GEOPHYSICAL WORKS, GEORADAR SURVEY
‘WATCHING’ THROUGH THE AGES INSIDE WALLS, TUNNELS AND SOILS
Geophysical work is a set of methods for investigating underground structures without excavation, used to study soils, search for minerals, and assess the condition of buildings and engineering structures.
GPR survey is a method of geophysical work that uses radio waves to detect underground objects, voids, utilities or changes in the ground. It provides accurate data on underground structures without disturbing the surface.
WORK METHODOLOGY
PLATFORM PROFILING
PLATFORM PROFILING
GROUND PENETRATING RADAR
DATA PROCESSING
PROFILING RESULTS
SOME OF OUR WORK
WORK PROCESS
During the works the geophysical complex (GPR) ‘Loza-1V’ with 100cm (180MHz) antenna complexes with 1MW (5kV) pulse power transmitter is used;
Methodology of works:
Profiling by separate profiles at the surveyed site – to study the geological structure of the ground vertically, the GPR with receiving and transmitting antennas is moved along the profile. The spatial step of measurements along the profile is chosen depending on the required detail of the object investigation, from 10 to 20 cm.
In the process of measurement GPR antennas are moved along the ground surface with fixation in each point of survey – this ensures high quality of profiling. The measurement results are displayed on the screen of the device in the form of a radarogram, which fixes the arrival time of the signal reflected from the interface for each measurement point. The arrival time of the reflected signal depends on the depth of the reflecting surface and the speed of wave propagation in the ground.
Processing and interpretation of the obtained data:
The results of profiling with GPR ‘Loza-1B’ are presented in the form of waveforms of individual measurements with a dynamic range of up to 120 dB. This potential is obtained by using ultra-high-power transmitters with a peak pulse power of 1 MW. Such a range means that the radiated signal can be attenuated by a factor of 1,000,000 and can be received at the level of natural and instrumentation noise. By emitting such a signal, we get a reflected signal with such a signal-to-noise ratio that its separation does not require complex mathematical methods and software tools. The main centre of processing is transferred to the area of separation of layers and layer boundaries and bringing the radarogram to the present depth scale taking into account the signal velocity in each of the geological layers. The boundaries of geological layers are selected by means of the procedure of signal maxima and minima extraction and representation of the section in the mode of the derivative function of the signal amplitude. The GPR section constructed from the sounding results and reduced to the depth scale corresponds to the geological section. Confirmation of the presence of possible disturbances of the structure, discontinuities and faults are purely radiophysical signs – peculiarities of signals in these zones: the presence of numerous shifts in the phase of the signal and the presence of multiple reflections of the signal.
Pic.1.
GPR complex Loza
The new technology allows to detect foundations of buildings, underground structures, minerals, water at depths from 10 cm to 100 m, and there is also a model that detects in concrete structures reinforcement and voids, wet and emergency areas. the ability to ‘look’ through and inside walls, tunnels, soils.
Progress of work
The field work was carried out on 23 September 2018. During the work, profiling was carried out at 2 sites and a road, a total of 51 profiles were carried out to obtain an area survey (3D), totalling 904 (linear metres) of geophysical profiles. The profiles were run parallel to each other at a distance of 1m to 1.5 metres (see profile layout).
Photo 1.1. Photo of the study site 1.
Photo 1.2. Photo of the study site 2.
3.Analysis of the obtained resultsProcessing and interpretation of the GPR data was carried out in a specialised computer programme. The processing was carried out in such a way that the main emphasis in processing the profiling results was placed on identifying large underground objects that can be interpreted as a cavity (an object with regular geometric outlines within a rock massif), such as vertical walls of a room and a vaulted ceiling. Bringing the radarogram to the present depth scale, taking into account the electromagnetic signal propagation velocity in each of the highlighted cavity anomalies, as well as geological (engineering) layers. The boundaries of the geological layers are highlighted using the section representation as signal amplitude. Confirmation of the presence of possible voids, structures, discontinuities, weak signals from subsidence and other deformations in the layer of overlying or host rock are purely radiophysical signs – the presence of hyperbolas, and the presence of multiple reflections of the signal in places where a void object is present, such as an underground passage or gallery.
Figure 3. An example of a GPR transect. The section shows a thick near-surface layer showing changes in the layered structure of the soils, several hyperbola wings are visible at 2m, 6m and 20m and a regular hyperbola at 15m, the depth of the object is 2.3m. Also shown is the waveform for this transect point, the waveform shows the behaviour of the wave that travels through the thickness of the layers and the object. The waveform can be used to judge the properties of soils and objects.
