GPS AND SUBTERRESTRIAL DETECTION TECHNOLOGIES

4TH INTERNATIONAL CONFERENCE
RECENT PROBLEMS IN GEODESY AND RELATED FIELDS WITH
INTERNATIONAL IMPORTANCE
February 28 - March 2, 2007, Inter Expo Centre, Sofia, Bulgaria
GPS AND SUBTERRESTRIAL DETECTION TECHNOLOGIES
- DETECTION OF ASH-COVERED DRAINAGE SHAFTS
ALEKSANDAR RISTIĆ, DUŠAN PETROVAČKI,
MILAN VRTUNSKI (SR)
ABSTRACT
The basic characteristic of a specific drainage structure of ash depots in large steam power plants is
the multilevel structure with several covered drainage systems. The central element of every
drainage system is the shaft that collects all water from ash and hot water mixture arriving from the
plant. The ash-covered shafts with unknown position are ecological threats and make further use of
depots impossible.
The aim of this paper is to show how Ground Penetrating Radar (GPR) and GPS technologies could
be used for detection of the unknown position of ash-covered drainage shafts.
Since the ash depots were very large, it was necessary to determine smaller area that will be
scanned. The reduction of the area provided additional excavation and made scanning with GPR
possible, in terms of reducing depth.
All principles specified above were successfully applied during the ash-covered shaft detection
project done for the "Kolubara A", one of the biggest steam power plants for electricity production
in Serbia.
Key words: Subterrestrial detection, GPS, Ground Penetrating Radar, ash depot, drainage shaft,
Pipeline route, Penetration depth,
INTRODUCTION
Energy production in steam power plants exacts the combustion of large amounts of lignite
powder, in boiler bearings specially designed for saturated steam production. Saturated steam is
used for propelling steam turbines and electrical energy production. Coal, burning in boiler, is
ground into powder in order to provide better combustion and a higher efficiency factor. After one
charge of coal is burnt, it is necessary to remove the combustion residue remaining from the boiler
grid, and reload the coal powder. The remaining combustion residue is washed out from the boiler
grid using water. The hot mixture contains fifteen parts of water and one part of ash. It is
transported through the pipeline to the ash depot. Ash depot is in the shape of a valley, with its
borders strengthened with a dike. It is built on suitable soil, using heavy machinery. The height of
the dike is between 3 and 5 meters, and its role is to hold the water from the hot mixture until the
drainage system discharges it from the depot. The pipeline for the hot mixture transport is ringshaped, in order to burden the dike evenly, and to maximize the depot layer drainage. The drainage
system is star-shaped from the borders and peripheral drainage shafts to a single or more central
drainage shafts (image 1). The central drainage shafts are connected with the collector pipeline
which transports the filtered water to the pump station, and then, after the treatment, the water is
released to the river.
When the level of ash reaches the edge of the dike (about 3 meters) and the usage of the
depot has to be continued, it is necessary to build a new dike and drainage system, independent
from the old one. The new drainage system needs to be independent in order to increase the
efficiency of drainage and to facilitate easier maintenance. The central drainage shaft of the old
drainage system is sealed, because there is a possibility of collapsing caused by the weight of new
layers of ash and the hot mixture.
Every upgrade of ash depot reduces the space for ash, decreases the stability of the dike,
with the possibility that the hot mixture breaks through the dike or drainage shafts used earlier.
Since the main collector and river are connected, and due to the possibility flooding of fertile soil
and factory, it is clear that the information about the position of the old main drainage shaft and its
facilities and the status of the dike is extremely important.
A
B
C
D
Image 1. Ash depot characteristics
A – Cassette 1, discharge pipeline, new main drainage shaft
B – Cassette 1, discharge pipeline, several peripheral drainage shafts
C – Cassette 1, new main drainage shaft
D – Cassette 2, new main drainage shaft, several peripheral drainage shafts
GPS AND SUBTERRESTRIAL DETECTION TECHNOLOGIES
The Ground Penetrating Radar (GPR) is a device used for non-invasive scanning and precise
detection of underground utilities. The GPR can be equipped with a GPS (Global Positioning
System) rover which is used for measuring spatial coordinates of the projection of the pipeline route
on the site surface [1]. The measurement of the pipeline parameters with GPR and GPS
measurement coordinates on the site surface are with centimeter accuracy. This measurement
accuracy satisfies geodetic mapping laws [1].
When the survey cart moves on the site surface, the transmitting antenna sends polarized,
high frequency electromagnetic (EM) waves into the ground. Because of different existing
inhomogeneties in the ground, e.g. soil layers, underground utilities, stones, gravel, cavities and
other anomalies, a part of the EM waves is reflected from the dielectric boundary between different
materials and the other part is refracted and goes to the deeper layers [4]. The described process is
repeated until the EM waves become too weak. The reflection of EM waves from the dielectric
boundary is the consequence of differences in the electric and magnetic properties of materials of
infrastructural objects and soil layers.
The time necessary for the propagation of EM waves from the transmitting antenna to the
boundary surface and their reflection back to the receiver antenna is defined as a two-way travel tR
[ns] time. The GPR measures tR, and finally calculates the relative depth of the underground object.
Because each location has its specific soil structure, εR has to be recalculated for each site. Usually,
the GPR recalibration method is used on site. This method is based on a GPR scan of an
underground object with known depth [2].
The maximum penetration depth is usually 3.5 to 7m (400MHz and 200MHz antenna,
respectively). Vertical resolution is usually 3 to 7cm (400MHz and 200MHz antenna, respectively)
[4].
The parameters are detected by 2D and/or 3D scans of the site. Complexity, type of the
underground objects and the amount of data determine which method is used for scanning.
Apart from the standard procedure of positioning of the underground objects by GPR
scanning, and the subsequent mapping of characteristic points with GPS, in certain situations it is
possible to apply inverted procedure [5]. This procedure is used when the area of interest needs to
be determined. Area of interest is specified for scanning of large areas in terms of reducing time of
scanning and/or when the depth of the underground object exceeds the maximum GPR antenna
penetration limit. In this situation, the scanning area is adjusted using heavy machinery to meet
measuring equipment possibilities.
Hence, the inverse procedure characteristic, apart from the altered sequence of equipment
application, also involves the use of GPS for the purpose of navigation to specific locations
determined in the previous analysis.
SURVEY AREA - GENERAL DESCRIPTION
There are several ash depots near the steam power plant ’Kolubara A’. The analyzed depot
consists of two sections - Cassette 1 and Cassette 2, which have been out of service for the last 20
years. During the exploitation, two layers of ash were formed, 6 to 10 meters thick. Considering the
production requirements, in both cassettes new dikes were built in order to restart exploitation. New
drainage system was also built (drainage pipelines, several local and one main drainage shaft in
each cassette). Reconstruction was done in cassette 1 first, but after a short period of time, a bigger
amount of ash and hot mixture broke through the old main drainage shaft, causing serious damage
and ecological threat. After that, Cassette 2 was not put to work. Location and depth of the old
drainage shaft was unknown, since the real situation and projected situation did not match, the
project documentation was not updated, and it was the only paper available. According to the
project documentation, there would have to be two main drainage shafts in each cassette, connected
to the main collector pipeline leading to the pump station. Cassette 1 area was approximately
178,000m2 at first, and Cassette 2 ≈188,000m2. It has to be mentioned that the areas of cassettes
were reducing along with the upgrade of the dike. Image 2 shows initial characteristics of the
analyzed depot.
Image 2. Initial ash depot characteristics
SHAFT DETECTION IN DEPOT 1
According to the new situation, Cassette 1 was analyzed in order to check if there was any
other drainage shaft whose location was defined in the project documentation, or on some other
specific location.
At first, GPR scanning was done (Image 3), on area with ca 350 meters in length, from exit
shaft UPŠ1, projected collector pipeline φ1100mm from drainage shaft PŠ1 to the pump station (at
94 meters msl), with the maximum scanning depth of 2.5 meters (scanning area colored orange on
image 3). It was in this way that the number and usage of facilities exiting Cassette 1 was
determined.
Following this, the geodetic coordinates for points UPŠ1 and UPŠ2 were taken from the
project documentation, GPS device Trimble 5800 was used to navigate to these points and the zone
around them was scanned thoroughly [3]. No object specified in project documentation was found
in these zones. GPS navigation and GPR scanning determined that UPŠ1 and UPŠ2 did not exist,
that there was no pipeline from UPŠ1 to PŠ1 and that there was no pipeline from UPŠ2 to PŠ2.
On that location, only smaller drainage pipes which drain Cassette 1 were detected, and
these were two steel pipes φ400mm and two smaller concrete square-shaped canals. Accordingly,
on location A (image 3), the investor excavated the connection shaft of the pipeline from the broken
shaft in Cassette 1, the pipeline route on location B was checked on the edge of the road to the dike,
and it was determined that the pipeline is on its route until it reaches the dike. Also, it was
determined that the real route of the collector pipeline from UPŠ1 to the pump station was displaced
in comparison to the project, and that the pipeline goes from location A and has horizontal turns that
were not foreseen in the project.
A GPS device was used to navigate to the broken shaft, which was covered after sealing.
While the shaft was being sealed, it was determined that there were no other connection pipelines
on the shaft. In further analysis, it was determined that the shaft was situated in the geometric center
of the basic contour of Cassette 1. When all these facts are considered, it was supposed that there
was only one main drainage shaft in Cassette 1.
Regardless of slight possibility of the existence of another shaft, in order to prevent any
possible damage to the new pipeline collector as a result of a cave-in, an area of ca 4500 m2 was
scanned (up to 3 meters in depth). The area was in the zone of the newly-built pipeline collectors
and shaft in Cassette 1 (scanning area colored purple on image 3). It was 15 meters wide on each
side of the collector and 150 meters long, to the dike between cassettes 1 and 2. The area included
the location of the broken shaft and its nearby surrounding. No other drainage shaft was found in
that area [6].
The survey was done using RTK method, with permanent network station AGROS used for
corrections [7]. Since centimeter level accuracy was not required, no local site calibration was done.
It would be unjustified to consume additional time for measurements and processing.
Image 3. Survey methodology in Depot 1
SHAFT DETECTION IN DEPOT 2
Facts and considerations included in the analysis of Cassette 1, were applied in the analysis
of Cassette 2. GPR scanning near the pump station determined that there was only one pipeline
leading to Cassette 2. This fact supported the supposition that there was only one main drainage
shaft in Cassette 2, most likely situated in the geometric centre of the basic contour of Cassette 2.
The ash layer thickness on the route from the pump station to the covered shaft was bigger
than the maximum penetration depth of both GPR antennas, so it was impossible to scan the
pipeline route at any location, apart from the specified, previously prepared location. Also, the large
area was a problem (it was ca 188,000m2, current size ca 133,000m2, and the area of interest at least
50,000m2).
Scanning and georeferencing of the existing plans in AutoCAD project was done,
considering the basic supposition. It provided the calculation of the geometric centre of the basic
contour of Cassette 2 (point ’center’ on image 4). Linking the calculated geometric centre point
(center) and the point at the end of the pipeline at the pump station (’CS’) determined the pipeline
route through Cassette 2. In addition, several points were generated, along the route (points T1, T2,
T3, T4, T5, T6, T7).
Image 4. Survey methodology in Depot 2
Following the navigation to the calculated point of the geometric centre in Cassette 2,
excavation started, using heavy machinery, in order to decrease the ash layer thickness.
During the excavation, GPR scanning was done in trench in Cassette 2 prepared earlier,
close to the dike. Considering the determined direction and measured point Tx, it was found that
point Tx was on the route defined according to the supposed shaft location. Further, points on the
route (T1, T2, T3, T4, T5, T6, T7) were marked [3]. Digging to the depth previously specified with
GPR [6], the buried drainage shaft was found on defined location. In addition to this, it was
determined that there was no other pipelines connected to the shaft, and this marked the end of
Cassette 2 analysis.
Image 5. GPR Survey and the detected shaft in Depot 2
CONCLUSION
In this work, new approach and successful usage of GPR and GPS in specific conditions for
the detection of underground infrastructure facilities was represented. Inverse procedure for
detection was applied to locate buried drainage shafts in ash depot of the steam power plant
’Kolubara A’, one of the biggest in the Republic of Serbia. Defining the zones of interest over a
large area provided conditions for fast and efficient usage of GPR, considering that the ash layer
thickness was beyond the maximum scanning depth.
Fast and efficient detection of drainage shafts provided uninterrupted production, in terms of
ash deposits and regarding ecological criteria.
To efficiently maintain depot in adequate state, it is necessary to periodically scan the dike
around the depot, which will prevent the possibility of the breakdown of the dike, flood and
ecological damage. Additional possibility, considering maintenance, would be to devise a simpler
GIS application which would provide a clear insight of all the necessary parameters (position and
status of the depot’s dike, for instance).
LITERATURE
1. D. Petrovački, A. Ristić, "Application of GPS and remote sensing technologies for mapping
of mid-pressure gas line network in area of Novi Sad", 5th InterGEO East Conference,
Belgrade, Serbia, February 22nd-24th 2006.
2. A. Ristić, D. Petrovački, M. Vrtunski, "Using of GPR and GPS technologies for detection of
underground utilities and soil characterisation", 14th conference of Serbian Association for
Hydraulic Research, Fruška gora, Serbia, November 13th-15th 2006. (in Serbian)
3. D. Petrovački, M. Vrtunski, A. Ristić, "Using of GPS Technology for Mapping of
Underground Infrastructure", 14th conference of Serbian Association for Hydraulic
Research, Fruška gora, Serbia, November 13th-15th 2006. (in Serbian)
4. D. J. Daniels, "Surface penetrating radar", The Institution of Electrical Engineers, London,
GBR, 2004.
5. D. Petrovački, A. Ristić, "Principles Of Using GPR And GPS Technology For Detection Of
Underground Utilities", 49th ETRAN conference, Montenegro, June 5th-9th, 2005 (in Serbian)
6. GSSI Inc., "Radan 6 User Manual", North Salem, USA, 2004
7. http://gpsweb.ns.ac.yu
AUTHORS:
Aleksandar Ristić, M.Sc.,
Faculty of Technical Science
Center for geo-information technologies and systems
D. Obradovića 6, 21000 Novi Sad, Serbia
+381 21 485 22 58
[email protected]
Dušan Petrovački, Ph.D.,
Faculty of Technical Science
Center for geo-information technologies and systems
D. Obradovića 6, 21000 Novi Sad, Serbia
+381 21 485 22 59
[email protected]
Milan Vrtunski, B.Sc.,
Faculty of Technical Science
Center for geo-information technologies and systems
D. Obradovića 6, 21000 Novi Sad, Serbia
+38121 485 22 60
[email protected]