Description & Limitations

Utility Locating Information

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Electromagnetic Locating

What is it?

     Electromagnetic Locating is the cutting-edge technology used for tracing the locatable utility lines, metallic pipes as well as for clearing drilling locations. Electromagnetic Locators are usually made of two parts- a transmitter and a receiver. The transmitter emits a selected frequency, while the receiver detects these radio frequencies, which enables the operator to accurately locate utilities. A Trinity Technician designates the utility with paint and flags to show its location. Direct connection(such as a gas line by way of the tracer wire) is the best way of using an Electromagnetic locator. This requires physical access to the pipe or service, usually by way of a surface feature that is connected to the pipe. Induction is another method of locating utilities. The transmitter can be used to induce a selected signal on to the utility, which can be detected by the receiver. Passive locating with EM locators can also be performed. Passively detecting utilities involves utilities that emit a frequency naturally(such as high voltage electrical cables which emit a frequency of 60hz). The best signals are obtained from direct connections where physical access to the pipe is necessary.

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Limitations

UTILITY DEPTH

     Electromagnetic fields degrade as they move away from their source, especially when they move through a conductive material such as soil. A sufficient amount of soil between the utility and the Technician’s receiver will prevent the Technician from detecting the utility.

utility composition

     Technicians can only detect utility lines made of electrically conductive materials, which generally means metal. Pipes made from materials such as plastic or terra cotta, therefore, can’t be detected by electromagnetic methods. A metallic line can also be difficult to detect if it has nonmetallic connectors or is corroded to the point that
it impedes the flow of electrons. This scenario causes the equipment to show breaks in the utility, especially where it makes turns.

Commonly Bonded Utilities

     Utilities are sometimes bonded together to provide a common ground that will divert a high-voltage current to the Earth. This arrangement will cause an electrical current
to flow on all of the utilities connected to that ground point. The Technician will then be unable to induce an electromagnetic signal only in the target utility, making it difficult to identify the signals that indicate the target utility’s location. The most effective solution to this problem is to temporarily break the common bond of the utilities(where available), while the technician performs the utility designation.

Utility Congestion

     A large number of utilities buried in the same area can make it difficult to distinguish one line from another. This problem occurs when an induced signal travels from
the target utility to a nearby utility rather than to the technician’s transmitter. The current flowing to the nearby utility will have an electromagnetic field that oscillates
at a particular frequency. This electromagnetic field will then interact with the electromagnetic field of the utility it has reached, distorting the target utility’s electromagnetic field. The location of greater signal strength for the target electromagnetic field will no longer be directly over the target utility, providing a false location for the target utility.

     This problem becomes particularly acute when two utility lines run parallel to each other for a long distance. In these cases, the interaction between the two electromagnetic fields can become so strong they create a single electromagnetic field. When this occurs, the technician will be unable to differentiate between the two utility lines and will only see a single utility line.

Poorly Insulated Utility

     Poorly insulated utilities are more difficult to detect since the strength of their induced signals decrease as they travel along the line. This occurs when a metal surface from the utility line comes into direct contact with the soil, causing the signal to travel through the soil instead of along the cable. The degree of signal loss is directly proportional
to the surface area of metal in contact with the soil. The difference in signal strength between well-insulated and poorly-insulated utility lines is particularly great for long distances, where poor insulation has greater opportunity to degrade the signal. A line that was well-insulated when it was first installed may also be difficult to detect
if the insulation has deteriorated over time.

Depth of Utility

     In most cases, utility depths given from Electromagnetic equipment are approximations, and should be treated as such.

Split Box

What is it?

     The Model A6, The Tracer, is a transmitter-receiver type metal locator for locating metallic objects that are buried (cables, pipes, vault lids, manhole covers, etc.).

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Limitations

Sensitivity

     The split box is a sensitive piece of equipment, therefore, if there are metallic objects in the vicinity of the scan area (metallic debris, automobiles, fencing, guardrails, reinforced concrete, etc.), there may be interference in its ability to detect metallic objects beneath the soil.

GPR

What is it?

     Ground Penetrating Radar uses a high frequency radio signal that is transmitted into the ground and reflected signals are returned to the receiver. The computer measures the time taken for a pulse to travel to and from the target which indicates its depth and location. The reflected signals are interpreted by the system and displayed on the unit.

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Limitations

Soil Type

     Potentially the biggest geophysical factor in ground penetrating radar limitations is that of soil type. Highly conductive soils, such as clay, have high signal attenuation. This severely inhibits the effective penetration depth of a given antenna frequency.

Surface Area

     The surface being scanned becomes a factor for GPR data when the terrain is rough enough to cause the equipment to separate from the surface more than 2 to 3 inches. The quality of the data when this happens is then compromised. Ideally, the surface needs to be reasonably flat with a good amount of clearance. Obstacles, like trees, bushes, cars, construction material, debris, etc. that the technician would have to scan around, may cause utilities in the area to be missed. If a surface is reinforced concrete with densely packed bars, the signal will suffer from dispersion. Rocky or made-up ground provides the same result. This greatly reduces the ability of the user to identify potential targets. The same is true when the top surface is completely saturated due to a period of rain or if the top surface is made of coarse gravel.

Moisture

     Since moisture raises the conductivity of the ground, heavy precipitation can affect the reading. Once the radar touches ground retaining moisture, the signal bounces right back, making it difficult to read anything below the water table.

Utility Depth

     The deeper the utility, the more likely factors like soil conditions, size, and orientation of the utility will cause limitations that can prevent the utilities from being located. The deeper the radar signal has to go, the higher probability that the signal will disperse before reaching the utility, causing minimal to no results returned.

Utility Size

     Different types of ground penetrating radar units will utilize different types of frequencies. Higher frequencies will typically provide a higher resolution to detect smaller targets at shallower depths. Lower frequencies will tend to provide results on larger targets, such as pipes and cables, at deeper depths. Smaller targets, such as cable TV service lines, are often difficult to locate with the typical GPR system. High frequency systems will have difficulty detecting larger utilities, which are commonly at deeper depths. This is because high frequency GPR systems cannot penetrate the ground much deeper than 24 inches.

Limitations Not Related To Technology

Client Provided Prints

     Technicians use utility prints, as-built drawings, etc. as a guide to aid in utility designation. These job aids help make sure utilities within an area are accounted for even if there are no visible surface features. If the technician isn’t provided with prints or given prints that are outdated, it can cause utility designation to take longer to complete, and causes equipment limitations to become more of a factor.

Access To Building Feeds

     Gaining access to buildings is a critical step to any utility investigation. When a technician has building access, they are able to see the utilities running into the building, even if that utility doesn’t have a visible surface feature on the exterior of the building. If the utility is made of the appropriate material, interior feeds can be used as a connection point.

Public Utilities

     811 should be called and a one call ticket should be created for any area that has public utilities present. Public utility owners have contracts with public utility locating companies, who are solely responsible for designating their utilities. Public utility locating companies are given information about public utilities including measurements for utilities that can not be detected through electromagnetic locating methods.