What is a ground penetrating radar GPR

Basics of radar technology

What is GPR?

Ground penetrating radar

Radar processor
and display
Housing for
Channel,
Recipient and
Antennas

Figure 1: Ground penetrating radar in action,
© 2010 Swedish History Museum, Stockholm

Radar processor
and display
Housing for
Transmitter-receiver
and antennas

Figure 1: Ground penetrating radar in action,
© 2010 Swedish History Museum, Stockholm

Fig. 2: The center frequency used is always a compromise between penetration depth and resolution

Fig. 2: The center frequency used is always a compromise between penetration depth and resolution

Ground penetrating radar

The Ground Penetrating Radar (GPR) (German: Bodenradar) is a geophysical investigation method to locate and close buried or hidden objects or layers in an optically opaque structure (e.g. soil, concrete, masonry, asphalt, stone, wood or ice) map. This non-destructive investigation method has been used since the 1970s for investigations into the environment, in mechanical engineering and in archeology. GPR is a successful example of its application as an ultrawideband radar. A typical ground penetrating radar with a penetration depth of up to one meter could operate in the frequency range from 300 MHz to 3,300 MHz. It allows faster, easier and cheaper examinations than other examination methods. The radiation density is absolutely harmless. The typical average transmission power is in the range of a few milliwatts.

Working principle

A ground penetrating radar uses electromagnetic energy to obtain information about the subsurface. The energy is radiated downwards into the ground and reflected back to the receiver at inhomogeneities and boundary layers with different conductivity. The depth of this inhomogeneity can be calculated from the transit time between the transmission signal and the echo signal. Depending on the type of radar and the propagation conditions, ground penetrating radar is able to map a cross section of the subsurface up to a depth of several tens of meters.

(interactive picture)

Technically, ground penetrating radars can be divided into two different classes. When the ground penetrating radar sends out a pulse-shaped signal, it is called a pulse radar. These radars have a receiver with a high sampling rate and can work directly in the time domain. In order to achieve a large bandwidth, these pulses have to be extremely short, typically in the range of a few nanoseconds or even shorter. Often the lower cut-off frequency is brought closer to zero in order to obtain a large bandwidth. In this case, a carrier frequency is no longer modulated in the transmitter, but a high-voltage pulse (typically between 20 and 200 volts) is sent directly to the antenna. The resulting type of pulse is called a baseband pulse. Its shape resembles a "Mexican hat" and is mathematically described as a negative normalized second derivative of a bell curve (Gaussian function).

Ground penetrating radars, which emit a frequency that changes step by step or continuously, and which process the difference frequency (or difference phase) between the currently transmitted and the delayed received signal, work in the frequency region. They are mostly frequency-modulated continuous wave radar devices with a step-by-step frequency change (SFCW radar). Such radar devices are much simpler in construction and therefore cheaper than pulse radar, but much slower. The transit time is determined by an inverted fast Fourier transformation. Due to the possibility of integrating the echo signals over a longer period of time, they have a better signal-to-noise ratio than pulse radars.

The choice of the frequency range is a compromise between possible penetration depth and resolution. Soil material has a low-pass character. Lower frequencies can penetrate deeper than higher frequencies. However, higher frequencies offer better resolution. The transmission signal is also attenuated to different degrees in different layers of the earth. The hardest to penetrate is wet clay, the easiest dry sand. The smallest object size that can be seen by the radar increases when the frequency is decreased. Figure 2 gives a rough overview: At 1 MHz, the radar could locate objects down to a depth of 30 to 40 meters, but only if they are more than 2 meters in size. The decisive factor for the vertical resolution is the effective bandwidth of the received signal. The horizontal resolution is determined by the radiation characteristics of the antenna used. This can be improved slightly by using methods of radar signal processing that, similar to Synthetic Aperture Radar (SAR), combine the echo signals from several positions of the ground radar. Unfortunately, these options are not as successful with ground penetrating radar as with SAR, since the speed of propagation in the ground is not constant and necessary phase corrections can only be estimated.

There are also many cellular networks within the frequency range used, the radiation fields of which can seriously disrupt the GPR. The user should ensure that his mobile phone is switched to flight mode during the measurement. One way of minimizing such interference is phase-coded modulation. However, this type of modulation is only possible with narrowband FMCW radar.

Image 3: Here, many A-scans have been merged into a simplest B-scan. The positive impulse component has been filled in with color and can now simulate the brightness modulation of the B-scope.

Image 3: Here, many A-scans have been merged into a simplest B-scan. The positive pulse component was filled in with color and can now simulate the brightness modulation of the B-scan.

