GPR – how it works?

 

The impulse Ground Penetrating Radar (georadar) is a precise transmitting - receiving measuring device, which implements the phenomenon of reflection of electromagnetic waves. The transmitting antenna sends an interrupted sinusoidal impulse of the length of one and a half period. The electromagnetic wave travels with the velocity, which is dependant on ectromagnetic properties of penetrated material. The second - receiving antenna, mounted at a certain distance, receives reflected signals, which are delayed from the transmitted signals from tens to thousands nanoseconds. The delay results from the distance between the transmitting antenna, underground reflectors (any material with different electrical properties to source material, which reflects a part of the energy of electromagnetic wave), and the receiving antenna.

The record of the signal forms a singular path, comparative to a single bore-hole. This kind of image was observed on an oscilloscope screen in distant times. Once multiple paths are compound, a 2-D image emerges. X corresponds with distance, while Z with GPR interception time, allowing us to determine the precise depth of buried obstruction. Creation of 3-D shapes involves the Y coordinate.

 

A single GPR trace consists of the transmitted energy pulse followed by pulses that are received from reflecting objects or layers. A scan is a trace where a colour or grey scale has been applied to the amplitude values. As the antenna(s) are moved along a survey line, a series of traces or scans are collected at discrete points along the line. These scans are positioned side by side to form a display profile of the subsurface.


A single trace, which can be compared to a single borehole, is the record of signal. After putting the traces one next to another, one can obtain a two-dimensional image, where X is the distance covered during the profiling, while Y is the time of “the listening in” by the GPR. The Z dimension appears during the formation of 3D solids.
The effective area being surveyed by the GPR has the shape of a cone (its shape is dependant on the properties of material being surveyed), thus, the signals coming to the receiving antenna come not only from objects located precisely under them, but also from objects in a close distance. The time needed for the signal to travel to an object and back varies depending on the distance from the antenna to that object. The shortest time is received in case, when the antenna is right above the object being surveyed. Thus, the cross-section of a pipe, for example, will be shown on the readout by the GPR as a hyperbola. Knowing the coefficient of the dielectric penetration of a centre being surveyed (the velocity of a electromagnetic wave in a given centre), the GPR permits to determine precisely the depth, on which a given object that causes the anomaly is located.
A single trace recorded by the GPR. One after another create a two-dimensional wave diagram .
Pipes will be imaged on the readout as hyperbolas by the GPR.
The manner of presenting survey results by means of various colour pallets.
Colourless two-dimensional diagram – the so called wiggle plot
The colour pallet from black to white, grey zero 50%.
The pallet, whose highest amplitudes were marked black, zero is white.
The wave diagram in colours set by the author.
The GPR operates in a wide range of frequencies from 10 MHz to 4 GHz and even higher. The choice of working frequency is dependant on the depth of penetration (because of wave attenuation along with the increase of depth) and the type of ground (silts and clays limit the range of electromagnetic waves as opposed to sands or gravels).
The electromagnetic waves of the highest frequencies are most deeply attenuated. Therefore, in case, when it is necessary to survey a deeper deposited lithological layers or objects, antennas operating in the lower range of frequencies from approximately 10 to 300 MHz are applied. However, obtaining higher depths is always paid for with the decrease of vertical resolution.
If it is necessary to carry out surveys on higher depths, then, they can be carried out from a borehole instead of the ground surface. In such a case, a borehole antenna is used. The antennas operate in different range of frequencies and, depending on the type of work being performed and the type of required information, one can use high– or low– frequency antennas. Our company offers borehole antennas of the frequencies of 100 MHz and 1 GHz.
Low- and medium-frequency antennas permit to detect larger objects, while high-frequency antennas, in the range from 1-5 GHz to 4 GHz, are characterized by high details and detect particular reinforcement rods in a concrete construction, for example.
The accompanied examples show the differences in profile resolution depending on the frequency used (antenna). In order to facilitate the comparison, each profile shows the same depth despite the fact that antennas of lower frequencies reach considerably deeper. The large anomaly (approx. 4 m deep) is a clearly visible insertion of another material – a wall in this case.
The comparison of profile obtained in the same location, but with the usage of antennas of different frequencies.
The profile achieved from the GPR with a non-shielded 500 MHz antenna.

 Different color palettes applied on radiagram
GPR wiggle plot

 

2D wave guide without colour

 

GPR gray scale

 

palette of colours from black to white, zero is gray

GPR black-white-black scale

 

a palette which the biggest amplitudes are marked black, zero is white

GPR color scale

 

wave guide in colours set by the author

 

GPR works within the range from 10 Mhz to 2 GHz. The frequency depends on eligible penetration depth. The electromagnetic waves are extinguished in loam and clay, while dry sand and gravel do it to lower extent.

The strongest fading effect occurs for the highest frequencies. Therefore they can be reduced to 10-300 Mhz, when deeper penetration is needed.

Lower frequency is the price for accessing high depths. GPR is only able to see bigger objects such as caves, tunnels, casts and the structure of lytological layers. Since we hardly seek cables, pipes or rock crumbs at these depths, this emerges as an advantage. These small object would produce noise, rather than useful information.
The specific examination of very deep parts can be done by placing the antenna in a bore-hole. Through this method, we can obtain information about the smallest objects.

The images show how used frequency influences the final resolution. Each profile is for the same depth (even though they are able to depict much deeper parts). The visible anomaly is a vivid part of other type of material, a brick lane in this case.