C. H. J. Jenks

Road surface and pavement condition assessment by high frequency GPR diffraction

The condition of roads and pavements near their surfaces is of interest to asset management, in particular of the roads and pavement assets, but also of assets buried under the roads or pavements such as cables or pipes. In many cases the decay of the near surface material results in small air or water filled regions within the tarmac/concrete/stone structure. The decay causes little change in the electromagnetic reflection or delay characteristics of these regions as measured by traditional GPR equipment. Signals launched along the surface are diffracted out of the surface by fractures and discontinuities within the surface. The change between a uniform and a fractured surface is seen to be more readily apparent in the direction normal to the surface, particularly at higher frequencies above about 3 GHz for fractures and for voids of the order of about 2 mm and above. Finite Difference Time Domain simulations and initial microwave frequency measurements indicate significant changes in diffracted signals levels are observed over sufficiently fractured regions.

Improved GPR image focussing with repetitive normalised Superimposition techniques

The Superimposition technique offers an alternative to Fast Fourier Transform (FFT) and Inverse FFT calculations. side lobe levels are reduced with little or no increase in main lobe width, as opposed to standard windowing techniques where side lobe level reduction produces an increase in main lobe width with a resulting loss in resolution. The new technique uses repetitive superimpositions showing improvements over spatially variant apodization (SVA) techniques. A new normalisation scheme enhances side lobe reduction even further with no increase in main lobe width indeed it can reduce the main lobe width. The technique is seen to be more resilient to noise when appropriate multiple evaluations are chosen. The technique produces responses from reflections in GPR data that are resolved to responses much closer to a delta function than FFT/IFFT or SVA evaluations. When used in focussing algorithms the traditional hyperbolic characteristics of a B-scan are focussed into responses whose width in depth and plan position that are slightly better than half a wavelength of the bandwidth used. This is seen in theoretical data and in both data measured by commercial GPRs and in experimental data from a step frequency continuous wave based GPR. Theoretically the technique produces a strong indication of the permittivity of the ground the GPR measures are taken over, while in measured data the identification of the permittivity of the ground is less clear.

Antennas for pavement void detection by scattering

Voids and cracks in asphalt or concrete can indicate the condition of a road or pavement. A decaying road may contain many very small cracks, and such small artifacts can be difficult to identify using conventional ground penetrating radar (GPR) techniques. A new approach is examined here to detect the scattering from the multitude of small defects in the ground. Consideration is given to the layout of suitable antennas to sensitise a GPR system to the laterally propagating cross-polarised signals that voids in a road will produce. A self-shielded antenna with a circularly symmetrical beam pattern is used to transmit vertically downwards. This is surrounded by a number of wide band dielectric wedge directional antennas, oriented to pick up the cross-polarisation of the scattered signal. The two antenna types and their groundplane configuration have reduced direct path coupling and therefore maximise system sensitivity. Tests over asphalt slabs with 7% and 3% changes in density and porosity show that quite significant changes in signal level make such changes in porosity detectable.

Investigation of soil contamination by iron pipe corrosion and its influence on GPR detection

It has been observed that the corrosion of iron pipes in soil can produce variations in ground conductivity around the pipe, and that the visibility of such pipes to GPR can be greatly reduced. This new investigation and measurement of the permittivity and conductivity of soil contaminated by iron pipe corrosion products produces more accurate knowledge of permittivity and conductivity data and their likely spatial variation with distance from the corroding pipe. The experimental data are the result of monitoring accelerated corrosion over a period of several weeks and using TDR and direct conductivity measurement schemes. FDTD simulations of GPR signals show how the corrosion induced variation in the visibility of the pipe varies with the thickness and shape of the new spatial variations permittivity and conductivity. The results indicate that in the earlier stages of pipe corrosion use of lower GPR frequencies will still detect the pipe, although at lower spatial resolution.

In-pipe GPR configuration and the determination of target depth and ground permittivity

Ground Penetrating Radar (GPR) is frequently used to survey roads, pavements and other sites. To determine the depth of the targets found the permittivity of the ground needs to be established or eliminated from the analysis of the GPR data. A novel way of using bistatic GPR equipment is to place the transmitters and receivers at the surface of the ground and in an available pipe. Measurements through the ground of signal delay can then be analysed to reveal the permittivity of the ground, and therefore establish the depths of targets in the vicinity of the pipe. This paper examines measurement and analysis techniques for such a scheme, and reports on a measurement system that has been developed. The influence of multiple reflections caused by the in-pipe location of antennas is examined and a relatively simple impulse response model is proposed that appears to be sufficient for deconvolving the multiple reflections out of the measured data.

UWB shielded teardrop monopole antenna for GPR and communications

Ultra-wideband characteristics are often sought for radar and communications applications. This paper investigates the properties of a re-sistively loaded teardrop monopole, embedded into a cavity in such a way that the antenna face has a flat profile. Finite Difference Time Domain analysis and measurements indicate ultra-wideband performance. Investigation of coupling between closely spaced antennas shows quite good isolation, and constriction of current flow to the immediate vicinity of the antenna. The basic causes of the low level late time ringing in the antenna are seen to be related to identified mechanisms, which can then be further investigated and optimised.

The use of a Software Defined Radio as an OFDM GPR

Commercially available off the shelf Software Defined Radios (SDR) have many of the components required to construct an OFDM or a stepped frequency GPR. Some additional components are required in order to make phase measurements however, as described here. There are also design features of the SDR used here which could make them unsuitable, but which can be mitigated as investigated here.