Along with automated BSEF analysis and the
creation of the attribute sections of the investigated subsurface environment
and the wave field reflected from it based on this analysis, the
GEORADAR-EXPERT implements a complete set of data processing methods that
should be present in every software for processing GPR information. These are
various types of signal transform, control of the geometry and visualization of
the GPR profile, working with manually created user layer boundaries, combining
two-dimensional GPR data into a three-dimensional assembly, and much more. In
other words, the GEORADAR-EXPERT software system has everything that a GPR
specialist is used to working with.
Along with the widely used methods of processing GPR data, algorithms and methods have been specially developed for the GEORADAR-EXPERT to effectively increase the resolution of GPR profile signals and suppress difficult interference.
The B-Detector (Boundaries Detector)
method, specially developed for the GEORADAR EXPERT, is designed to suppress
interference and increase the vertical resolution of the GPR profile, which
makes it easier to distinguish nearby, vertically, reflections from the
boundaries of subsurface layers. Using B-Detector, the user increases the
frequency and spectrum width of the GPR profile signals, thereby reducing the
duration of these signals and increasing the degree of detail of the GPR
As a result of processing by the B-Detector method, the GPR profile obtained using a low-frequency antenna looks as if the frequency of the GPR antenna for recording this GPR profile was tuned to a frequency several times higher than the actual frequency of the antenna. Compared to the original GPR profile, the vertical resolution of the GPR profile after processing by the B-Detector method is several times higher. If necessary, the B-Detector method can lower the central frequency of the GPR profile signals.
Further, as an example of the application of the B-Detector method, the GPR profile recorded during the survey of the road pavement with a 1200 MHz GPR is considered, see the figure below on the left. Starting from the 4 m distance along the profile, the asphalt concrete pavement of the highway consists of two layers. According to a priori information, the upper layer of asphalt concrete has an average thickness of 0.04 m, the lower boundary of the second layer of asphalt concrete lies in the depth range from 0.12 to 0.15 m from the surface of the pavement. Below is a layer of crushed stone base, the thickness of which, on average, is 0.15 m. Not all reflections from the boundaries of these layers are noticeable on the raw GPR profile. For example, it is difficult to detect a reflection from the contact boundary of asphalt concrete layers at a depth of 0.04 m. From this it can be concluded that the 1200 MHz antenna does not provide sufficient resolution of the GPR profile for a detailed study of the road layers.
In such a situation, it would seem logical to
use a higher frequency antenna that emits radio pulses of shorter duration,
which would improve the resolution of the signals. But with increasing
frequency, the attenuation of the radio pulse increases, which means that
choosing a higher frequency GPR antenna will lead to a decrease in the depth of
the GPR survey. For this reason, at the hardware level, it is impossible to
achieve an improvement in the vertical resolution of the GPR profile without
reducing the depth of the GPR survey. At the software level, to some extent,
this problem can be solved using the B-Detector method.
The figure above on the right shows the result of processing the considered GPR profile by the B-Detector method. As a result of using this method, the resolution of the GPR profile has significantly increased. Reflections from the boundaries of road layers have become compact, do not overlap each other, and are well distinguished. Phase distortions caused by the mutual influence of reflections from nearby boundaries have disappeared. Aligned reflection phases make the GPR profile well adapted to the process of automated boundaries picking, which is implemented in the GEORADAR-EXPERT software system. The availability of the automated boundaries picking option is important for processing large amounts of data, for example, obtained during road or railway GPR surveys.
Next, as another example, the result of GPR profiling of a highway with a 400 MHz GPR is presented. The upper figure shows the raw GPR profile, below is the result of its processing by the B-Detector method.
As a result of the application of the B-Detector method, the lower boundary of the road pavement is clearly visible on the processed GPR profile, the depth of which varies around the 0.2 m tick. This boundary is difficult to fully detect on an unprocessed GPR profile. The figure below shows on the left the amplitude-frequency spectrum of the signals of the original GPR profile, on the right is the spectrum of the GPR profile after applying the B-Detector method.
As a result of using the B-Detector method,
the central frequency of GPR profile signals increased approximately 3.5 times
– from 264 MHz to 911.4 MHz. The width of the signal spectrum has increased 3.7
times – from 74.7 MHz to 277 MHz. An increase in the width of the signal
spectrum indicates a decrease in its duration, which leads to an improvement in
the vertical resolution of signals on the GPR profile. Thus, as a result of
using the B-Detector method, the GPR profile looks as if it was obtained not by
a 400 MHz GPR, but by a hypothetical higher-frequency GPR with a central
frequency of 400 *3.5= 1400 MHz, providing a greater penetration depth of the probing
pulse, which is not typical for a high-frequency GPR.
