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Differential Soil Impedance Obstacle Detection
Project Number

The goal of this project is to design, fabricate, and test a prototype sensor system for detecting obstacles in front of, or nearby the head of a horizontal directional drilling (HDD) rig. The sensor will be designed to be sensitive to metallic, plastic, or ceramic obstacles embedded in the soil.


Gas Technology Institute (GTI) – project management and research products

Des Plaines, Illinois 60018


The use of guided directional drilling for the installation of gas services and mains by the natural gas industry is becoming more prevalent. However, the application of this technology is limited by the potential hazards associated with drill head collision and buried utility lines (e.g., gas, electric, water, telephone, and sewer). The development of this sensing technology to avoid unintentional contact with buried lines would benefit equipment contractors, owners of buried utility lines, and the customers these lines.

The obstacle detection system being developed in this project utilizes an impedance sensing technique. This technique can resolve small changes in the electrical impedance (objects of varying resistive and dielectric properties) of the surrounding environment. Plastic pipe and ceramic conduits represent discontinuities in the soil that should be easily discernable. An array of electrodes around the drill head and the body of the drill itself becomes the sensors in this concept. The body of the drill itself would be used to launch the sensing signal (in the frequency range below 500 kHz) into the soil, eliminating any blind spot ahead of the drill. Simple signal processing and multiplexing will be used to determine the direction and range of an obstacle. The goal is to detect and avoid the obstacle, not to image it, eliminating the need for high frequency time-of-flight signal processing. The normal rotation of the drill head will be utilized to scan the vicinity of the head for obstacles. The array could also be used to passively sense the 60 Hz signatures radiated from buried power lines.

Sensor in loam test bed
Sensor in loam test bed

The North American gas industry is increasing the use of guided directional drilling for the installation of gas services and mains. However, this increased use is limited by the hazards associated buried utility line collisions, such as gas, electric power, water, telephone and sewer. Drilling equipment users and their customers, and the owners of buried utility lines would all benefit from the development of sensing technology that could help avoid unintentional contact with buried obstacles. Such development could potentially increase the overall reliability of natural gas delivery as well as minimizing other utility interruption by decreasing third party damage incidents. The system also could enhance the safety of industry workers and the public during the installation of natural gas infrastructure.

Accomplishments (most recent listed first)
  • Surveyed existing methods of remote obstacle detection with a focus on those methods employing impedance bridge based sensors and described the current state-of-the-art of the technology being developed,
  • Examined the conductivity and dielectric properties of typical obstacles and representative soils, and
  • Constructed a model consisting of a steel rod with an angled tip and an electrode array and began testing in both a soil test bed with buried obstacles and an electrolyte tank with submerged samples of various obstacle materials.
  • Development of hybrid design with revised drive configuration (current flow from tool tip to drill stem), a capacitive pickup, revised conditioning circuitry, enhanced shielding and assembly capabilities.
  • Benchtop testing of hybrid configuration of sensor yielding enhanced results over previous version.
  • Testing of the hybrid configuration sensor in a loam test bed, yielding results better than previous versions but still insufficient for field application.
Final hybrid sensor configuration
Final hybrid sensor configuration

The revised final sensor configuration was developed from and incorporated the revised resistive drive configuration with current flow from tool tip to drill stem and capacitive pick ups. It also incorporated enhanced shielding, a revised overall assembly design and modified conditioning circuitry. This configuration proved to overcome many of the issues which had arisen with previous versions and eliminated much of the signal conditioning problems (including bias, signal drift, and signal strength), but issues of greater than expected complexity of soil properties and contact issues with air gap formation between sensor and soil prevented testing from proceeding to the final stages of in ground testing.

Current Status

and Remaining Tasks:
Project work has reached an end but the project may continue in some form through alternative funding by the American Water Works Association (AWWARF). AWWARF originally allocated funding just for commercialization, but after presenting the results from the project, they have expressed that they may be open to potentially allowing a portion of the funding to be used for additional basic research. The project demonstrated that there is some promise in the technology but that additional issues need to be addressed before it is ready for dependable and accurate field application. Project final report is in progress and is to be completed by early Spring 2005.

Project Start
Project End
DOE Contribution


Performer Contribution


Contact Information

NETL – Richard Baker ( or 304-285-4714)
GTI – Christopher J. Ziolkowski (847-768-0500)

Additional Information