε1 corresponds to the dielectric constant of medium 1. Applying the principle to studies of reinforced concrete, the existing media will be concrete and the rebar of the reinforcements, these acting as short-circuiting for the emission of the georadar. for which they are characterized by maximum amplitude. In the examples studied, the decrease in this maximum amplitude reflection that must occur in the metallic elements (rebar) should be indicative of the degree of oxidation / corrosion.
A concept that we must bear in mind is that of Electrical Resistivity (
The electrical resistivity of a particular conductor material is a measure of how strongly the material opposes the flow of electric current through it, and is inversely proportional to conductivity (σ). This variable parameter depending on the material and its density, is a concept used in GPR measurement, being the more effective the greater the difference between the different materials that make up the sample analyzed. In this way, sands and sandstones have resistivity values between 100 and 5000 Ωm and clays between 3 and 100 Ωm, compared to 0.0172 Ω.mm2/m for copper or 0.0971 Ω.mm2 /m for iron.
This technology is widely proven as a non-destructive test of special efficiency for its use in the construction sector, from the verification of the thermal conditions of the building envelopes to the detection of various deterioration processes caused by physical and chemical agents in the structures and their elements. They are included as analysis tools to raise planimetries identifying their injuries (
The development of easy-to-use portable equipment and its popularization at affordable costs has allowed its use to be generalized. Likewise, it makes it possible to show the real temperature of an object and observe the variations in temperature and emissivity on its surface by measuring the variations in infrared radiation. The image we obtain is called a thermogram, which instantly shows the temperature at each point on the surface of the object, whether it is stationary or moving. The system can be applied in two main ways, the active and the passive technique.
Active Thermography. It uses a stimulation by means of an external infrared radiation source that heats the surface of the médium, producing a heat radiation that depends on the temperature distribution, allowing the detection of anomalies on the surface or under the surface.
Passive Thermography. In this case, the system analyzes the object’s own emissions without the need for external stimulation, measuring and interpreting among other parameters the surface temperature patterns.
Although aerial photography systems have been used to identify and survey land uses since the mid-nineteenth century (
There are several types of aerial photos (
Vertical photography, which keeps the axis of the machine vertical. They are the most used in the elaboration of maps.
Oblique photography, in which the optical axis of the camera forms an angle with the vertical. This method covers a greater surface than the vertical, forces to make corrections that rectify the distortions of the image, preventing stereoscopic vision.
Panoramic photography, in which the angle of view of the terrain is greater than 100º, which generates large distortions.
Among the film typology, in addition to panchromatic, which provides images similar to those captured by the human eye, and color, infrared photographs distinguish between black and white photographs, and color infrared, or false color, which have many advantages since in addition to being sensitive to the same spectrum band as black and white, they assign assigned colors to objects. with certain properties or characteristics.
In the technology of infrared color photography, the blue color is completely eliminated, thus filtering the effect of atmospheric light scattering, providing information beyond the spectrum of visible light, between 700 and 1200 nanometers. This type of photography has its origins in 1910, when the American physicist Robert W. Wood published his research to demonstrate the reflection of chlorophyll, by including cryptocyanin as a dye applied to photographic plates. Later, during the First World War, the United States used plates sensitive to the infrared spectrum to detect camouflaged elements. At present, for use with a digital camera we will only have to use an infrared filter.
The evolution of systems that traditionally allowed capturing images for cartographic uses now allows us to analyze time series of physical phenomena that occur on earth. The development of complex algorithms and specific techniques and processes now allow a large amount of data about the interrelation of objects with electromagnetic radiation.
The spectrum is the distribution of the intensity of radiation as a function of a characteristic quantity, such as wavelength, energy, frequency, or mass. In this way, the spectral image is the one that reproduces an object depending on the wavelength that it is emitting or reflecting. This radiation allows a substance to be identified. Electroscopes, in addition to allowing the observation of spectra, make it possible to carry out measurements on their wavelength, frequency and intensity of radiation. The electromagnetic spectrum ranges from the lowest wavelengths of radiation, such as gamma rays, to the highest, such as radio waves. Within this spectrum, the range of radiation visible to the human eye is between 400 nm and 700 nm. Below this band is ultraviolet radiation, and above infrared.
