1、Standard Practice for Application of Ground Penetrating Radar (GPR) to Highways AASHTO Designation: R 37-04 (2013)1American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-5a R 37-1 AASHTO Standard Practice for Application
2、of Ground Penetrating Radar (GPR) to Highways AASHTO Designation: R 37-04 (2013)11. SCOPE 1.1. This standard practice provides guidance to the highway engineer in the application of noncontact ground penetrating radar (GPR) to transportation facilities. It is intended to instruct the engineer regard
3、ing the specific uses of noncontact radar for pavement layer thickness surveys, quality control of new pavement construction, evaluation of granular base material, identification of zones of asphalt stripping, and assessment of bridge decks. GPR has numerous applications for the transportation indus
4、try, but at this time requires extensive training in its use and interpretation of the data output, as well as experience in local pavement conditions. 2. REFERENCED DOCUMENTS 2.1. ASTM Standard: D4748, Standard Test Method for Determining the Thickness of Bound Pavement Layers Using Short-Pulse Rad
5、ar 2.2. Federal Highway Administration Standard: FHWA/TX-92/1233-1, Implementation of the Texas Ground Penetrating Radar System, Texas Transportation Institute with the Federal Highway Administration, 1992 3. SUMMARY OF METHOD 3.1. Principles of GPRGPR utilizes radio waves as an energy source. They
6、are transmitted into the pavement and reflected at layer interfaces. Radio waves have free space wavelengths on the electromagnetic spectrum ranging from 0.001 m to 10 m. GPR operates in the range of 0.1 m to 10 m, which is the low end of the radio wavelength spectrum. As with all electromagnetic wa
7、ves, radio waves travel through a vacuum at the speed of light. When the radar waves pass through a medium other than a vacuum, the velocity of propagation becomes a function of the dielectric constant of the medium. A dielectric is defined as an insulator between two electrical conductors; the diel
8、ectric constant for any material measures its effectiveness when used as the dielectric of a capacitor. For example, air has a dielectric constant of one. If air in a capacitor is replaced by mica, the resulting capacitance is six times greater, so mica has a dielectric constant of six. Some represe
9、ntative dielectric constant values for earth materials are given in Table 1. 2016 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-5a R 37-2 AASHTO Table 1Dielectric Constants for Construction Materials (R
10、eference 8.5) Material Relative Dielectric Constant (r) Air 1 Water (fresh) 81 Water (salt) 80 Sand (dry) 35 Sand (wet) 2030 Silts 530 Clays 540 Granite 46 Limestone 48 Portland cement concrete (cured) 611 Bituminous concrete 36 3.2. The velocity of a radar wave through a given medium varies in inve
11、rse proportion to the square root of the materials relative dielectric constant r. For example, if a material with a dielectric constant value of 4 has a radar wave passing through it, the wave travels half as fast as it does through air (r= 1) and twice as fast as it would through a material having
12、 an rvalue of 16. In general, radio waves propagate through dielectric materials, but are reflected from conductive materials. When there is a boundary between two materials having different dielectric properties, some of the radar energy will be reflected, and a portion will pass through the bounda
13、ry. The time required for a radar pulse to travel from the source to an interface and back is the two-way travel time, and is dependent on the depth of the interface and the dielectric constant of the material overlying the interface. Converting two-way travel time to information about the depth to
14、the interface can be done by means of the following formula: d = v t/2 (1) where: d = depth, v = velocity, and t = two-way time. 3.3. The velocity of the radar wave is primarily dependent on the dielectric constant of the medium, and can be calculated with the following equation: v = /rc (2) where:
15、c = speed of light. As can be seen from the data in Table 1, the moisture content has a large influence on the dielectric constant, and therefore affects the two-way travel time, so that the greater the amount of water saturation, the lower the radar wave velocity. 3.4. There is another electrical p
16、roperty upon which GPR depends, and that is conductivity. Attenuation of the radar waves (which causes the waves to decrease in amplitude and energy) is caused by higher conductivity of a medium and results in less depth of penetration. Attenuation is related to the frequency spectrum emitted by the
17、 radarthe higher the frequency, the greater the attenuation of the signal. For most pavement materials in dry condition, attenuation of the wave is not a serious problem. However, with some materials, particularly new concrete (within at least 180 days of placement), signal attenuation can have a si
