The use of high frequency transducers, MHz, allowing the resolution to target a few cm thick in the first half meter suspect.

Transcription

1 METHODOLOGY GPR (GROUND PROBING RADAR). In recent years the methodology GPR (Ground Probing Radar) has been applied with increasing success under the NDT thanks to the high speed and resolving power. As a secure effectiveness is the ability to change the definition and penetrate deep inside the half suspect changing only transducers (antennas), maintaining unchanged the mode of acquisition. The use of high frequency transducers, MHz, allowing the resolution to target a few cm thick in the first half meter suspect. The use instead of antennas at low frequency, Mhz, with a high power transmission, allows reaching depths in excess of 10 m, in which case, however, the wavelength of the signal to obtain a resolution target of some decimetre thick. Finally, there is the possibility of using a wide range of transducers intermediate frequency choices based on the purpose of the investigation and ensuring the best resolution / penetration signal. Methodology Time Slices The methods of interpreting classical provide for the reconstruction of sections radar-stratigraphical every profile acquired, the correlation between the anomalies located on adjacent profiles is based on the coincidence of a target on multiple profiles pe on the operator. In order to optimize both the interpretative phase is to obtain a greater readability of the results, was developed software (Gea - Applied Geophysics) simultaneous processing of all profiles acquired on the investigation. This methodology of compilation, called Time Slices, provides high definition of anomalies in the main horizontal development through the construction of sections parallel to the surface investigation located at depths growing. The main advantages of this type of development can be summed up in a reliable correlation of anomalies on adjacent profiles and a precise definition of their extension both horizontal and vertical. Instrumentation The basic instrumentation is a digital SIR System 2 of GSSI (Geophysical Survey System Inc..) North Salem USA, made up of a central unit within which generates the signal sent to the transmitter section is processed and the received signal by section receiving unit. An LCD display allows all operations System calibration and parameter acquisition which recording time, gain and type of filter applied to electromagnetic signals. The same display also allows the display profile while capturing allowing a rapid assessment, during field operations, quality of the signals recorded.

2 Central Unit SIR System 2 All'unità Central is connected through a cable multipolar, the antenna which is designed to transmit electromagnetic signals within the subsurface and to receive the reflections caused by electromagnetic interfaces present at different depths of investigation. The frequency of transmission is determined by the construction unit itself and can vary from 14 MHz to over 2.5 GHz. The data is recorded on a magnetic media (Hard Disk) internal to the Central from which can be transferred to a PC for further processing. Size and weight extremely low enable their use in condisioni criticism where necessary speed in handling instrumentation request within the scope of specific. Data Acquisition field As already anticipated the choice of frequency transducer is closely linked with the specific aim of the survey. Research indirect, based on the effects caused by damage of the artefacts, have as their objective abnormalities medium-sized placed at depths that can reach values close to 10 m. The configuration is typical instrumental thus constituted: -- Transducer from 300 MHz -- Time to full scale than 200 ns (nanoseconds) If the depth is reduced to a maximum of 5 m is preferable to use a transducer to 500 MHz with a time of full scale exceeding 100 ns. For direct inquiries on artefacts is required a level much higher resolution.

3 The optimal configuration is also instrumental in this case in light of the objectives of the survey. For the evaluation of thicknesses: -- Transducer from 1500 MHz -- Time to full scale 20 ns The assessment of the size of the artefacts requiring greater depth investigation, unfortunately achieved at the expense of the resolution, is characterized by: -- A 500-MHz transducer -- Time to full scale 75 ns Obviously configurations proposals represent a standard that must be, in any case, optimized through calibration tests prior to the survey. Where possible configuration must be maintained consistently in order to achieve results comparable across the structure investigated. The geometry of acquisition profile also varies depending on the target; direct investigations require a high coverage of the investigation while indirect may require more spacing between adjacent profiles. FIELDS OF USE Applications to the field geological: Stratification of soil Bedrock Depth Identification of voids Identification of groundwater table Existing cuts (or ditches) Engineering applications: Public utilities mapping Bars surveying Roads, foundations, dams, embamkments surveying Bridges and flyovers Coatings Jet grouting Tunnels (thickness of the vault, infiltration, fractures, ) Airports Masonries Archeology applications: Identification of buried structures Check Status conservation masonry Applications to environment: Landfills Identification leaks liquid contaminants Coatings

4 Engineering applications The search bars (left) and the hanging outside (below) of vertical walls are operations of rapid execution and totally without risk of damage to the surface investigated. Fig.1 The figure Fig.1 presents an incorrect position of the second bar from the left which is not aligned with the rest. Fig.2 The figure FIG. 2, acquired on a platform of concrete, presents the formation of some air pockets over the electrowelded wire mesh, the strong reflections also mask the presence of the wire mesh. Fig.3 Pipes, also small, can be easily localized even if covered by electrowelded wire meshes. The figure on the left presents a profile GPR at 1500 MHz. The anomalies' to hyperbole 'of Fig.3 are caused by two heating pipes inside a wall.

5 Fig.4 The right Fig.4 present the position of five tanks in an industrial area. The determination of position and depth makes it much faster reclamation of abandoned areas. The three reflections' to hyperbole 'under the tanks are due to signal interference additive. Investigations on roads The definition of thicknesses of the different layers is now possible through the use of high frequencies. It starts from 1 GHz up to 2.5 GHz. With these frequencies detail of the GPR reaches levels until a few years ago unexpected. The profile in hand, acquired with an antenna from 1.5 GHz, is on a survey carried out on the road pavement of a large urban center. The information campaign so high frequencies are often limited readability, you must then process the data in order to highlight relevant information to the definition of the objects of investigations. Fig.5

6 In the previous example, for defining any anomalies contact the background - paving asphalt, the application of a pass filter - low has allowed the removal of all high frequency components of reflections, maintaining unchanged useful information. The definition of speeds of propagation of electromagnetic waves is crucial for the exact definition of thicknesses of the various levels constructive. The analysis carried out on the sample data showed different speed in layers, characterized by a dielectric constant ε r = 3.3 which represents a speed of 16.4 cm / ns, and part of the background, characterized by a dielectric constant ε r = 7.8 with a speed of 10.7 cm / ns. These values are used for the conversion of radar-stratigraphic section, according times, in the section stratigraphic expressed in deep. The display data mode wiggle, where the profile is represented by a sequence of waveforms in a graph time / amplitude, is often very useful for defining the thickness as it allows to follow with greater ease phases of the signals relating to individual reflectors. The figure above shows the previous example on which you can detect reflection of binder thickness of 6 cm, in red, and the background, colour green. In blue is shown a delamination of the background. At the bottom, about 50 cm in depth, there is an electric pipe indicated with yellow. As part of it is important to note that the thickness of the binder remains constant throughout the section constant The ability to define any anomalies layer of foundation is extremely useful for proper management of maintenance, combining relief GPR continuously measures with the deflection for sections homogeneous through HWD or FWD. The importance of surveying roads with GPR + HWD is demonstrated by the need to identify the real cause of the damage of the pavement between lack of load-bearing capacity of layers of foundation or end of useful life of the layers of pavement surface related. The responses obtained from this procedure is useful for choosing the correct method of intervention, for example cold recycling with foam bitumen in the first case or cold recycling with bitumen emulsion in the second.

7 Depth Types of antennas and optimum depths for surveys of road pavements.