The colors that the human eye appreciates correspond to the wavelengths that objects transmit or reflect, and not those that they absorb, being these complementary to the former. For this reason, if it is intended to observe the radiation absorbed by an object, a suitable source must be used, and study the image band covering the region of the spectrum closest to the used source.
We can then deduce the differences between a multispectral and a hyperspectral image (
While to take color photos as seen by the human eye, RGB cameras (red, green, blue) are mounted, multispectral cameras allow us to visualize radiation that goes beyond RGB, with this type of camera we will be able to capture the red edge (0.68 to 0.75 microns) and the near infrared (0.75 to 1.7 microns).
There is a wide variety of multispectral cameras and NIR (near infrared) cameras on the market. The choice will depend on the possible use that is going to be given. The use of these systems is wide, and they go as far as the search for the location of hydrocarbon deposits through the exploration of small superficial leaks, constituting a direct method of exploration (Hörig, Kühn, Oschütz and Lehmann 2001).
Also call Electromagnetic Profiler, the GEM-2 is an active method that uses an electromagnetic signal (EM) to detect variations in the electrical conductivity of the surface being analyzed, providing a fast multi-frequency technique for surface geophysical explorations that allows to visualize the geomorphological and geological structure of the subsoil, and the spatial and volumetric positioning of internal anomalies, provided they have a certain entity (
This secondary field is separated into two orthogonal components, which provide, on the one hand, a measure of the apparent conductivity of the analyzed area, and on the other hand, variations that indicate anomalies of different consideration.
A big advantage of the system is its portability, which linked with active GPS tracking allows an operator to collect around 20,000 data points per hour at five frequencies. The depth of penetration into the material that is achieved depends, among other factors, on the conductivity of the material and the chosen EM wave frequencies
Two projects of special interest carried out by the specialized company Falcon High Tech are briefly described below, which, with different objectives, allow a broad vision of the capabilities of the use of the different systems, taking advantage of their complementarity.
The buildings presented some deficiencies in the waterproofing of the roofs caused by a concatenation of mistakes in their construction, which caused repeated pathologies. During the development of the repair work, atmospheric phenomena of an explosive cyclogenesis type occurred, which caused the formation of water pockets between the various layers of roof formation. That problem occurred in a greater and different number of buildings than those reflected in the project. of construction.
These bags produced pathologies of diverse consideration because of the leaks into the houses. The property made, with little success, an attempt to locate and eliminate the bags by opening on site test holes. The method was unsuccessful so, the work was entrusted to a specialized company with the aim of locating, spatially identifying, and, where appropriate, quantifying the possible existence of dammed rainwater.
To carry out the work, the company used the following coordinated systems:
Thermography and infrared aerial photography (TIR / IR) on the entire surface analyzed.
Realization of longitudinal and perpendicular profiles (X, Y) by means of GPR in the whole grid. (1.20 m thick grid mesh).
Carrying out longitudinal and perpendicular profiles (X, Y) using a grid GEM with the same mesh pattern as the GPR.
The GPR equipment was used manually following the same grid mesh methodology, with the advantage of its resolution greater than 20,000 information points per square meter.
The use of the multifrequency electromagnetic GEM during data collection was carried out by means of a sweep at an approximate height of 0.20 cm above the surface. The equipment allowed to carry out an exact positioning of the anomalies that were detected, since it was equipped with a GPS differential and three frequencies, in addition the data obtained was reliable even at depths greater than 1 m for this investigation. Both technologies were carried out under very complex conditions, given the location of the prospecting.