18、gnificant impact on the amount of energy reflected from the pavement structure. 2016 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-5a R 37-3 AASHTO 3.5. In discussing frequency of GPR antennae, it is to
19、 be noted that the signal output is not a single frequency, but rather is a frequency spectrum (bandwidth). An antenna that has a 1-GHz output actually produces a bandwidth frequency distribution having a mean value of 1 GHz. This signal is produced in short pulses, with a comparatively long period
20、of time between pulses, so that the reflected signals may be recorded. The electromagnetic signal pulse is on the order of 1 ns in duration, while the time between pulses may be tens of thousands of ns. Higher-frequency signal pulses have shorter pulse periods; a 1-GHz signal will have a 1-ns pulse,
21、 while a 2-GHz signal will have a pulse of 0.5 ns in duration. 4. SIGNIFICANCE AND USE 4.1. GPR is a geophysical technique that uses radio waves to acquire subsurface information. The system operates by transmitting energy waves into the earth and recording the waves that are reflected off interface
22、s between layers of material with different electrical properties. GPR has been used for several decades as a tool for geologic investigations, particularly for environmental and groundwater applications. In more recent times, a type of radar system developed specifically for transportation applicat
23、ions has evolved using high-frequency, noncontact antennas that can travel over highways and bridges at the speed of traffic while acquiring data. The information from these GPR systems can be used to find voids under pavement, study pavement layer thicknesses, evaluate moisture or density variation
24、s, and assess the condition of bridge decks. 4.2. Agencies planning to use a great deal of GPR data; e.g., using GPR as part of a network-level pavement management system, are likely to find it most cost-effective to purchase one or more GPR systems and invest in the personnel training necessary for
25、 its usage. The cost of training is largely a function of the data processing software used to evaluate data. As the software packages become more user-friendly, there will be progressively less training required for the user. 4.3. Transportation agencies that plan to use a limited amount of GPR dat
26、a, such as occasional distressed pavement examinations on discrete stretches of highway, may find it advantageous to hire consultant contractors to conduct the surveys and perform the analysis, rather than investing in the equipment and training necessary to do it themselves. 5. EQUIPMENT 5.1. Types
27、 of GPR SystemsThere are two basic types of GPR systems, the difference being in the type of antenna used to produce the radar waves. Ground-coupled antennas are used primarily in geologic and environmental applications. They generate radar waves with frequencies of 50 to 500 MHz, although some syst
28、ems can produce radar wave frequencies of greater than 1 GHz. As the name implies, ground-coupled radar antennas must keep in contact with the ground during a GPR survey so that they are dragged by hand or towed slowly (at speeds less than 10 kph). The antenna in a ground-coupled system is generally
29、 a bowtie (dipole) antenna that generates a signal that covers a wide area and has scattered reflections. The system is capable of providing subsurface information to a depth of up to 15 or 16 m, depending on the geologic conditions encountered and selection of antenna frequency, but often will yiel
30、d little information in the uppermost meter of the surface layer because of a phenomenon known as “ringing” caused by a bistatic coupling mismatch. This problem can be minimized by selecting a high-frequency GPR system if the desired target is known to be near the surface. Ground-coupled GPR systems
31、 are used for mapping bedrock and soil strata, detecting buried drums and pipelines, and tracing contaminant plumes. 5.1.1. The type of GPR system most used for transportation facilities, and which is the main focus of this standard practice, utilizes the noncontact horn antenna, which is suspended
32、over the surface of the ground and which can perform surveys at speeds of up to 80 km/h. Usually, the antenna in this system is designed with a narrowly focused beam of electromagnetic energy, rather than the larger 2016 by the American Association of State Highway and Transportation Officials. All
33、rights reserved. Duplication is a violation of applicable law.TS-5a R 37-4 AASHTO beam width used in ground-coupled GPR, resulting in less backscatter from the surrounding area. The operating center frequency range is typically around 1 GHz, but some systems are now available that have frequencies a
