Ground radar
A ground penetrating radar, also georadar, engl. Ground Penetrating Radar (GPR) or Radio Echo Sounding (RES) allows a non-destructive characterization of the subsurface with high-frequency electromagnetic waves . In geophysics , it is mainly used to study the upper layers of the earth's crust . In military applications it is used to detect land mines.
description
Various radar technologies can be used as ground penetrating radar . The pulse radar using an amplitude modulation and ultrawideband method in the time domain. The continuous wave radar method is also possible - either continuously or discretely frequency modulated ( SFCW ). With pulse radar , the distance results from the time difference between the transmission of a short pulse and the arrival of the reflected signals at the receiver. With FMCW and SFCW radars, the difference frequency or difference phase between the transmitted and received signals is evaluated.
In general, the greatest possible bandwidth of the transmission signal is sought. One possibility for the technical realization of a large bandwidth is to let the lower limit frequency tend towards zero. The signal form is then not modulated to a carrier frequency, but only an extremely short high-voltage pulse in the range from a few picoseconds to a few nanoseconds is sent to the transmitting antenna. The resulting type of transmit signal is called a baseband pulse . A pulse shape is then sent as a so-called "Mexican hat" , which is formed by the second derivative of a bell curve . The bandwidth results from the shortness of the high voltage pulse. The receiver is then tuned to the center frequency of the bandwidth. An electrically shortened dipole or spreading dipole is usually used as the antenna , which is sometimes even in direct contact with the earth's surface in order to reduce coupling losses at the transition from the air to the earth. The entire ground penetrating radar is then dragged over the ground on a tarpaulin, for example. If the distance to the earth's surface is greater, short TEM horns are also used, which then have relatively large dimensions due to the low frequency range.
With continuous wave radar devices, separate transmitting and receiving antennas are used with a certain distance from one another. Continuous wave radars are usually narrow-band and achieve the necessary bandwidth by gradually or continuously changing the center frequency. These radars can use any broadband antenna that covers the spectrum of the transmitter. They transmit with less power, but have better noise suppression due to the integration of the echo signals over time. The measurements therefore take longer, the ground penetrating radar may only be moved over the ground at walking pace. Pulse radar devices, on the other hand, can be built into vehicles and used during fast-moving traffic.
The working frequencies are in the range from 1 to 2400 MHz. The choice of the frequency range is a compromise between the desired resolution, the penetration depth to be achieved and the frequency-dependent effect of soil inhomogeneities. Special mine search radar can also use higher frequencies up to 4 GHz, as most mines are only deployed to a depth of 20 cm.
Radar gram
The picture shows an example of a ground penetrating radar image. The distance between the dark green transmitting and receiving antenna is constant. Each measurement creates a column in the image. By moving the transmitter and receiver unit along the profile line, a two-dimensional representation of the reflections in the subsurface is obtained.
The propagation of electromagnetic waves in the subsurface is heavily dependent on the structures in the ground that cause reflection , scattering , diffraction and transmission of the radiated wave. Because of the low frequencies, the antennas cannot have a great directional effect. A reflective object in the ground can therefore be located from a great distance. When they pass over the object, these signals result in a hyperbolic shape in the radar image.
For the conversion of the transit time of the signal into a depth specification (time-depth conversion), the respective speed of propagation of the electromagnetic signal in the media it traverses is required, which depends on the specific electrical properties of the respective media. When calculating the depth, the soil structure must therefore be taken into account. There are tables for this which indicate the speed of propagation in meters per nanosecond , depending on the type of soil and moisture. For the time-depth conversion, further geological information (e.g. from core drilling) must therefore be used. However, this is not the only way to determine the speed of propagation. By analyzing the hyperbola that a simple reflector, such as a pipe, leaves in the radar gram, one can also deduce the speed of propagation.
Areas of application
To explore the flat subsurface, ground penetrating radar is used as a non-invasive method for geological and geotechnical issues, such as dike monitoring and flood protection, as well as in raw material exploration (sand, gravel), pipeline exploration and for engineering geological investigations. Further application possibilities are in the area of archeology , the technical investigations of contaminated sites (underground installations and cavities, fillings, pipes and soil layers) as well as in mining and tunnel construction.
