GROUND PENETRATING RADAR (GEORADAR) INSPECTION

Transcription

1 - CIVIL ENGENEERING - GEOLOGY AND ENVIRONMENT - GROUND PENETRATING RADAR - LOSSES DETECTING RADAR SYSTEM - ARCHEOLOGY & CULTURAL HERITAGE - CARGO INSPECTION - LOSS CONTROL - CHEMICAL ANALYSIS - INDUSTRIAL SYSTEMS AND TECNOLOGY - RESEARCH GROUND PENETRATING RADAR (GEORADAR) INSPECTION PREFACE... 2 GEORADAR OPERATING PRINCIPLE... 5 OPERATING MODE ON LINES AND PIPELINES... 8 IDENTIFICATION OF SPILL USING GAS TRACERS OPERATING MODE FOR STRUCTURAL CHECKS OPERATIONAL MODE FOR ATMOSPHERIC TANKS REFERENCES Head Off. I Catania, Via Cagliari, 56 T: +39 (095) F: +39 (095) M: info@pccgroup.it T: +39 (095) Via Delle Ginestre Zafferana Etnea (CT) P.IVA REA CT

2 PREFACE Geoadar is an electronic system, able to indicate grounds and materials with considerable details, using the reflection of electromagnetic waves produced by the system. The analysis result is made from tomograms that visually illustrate trends of the discontinuities in the subsurface, thus allowing to detect the presence of metal materials, concrete foundations, pipes, cables, cavities, but also leakage of fluids (spills) in general (water, hydrocarbons, waste water, sewage oily...) and in-homogeneities of various kinds. The survey is carried out using the MCH SMA ISV system, comprising: Radar Control System Antenna sensors Radar Control system The radar control system is composed by: Radar control unit DAD (Digital Antenna Driver) Responsible for the generating of antennas control pulses. Technical characteristics of the DAD are as follows: Scan Rate:> 850 Scan / rate Range:> 9999 nsec. Hw Architecture: 16bit Number of samples per scanning: Number of radar channels: 4ch Lan, Battery 2

3 Figure.1 DAD Laptop For control and saving of radar data using the K2 software in charge of radar information management. Figure.2 Laptop for management of radar data. Antenna Sensors Antenna sensors used for prospecting consists of: TR SMA Antenna Consists of 4 radar sensors with 200MHz and 600MHz antennas for surface investigation and suitable for evaluation of underground utilities presence and for the ground saturation condition assessment. 3

4 Figure.3 TR SMA Antenna used for radar prospecting Figure.4- TR SMA Antenna during investigation 4

5 GEORADAR OPERATING PRINCIPLE All real objects absorb electromagnetic waves depending on own electrical characteristics. A homogeneous medium, partially conductor, is defined, as electric point of view, with a pair of values: dielectric constant conductivity Subsoil consists of a heterogeneous medium whose dielectric characteristics are determinant for the identification of well-defined and interpretable signals. The electromagnetic wave generated by the radar is emitted into the ground by a transmitter inside the antenna. When the electromagnetic waves encounter a physical discontinuity, part of the incident energy is reflected, generating a pulse, which is collected by a receiver, with a shape similar to that transmitted but attenuated and distorted in phase and frequency. The transmitted pulse shape is appropriately calibrated so as to obtain a spectral distribution of Gaussian type with the central value that represents the characteristic frequency, or center frequency of the antenna, which corresponds to the dominant frequency of the pulse. The central frequency of the antenna determines the characteristics of resolution and maximum achievable exploration depth. Antennas can operate in three main ways: monostatic arrangement; bistatic arrangement; cross-polar arrangement. 5

6 With the monostatic arrangement, transmitter and receiver are assembled into a single structure, allowing to obtain information throughout the investigated area and to determine the depth of targets. This arrangement is recommended for superficial information such as services and archaeological finds with medium-high-frequency antennas ( MHz). With the bistatic arrangement, transmitter and receiver are separated and placed at a certain distance from each other. The benefit is a more detailed response in the deeper areas, while the disadvantage is the absence of response when the two components are too far from each other. This arrangement is recommended to obtain information from deep areas and is generally used with medium-low-frequency antennas ( MHz) and geological purposes. With the cross-polar arrangement, transmitter and receiver are mutually orthogonal. This arrangement is particularly useful in the detection of targets oblique inclined relative to the direction of antennas movement and, in general, also for special scientific applications. The radar sections represent the results of GPR surveys. The horizontal axis reproduces the direction of the antenna advancement, while the vertical axis represents the direction of pulses penetration. This distance is expressed as delay between the pulse emitted and the reflected one and is equal to two times the antenna-target distance. The delay value is converted into metric value by using the signal propagation speed in the medium. 6

