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GPR surveys on a dike near St. Jacobiparochie

Tomi Herronen & Timo Saarenketo 2008

1. Introduction In April 2008 Roadscanners Oy survey group from Finland did Ground Penetrating Radar (GPR) surveys on a dike in the Netherlands, in Friesland, near the village of St.Jacobiparochie. The primary goal of these surveys was to test what is the best way to utilise GPR technique in locating areas of disintegrated asphalt and other structural defects in the asphalt layer on the top of dike and the layers beneath it. This survey was a continuation of the tests, done on the Friesland and Hellegatsdam test sections in summer 2007, which produced promising results. An additional goal was to test recently developed signal processing techniques to see if they could be used to improve the test results.

The surveys were done utilizing two different types of GPR systems, one a pulse radar horn antenna system with two central frequencies and the other was a stepped frequency 3d GPR system with full 31 antenna setup.

The survey site location is presented in figure 1.

The surveys were ordered and the test site chosen by KOAC-NPC, contact person Arjan De Looff and the final customer was Rijkswaterstaat, contact person Bernadette Wichmann.

This report provides a summary of the key research results from the surveys.

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Figure 1. Location of the test site. (Maps from http://www.viamichelin.co.uk and KOAC-NPC)

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2. Background for GPR techniques 2.1 Basic principles of the method The ground penetrating radar antenna transmits a short electromagnetic pulse of radio frequency into the medium. When the transmitted wave reaches an electric interface, part of the energy is reflected back while the rest continues its course beyond the interface. The radar system will then measure the time elapsed between wave transmission and reflection. This is repeated at short intervals while the antenna is in motion and the output signal (scan) is displayed consecutively in order to produce a continuous profile of the electric interfaces in the medium (figure 2). The profile is shown in grey or colour scale, where different shades or colours equal different magnitudes of the reflected amplitudes.

Figure 2. Example of 3D-GPR profile on the left and a single scan on the right side.

In general, the propagation speed of the wave and its reflection are affected by the dielectric value, the magnetic susceptibility and electrical conductivity of the medium. The electrical material properties relevant for radar on dikes are the dielectric permittivity and electric conductivity of the material. They display variability according to aggregate type used in the asphalt, type of binding material (bitumen) and presence of conductive minerals, presence of porosity and fractures, and finally the effect of salt water and accumulation of material in the fillings of the pores and fractures. The most important electrical property affecting the GPR signal is dielectric permittivity, which affects the GPR signal

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velocity in the material. This permittivity can be electrically dispersive especially with problem materials which means that lower frequency components on the GPR pulse face a lower dielectric value in the asphalt which means that higher frequency components are propagating faster in the material than lower frequency components. The results of this is that the pulse is not in the same shape and phase in the asphalt (Saarenketo, 2006) The increasing electrical conductivity of the medium, caused by seawater salts in asphalt, also contributes to the attenuation of the wave and to some extent its reflection.

The antenna wavelength affects the ability of the system to identify objects of different sizes. For example, high frequency antennas with short wavelength have better resolution, but shallow penetration depth, while low frequency antennas with longer wavelength have a coarser resolution, but penetrate deeper into the medium. The degree of saturation with water, the salinity of water, and variation in porosity will also affect the net propagation of radar waves in the material.

The electrical conductivity of a medium influences the degree of attenuation in the amplitude of the electromagnetic waves. Significant attenuation takes place when electrical conductivity becomes greater than 0,010 S/m. If the conductivity is low and the number of electrical interfaces is high, multiple reflections will reduce penetration depth, while poor conductivity combined with a small number of interfaces will cause the wave to be attenuated as a function of the distance between the antenna and the reflecting interface.

(Momayez et al., 1998; Saarenketo, 2006; Saksa et al, 2005).

2.2 GPR equipment GPR systems use discrete pulses of radar energy. These systems typically have the following four components (e.g. Saarenketo & Scullion, 2000): 1) 2) 3) 4)

pulse generator which generates a single pulse of a given frequency and power transmitter antenna, which transmits the pulse into the medium to be measured receiver antenna, which collects the reflected signals and amplifies the signal sampler which captures and stores the information from receiver antenna.

