Experimental Validation of Enhanced GPR Signals by A Broadband Metasurface

In this paper, the potential of a broadband metasurface for ground penetrating radar (GPR) signal enhancement is investigated by simulation and experiments. Simulation results show that the reflection at the air-MUT interface can be reduced from 35% to 5% over a broad frequency range (relative bandwidth up to 44%) when the broadband metasurface is in place. Measured reflectance is consistent with the simulation results. Meanwhile, the electric field strength measurement results demonstrate that the transmitted electromagnetic signals can be amplified when the reflection is reduced over the same frequency range. GPR experiments verified that clear hyperbolic signals emerged for commonly undetectable pipes when the high-frequency signals are enhanced. The proposed broadband metasurface can be an effective solution for the detection of nonmetallic inclusions in high-lossy media.


Inrtoduction
Ground Penetrating Radar (GPR) technology is broadly implemented on-site by civil engineers with the aim of detecting buried dielectric interfaces (buried pipes, concrete delamination, steel rebars, cables, etc.) or measuring the depth or thickness of underground layers [1].However, it is a significant problem that partial energy will be reflected when electromagnetic waves travel from air to the ground surface in the air-coupled GPR systems.This kind of reflections makes the signals suffer a severe loss before transmitting into the underground.Thus, the effective maximum transmission or minimum reflection at this air-ground interface is desirable in many non-destructive testing systems including GPR.
In recent years, metasurfaces, the two-dimensional analogues of metamaterials, are used to gain arbitrary control of electromagnetic waves.Metasurfaces usually consist of periodic unit cells with a specially designed pattern [2], can efficiently reshape the reflection, refraction and transmission of incident waves at designed frequency range [3].Previous works have demonstrated that subwavelength metasurface coatings can achieve near perfect antireflection for semiconductor materials [4], where an interference theory was developed to reveal the underlying mechanism.
We recently demonstrated the enhanced detection of GPR under impendence matching condition using electric Closed Ring Resonators (CRRs) and mesh array structure [5].However, the experimental results show that the enhancement is limited because that the bandwidth of the metasurface is less than that of GPR, which is usually about 100%.Thus, we subsequently developed an electrically thin metasurface with increased relative bandwidth (RBW) of 38% [6].In this paper, we further widen the bandwidth by implementing the optimization of the unit cell design, with the numerical and experimental demonstrations of how much electromagnetic transmission enhancement can be achieved for a given ground condition.The on-site GPR experiment results successfully validated the improvement of the identification of the hyperbolic signals by the broadband metasurface, with the RBW up to 45%.Compared to the previous works on impedance matching for GPR applications, our proposed broadband metasurface demonstrates a better performance in enhancing the high frequency signals.What's more, it is easy to fabricate and implement as well as being light-weight compared to other broadband metasurface and structure we proposed before [6,7].To the best of our knowledge, this is the first experimental demonstration of broadband impedance matching for GPR applications.

Unit cell design and simulation
The unit cells of a typical narrowband and a broadband metasurface are presented in Figure 1a and Figure 1b, respectively.We have investigated the transmission enhancement of GPR signals for Figure 1a and achieved significant improvement of the detection depth [5].The unit cell in Figure 1b is a broadband antireflection structure based on the unit cell we proposed in [6].The illustration of the destructive interference process and the corresponding variables are shown in Figure 1c.When a welldesigned metasurface is placed on the surface of materials under test (MUT), the conditions 12 21 23 r r r == and 21 23 Φ +Φ +2β=0 are satisfied to sustain a destructive interference at the air-spacer interface and the transmission into MUT can therefore be enhanced.Here, β is the phase delay introduced when the electromagnetic waves propagate through the spacer [4].The reduced reflectance at the air-spacer interface is shown in Figure 1d where the relative permittivity of MUT is MUT 15  = .We can see that the reflectance of bare MUT can be largely reduced from 35% to 5% with an RBW of 44% (1.05-1.65 GHz) when the broadband metasurface is used, which means that the reflection of the ground surface can be largely reduced over the GHz range for GPR applications.

