Radar (Overview)

Florian Moosbauer, winter semester 2015/16

Deutscher Artikel

RADAR stands for Radio Detecting And Ranging. It is a method using electromagnetic waves for the examination of structures and determining the position and velocity of objects. ^[1]

Introduction

The measurement method RADAR was developed at the beginning of the 20th century. First, it was developed for military and marine to determine the distance of objects. RADAR systems send an energy pulse at high frequencies via an antenna. A computer records the intensity and the required time of the reflected signal. Objects in the air or differences in the nature of the examined materials produce reflections. These reflections are caused by moisture differentials or material transitions. ^[2]

In geoscientific field, RADAR is used as Ground Penetrating Radar (GPR) or electromagnetic reflection measurement (EMR). GPR is used non-destructively determine voids, inclusions and much more in the underground. ^[1]

Physical basics

The radar measurement is based on electromagnetic waves.

Resolution and penetration

Resolution

The resolution of a radar measurement is divided into horizontal and vertical resolution. If there is a good resolution in the horizontal direction, then two objects closed together lying (horizontal) can well differentiated. If the horizontal resolution is low, two objects cannot be perceived as separate. The resolution depends on the wavelength $//<![CDATA[ \begin{array}{l}λ\end{array} //]]>$. The resolution decreases with increasing measurement depth.

$//<![CDATA[ \begin{array}{l}r=\sqrt{\frac{\lambda \cdot z}{2}+\frac{{\lambda }^{2}}{16}}\end{array} //]]>$

$//<![CDATA[ \begin{array}{l}r\end{array} //]]>$ = horizontal resolution [m]

$//<![CDATA[ \begin{array}{l}λ\end{array} //]]>$ = wavelength [m]

$//<![CDATA[ \begin{array}{l}z\end{array} //]]>$ = measuring depth [m]

For the vertical resolution especially the permittivity $//<![CDATA[ \begin{array}{l}ε_R\end{array} //]]>$, a material-specific characteristic value, is crucial. Is this high, resolution is good. Furthermore, the transmit pulse length $//<![CDATA[ \begin{array}{l}T\end{array} //]]>$ is of great importance. The following formula applies:

$//<![CDATA[ \begin{array}{l}d=\frac{T\cdot c}{2\sqrt{{\epsilon }_{R}}}\end{array} //]]>$

$//<![CDATA[ \begin{array}{l}d\end{array} //]]>$ = vertical resolution

$//<![CDATA[ \begin{array}{l}T\end{array} //]]>$ = Transmitted pulse length

$//<![CDATA[ \begin{array}{l}c\end{array} //]]>$ = speed of light

$//<![CDATA[ \begin{array}{l}ε_{R}\end{array} //]]>$ = permittivity ^[3]

Penetration

GPR is applicable with different frequencies. The higher the used frequency, the better the horizontal resolution is and the lower is the penetration depth. Particularly large penetration depths can be achieved when the underground badly conducts (less damping). ^[3]

Principle of the measurement

The GPR is an electromagnetic impulse-echo method. For this purpose, a pulse generator emit very short electromagnetic waves via a transmitting antenna (transmitter) at a high repetition rate in the medium to be examined. A portion of the energy of the electromagnetic waves are reflected at interfaces between layers having different permittivity. The reflected signal is registered by a receiving antenna (receiver).

In radargram, on the X-axis is plotted the measurement path, on the Y-axis the duration of the measurement.

The antennas have because of the low frequency and the necessary bandwidth low directivity. So there is an echo signal of objects, which aren´t just below the antennae. This echo signal is still accurately represented under the radar. The echo is shown at a greater depth, so the typical hyperbole arise (see radar gram). The actual position (in X and Y direction) is shown by the apex of the hyperbola. ^[3]

Working principle

Pulse Radar

Pulse radar is an imaging Radar, which transmits pulses in the microsecond range to achieve a high bandwidth. Reflective echo is detected and evaluated. The pulse radar provides information about the distance and approximate size of objects.

Stepped-Frequency-Radar

The SFR-method change stepwise or continuously the frequency and let the amplitude of the signal constant. This method provides by controlling the measuring rate better resolution and penetration as the pulse radar. Because the power of the transmitter is very low, this method is recommended for other technical arrive.

Radar Interferometry

In this method, two different transmitter positions are taken. This phase information is additionally obtained. The radar interferometry can be used to detect moving objects, or for the localization of changes in the centimeter range.

FMCW Radar

FMCW stands for Frequency Modulated Continuous Wave. It is a modeled continuous wave radar. This means the transmitter is operating permanently, compared to pulse radar, and varies its frequency. ^[3]

Application of Ground Penetrating Radar

Application of Ground Penetrating Radar is a modern and non-destructive testing for structures and buildings. There are many topics to analyse ^[4]:

• Locating pipelines, cables and anthropogenic debris (e.g. barrels, foundations), also with Unmanned Aerial Vehicles
• Cavity detection
• Detection of gaps and weaknesses in roads, dams and foundations
• Investigation of sediment and soil structures, distinction between anomalies and homogeneous areas
• Detection of faults, fissures and cracks in hard rock
• Finding clay lenses, ice wedges, peat, etc.
• determining the position of the groundwater surface in gravel, sand and sandstone
• examination of the state of landfill liner systems

Literature

1. V. Dundelach: Georadar. www.bgr.bund.de, 23.11.2015
2. Ground Penetrating Frequently Asked Questions. www.geophysical.com, 23.11.2015
3. Radar, Große, C.: Grundlagen der Zerstörungsfreien Prüfung; Arbeitsblätter im Rahmen der Lehrveranstaltungen des Lerstuhls für Zerstörungsfreie Prüfung der Technischen Universität München
4. Georadar Aktuell; 2005. www.dr-tillmanns-und-partner.de, 23.11.2015