There are two main types of GPR systems: traditional time-domain pulse systems and SFCW (stepped-frequency continuous-wave) frequency-domain systems. The radar antennas are normally categorized in two groups: airlaunched horn antennas and ground-launched dipole antennas.

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The air-launched systems operate around 1-2 GHz. The penetration depth of the horn antennas is limited to approximately 1 m. During the data collection antennas are suspended approximately 0.3-0.5 m above the measured surface. The speed of the measurements is high, even up to 90 km/h.

On the contrary, the ground-coupled antennas operate in a wide range of frequencies from 40-2500 MHz. The advantages of ground-coupled antennas are better penetration depth and vertical resolution. The antennas need to be in contact with or near the surface and therefore are not optimal for larger scale measurements due to slow measurement speed.

The stepped-frequency 3D-GPR system can be considered, in these tests, to be an air-coupled antenna. 3D-GPR antenna arrays may consist of several (up to 63) transmitter-receiver dipoles with the 7.5 cm lateral distance between antenna elements. The antenna array can be elevated up to 1 m from the measured surface. The frequency-domain 3D-GPR system is efficient especially in applications where detailed information from a wider area is required. The dense measurement grid provides information both in longitudinal and transverse direction. This makes it an optimal system for measuring large areas quickly. On the other hand the post-processing software and interpretations is more time-consuming than with the traditional measurements.

The 3D-GPR system emits a step-frequency continuous-wave (SFCW) waveform, which is a sinewave with constant amplitude and stepwise frequency variation. Figure 3 shows an example of an SFCW waveform. The waveform is specified by determining a start frequency (f min ), a stop frequency (f max ), a frequency step (∆f), and a dwell time (Td). (3D-Radar, 2005)

Figure 3. Step-frequency waveform.(3D-Radar, 2005).

Figure 4 shows an analogy between the time-domain and the frequencydomain GPR methods. The time-domain system transmits a short pulse which is recorded as a pulse. The frequency spectrum of the time-domain system is bell-shape with the centre frequency of antenna. In contrast, the stepped frequency-domain system transmits the same power and time each frequency step (e.g. 100-2000 MHz with 2 MHz steps), which produces a rectangular GPR surveys on a dike near St.Jacobiparochie

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shaped frequency spectrum. The time-domain signal is then calculated from frequency information using the inverse Fourier-transformation. The uniform frequency spectrum in frequency-domain systems enables the efficient signal analysis of the measured data.

Figure 4. Principles of a) time-domain and b) frequency-domain GPR systems.

(Modified from Passi, 2007).

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3. Surveys conducted 3.1 Equipment 3.1.1 GSSI system The GSSI SIR-20 represents a traditional GPR technique; a pulse radar.

During data collection the transmitter antenna sends a pulse into the ground and the reflections from the layer interfaces are received by a receiver antenna and its’ amplitude and two way travel time is measured. This process is repeated rapidly as the antenna is moved along the profile. The result of this procedure is a GPR profile with time as y-axis and distance as x-axis.

The two antennas used were air-coupled (horn) with centre frequencies of 1.0 GHz (Model 4108) and 2.2 GHz (Model 4105) (see figure 5). The 1.0 GHz horn antenna has been tested extensively and used widely for different types of quality surveys because of itsâ&#x20AC;&#x2122; reliability and stability. The data collected with this antenna type is almost always comparable with earlier results or results from the same type of surveys done elsewhere. The higher center frequency antenna (2.2 GHz) gives better resolution especially from the top layers (< 5 cm) but suffers from lower transmitting power which results in higher noise level and instability of the signal compared to Model 4108.

Figure 5. Survey van equipped with GSSI 1.0 GHz and 2.2 GHz horn antennas mounted on the same frame.

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3.1.2 3d radar system There are several multichannel GPR systems available on the market which can be called 3d GPR. In most of the 3d GPR technique feasibility surveys done during recent years, Roadscanners Oy has been using a Geoscope 3dradar system which has been developed and manufactured by the Norwegian firm 3d-radar As (figure 6). To analyse the data, Roadscanners has developed a special 3d Module for its’ Road Doctor software.

