Every underground utility locating technology has its own limitations. There is no single technology that can be used for every type of utility, soil type, and site. Many factors, including characteristics of expected underground utilities, geological conditions at the site, environmental and social factors, and experience of the operators should be considered as criteria for the appropriate selection of technologies.
- Type of Surveyed Utility
- Material of Surveyed Utility
- Depth of Surveyed Utility
- Internal Condition of Surveyed Utility
- Access to Surveyed Utility
- Type of Soil at Survey Site
- Ground Surface Cover of the Survey Site
- Utility Density at the Survey Site
- Experience and Knowledge of the
Type of Surveyed Utility
- Certain locating technologies are accurate for locating specific types of utilities.
- Acoustic or thermal characteristics of the surveyed utilities dictate the accuracy of the locating surveys in parallel with other factors.
- Acoustic surveys can be used effectively for water and gas pipelines, which create vibrations that can be captured by a receiver.
- Thermal surveys can be used only for warm utilities such as sanitary sewers and high-voltage power lines to detect anomalies in the temperature field from the surrounding ground.
Material of Surveyed Utility
- Some locating techniques are limited or more effective for specific materials.
- A limited number of locating technologies are available for nonmetallic utilities. Magnetic surveys are not applicable to nonferrous metallic materials such as copper, plastic, and concrete materials, but are applicable to ferrous metallic materials, including steel, cast iron, and ductile iron.
- Electromagnetic methods, such as ground penetrating radar (GPR) or the terrain-conductive survey, have great benefits that can locate both metallic and nonmetallic materials.
Depth of Surveyed Utility
- The penetration limitation of the signal of each technique is an important factor for the selection of techniques.
- The resolution and accuracy of the results decreases with increasing depths.
- The applicable depth of metal detectors is less than 0.6 m, whereas that of pipe and cable locators is up to 5 m.
- The applicable depth of acoustic surveys varies in relation to target utilities.
Internal Condition of Surveyed Utility
- The internal condition of utilities refers to the flowing materials and fill level of the surveyed utilities.
- Specific techniques, such as the acoustic surveys, work better depending on the fill levels of utilities.
- the acoustic survey is more applicable when the pipeline is filled with water or gas because the method is on the basis of the pressure transporting the sound wave.
- The internal conditions of utilities also affect the density anomalies of the gravity survey. The gravity survey detects different densities because of the presence of underground pipelines from surrounding areas.
- For the gravity survey, an empty water pipeline is more detectable than a filled water pipeline because of the density difference between the air and surrounding soils.
Access to Surveyed Utility
- The site accessibility affects the accuracy of the surveys.
- Certain sites require traffic control to conduct surveys. In such conditions, it is beneficial to evaluate locating technologies, which would give the most accurate results in the shortest time.
- Certain locating technologies require direct contact with the utility and/or surface appurtenance.
- For acoustic surveys, prior knowledge about the surface appurtenance of the target utility is necessary because the transducer introduces sound waves into the utility through the surface appurtenance.
Type of Soil at Survey Site
- The signal penetrations of some locating technologies depend on the properties of the soil.
- Soil properties have a direct effect on signal penetration depth and accuracy.
- High conductivity in clays or highly saturated sand causes rapid dissemination of GPR signals so that the penetration of the GPR signal is reduced to less than 1 m.
- The loss of GPR penetration depth is significant in comparison with 2 m in low- conductivity soil.
- A terrain-conductive method is more effective in highly conductive soils, whereas a resistivity method works well in highly resistive soils.
Ground Surface Cover of the Survey Site
- Many underground utilities are buried under surface pavements with asphalt or reinforced concrete, which limits the penetration of electromagnetic signals.
- Acoustic surveys and thermal surveys also may have some difficulty capturing vibration and heat flux depending on the cover of the surveyed site.
Utility Density at the Survey Site
- Proximity and density of nearby buried objects may interfere with accuracy of the surveys. Locating technologies needs to be carefully evaluated for such sites.
- High utility density increases the possibility of accidents because of hitting the utilities.
- Surrounding ferrous features, such as guardrails, can significantly affect the accuracy of certain magnetic or resistivity surveys.
Experience and Knowledge of the Survey Crew
- Qualified underground utility locating consultants that are both knowledgeable and experienced with all applicable geophysical techniques are necessary for surveying underground utilities and interpreting the results of the surveys.
- The accuracy in application of underground locating technologies and interpretation of survey results are greatly influenced by the experience and knowledge of the surveying crew.
- Surveying crews should be able to effectively evaluate the site conditions, capabilities, and limitations of the locating technologies for accurate surveys