NDT Basics

The earliest use of radar for detecting hidden objects is accredited to Hülsmeyer in 1904; Whilst the first text describing the use of radar in detecting buried objects appeared in 1910 by two Germans, Leimbach and Löwy.

Radar NDT involves the generation, transmission and reception of Electromagnetic Waves (EM waves) to determine the presence of objects. EM waves comprise of an electric field and a magnetic field that are in the form of cosine waves, which travel perpendicular to both each other and the direction of propagation of the wave.

All EM Waves travel at 3x10^8 ms-1, the speed of light in free space. The velocity of the EM wave through a medium will depend on the properties of that medium. Radar used in NDT operates at the Microwave frequency range, which is roughly between 300MHz-300GHz in the EM Wave Spectrum.
EM waves are transmitted as a signal into a medium and portions of the signal's energy are reflected and refracted whenever it encounters an object, or a boundary, which represents a new medium with contrasting EM properties. The scattered energy is eventually detected at a receiver location.

The above diagram shows the basic principle of radar testing.

The velocity of a propagating wave is related to both its frequency and wavelength, given by the following equation:

Where v is the velocity of the wave, f is the frequency of the wave and lamda represents the wavelength.

This phenomenon where the amplitude of an EM wave reduces as it travels through a medium is known as attenuation. The degree of attenuation is governed by the EM properties of the medium. Attenuation, therefore, is related to depth of penetration; The higher the attenuation, the less distance the wave can travel before it becomes too weak for detection and therefore the shallower the depth. It must be noted that for an impulse radar test the two-way travel distance is the penetration depth.

Resolution defines the size of objects that radar can detect. The theory of resolution is shown below.

The above diagram shows three different frequency radar waves travelling through a medium. The diagram is designed to show how the shape of the resulting wave detected at the receiver is formed and the ability of the formed wave to enable resolution of the interfaces. Traces A, D & G represent the reflection from the top interface, whilst traces B, E & H represent the reflection from the bottom interface. Traces C, F & I show the resulting traces as seen on the display equipment. It can quite clearly be seen that the high frequency wave resolves both interfaces easily. The medium frequency wave also manages to resolve both but is at the limits of its resolution. The low frequency wave manages to resolve the top interface, but as the lower interface is within one wavelength of the top interface, resolution of the lower interface is not possible; although the resulting wave indicates there is something there that can not be located.

The higher the degree of resolution the smaller the objects which can be detected. However, high resolution demands a high frequency and the higher the frequency the smaller the penetration. Therefore a trade-off is struck between resolution and penetration and the extent of this compromise will depend on the priorities of the investigation. In this case, our required depth of investigation changes, but because we are varying an air gap, there is little difference in attenuation. This is due to the fact that the relative permittivity of air is 1 F/m and this equates to negligible increase in attenuation of the EM wave.