Ground-penetrating radar
A ground-penetrating radar, ground penetrating radar also, Eng. Ground Penetrating Radar (GPR ) or Radio Echo Sounding (RES ), measures disturbances in the upper layers of the earth by reflecting electromagnetic radiation. In geophysics it is used to examine the upper layers of the earth's crust and the military to detect land mines of all kinds
With this radar, it is an ultra-wideband techniques, up to a few nanoseconds length emits very short pulses of a few picoseconds of the surface into the ground, and after being reflected at a layer boundary and objects, as well as the dispersion of inclusions resumes. The propagation of electromagnetic waves in the substrate is highly dependent on the cause in the soil structures, reflection, scattering, diffraction, and transmission of the incident wave. Recorded are the running time, the phase and amplitude of the reflected wave.
For the emission and reception of the short pulses are wideband antennae, and in particular more suitable for ultra- short pulses Vivaldi antenna was used. The operating frequencies are in the range of 1-1000 MHz. Higher frequencies improve resolution. On the other hand, the signal attenuation, and thus to the depth of penetration from.
Radargram
The picture shows an example of a ground radar image. The distance between dark green transmit to the receive antenna is constant. Each measurement generates a column in the image. By moving the transmitting and receiving unit along the profile line is obtained, a two-dimensional representation of the reflections in the subsurface.
The first signals set by an approximately 5 ns. They have taken the direct route through the air, hence the antennas were at a distance of about 2 m.
For the conversion of the period of the signal to a low value ( time - depth conversion ), the respective velocity of propagation of the electromagnetic signal in the media traversed by it is required, which depends on the electrical properties of the media. At the time - depth conversion, therefore, further geological information must be (eg from core drilling ) were used.
Applications
To explore the shallow subsurface, the ground radar is used as a non-invasive method in engineering geology and geotechnical issues in mineral exploration (sand, gravel), the pipe run exploration and for engineering geological investigations. Other possible applications are in the field of archeology, technical investigations of contaminated sites (underground installations and cavities, fillings, wires and ground layer gradients) as well as in mining and tunneling.
The Mars Express spacecraft studied with the radar MARSIS Mars soil to a depth of up to 5 km. The distance between the antenna and the ground is more than 300 km to 800 km. MARSIS is also able to probe the ionosphere. The measurement frequency is 1.8 to 5 MHz, 0.1-5 MHz at Ionosphärenmessung.
The U.S. Mars Reconnaissance Orbiter carries a similar ground radar, the SHARAD (Shallow Radar ). The higher measurement frequency of 15-25 MHz provides a higher resolution than MARSIS, but a lower penetration depth. The picture on the right shows a SHARAD radar grams along the track shown in the lower part of image. The colors characterize the vertical profile of Green (sink) to red ( survey). The estimation of the depth profile was carried out by an estimate of the signal propagation speed in the rock.
Borehole radar
Another application of the ground radar, the borehole radar which is specially designed for drilling holes. A method of geophysics are test drilling for material analysis. In this context, the borehole radar systems provide a significant opportunity to characterize the environment of the holes using a non-invasive ground penetrating radar system. Radar systems are used mainly wellbore with omnidirectional receiving antennas measure the distance of the reflectors, but no information on the azimuthal angle. Direction -sensitive antenna, however, possible to measure the distance and direction of the reflected echoes.