7 Figure.5- Georadar Operating principle The radar display of a target present in the subsoil is shown in Figure 5. The radar signal received by the antenna is always characterized by the presence of "noise" that must be removed in order to emphasize as much as possible the visibility of the wanted targets. Then there are several processing procedures where the signal is subjected before the data interpretation stage. The following are some of the main processes used: Vertical filtering: Every single radar track contains a set of peaks that have a specified period. The purpose of the filtering in the time domain (vertical filtering) is to remove all spurious frequencies, that are not connected to targets present in the subsoil. Horizontal filtering: The set of traces connected with a given structure defines a horizontal frequency; the aim of the horizontal filtering (spatial domain) is to remove the low frequencies (parallel bands) which are not connected to any structure, but which are generated in the area between the antenna and the surface. Migration: Operation used for underground utilities search. In fact, thanks to the strong peripheral sensitivity of the radar antennas, the underground utilities generate reflection with hyperbolic trend and the migration process removes the tails of these hyperboles, keeping only the top point corresponding to the target locatio. In this way it is possible the detection of very near services, which otherwise would provide a very confusing reflection due to the multiple reflections. 7

8 OPERATING MODE ON LINES AND PIPELINES Once the area in which the line to be investigated is situated, as a function of the length, it is divided into sectors to facilitate the software processing of the acquired radar data (see eg. Figure 6) Figure.6- Example of subdivision into different sectors Then, are carried out a number of longitudinal and transverse scans along the road or the surveyed area, sufficient to obtain a good cover (usually scans with arrays of 1 meter both longitudinally and transversely). x y Figure.7- Individual scans viewing The volume investigated is subsequently subjected to tomographic analysis that returns images at different depths on the x, y plane (see Fig. 8). The spacing between subsequent scans may vary depending on the aims. 8

9 Figure.8- Radar tomography at different depths. Fig.9- CAD identification of subservices 9

10 Figure10- Radar map identification of three different underground services Figure.11- Example of 3D display of identified underground utilitie. 10

11 11 Figure.12- Display of longitudinal and transverse scans.

12 IDENTIFICATION OF SPILL USING GAS TRACERS This technology is used to support Georadar. Tracer Tight is the precision integrity testing service that identifies and locates leaks to a high level of sensitivity. The system is able to identify leaks as small as one liter per day. It identifies and locates leaks in underground piping and bulk storage tanks. It uses inert tracer gases that are placed in the contents of the tank or in the line to be check for leaks. In case of leaks, these gases give response thanks to special sensors, properly installed, which qualify the extent and exact location of the spill. This test consists of the following steps: Emptying line; Identification of the line with Georadar system and / or Locator Probes making Measuring the presence of tracer gas in the environment; Insufflation of tracer gas; Relief on ground level to locate the leak. Measurements are made with a mass spectrometer, autonomous and portable, designed so as to have the same sensitivity and selectivity of a laboratory common mass spectrometer. The spectrometer draws a certain amount of air per second, it ionizes all the molecules and allows only ionized trace gas molecules to pass through an electromagnetic mirror system, generating an ion current. These ionized molecules are collected on a collector and the electric current that follows is amplified and digitized to obtain the result. Fig.13- Leaks verifing 12

13 OPERATING MODE FOR STRUCTURAL CHECKS Using special radar antennas is possible to investigate concrete structures such as basin, bridges, tanks, pillars, walls as well as submerged structures. The greater detail of radargrams obtained with very high frequency antennas enables to identify micro-cracks inside the structure and possible hidden deteriorations. Figure.14- Concrete reinforcement detection Figure.15- Structural check of submerged wall 13

14 14 Fig.16- Structural check of submerged wall

15 OPERATIONAL MODE FOR ATMOSPHERIC TANKS After installing a series of piezometers below the investigated tank, a series of scans are performed with a special radar antenna designed for the specific purpose. These scans have the main purpose of identifying ground areas at different dielectric capacity where tank is installed and through the display and overexposure of tomographic maps identify any spills. The following figures show the investigative mode. Figure.17- Probes Arrangement Figure.18- Containment basin inspection 15

16 Fig.19- tank bottom scanning Figure.20-tomography of tank bottom scans 16

17 REFERENCES ENI SpA VERSALIS SpA ERG SpA ISAB srl SARAS SpA API SpA ENIMED SpA CONSORZIO DI BONIFICA CATANIA RAFFINERIA DI GELA SpA RAFFINERIA DI MILAZZO SpA KUWAIT PETROLEUM ITALIA SpA ESSO RETE DISTRIBUZIONE PRIOLO SERVIZI ENEL DISTRIBUZIONE SpA ALFA CONSULTING SASOL 17