– as a result of interpretation of geophysical profiles anomalies were detected which may correspond to underground structures like tunnels or galleries, it is impossible to describe the object in more detail from the radarogram, but it is possible to judge the presence of the object and its parameters (volume, depth, etc.). GPR transects in the report represent a geoelectric section, the main criterion for search – violation of continuity of geophysical horizons, radio images in the form of hyperbolas or clusters of hyperbolas, which can be formed by more complex underground objects. The sections show local anomalies that GPR detects in the form of ‘hyperbolas’, layers of continental sedimentary horizons, as well as local ‘objects’, probably the result of natural collapse of individual elements of underground structures or local objects such as basements or communications.
Description of GPR transects Area 1 No underground anomalies or hidden burials were found in the surveyed areas in the area of the proposed burials. The processing of the footage in the Mole programmes also did not reveal any burials or hidden underground anomalies. And, in view of this, we can assume that the real burial is located elsewhere.
Figure 3.1.1. GPR transects. There are no obviously pronounced anomalies.
Picture 3.1.2 Horizontal slice across the area (top view), the slice shows a depression, possible objects at depths from 1.3 metres.
Area 2
On the surveyed areas in the area of the cinema, where the ruined church was located, at the beginning of the 7 – 11 profile a pronounced underground anomaly of rectangular shape, measuring 9 by 6 metres with a depth from the surface of 2.3 metres was found. We recommend excavation at this location to determine the true size and purpose of this anomaly.
Picture 3.2.1 GPR transects 7,8,9,9,10,11 profiles. Example of identified anomalies on profiles along Site 2. On profiles 7-10 first from 0 to 9 metres an extended underground anomaly was detected, most likely a radiosample from an underground structure.
Picture 3.2.2 Horizontal slice across the area (top view), the slice shows objects at a depth of 2.4m. (this anomaly is approximately 9m long and 6m wide).
Picture 3.2.3 Horizontal slice across the area (top view), the slice shows objects at a depth of 3m.
(This anomaly is approximately 9 metres long and 6 metres wide).
Picture 3.2.4 Horizontal slice across the area (top view), the slice shows objects at a depth of 3.5m.
(this anomaly is approximately 9m long and 6m wide).
Picture 3.2.5 Horizontal slice across the area (top view), the slice shows objects at a depth of 4.5m.
(this anomaly is approximately 9m long and 6m wide).
Picture 3.2.6 Horizontal slice across the area (top view), the slice shows objects at a depth of 5.5m.
(This anomaly is approximately 9m long and 6m wide).
Area 3 Road.
Picture 3.3.1. GPR cuts. An example of anomalies detected on profiles along the road.Anomalies detected on the profiles, 1, 2 and 5 are likely to be a radio image from an underground structure,3 is likely to be a radio image from an underground structure that has been collapsed and backfilled,4 is likely to be a radio image from a backfilled pit.
Conclusions:
Based on the results of the research conducted, the following was established:
Area 1. Presumed burial site.
There are no underground anomalies or burials in this area.
Area 2. The area in front of the cinema (the site of the ruined church).
A local object measuring 9 metres by 6 metres was found in this area as a result of processing, located in the area of the 7-11 profile at the beginning of the site. It was 2.3 metres deep from the ground surface.
Road. Three linear passes were made, parallel to the main street, with a distance away from the street to the side, towards the synagogue. After line scanning and processing, we see on the first pass the presence of two anomalies at 54 and 60 metres of the profile with a depth of 2.2 metres from the surface.
On the second section there are two anomalies present adjacent to each other with depths ranging from 1.75 metres to 2.10 metres from surface.
In the third area near the synagogue at 9 metres and 42 metres there are two anomalies present with a depth of 2.2 metres from surface.
An area survey of this area is recommended to identify and detail the anomalies present.
On the two investigated sites where the GPR 3D survey was carried out, signals from underground objects were detected, the presence of historical references confirms the presence of an extensive network of underground tunnels in this part of the old town. The main purpose of GPR application is non-destructive study of large underground objects of man-made origin. Signals from such objects were detected on most of the GPR profiles, and later three de-models were constructed based on these data. Each of the identified structural anomalies was marked on the section and given a parametric and geological characterisation based on the radio image of this structural heterogeneity in the GPR field.
The broad zones of anomalies identified at Site 2 deserve special attention due to their apparent man-made origin and historical and cultural interest as a well-preserved underground feature. See diagrams and figures for the location of the anomalies. Monitoring is recommended for other structural heterogeneities identified during this survey.