The thermal differences in the analyzed grid were an indicator of the existence of deficiencies inside different geo-morphological structures (contamination, cavities, water, decay, etc.), since the facing acts as a heat collector during the day and from dusk it radiates the energy by thermal dissipation to the colder air. As there is a layer of different density and composition, a variation in the temperature of the facing could be detected in these areas. The presence of moisture on the surface and / or shallow levels would delay thermal dissipation and, in any case, would considerably lower the detected temperature (
During the collection of thermographic data, the daytime atmospheric temperature remained at very low thermal values, so there was no great difference in thermal dissipation, which slightly impaired the thermal differentiation capacity of the structure of the area.
The primary anomalies were verified using high-resolution ground geophysical methods. In general, the surfaces presented a very low profile gradient and a heterogeneous thermal level.
Likewise, both the GEM and the GPR detected electromagnetic anomalies associated with the existence of water inside the roofs of the building envelopes, most of them grouped in the upper and central layers, between 0 and -30 centimeters deep, medium-sized and current in nature, very difficult to identify due to the presence of rainwater on the surface. The areas with the greatest accumulation were positioned, showing maximum accumulations of water in the free phase of up to 2 cm.
However, the accumulation of constitutive layers of the roofs, with a very marked difference in their dielectric constants and their geophysical properties, significantly hindered the interpretation of the final data, and therefore the determinations of the conclusions.
Once the field work was finished, the data was dumped into a computer for subsequent export, treatment, representation and interpretation, using specific software for each task. The GEM-2 software: WinGEMv3 was used for the data dump and export, while the Surfer9.0 and Matlab6.5 programs were used for the treatment and graphic representation. Within the data processing, several processes were carried out. Depending on the results that were obtained, decisions were made in the processing of the records, that is. Ranges of values were eliminated, certain contrasts or zones were accentuated, filters, operators were applied, using various color scales, etc. In this way, maps of magnitudes were obtained in which we will be able to appreciate, in the best possible way depending on the sedimentary environments, the most interesting contrasts or anomalous areas according to the analysis objectives.
The data obtained was treated by the specialized company with its own system for its processing, mostly by using the following commercial software:
Georadar: Radan 6.6 And Radan 7.0. Surfer 10.0
Electromagnetic Profilometer: Magmap, Surfer 10.0
Proton Gradiometer: Magmap, Surfer 10.0
Infrared-Multispectral Sensors: Flir Reporter 8.0, Flir Quick Report, Adobe Photoshop, Pix 4-D
In one of the supports of the isostatic central slabs, a serious accumulation of residual moisture was detected that caused advanced internal corrosion and delamination, recommending the adoption of immediate safety measures, as well as the realization of intrusive analysis, since it could not be ruled out the sudden collapse of that point, given the degree of its structural affection (
As a brief summary of the study’s conclusions, we can point out that the analyzes carried out are conclusive in that both slabs are clearly differentiated in their degree of internal density, and even in the internal composition of concretes and rebar. Significant electromagnetic anomalies were detected in both bridges, very marked in the decks, and less important in the piers and abutments, associated with internal corrosion, delamination and fractures, due to the erosive action of residual moisture.
In relation to the main points with anomalies and / or pathologies, the main problem detected was internal corrosion, a decrease in the density of the concretes due to the massive entry of residual moisture, particularly serious in three of the four connection areas between the concreting and the two isostatic zones. These isostatic areas are in good condition except for the almost superficial layer, where corrosion is beginning to act.
We can summarize that the GPR has a limited penetration capacity, approximately between 4 and 5 meters of maximum depth in a medium soil, in relation to the high resolution antennas used, the only ones capable of detecting dielectric constant associated with anomalies or internal pathologies in a structure, as well as areas affected by hydrocarbons, water, heavy metals or exogenous chemical compounds. In this way, the data obtained can be interfered with, and therefore its readings altered, by strong electromagnetic fields or the massive presence of saturated clays in the subsoil, or the presence of salt water.