Figure 6. Geoscope 3d –GPR with 2.4 m wide and 31 channel antenna array mounted on a survey vehicle as a lifted mount.

During 3d Radar data collection the transmitter antenna sends a pulse into the road and the reflections from the layer interfaces are received by a receiver antenna and its’ amplitude and frequency are measured. This process is repeated rapidly through a frequency range of, for example, 100 MHz to 2000 MHz at 2 MHz steps (range and steps depending on the antenna) and through all the channels (antenna elements). The resulting frequency domain data will later be processed into time-domain format and interpreted as a conventional pulse radar profile.

During data collection the Geoscope’s wheel mounted encoder controls the interval at which scans are recorded. In this survey antenna model B2431 was GPR surveys on a dike near St.Jacobiparochie

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used. It is 2.4 m wide and has 31 channels in total: 16 small, 8 mid-size and 7 large antenna elements (figure 7). The resulting files have 31 parallel longitudinal profiles that present the possibility to easily compare differences (anomalies) in the layers below.

Figure 7. Schematic picture of a 3d antenna array. The antenna consists of several transmitter-receiver antenna pairings.

3.2 Surveys conducted The survey area was 18x105 m in size. The GSSI surveys were made with 0,5 m line separation and 105 m long lines totalling 36 lines (3,8 km), same scheme with both antennas. The 3d radar surveys consisted of 7 runs totalling 200 lines, 105 m each.

The antennas were mounted on a van and positioning was done using the markings on the dike, survey wheel and a GPS as a backup. Also a digital video was collected over 3 lines.

The antenna setup for the GSSI horn antennas included 10 scans per meter and using single point gain settings. Metal plate calibration files were taken at the beginning and at the end of the survey.

The 3d radar antenna was elevated to 50 cm above the ground in order to produce better surface data response. The full frequency range of the antenna was used (100 MHz â&#x20AC;&#x201C; 2000 MHz) with 2 MHz steps. The sampling interval used was 7.5 cm (equals approximately 13 scans per meter).

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4. Processing and interpretation After data collection the data was then processed and interpreted using Road Doctor-software’s 3d module. With this module GPR data can be viewed as longitudinal lines, cross sections and as time slices from the different depths.

In the depth and time slice calculations a dielectric value of 6 was used.

The following signal processing tests to improve the quality of the GPR signal were made: 1) Zero-level correction (elevation correction) (figure 8): Because the horn antenna is about 50 cm above the ground a special process for elevation correction has to be done to remove the bouncing of the antenna and its’ effects. By using the surface pulse from the received data and special elevation correction functions, the surface of the ground is levelled to a flat ”zero-point” of data throughout the profile.

For the 3d-antenna data the same correction is done with a slightly different process, but the idea is the same

Figure 8. Example of AC antenna data showing unprocessed data without elevation correction on top and corrected profile below. The white strong reflection on the top profile (at approximately 6 ns) is the surface.

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2) Metal pulse calculation for dielectric value o The dielectric value calculation is done usually at the same time as the elevation correction. It is based on comparison between a â&#x20AC;?perfect reflectionâ&#x20AC;? from a metal plate and the actual measured surface. Currently, metal pulse calculations are possible only for the GSSI horn antennas.

3) Matching parallel lines o When the survey is done as a grid, some time has to be spent to match the parallel lines. Even when the work in the field is done carefully, there might be minor differences between line synchronizing.

4) Background removal o To remove constant noise from the data, a background removal can be done. This process removes a sum pulse, calculated average from all of the scans, from the data.

5) Vertical filters o To remove low or high frequency noise from the data. Can be also used to examine low or high frequency energy behaviour in the media.

6) Horizontal lowpass filtering o Basic idea is somewhat similar to background removal, but this is used on shorter sections.

In the figure 9 a comparison between 1.0 GHz data and 2.2 GHz data is presented. The 2.2 GHz data clearly has better resolution. Some reflections from the layers below (dike filling) are also visible in both profiles.

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Figure 9. A RoadDoctor view with 1.0 GHz antenna data on the top and 2.2 GHz antenna data below.

