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33 a p p e N D I x a accuracy The surface dielectric values calculated from the air-coupled ground-penetrating radar (GPR) data are used to determine where to test with in-depth nondestructive testing (NDT) devices. The surface dielectric is calculated as follows: A A A A a m m 1 1 1 1 2 ε = + ï£«ï£ ï£¶ï£¸ â ï£«ï£ ï£¶ï£¸       where ea = dielectric of the lining surface, A1 = amplitude of reflection from the surface in volts, and Am = amplitude of reflection from a large metal plate in volts (this represents the 100% reflection case). The accuracy of these amplitudes is critical in calculating the surface dielectric. The Texas A&M Transportation Institute (TTI) developed an air-coupled GPR hardware specification that contains the requirements for such systems (see Appendix T for a discus- sion of this specification). The specification addresses the accu- racy of the system. The distance measuring indicator (DMI) used with the air-coupled GPR system in this study is accurate within 1 ft. Air-coupled GPR data should not be relied on for accu- rate measurements of the depths of defects in tunnel linings. The researchers believe that the surface dielectric value can indicate where such defects might be located. Testing conducted during this study indicated that the air- coupled GPR data could indicate 1-sq-ft air voids (1-in. thick) up to 3 in. from the lining surface for reinforced linings, and up to 7.625 in. from the lining surface for plain unreinforced linings. The calculated depths of the defects from the TTI air-coupled GPR data analysis software was 2.6 in. and 7.7 in., respectively. Therefore, the system was accurate within 0.4 in. for the shallow void and approxi- mately 0.1 in. for the deeper void. The testing indicated that the air-coupled GPR data could indicate a 1-sq-ft water-filled void at 3 in. from the lining surface. The calculated depth of this defect from the TTI air-coupled GPR data analysis software was 2.7 in. Therefore, the system was accurate with 0.3 in. for this water- filled void. precision The air-coupled GPR hardware specification also addresses the precision of the system to ensure that the surface dielectric mea- surement is precise. The measurement results of the DMI used with the TTI air-coupled GPR system are repeatable and repro- ducible within 1 ft. Calibration procedures The air-coupled GPR hardware specification is also used for calibrating the system. The DMI should be calibrated every 3 months. This is done by traveling over a known distance (minimum 500 ft) and comparing the DMI measurement to the known distance measurement. testing procedures Air-coupled GPR antenna manufacturers have their own GPR-system-specific user manual that should be followed when collecting data. Before collecting data on a tunnel lining, personnel should collect at least 50 air-coupled GPR waveform traces over a minimum 16-sq-ft metal plate (4 ft long by 4 ft wide) at the operating height of the antenna (between 12 in. and 18 in.). These data will be used to calculate the surface dielectric. During data collection on the tunnel lining, the air-coupled GPR data should be collected at 1-ft intervals or less. Air-Coupled Ground-Penetrating Radar Testing Criteria
34 Cost The price for a complete air-coupled GPR system with survey van and mounting is usually between $180,000 and $200,000. Limitations The air-coupled GPR surface dielectric is recommended for use in determining where to test tunnel linings with in-depth nondestructive testing devices. At present, only one company manufactures Federal Com- munications Commission (FCC)âcompliant air-coupled GPR systems for sale in the United States. However, several air- coupled GPR service providers in the United States provide data collection and interpretation services. They may use the FCC-compliant systems or grandfathered systems similar to the 1-GHz system used by TTI in this study. Air-coupled GPR data should not be relied on to accurately measure the depths of defects in tunnel linings. Air-coupled GPR can detect 1-sq-ft defects up to a depth of 3 in. for rein- forced linings and 7 in. for plain unreinforced linings if they contain a significant amount of air (such as a 1-in.-deep air gap) or a significant amount of moisture (such as a 1-in.-deep water- filled void). External electromagnetic radiation such as cell phone antennae, radio antennae, and television station antennae can cause signal degradation. Salts (either from deicing opera- tions or from seawater) in the concrete may result in signal penetration problems. Also, steel fibers in shotcrete prevent air-coupled GPR signal penetration. Concrete containing steel slag can prevent air-coupled GPR signal penetration as well. Data Management Commercially available air-coupled GPR systems (such as the FCC-compliant system described at the webpage http://www .geophysical.com/antennas.htm) come with data collection and management software. Some air-coupled GPR service providers also have their own data management software. Data analysis and Interpretation Data analysis software is provided by the manufacturer of such systems. Air-coupled GPR service providers may also provide data analysis and interpretation services. The surface dielectric data are easy to calculate using avail- able software. However, data interpretation for locating sub- surface defects can only be done by experienced, trained users and usually demands engineering judgment.