Radar signal processing

A raw image is generated from the radar data along the direction of movement. They represent a section through the terrain. Since the antennas cannot have a great directional effect due to the low frequencies and the necessary bandwidth, all objects in the ground already give an echo signal if they are not yet exactly under the ground radar. This echo signal is still displayed exactly under the ground penetrating radar. Due to the greater slanting distance, this echo is shown at a greater depth than it actually is. When driving over the reflective object, a hyperbolic target sign is generated. The actual position is the vertex of the hyperbola. In the raw image there is always at least one continuous line in the vicinity of the surface, which results from the direct connection of the transmitting antenna to the receiving antenna through the air, followed by a second line, which results from the direct wave in the ground.

Using suitable software, this image can be reduced to the actual positions of the objects with the help of a large number of filters. These filters reduce scattering losses, noise and the phenomenon of interference, also referred to here as floor clutter, due to various inhomogeneities that are actually not objects at all. At the same time, the gain factor can be increased depending on the running time in order to compensate for the attenuation in the ground.

Conversion of the running time to a distance / depth is not so easy, however. The speed of propagation of electromagnetic waves in the ground varies greatly. In dry soil the speed of propagation is only half as great as in the air. In the case of moist sand it is even reduced to only a quarter, in the water even to a little more than a tenth. These differences must be known and can, for example, be estimated by means of a resistance measurement or compared with seismological measurements. However, only a core drilling can provide final certainty.

The result of a single radar measurement can be displayed as an A-scan on an oscilloscope. This shows a time line on the X-axis and the amplitudes of the delayed pulses on the Y-axis. A B-scan can be generated from several such individual measurements. The distance covered by the radar is plotted on the X-axis and the timeline on the Y-axis. The echo signals are now modulated in brightness or made visible on this surface using false colors. This B-scan is the section through the earth along the path that the radar travels. A map with the top view of the test area can be generated as a so-called C-scan from several such parallel sections, the axes now being the two-dimensional distances on the test area. All of these representations are referred to in the publications as radar gram. The terms A-Scan, B-Scan and C-Scan have a peculiar meaning for GPR and should not be confused with A-Scope, B-Scope, and C-Scope as vision devices of reconnaissance and targeting radar devices.

conical rods
as a spotlight
Ohmic resistance
between feed
and parabolic reflector

Figure 4: Structure of an Impulse Radiating Antenna (IRA)

conical rods
as a spotlight
Ohmic resistance
between feed
and parabolic reflector

Figure 4: Structure of an Impulse Radiating Antenna (IRA)

Pulse-emitting antennas

If the GPR works in the frequency range, the choice of antenna is not critical. With its broadband, it should only cover the frequency range used. In the case of pulse radar devices operating in the time domain, the choice of antenna is more difficult. The antenna must have the same phase center for all frequencies in the range in order not to change the pulse shape. Thus, for example, the log periodic antenna is less suitable. In addition, small geometric dimensions must be observed for portable devices so that the devices do not become too unwieldy.

Impulse radiating antennas (IRA) have been specially developed for ultra-broadband applications. They consist of a dipole made of, for example, conical round material in front of a parabolic reflector. They have a good directivity and allow a very high peak field strength. With a diameter of 0.8 m, their bandwidth ranges from 0.2 to 6 GHz. The transition between the two halves of the radiator and the parabolic reflector, which is provided with a resistance coating, prevents reflections of very high frequency pulse components on the radiator. Compared to an electrically equivalent linear conical horn antenna, the IRA is much more compact.

Applications

GPR is available as a handy device that can be operated manually on the floor or on walls. These devices are also suitable for examining non-metallic materials for cracks or cavities. Larger devices can either be dragged or dragged across the ground on runners or built into a small cart that is pushed by a single person. The housing and the means of transport should be constructed of non-metallic materials as far as possible so as not to impair their function. Installation in vehicles is also possible, but is mounted here with a non-metallic boom, either far in front of the bonnet or far behind the tailgate. Ground radar can also be transported as a trailer load with a helicopter, such as the "HERA" (HE licopter RA dar) from the Swiss company RST.

The GPR MARSIS is an integral part of the Mars Express spacecraft and examines the Martian soil to a depth of 5 km with frequencies between 1.8 and 5 MHz. The distance between the antenna and the Martian soil is between 300 and a maximum of 800 km.

The GPR is used, among other things, for the investigation of water-bearing layers, for measuring the thickness of rock massifs, sedimentation layers and water depths as well as for the detection of caves. Other uses include mapping pipes, manholes, cables, and boulders. The combination with other investigation methods, such as seismological and resistance measurements, reduce the uncertainties when characterizing the cause of reflection.

Image gallery of an archaeological application of a ground penetrating radar

A radar survey conducted by Dean Goodman in Japan shows a circular burial mound with a burial in it, which is clearly shown in the lower layer of Figure 6. Image 7 is a 3-dimensional section of a burial chamber that was discovered by Dean Goodman in a burial mound on the island of Kyushu in Japan. It contained the remains of a samurai with a variety of funerary items including bronze swords. (Image sources: Dean Goodmann)

Image 5: an older ground penetrating radar in action

Photo 6: layers

Fig. 7: 3D representation