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5. Results 5.1 About dike structures and visual inspection According to the drill core results provided by KOAC-NPC, the structure of the dike at the test site is as follows: Surface treatment about 5 mm, beneath that asphalt concrete (AC) for 200-300 mm and below AC the filling material.

Table 1. Drill core results (Provided by KOAC-NPC). See also Figure xx.

Distance from starting point (m)

Core number

Distance from lower edge revetment (m)

Thickness surface treatment (mm)

Thickness asphalt concrete (mm)

Total layer thickness (mm)*

Depth deteriorated asphalt (measured from surface) (mm)

Details**

1 2

2,6 12,4

2,5 12,1

4 4

292 225

296 229

0 0

3

27,1

1,9

4

234

238

70

4 5

40,8 54

4,4 9,8

4 4

213 211

217 215

140 0

6 7 8 9

63,1 74,1 82,5 86,4

2,6 1,4 3,1 10,8

4 4 4 4

207 230 187 207

211 234 191 211

100 0 0 0

10

89,3

2,8

4

235

239

80

11 12

95,6 101,1

10,1 4,3

4 4

210 196

214 200

90 0

upper 20 mm loose material upper 30 mm loose material upper 30 mm loose material

upper 20 mm loose material upper 20 mm loose material

*Total layer thickness is sum of thickness surface treatment and asphaltic concrete. Layer thickness is for each core the average of 4 measurement. ** Drilling caused damage to the deteriorated cores. Loose parts (separated from the rest of the core) are mentioned apart.

The drilling results are presented in all the following images as black and white dots (figure 10). White dots represent a core in good condition (core numbers 1,2,5,7-9 and 12) while the black dots are deteriorated at different levels (core numbers 3,4,6,10 and 11). Using these maps it is easy to compare and study which processing analysis methods and techniques were producing useful information.

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Figure 10. Drill cores on a map. Numbers refer to Table 1.

In the following figures the seaside is on the top (0 m) and the starting point is the spray paint marked starting point at the test site (by the client).

The depth chart of the pavement bottom is shown in the figure 11. The upper part of the dike seems to be thicker than the lower part. Also close to the starting point, a thicker layer of pavement is in transverse direction.

Figure 11. Depth chart based on 1.0 GHz antenna data. Scale 150 to 350 mm.

The roughness map calculated from GPR horn antenna elevation file (figure 12) shows interesting details from the trends of the asphalt surface. This surface seems to have a â&#x20AC;&#x153;wavyâ&#x20AC;? profile with 5 - 8 m wavelength. This must have an effect on the water flow down the dike surface.

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Figure 12. The antenna bouncing map (see Chapter 4, 1-elevation correction and figure 8). The scale is Âą 75 mm. The pattern of up-down structure is clearly visible.

5.2 Dielectric values Figure 13 presents a map of dielectric values of the surface (approximately top 30-35 mm) based on 1.0 GHz horn antenna data. The high values could indicate moisture concentration while low values possibly indicate porous or deteriorated surface. The high dielectric values close to the starting point and at the ending point are reflections from the aluminium marker tape. The high values at about 6-8 meters from the start are probably related to a patching or some other operation. The high values could indicate saline concentration while low values possibly indicate porous or deteriorated surface. The high saline concentration could be explained by porous pavement filled with salines.

Figure 13. Map of dielectric values of the surface (approximately top 30 mm) based on 1.0 GHz horn antenna data, scale 3 to 6 (blue to red). The red vertical line in the beginning and in the end are reflections from the aluminium tape used as a marker.

The anomaly around the drill core numbers 8 and 10 is also visible in the video data (figure 14). The poor condition of the pavement is obvious. When compared to depth map (figure 11), it can be also noticed that the pavement is at itsâ&#x20AC;&#x2122; thinnest on that location.

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Figure 14. Still video image of the defects in the pavement surface around core samples 8 and 10.

In figure 15, a map of dielectric values based on 2.2 GHz antenna data is shown. The map has similarities when compared to map of 1.0 GHz Er values (figure 13). The differences can be explained that the frequency range of 2.2 GHz antenna measures a thinner slice (down to 18-20 mm) from the surface than the of 1.0 GHz antennaâ&#x20AC;&#x2122;s (down to 30-40 mm) and the information is collected closer to surface. Notice also that 2.2 GHz antenna has a bit lower average in Er which can be explained as the effect of the rough surface texture. See figure 16 for more.

Figure 15. Map of dielectric values of the surface (approximately top 30 mm) based on 2.2 GHz horn antenna data, scale 3 to 6 (blue to red).

In figure 16, the difference in Er-values between two horn antennas is calculated. An interesting pattern is created in the map, when differences in Er-values are changing.

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This tells the differences in the properties from roughly 20 mm to 40 mm. This map has similarities to the surface shape pattern presented in figure 12. It seems that in the bottom of the â&#x20AC;&#x153;wavesâ&#x20AC;? of the surface the moisture defects penetrate deeper.

Figure 16. The difference between Er-values measured with 1.0 GHz and 2.2 GHz antennas. The red vertical line in the beginning and in the end are reflections from the aluminium tape used as a marker (approx 1 m wide).

The deviation map of dielectric values is presented in figure 17. The distance interval for deviation calculation is 1 m. In the map blue colour presents low deviation and red colour for high deviation. The high deviation values indicate heterogenic response from the top layer with indications of surface cracking. The resulting map matches quite nicely to observations of drill core data with an exception of drill core 11. For some reason core 11 does not match with any of the maps. Possible reason for this might be, that the defect is very local and the antenna did not pass directly over it.

Figure 17. Map of Er deviation based on 1.0 GHz antenna data. The red lines in the beginning and in the end are affected by the aluminium marker tape.

The dielectric values of the pavement bottom might be useful to give some indication on the behaviour of water inside the dike structure. The map (figure 18) follows the trends of the surface dielectric values, so not much difference in here (see figure 13).

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Figure 18. Dielectric values of the bottom of the pavement based on 1.0 GHz antenna data. Again the higher Er-values are reached at the lower part of the dike, close to sea. Blank areas are missing data.

5.3 Amplitude analysis The amplitude analysis is based on slicing the three dimensional data grid at different depths (times). The resulting map shows anomalies based on reflection amplitude. The slices can be done using maximum, minimum, average or absolute values of the amplitudes at a certain time range. For example the slice in figure 19 is done from the very surface with a width of 0.2 ns. This means that the reflections from a depth of -0.1 ns to 0.1 ns are taken into account and the maximum value of those amplitudes is printed on the map.

The resulting maps are very promising but at the same time more samples are needed to verify the results. These maps could be used to guide sampling to most interesting places, or, in the first place, just take samples from high red or blue areas and see what they show and if they correlate with these results.

Figure 19. Absolute average amplitudes from the very first reflected pulses from the surface. Level of 0 ns (width 0.2 ns). Data from 1.0 GHz horn antenna.

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The map in figure 20 is from a depth of approximately 40 mm. There is good correlation between deteriorated core sample and red, high average amplitude at core samples 3, 4, 6, 10 and 11, but also some irregularities as with samples 8 (at 82,5 m) and 12 (at 101,4 m).

Figure 20. Absolute average amplitudes from the level of 0.6 ns (width 0.2 ns).

Equals depth of 40 mm using dielectric value of 5. Data from 1.0 GHz horn antenna.

In the following two figures (21 & 22) an interesting feature is shown. It seems that a crack pattern with about 3m interval has been formed in the bottom of the AC layer. This is most probably so-called shrinkage cracking, which follows when the bound layers shrink due to temperature changes or due to shrinkage of the fill beneath the asphalt layer.

Figure 21. Shrinkage cracking in the time slice (red colours) over the whole section.

In the subwindow on top a GPR profile and on the left subwindow a transverse profile.

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Figure 22. Zoomed in to the latter part of the time slice to make the shrinkage cracking more visible.

5.4 Frequency spectrum analysis Frequency spectrum analysis is a method that is used to analyse the behaviour of different frequencies of electromagnetic waves in the material.

Because different frequencies give a little different response over the same material, the changes in the spectrum might work as an indicator for structural changes.

The map in figure 21 shows the behaviour of high frequency energy on the top 4 ns.

Figure 21. Frequency power spectrum over 0-4 ns time scale (0-25 cm with er = 6), the frequency range of 1000 to 2000 MHz. Data from 1.0 GHz antenna, not filtered.