34、s low as 0.5 GHz and as high as 2.5 GHz. These high-frequency broadband radar signals yield excellent resolution, permitting identification of very thin layers. The noncontact systems have the advantage of providing near-surface information, unlike most ground-coupled GPR systems. The drawback of hi
35、gh-frequency signal generation is that, under typical conditions, the depth of penetration is limited to about 0.6 m. Therefore, noncontact GPR can yield information about pavement and sub-base strata but will yield very little information below that. 5.2. GPR EquipmentA GPR system consists of the f
36、ollowing components: a signal generator; an antenna that transmits and receives the radar pulse signal; a sampler/recorder that collects the returned signal and stores it; a signal processing system to convert the pulse data into a waveform; a monitor for viewing the data; and a recording system for
37、 storing data. The antenna is on a boom directly attached to the front or the back of the vehicle. All the other equipment is contained within the survey vehicle. A distance calibration system is tied into the GPR survey so that anomalies can be located precisely. It is useful to operate a synchroni
38、zed video imaging system during the GPR survey so that a visual record of the pavement surface is available during the data processing. Two operators are recommended during the survey, one to devote full attention to the safe operation of the vehicle, and one to operate the equipment. 5.2.1. The min
39、imum GPR configuration is a single antenna radar system. For pavement surveys, a single antenna pass per lane is adequate for thickness evaluation. For condition surveys (void mapping and bridge deck evaluation), multiple antenna passes per lane are preferable to achieve the required level of detail
40、. This can be achieved by making multiple passes with a single antenna vehicle or by using a radar vehicle equipped with multiple antenna systems (Figure 1). Obviously, in a high-traffic area with limited highway access, the multiple antenna vehicle is more efficient to use for this type of survey a
41、s it requires less traffic protection and fewer cycles of entering and exiting the highway. Figure 1Multiple Antennae on a GPR Survey Vehicle 5.3. Data OutputWhen the GPR electromagnetic energy is directed into the ground, some of it is reflected from interfaces between materials of differing dielec
42、tric values and returns to the GPR receiver; the reflected pulses are the only part of the transmitted signal that is seen on the GPR monitor. The energy of the reflected radar pulse is converted to a waveform that is displayed on a monitor as a graph of amplitude (in volts) against time. Reflection
43、s are in phase with the emitted 2016 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-5a R 37-5 AASHTO signal if the pulse travels from a layer with a low to a higher dielectric and out of phase if the sig
44、nal goes from high to low dielectric (Figure 2). The waveforms can then be displayed in a variety of formats; one method is to use the stacked waveform approach (Figure 3). The most common way of viewing GPR data is by the use of a color transform system. The line scans shown in Figure 2 are color-c
45、oded based on signal amplitude; the individual waveforms are then stacked side by side to form a color-coded image of subsurface condition as shown in Figure 4. Figure 2Typical GPR Reflections 2016 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplic
46、ation is a violation of applicable law.TS-5a R 37-6 AASHTO Figure 3Stacked GPR Waveforms from a Section of Highway Note: The depth scale is on the right of the figure and the distance along the highway is shown in the lower scale. Figure 4Color Transform Output 2016 by the American Association of St
47、ate Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-5a R 37-7 AASHTO 5.3.1. The amplitude of the displayed waveform is a function of the difference in dielectric constant between two material layers; the greater the contrast, the larger the
48、waveform amplitude of the returned signal. Two layers may have different material characteristics, but unless they have differing dielectric constants as well, they will not yield a noticeable GPR reflection. The reflected amplitude is also a function of the conductivity of the material through whic
49、h the radar pulse travels. Lossy dielectrics (i.e., material having higher conductivity) tend to attenuate the signal, reducing its amplitude and yielding a less distinct contrast between layers. 5.4. Data ProcessingCurrent GPR units may produce waveforms at the rate of 50 per second, or 180,000 scans per hour and greater (Section 8.1). It is recommended that computers are used to store the data output onto hard drives or CD-ROMs. 5.4.1. While great strides have been made in developing computer software for signal processing and storage, it should be