In the case of the GEM, its penetration capacity is limited, approximately between 8 and 10 meters maximum depth in a medium soil, in relation to the high resolution frequencies used, between 50,000 Hz and 1,000 Hz, the The only ones capable of detecting dielectric constant associated to the affection of hydrocarbons, water, heavy metals or exogenous chemical compounds, and in the same way as in the georadar, the readings can be interfered with and altered by electromagnetic fields of great intensity.
As for the TIR and IR, they have a limited penetration capacity, approximately between 1 and 2 meters of maximum depth in medium soil. Their readings can also be interfered with and therefore altered by architectural structures in the subsoil, cavities with air or water, service pipes and pipes with fluids under pressure or high temperatures or dense geological layers such as rock outcrops.
The GPR is the geophysical system with the highest resolution, and the profilometer is a complement through conductivity. The two systems together are unbeatable as non-intrusive and non-destructive methods (European GPR Association). The georadar basically detects the dielectric coefficient of the analyzed element, and in the event that the sample presents internal pathologies, such as fractures, fissures, repositioning, decay, etc., the constant of its dielectric coefficient varies, as there are other compounds (air, water, etc). The use of the Profilometer system is completely complementary, since it indicates the conductivity values. And although it provides a lower resolution, it has the great advantage of being easy to deploy in the data collection phase.
While some materials used in construction have ranges of variation of their electromagnetic parameters that we can qualify as small, such as asphalt with a dielectric permittivity (ξ) between 3 and 8 and a conductivity (σ) between 0.1 and 1 mS / m, or a metal pipe with values of 1 and 108 respectively, in the case of concrete (
On the other hand, the porosity of the material and the fluid contained between the pores largely determine the electromagnetic parameters of the material to be studied, which affects the speed of propagation of electromagnetic waves (
Although water also has differences in the parameters of its relative permeability depending on its state and even on the temperature, a value of 0.81 is usually taken to simplify the calculation of the speed, which corresponds to an ambient temperature. of about 180. Ice and snow present lower values due to the percentage of air they contain.
The presence of water will therefore be decisive for the determination of speeds (
The thermal differences in the analyzed structures are an indicator of the existence inside of a different geo-morphological structure (cavities, water, contamination, decay, corrosion, etc.) since the facing acts as a heat collector during the day and from at dusk they radiate the energy by thermal dissipation to the colder air. When there is a layer of different density or a pathology, a variation in the temperature of the facing is detected in these areas. The presence of moisture on the surface and / or shallow levels delays thermal dissipation and always considerably reduces the detected temperature. The existence of internal cavities in a concrete structure causes an increase in temperature in these layers, which can be detected and differentiated by infrared.
During the collection of thermographic data, it is important to highlight that a difference in daytime and nighttime temperatures and humidity benefits the thermal differentiation capacity of the structure of the area to be analyzed, the better the greater the difference in thermal dissipation or thermal gradient.
As it is an eminently practical infrared, GPR, sonar and GEM inspection, prior but closely linked to a specifically technical interpretation and remediation of the problem suffered by the sample, and with the purpose of showing that the results of a GPR inspection and / or GEM are easily usable by professionals outside Geophysics, the reports do not include explanations of a mathematical and technical nature about their theoretical operation, methodology and post-processing techniques used.
The GPR and GEM technology, used in a complementary way, are non-intrusive and non-destructive technical instruments to analyze concrete structures and detect possible pathologies such as cavities, corrosion, internal fractures or delamination. The deterioration of the external and internal structure of the constructions is the cause of the combination of several complex phenomena, the physically induced (thermal gradients, volumetric changes, abrasion, erosion, cavitation,) and the chemically induced (chlorides, carbonates, sulfates and attacks acids) that cause internal corrosion, generally due to the entry of moisture. The internal corrosion of the bars also causes the deterioration of the concrete structure, and is detected by these systems due to the increase in its conductivity.