When the timescale is increased, the AC layer is included in the calculations as whole (figure 22).

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Figure 22. Frequency power spectrum over 0-6 ns time scale (0-37 cm with er = 6), the frequency range of 1000 to 2000 MHz. Data from 1.0 GHz antenna, not filtered.

The frequency analysis suffered a problem that could have been avoided by using a higher number of samples per scan. Because the goal was to concentrate on the very surface of the pavement, only few samples of those 512 collected are from this layer. That is why the maps above are from a wider time range (the range of 0-4 ns equals 25 cm with Er-value of 6) and not only for example the top 5 cm. In the future it is recommended to take 1024 or even 2048 samples per scan.

5.5 3d-radar data analysis The results of the 3d radar data analysis, used as air coupled antenna, were disappointing, even though the authors tried numerous techniques to obtain a clear image of the problems. Figure 23 presents an example of the 3d data, an amplitude map calculated from the surface part of the asphalt using a single antenna element size (small). This map shows different amplitude levels at different passes indicating possible temperature drift in the hardware and thus this kind of amplitude analysis cannot be made.

Figure 23. An example of 3d antenna data.

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Another possible reason for the failures of the 3d-radar data can be found in the fact that, based on experience, the 3d-radar seems to be very sensitive to material with a high imaginary part of the dielectric value. And in this dike, the imaginary part of the asphalt dielectrics are quite high because of the sea water.

However Roadscanners software team is working to improve the processing technique of the 3d radar data and the data collected in this survey will be reevaluated when new processing algorithms are ready.

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6. Conclusions and suggestions It still seems that the most reliable indicator of defects in the pavement surface is the dielectric value calculation based on the surface reflection. In this survey the high dielectric values correlated well with the results of the core sample data. Also Er-deviation indicates surface anomalies. These results can be produced with 1.0 GHz or 2.2 GHz horn antennas. This also verifies the conclusions drawn after the first surveys in Friesland and Hellegatsdam.

The 3d-radar system is still under development. The main problems around this project were with the elevated antenna. The unevenness of the dike surface combined with the bouncing of the survey van resulted in a software headache. The idea of the 3d-radar survey was the same as it is with the horn antennas (levelling the surface â&#x20AC;&#x201C; elevation correction). But the raw data collected with this system has a much higher noise level and this is why the processing will not follow the same rules. There have also been hardware changes in the antenna, that did not improve the data quality. This has been acknowledged by the manufacturer and is being corrected.

Based on these and previous tests both GSSI 1.0 GHz and 2.2 GHz horn antennas can be recommended to be used in the asphalt damage detection.

Their integrated analysis also provides information about the depth of the damage. The sampling density (samples per scan) should be high, 1028 or 2048 in the future to make detailed frequency spectrum analysis possible.

For future considerations, a very promising new product has now entered the market when GSSI released new 2.6 GHz ground coupled antenna. The antenna is under testing at Roadscanners and the plan at the moment is to use it to locate cracks inside the pavement. The test plans also include bridge decks and concrete structures because of itsâ&#x20AC;&#x2122; high resolution.

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REFERENCES

Momayez, M., Hassani, F.P., Hara, A. & Sadri, A., 1996. Application of GPR in Canadian Mines. CIM Bulletin Vol 89 no 1001, 1996.

Passi, T., 2006. Maatutkatekniikan hyödyntäminen radan tukikerroksen kunnon arvioinnissa. Diploma thesis. Technical University of Tampere, 80 p, 4 app.

Saarenketo, T., Scullion T., 2000. Road evaluation with ground penetrating radar. Journal of Applied Geophysics, vol. 43, 2000, pp. 119-138.

Saarenketo, T., 2006. Electrical properties of road materials and subgrade soils and the use of ground penetrating radar in traffic infrastructure surveys.

PhD thesis. University of Oulu, 121 p., 5 app.

Saksa P., Heikkinen E., Lehtimäki T., 2005. Geophysical radar method for safeguards application at Olkiluoto spent fuel disposal site in Finland. STUKYTO-TR 213. STUK, Helsinki

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