The Mars Express space probe uses the MARSIS radar to investigate the Martian soil at a depth of up to 5 km. The distance between the antenna and the ground is more than 300 km to a maximum of 800 km. MARSIS is also able to probe the ionosphere . The measurement frequency is 1.8–5 MHz, 0.1–5 MHz for ionosphere measurements.
The American Mars Reconnaissance Orbiter probe carries a similar ground radar, the SHARAD (Shallow Radar). The higher measuring frequency of 15-25 MHz provides a higher resolution than MARSIS, but a lower penetration depth. The picture on the right shows a SHARAD radargram along the track shown in the lower part of the picture. The colors characterize the height profile from green (depression) to red (elevation). The profile depth was estimated by estimating the speed of signal propagation in the rock.
GPR can be used to measure the thickness of glacier ice.
One example is the discovery of an aircraft that was lost in 1942 91 m under the Greenland ice using GPR on board a drone.
Recent developments make it possible to use ground radar with a helicopter in inaccessible areas, for example to map the water table, glaciers or sediment layers. The typical working height of the ground penetrating radar is 15 to 25 m above the surface. In the data processing, the exact current position of the radar is recorded by GPS and allows the measurement results to be stored with digital map material. The measurement process allows a flight speed over the ground between 40 and 80 km / h.
Borehole radar
Another application of ground penetrating radar is borehole radar, which is specially designed for boreholes. One method of geophysics is test drilling for material analysis. In this context, borehole radar systems offer an essential possibility of characterizing the surroundings of these boreholes using a non-invasive ground radar system. Mainly borehole radar systems with omnidirectional receiving antennas are used, which measure the distance from reflectors, but no information about the azimuthal angle. Direction-sensitive antennas, on the other hand, enable the distance and direction of the reflected echoes to be measured.
literature
- Jürg Leckebusch: The application of ground penetrating radar (GPR) in archaeological prospection - 3D visualization and interpretation. Leidorf, Rahden 2001, ISBN 3-89646-403-5 .
- DJ Daniels: Ground-penetrating radar. Inst. Of Electrical Engineers, London 2004, ISBN 0-86341-360-9 .
- CS Bristow: Ground penetrating radar in sediments. Geological Society, London 2003, ISBN 1-86239-131-9 .
- Harry M. Jol: Ground Penetrating Radar - Theory and Applications. Elsevier, Amsterdam 2009, ISBN 978-0-444-53348-7 .
- Günter Schlögel: Modeling and localization of small-scale deposits (war relics) underground with georadar. Dipl.-Arb., Montanuniv. Leoben 2007, [1] (pdf, 3.5 MB, accessed March 9, 2009).
- Olaf Borchert: Receiver Design for a Directional Borehole Radar System. Dissertation . Bergische Universität, Wuppertal 2008, [2] (pdf, 8.2 MB, accessed October 12, 2009).
- Jan-Florian Höfinghoff: Investigations into the applicability of georadar in the drilling assembly. Dissertation. Leibniz University, Hannover 2013, ISBN 978-3-944586-23-6 .
Individual evidence
- ^ Merrill Skolnik: Radar Handbook, Third Edition McGraw-Hill Professional, 2008, ISBN 978-0-07-148547-0 , p. 21.20
- ↑ a b c D. J. Daniels: Ground-penetrating radar. Inst. Of Electrical Engineers, London 2004, ISBN 0-86341-360-9 , p. 177f ( limited preview in Google book search)
- ↑ Martin Fritzsche, Application of Pattern Recognition Methods to Detect Landmines with Georadars in “Research Reports from the Institute for High Frequency Technology and Electronics at the University of Karlsruhe”; Karlsruhe, Univ., Diss., 2001 Volume 30 page 7
- ↑ Knödel, Klaus: Geophysics: with 57 tables . 2nd edition Springer, Berlin 2005, ISBN 978-3-540-22275-0 .
- ↑ Georadar - Determining the stability of dykes, ( Memento from November 18, 2015 in the Internet Archive ) “planeterde” portal, accessed on November 18, 2015.
- ↑ https://crev.info/2018/08/ww2-aircraft-found-300-ft-greenland-ice/ David F. Coppedge: WW2 Aircraft Found Under 300 Ft of Greenland Ice , accessed on Oct. 20, 2019
- ↑ technical specification of the HERA (HElicopter RAdar) from RST in Switzerland.
