The figure below shows the main window of the GEORADAR-EXPERT software system, where the red frames indicate the location of the histogram of the Probability attribute and the graph of the Permittivity – Probability corrective function.
The Probability attribute provides an estimate of the probability that a combination of GPR profile signals identified by the BSEF analysis as a reflection from a subsurface object is indeed a reflection from a real-life subsurface object. The measure of probability is the ratio of the number of in-phase signals on the line passing through the maxima of the diffracted reflection to the length of this line. If all diffracted reflection maxima consist of signals of the same phase, the probability value is 100%.
Based on the information provided by the Probability attribute histogram, the user can exclude field analysis points with low probability values from the section calculation, thereby increasing the truth of that section. Note: a field analysis point is a point on a GPR profile with X[m] and Y[ns] coordinates, which indicate the position of the top of the diffracted reflection, the measured kinematic and dynamic characteristics of which are attributes of this point. As attributes, also, are the values calculated on the basis of the measured characteristics.
The figure below shows an example of a histogram of the Probability attribute. The Probability range is set using the input boxes located under the axes of the histogram.
Using the Permittivity - Probability corrective function, the user can minimize possible errors of automated BSEF analysis, and using the Permittivity - Probability corrective function plot, one can evaluate the reliability of certain dielectric constant values. For example, from the corrective function plot shown below, it can be inferred that high values of permittivity have low Probability attribute values.
The following is a comparison of the final results of a GPR survey obtained without using the Probability attribute range constraint and with such a constraint. GPR survey was carried out in order to study the landslide situation in the area of the road. Below is a photograph of a site of the road with a landslide displacement of soil on the side of the road, and a GPR profile obtained during the study of this section using a 150 MHz GPR. In the photograph, the manifestation of the landslide process is marked with a rounded frame, the black arrow shows the movement of the the GPR unit.
Below is the result of processing the above GPR profile using automated BSEF analysis as a section of the Central Frequency attribute (central frequency of reflected signals). Two variants of the section are presented: without limiting the range of the Probability attribute and with the application of a limiting. As a result of the limitation of the Probability attribute range, only those field analysis points whose Probability attribute value was equal to 100% were used to create the section.
It can be seen that in the section created with the Probability attribute limiting, the boundary between the fill and bedrock is traced much better than in the section where the Probability attribute limiting was not applied.
The following shows the results of creating a section of the Re(permittivity) attribute (the real part of the complex relative permittivity) without limiting the range of the Probability attribute and with the limiting applied. As a result of the limitation of the Probability attribute range, only those field analysis points were used to calculate the section, the Probability attribute values of which lay within the specified probability range of 85 - 100%.
On both sections, the cross section of the landslide body is colored red, however, on the section on the left, the landslide body is displayed fragmentarily. Further, to obtain more complete information about the subsurface, the sections of the Central Frequency and Re(permittivity) attributes were summed in the Summation module of the GEORADAR-EXPERT software system. The results of this summation are shown below. On the left - the result of the summation of the sections calculated without applying the Probability attribute range limiting, on the right - the result of the summation of the sections calculated using such a limiting.
The summation result obtained with the Probability attribute range limiting fully shows the cross section of the landslide body, which is not the case with the summation result of sections calculated without the probability range limiting. It is noticeable that on the summation result shown on the left, there is no fragment of the landslide body located in the zone of manifestation of external signs of the landslide process, and on the summation result shown on the right, one can observe the exit of the landslide body to the surface in this place:
Undoubtedly, the ability to create a section using only those field analysis points whose Probability attribute values are high enough increases the truth of this section, but such a restriction does not always lead to such a result. Real reflections from subsurface objects can also have a low value of the Probability attribute. This happens when a local object that generates a diffracted reflection is at such a depth that the reflected signals from this object are comparable in amplitude to random interference and noise. In this case, the application of the range limitation of the Probability attribute will lead to the removal of field analysis points from the section calculation that do not contain BSEF analysis errors, which will lead to a deterioration in the quality of the final result.
The decision to apply the limitation of the attribute range should be made on the basis of the principle of the best correspondence of the created section to the a priori information about the object under study and / or the greatest consistency in the shape and location of the section elements for a given type of the subsurface. To do this, a section is created without limiting the range of the Probability attribute and several sections with different values of the boundaries of the range of the Probability attribute. As the final result, the section that best meets the above criteria is selected. As experience shows, the lower limit of the range of the Probability attribute can be in the range of 70 - 100%, and the upper limit of the range is 100%.
Previously, in the list of corrective function (CF) creation modes, which were selected using the drop-down list located in the lower right corner of the CF panel, there were only two items - Auto and Manual (automatic and manual CF creation modes). Now several options for calculating the corrective function in automatic mode have been added - see the figure below.
The drop-down list items with the name Auto or consisting of the abbreviation CF and a serial number represent different ways of automatically creating a corrective function that take into account various features of the subsurface structure. The choice of the method for automatic creation of CF is carried out according to the principle of the best correspondence of the created section to a priori information about the object under study and / or the greatest consistency in the shape and location of the section elements for a given type of the subsurface. Index R in the name of the list item means reversal of the corrective function. For example, the corrective function created when selecting the list item CF 1R is calculated in the same way as the corrective function from the list item CF 1, and then reversed.
As practice has shown, after it became possible to apply various options for creating a corrective function in automatic mode, the Manual CF creation mode using the mouse became used quite rarely.