CAPACITIVE TOMOGRAPHY FOR THE LOCATION OF PLASTIC PIPE Page: 3 of 30
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ABSTRACT
Throughout the utility industry, there is high interest in subsurface imaging of plastic, ceramic,
and metallic objects because of the cost, reliability, and safety benefits available in avoiding impacts with
the existing infrastructure and in reducing inappropriate excavations. Industry interest in locating plastic
pipe has resulted in funding available for the development of technologies that enable this imaging. Gas
Technology Institute (GTI) proposes to develop a compact and inexpensive capacitive tomography
imaging sensor that takes the form of a flat plate or flexible mat that can be placed on the ground to image
objects embedded in the soil.
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.
Ground penetrating radar (GPR) has been applied to this area with limited success. Radar requires
a high-frequency carrier to be injected into the soil: the higher the frequency, the greater the image
resolution. Unfortunately, high-frequency radio waves are more readily absorbed by soil. Also, high-
frequency operation raises the cost of the associated electronics. By contrast, the capacitive tomography
sensor uses low frequencies with a multiple-element antenna to obtain good resolution. Low-frequency
operation lowers the cost of the associated electronics while improving depth of penetration.
The objective of this project is to combine several existing techniques in the area of capacitive
sensing to quickly produce a demonstrable prototype. The sensor itself will take the form of a flat array of
electrodes that can be inexpensively fabricated using printed circuit board techniques. The image
resolution is proportional to the number and spacing of the electrodes in the array. Measuring the complex
impedance between adjacent electrodes at multiple frequencies forms the image. Simple location of
plastic pipe with a two-electrode array has already been demonstrated.
Thus far, 4-element and 16-element sensor arrays have been fabricated on 24" by 24" printed
circuit boards and tested. The sensor arrays have been tested with buried plastic piping at GTI both in soil
boxes and an outdoor facility. Sensitivity to the presence of plastic pipe in soil has been demonstrated
with 2", 4", and 6" diameter pipes at depths greater than 4 feet. This sensitivity is unaffected by soil
moisture conditions. A 64-element array is currently being fabricated to provide greater spatial resolution
of buried objects.iii
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Huber, Brian J. CAPACITIVE TOMOGRAPHY FOR THE LOCATION OF PLASTIC PIPE, report, July 29, 2003; United States. (https://digital.library.unt.edu/ark:/67531/metadc785747/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.