Automated BSEF Analysis

The fact that after suppression of interference on the GPR profile, reflections from the boundaries of the layers do not become noticeable does not mean that they are absent. When useful reflections are close in their characteristics to interference, during the filtering process, these reflections are removed together with interference. The BSEF automated analysis algorithm works more selectively, and is able to separate useful signals from interference close to them. 

The figure below shows a section of the Resistivity attribute created based on the results of automated BSEF analysis of the GPR profile in question. In contrast to the visually uninformative profile, the attribute section displays the structure of the subsurface environment well. Within the investigated subsurface, two main layers are distinguished. The upper layer is characterized by reduced resistivity values, this layer is displayed mainly in blue colors on the section. The lower layer has higher resistivity values and is represented by yellow-red shades. According to the section, it is possible to trace how, as you move away from the coastline, the thickness of the layers of coastal soil and the electrical resistivity inside these layers change.

A visual representation of the variability of the section attribute can be obtained using the statistical module of the GEORADAR-EXPERT software system, which provides information in the form of graphs and tables for 12 statistical indicators. Indicators can be calculated for each layer, for user-specified layers, for each layer boundary, or for the entire section as a whole. As an example, the figure below shows the result of using a statistical module in the form of a graph of changes in the average values of the electrical resistivity in the upper layer of the Resistivity attribute section. The area of the graph that exceeds the user-defined threshold is highlighted in red. At the bottom right is a thumbnail of the section, which shows the boundaries of the layers. The fill highlights the layer whose data is represented by the graph.

It is obvious that the switch from the representation of data on the subsurface environment in the form of a set of amplitudes of reflected signals (GPR profile) to the characteristics of this environment obtained as a result of the application of the BSEF automated analysis method (attribute section) significantly increases the informativeness of the GPR survey. The result, presented in the form of an attribute section, is more understandable to the customer of this survey. The list of attributes used in the GEORADAR-EXPERT software system is quite wide. The list of attributes and their description can be found in the Section List of Attributes section of the user manual, which is available for download at the FOLLOWING LINK.

One of the advantages of automated BSEF analysis over other methods of processing GPR data is the ability to study subsurface environment with this method, the electrophysical characteristics of which change vertically smoothly, without sudden jumps. Abrupt changes in the properties of the subsurface in the vertical direction create conditions for the reflection of probing pulses and their reception by GPR. Environments that have a smooth character of changing electrophysical properties cannot form reflections. In such places, only high-frequency noise and various kinds of interference are present on the GPR profile, provided that the studied environment in these places does not contain local objects. 

Local objects are objects in a subsurface environment whose linear dimensions are comparable to the wavelength of the GPR probing pulse, and the electrophysical characteristics of these objects differ from the electrophysical characteristics of their host environment. For example, stones in the ground can act as such objects. Local objects have an important feature. It consists in the fact that when the GPR probing pulse interacts with local objects, these objects become a source of diffracted waves, the kinematic and dynamic characteristics of which carry information about the properties of the subsurface environment. 

Due to the low intensity of diffracted waves, which is often comparable to the level of interference, visual analysis can detect only a small part of these waves on the GPR profile. As a rule, this amount is not enough to obtain a detailed information about the structure of the subsurface environment based on the measured characteristics of these waves. 

Further, the figure on the left shows the result of GPR profiling of soil with a smooth change in electrophysical characteristics. The profile was obtained by a GPR 250 MHz. The profile crosses the dry bed of a seasonal stream, the talweg of which is 25 m from the beginning of the profile. The electrophysical characteristics of soils in this profile change smoothly with depth, without sudden jumps forming reflections on the GPR profile. For this reason, the profile does not contain characteristic extended reflections, which are interpreted as reflections from the boundaries of the layers. Under these conditions, it is possible to obtain information about the structure of the studied environment using automated BSEF analysis. 

The figure on the below shows a section of the Resistivity attribute. The section of the attribute shows the position and shape of the channel stream deposits, as well as the distribution of resistivity within these deposits. The section shows that the boundary between dense soil and loose sediments has a concave shape and reaches a maximum depth of 5.5 meters in the talweg area of the stream. The lowered values of the section attribute indicate that the stream has not completely dried up, and the lowest resistivity, indicating the highest humidity, is located in the talweg area of the stream, at depths of more than 1 meter.

Sections Summation 

The GEORADAR-EXPERT software system has a fairly wide set of attributes for solving a whole range of GPR tasks. It happens that a section of one attribute does not provide complete information about the object under study, but various fragments of this information are distributed across several attribute s​ections. In this case, the summation of sections a​llows you to combine disparate information about the object of study into one summary section. Also, summation eliminates artifacts caused by the accumulation of errors in the process of collecting and processing GPR information. 

The figure below on the left shows the GPR profile recorded during the study of the paleodoline by a GPR 100 MHz. The center shows a set of attribute sections for summation, created based on the results of automated BSEF analysis of this profile. The result of summing these sections is shown on the right.

Visual analysis of the GPR profile shows that this profile is uninformative, starting from a depth of 2 meters. The base of the paleodoline, according to a priori information, is below this value. The attribute sections show some details of the buried relief, but each of the sections separately does not provide complete information about the structure of the studied subsurface. 

The summation operation combined this disparate information into one whole. The relief of the base of the paleodoline on the summation section is well tra​ced along its entire length. In the figure, the base of the paleodoline is indicated by a dotted line. This example illustrates how the use of the summation module allowed solving the problem of insufficient information content of the GPR profile and individual sections of attributes by this profile.