Often, in a road survey, it is required to obtain two GPR profiles for the same location. One GPR profile is recorded using a high-frequency antenna that provides penetration of the probing pulse to a depth of about 1 meter. This entry is used to study the pavement layers. The second GPR profile is recorded using a lower frequency antenna that provides a depth of 3 - 8 meters. Such an antenna is suitable for the study of soils under the road.
If a single-channel GPR is used, which does not provide simultaneous operation of two antennas tuned to different frequencies, then to record two GPR profiles in the same place, it is necessary to pass the same route with different GPR antennas twice.
In such a case, using the B-Detector method, it is possible to achieve a compromise between the time spent on recording and processing GPR data and the quality of the GPR study result. Using the B-Detector method and one midrange antenna, for example, a 400 MHz antenna, as in the example under consideration, will reduce the volume of field and cameral work by half. Taking into account the significant mileage of GPR profiling during road works, this is a significant economy.
Another argument in favor of using the B-Detector method. A small geophysical company may not have a full set of GPR antennas that cover the entire range of operating frequencies of georadolocation to solve a wide range of tasks. The acquisition of a large amount of geophysical equipment requires significant financial costs, and this is sensitive for a small company. The use of the B-Detector method will allow you to save on high-frequency antennas, allowing you to have one medium-frequency and one low-frequency antenna available for some time, for example, 500 and 100 MHz.
Along with the B-Detector method, an increase in the vertical resolution of the GPR profile in the GEORADAR-EXPERT software system can be performed using wavelet decomposition. This transform resembles a windowed Fourier transform, only in the Fourier transform the signal is decomposed into components in the form of sines and cosines, and in the wavelet decomposition the decomposition is performed using special functions - wavelets, the graph of which resembles a GPR probing pulse in shape. After the wavelet decomposition, the signals is restored by high-frequency decomposition levels. The central frequency of the restored signals and the width of its spectrum is greater than that of the original signals, which means that the restored signals is shorter than the original one. And if the signals is shorter, then the vertical resolution of such signals is better.
Below is a comparison of the results of using the methods of B-Detector (upper image) and wavelet decomposition of signals.
Each of the presented methods of increasing the
resolution of GPR data has its advantages. The user, depending on the features
of the GPR profile wave field and the tasks of GPR research, can choose which
of these methods to use in each specific case.
Interference and Air Reflections Suppression
One of the problems faced by a specialist in processing GPR data is the suppression of reflections from objects located on the surface. These so-called air reflections often have a high level of amplitudes, which allows them to mask reflections from subsurface objects well. Shielding GPR antennas does not allow you to completely get rid of these air reflections. To the greatest extent, air reflections are manifested on GPR profiles obtained using dipole low-frequency antennas, where shielding is not provided. Also, diffracted reflections from contrasting local objects lying at a shallow depth can act as interference.
The spatial filter implemented in the GEORADAR-EXPERT software system allows you to solve the above problem. As an example of suppressing intense masking interference using a spatial filter, a profile obtained by a 150 MHz GPR that crosses tram tracks is taken. The figure below on the left shows the raw GPR profile, in which intense reflections from metal rails and ground infrastructure objects are superimposed on weaker reflections from the boundaries of layers in the ground. The result of spatial filtering is shown on the right. Diffracted reflections-interference is suppressed and does not mask reflections from the boundaries of the layers.
Along with the spatial filter, the
GEORADAR-EXPERT software system implements interference removal by decomposing
GPR profile signals into components. If the GPR profile signal matrix is
decomposed into components, and then restored, having previously discarded
those decomposition levels that contain information about interference, then
there will be no interference on the restored GPR profile. Each level of decomposition contains its own
characteristic features of signals. The lower levels contain spatially extended
horizontally oriented components of the GPR profile. The higher the
decomposition level, the more compact the decomposition components become.
The following is an example of the application of decomposition into components. On the left is an example of a GPR profile that contains two types of waves. These are extended subhorizontal reflections and diffracted reflections, which look like hyperbolas on the GPR profile. The center shows the result of the restoration of this GPR profile by the lower levels of decomposition. It is noticeable that characteristic reflections in the form of hyperbolas have disappeared on the profile. On the right is the result of recovery by the higher levels of decomposition. In this case, information about subhorizontal extended reflections is discarded, and diffracted reflections are not affected.
The GEORADAR-EXPERT software system has
more than two dozen options designed for processing GPR profile signals. With
the help of these options, almost the entire range of tasks that a user may
have in the process of processing GPR data is solved. The user can save the
sequence of applying data processing options to a file and apply this sequence
in the future.
Automation of user actions for processing the same type of GPR data frees him from being at the computer. In batch processing mode, the user has to set processing parameters and select a group of GPR profile files, after which the downloading, processing and saving of the processing result is done automatically, without the user's involvement.
This review describes only some of the options for processing GPR data specially developed for the GEORADAR EXPERT. For a full description of the capabilities of the GEORADAR-EXPERT software system, see the user manual, which can be downloaded using this button: