Capacitive Tomography for the Location of Plastic Pipe
The goal is to help to ensure the safety, reliability, and integrity of the United States natural gas transmission and distribution network.
The objective of this project is to develop and test a system that can detect and image buried plastic and ceramic pipe. The system would be designed to detect variations in the electric permeability of soil, corresponding to the presence of a buried plastic pipe.
Capacitive tomography is the detection of the structural details of an object by monitoring changes in an electric field produced by that object. The electric field most often will be produced by an arrangement of capacitive elements. In the current capacitive tomography project, a number of electric field producing and electric field detection elements are configured in a planar array. The field producing elements or transmission elements will be driven by a 50-200 kHz signal. The planar array will be placed directly on or proximate to the surface of the ground. The goal of this system is to detect natural gas pipe structures, in particular those that are composed of non-metallic materials, such as plastic or ceramic pipe.
16-element CT sensor mounted on cart
Gas Technology Institute (GTI) – project management and research product
Des Plaines, Illinois 60018
Demonstrated at the “Underground Focus Live” conference at Manteno, Illinois.
A compact, low-cost sensor that can image objects through soil could be applied to multiple operations and will produce a number of cost savings for the gas industry. In a stand-alone mode, it could be used to survey an area prior to excavation. The technology would improve the accuracy and reliability of any operation that involves excavation by locating or avoiding buried objects. An accurate subsurface image of an area will enable less costly keyhole excavations and other cost-saving techniques.
Capacitive tomography (CT) imaging provides an alternative subsurface imaging to Ground Probing Radar (GPR). It has several distinct advantages. CT is not hindered by wet soils. The CT electronics can be produced at lower cost than that for GPR. The image produced by CT is easy to interpret and does not require extensive operator training. GTI will seek funding to take the CT from proof-of-concept to a prototype more suitable for commercial production.
- Completed design of prototype sensor array and electronics,
- Constructed field-ready prototype, and
- Successfully field demonstrated capacitive tomography (CT) sensor prototype.
Throughout the utility industry, there is a high interest in subsurface imaging of plastic, ceramic, and metallic objects because of the cost, reliability, and safety benefits that result from avoiding impacts with pipelines and reducing the need for excavations. A compact, low-cost sensor that can image objects through soil would produce cost savings for the gas industry. Ground Probing Radar (GPR) has only partially fulfilled this need in that it has difficulty in wet or mineralized soils and requires a well-trained operator to interpret the GPR results even under optimal conditions.
GTI's capacitive tomography (CT) sensor design takes the form of a flat array of electrodes that is fabricated using printed circuit board techniques. The image resolution is proportional to the number and spacing of the electrodes in the array. An excitation signal is injected into the soil at several frequencies in the 100 kHz to 200 kHz range. The complex impedance between adjacent electrodes provides the image data. An electronics package amplifies and scans the signals from the various electrodes. The sensor signals are then collected on a personal computer via a data acquisition board and the data is displayed in a graphical user interface written in National Instruments LabVIEW.
Bottom side of CT sensor array mounted to cart
Over the course of the project several 24-inch square printed circuit board sensor arrays were fabricated. A four-element array was constructed first and was readily able to detect a 4-inch plastic pipe buried in 4 feet of soil. A 16-element array was then built along with the necessary electronics for interfacing signals from the sensor array to the PC. This second generation design improved signal strength by as much as a factor of 100. Runs of 2-inch, 4-inch, and 6-inch polyethylene (PE) pipe were installed on GTI property as test targets. The 16-element array readily detected PE pipe in up to 5 feet of wet soil.
A simple imaging protocol was implemented, consisting of 16 square elements arranged on a grid on the computer screen corresponding to the elements in the sensor array. The elements would lighten or darken according to the strength of the received signal directly under the sensor element. Looking at the grid one can get a coarse “silhouette” type image of objects in the soil beneath the sensor array.
A two-wheeled car mounted with the 16-element sensor board was constructed. An operator can easily push the cart over an area of interest to perform imaging. This arrangement was publicly demonstrated at the Underground Focus Live Trade Show in summer 2003. This show allowed side-by-side comparison of the CT approach with several GPR imaging systems. It was determined that CT provides a much more intuitive image of subsurface objects than GPR.
Subsequent to this demonstration, a 64-element multilayer sensor board with surface mount components was constructed. This sensor board was found to increase the image resolution by a factor of 4. The use of surface mount components should eliminate sensor element cable loading and other parasitic effects.
CT imaging is an alternative subsurface imaging method that has three distinct advantages over GPR: (1) CT is not hindered by wet soil, (2) CT electronics can be produced at lower cost, and (3) the image provided by CT is easy to interpret and does not require extensive operator training.
- Capacitive Tomography provides a viable alternative to Ground Penetrating Radar for sub-surface imaging.
- CT has sufficient sensitivity to detect PE pipe in wet soils at depths greater than 4 feet.
- The technique uses a combination of phase detection and signal averaging to achieve the required sensitivity.
- The CT data display is easy for the operator to interpret. It does not require a great deal of operator training or post-processing.
- A 16-element array was demonstrated at the UFL trade show where it was compared side by side with GPR.
- Several manufacturers at UFL expressed interest in the CT technology.
- The signal to noise ratio is sufficient that a 64-element array was designed and fabricated. Preliminary tests show very good sensitivity even with smaller individual elements.
- There is further development work needed on the 64-element sensor array. GTI will be seeking additional funds to bring CT to commercialization.
Current Status and Remaining Tasks: This project is complete and all deliverables have been received. The next step would be to take CT from proof-of-concept to a prototype more suitable for commercial production.
Project Start: September 12, 2001
Project End: September 11, 2003
DOE Contribution: $348,905
Performer Contribution: $321,061
NETL – Tony Zammerilli (firstname.lastname@example.org or 304-285-4641)
GTI – Christopher Ziolkowski (email@example.com or 847-768-0549)
Final Report - [PDF-1821KB]
Fossil Energy Techline: "Pipe Locating Sensor Could help Prevent Natural Gas Leaks"
Technical Report [PDF-367KB] January, 2002