Experimental validation
Based on the design and corresponding reflectance in Figure 1, a broadband metasurface sample (500 mm × 500 mm × 14 mm) was fabricated.Considering the nondestructive testing applications, we choose foam bricks, a widely used building material, as MUT.The approximate NRL arch method [8] is adopted to obtain the reduced reflectance.Figure 2a displays that the reflectance is reduced mostly to 10% from 1.0 GHz to 1.48 GHz with an RBW of 39%, compared to the reflectance of 35% of the bare wet MUT under the normal incidence.Note that the reflectance ripples around 1.05 GHz and 1.22 GHz, which might be originated from the imperfect EMC condition at low frequencies due to our small-scale microwave anechoic chamber.After the reduced reflectance is validated, a near-field scanning system is used to measure the enhanced transmitted electric field at the backside of wet MUT.The measurement area was divided into 15 × 30 grids with spacing of 10 mm at two orthorhombic directions.The amplitude and phase of electric field strength at every point were incrementally obtained.We then calculated the percentage of increase (averaged value) for the transmitted electric field strength at every scanned point when the broadband metasurface was applied, as presented in Figure 2b.It can be seen that the transmission is largely enhanced over a broad frequency range when the broadband metasurface is in place.In order to confirm the signal enhancement for GPR survey, a GPR detection setup is conducted and illustrated in Figure 3. Layered MUT consists of two foam bricks, of which the top layer was prepared (by immersing it in water to increase the water content) to serve as the wet layer and the bottom layer was the dry foam brick.A cylindrical hole, located at the middle of the bottom MUT layer, was drilled to mimic an air pipe and a metallic pipe if a metallic pipe was placed in the hole, respectively.The OKO GPR system with a central frequency of 1200 MHz is used to execute the real-world validations.As can be seen from Figure 4a(i) and Figure 4b(i), when the pipe is located beneath the top MUT layer with severe attenuation, the hyperbolic signature of neither the air pipe nor the metallic pipe can be identified by the normal GPR survey.When the broadband metasurface was applied to match the impendence between air and the top MUT layer, the hyperbolic signatures of the air pipe and the metallic pipe can then be recognized much more clearly according to the results in Figure 4a(ii) and Figure 4b(ii), respectively.Furthermore, we applied the low-frequency (LF, 0.3-0.8GHz) and high-frequency (HF, 0.8-1.5 GHz) bandpass filters for Figure 4a(ii) and Figure 4b(ii), with the resulted GPR images presented in Figure 4 a(iii-iv) and Figure 4 b(iii-iv), respectively.Firstly, Figures 4(a-b)(iii), the LF part of enhanced GPR signals, are similar to Figures 4(a-b)(i), which makes sense because that our broadband metasurface wasn't designed to enhance the LF signals according to the simulation results in Figure 1d.Secondly, the hyperbolic signals become easier to be identified owing to the fact that the HF signals between 0.8 GHz and 1.5 GHz are enhanced, which can be validated by Figures 4(a-b) (iv).These observations directly confirm that our designed broadband metasurface can contribute in making the hyperbolic signature of the buried pipe clearer if its designed frequency band is closer to that of the GPR working band, which in this demonstration is the HF range.

Conclusions
In this work, the enhancement of a broadband metasurface for GPR surveys is experimentally investigated.Simulation and experimental results consistently show that the radar signals can be enhanced over a broad frequency range when the impendence mismatch between air and the MUT is alleviated by the broadband metasurface.Our real-world GPR experiment results successfully demonstrate that the unclear if not totally detectable hyperbolic signature of air and metallic pipe can be recognized after broadband metasurfaces are deposited atop the MUT.The ultra-wideband metasurfaces are being designed to match the traditional GPR systems with the RBW of 100%.

Figure 1 .
Figure 1.The design of the narrowband (a) and broadband (b) metasurface unit cell.(c) The interference model of the antireflection metasurface and the associated variables [4].(d) The simulated reflectance corresponding to (a) and (b).

Figure 2 .
Figure 2. (a) Measurement results of the reflectance (red solid line).(b) The average percentage of increase in the electric field strength (green solid line).The dashed dotted black line is the calculated reflectance of the bare substrate (ε substrate = 15, lossless).

Figure 3 .
Figure 3. Real-world GPR survey without (a) and with (b) the metasurface in place.

Figure 4 .
Figure 4. GPR B-Scan images when the air pipe (a) and metallic pipe (b) are located within the bottom layer foam brick.(a)(i) and (b)(i) are the B-scan images when no metasurface is in place, respectively.(a)(ii) and (b)(ii) are the B-scan images when the broadband metasurface is applied.(a)(iii-iv) and (b) (iii-iv) are the LF part and HF part of (a)(ii) and (b)(ii), respectively.