Underground utilities include a variety of specific lines, including electrical, gas, sewage, telecommunications and water. The task of locating these utilities in a noninvasive manner has never been easy, but it has grown more difficult in recent years due to the great increase in utility lines. Accurate location is essential for both the safety of construction crews and the construction company’s bottom line. The challenges of locating underground utilities generally consist of a signal that’s too weak or interference from other sources.
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Underground lines emit electromagnetic (EM) signals in a variety of ways. Electrical power lines generate their own signals since they carry their own electrical current. However, other lines also generate EM signals by induction, meaning that the locator transmits a signal that the utility line re radiates as a result of the metals it contains. Induction creates signals that are much weaker than those from power lines, but modern locators can still detect them at a useful distance in many cases. The frequency of the reradiated signal also varies significantly, depending on the composition of the utility line. Tuning a locator to the correct frequency is, therefore, a major part of the operator’s job.
Locating methods may be classified into active and passive types. Active locating methods involve detecting the route of a known utility line. The operator directly connects the locator to the line or transmits a signal at a particular frequency into the ground, where it’s reradiated by the utility line. Passive methods involve locating unknown lines, generally by sweeping the area with the locator’s receiver. The operator looks for EM signals that are being reradiated by utility lines over a range of frequencies. However, passive location doesn’t allow the operator to distinguish between different types of utility lines the way active location does.
Locators may generally be classified into single and multi-frequency types, with each type having its own advantages and disadvantages.
Single frequency locators have a simpler design and have been in use for decades. The transmitter emits an EM signal with a single high frequency, which is most effective for sites with a low concentration of underground utility line. The use of a high-frequency signal means that a single-frequency locator is very effective at detecting utility lines. However, its lack of discrimination also means that this type of locator can’t identify the specific type of utility line, nor can it determine the depth of the line with any accuracy.
Multi-frequency locators allow the operator to select a particular frequency to detect, with some models offering as many as five frequencies. Lower frequencies are more effective at identifying a particular line, although their overall detection ability is less than high frequencies. An operator will often begin a sweep with high frequencies to detect a line’s presence and then switch to lower frequencies to differentiate between two different lines in the same area.
Some locators also provide the current measurement index (CMI) of a utility line. This feature allows the operator to follow the route of a particular line with greater precision, especially if it crosses over another line. Depth measurement is also an important feature of modern locators, which they accomplish by using an algorithm to convert signal strength into an estimate of depth. However, the depth measurement is prone to error due to distortion caused by signals from other sources.
The general solution to the challenges of locating utility lines is to adjust the signals transmitted by the locator. The operator must often use a variety of techniques to induce a signal or alter the flow of existing current in the target utility line so that the locator’s receiver can detect it. Common techniques include changing the signal frequency or the method of inducing a current. Operators may also change the size of location of the ground antenna to detect a signal more effectively. Another possible solution is to attach the locator’s second lead to the far end of the utility line, rather than a grounding rod as is the standard practice.
Poorly Insulated Utility
Poorly insulated utilities are generally more difficult to detect since the strength of their induced signals decrease as they travel along the line. This process 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. Furthermore, a line that was well-insulated when it was first installed may also be difficult to detect if the insulation has deteriorated over time.
Proximity to Other Buried Utilities
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 locator’s transmitter. The current flowing to the nearby utility will have an EM field that oscillates at a particular frequency. This EM field will then interact with the EM field of the utility it has reached, distorting the target utility’s EM field. Furthermore, the location of greater signal strength for the target EM 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 EM fields can become so strong they create a single EM field. When this occurs, the locator will be unable to differentiate between the two utility lines and will only show a single utility line.
Commonly Bonded Utility
Utilities are often bonded together to provide a common ground that will divert high-voltage current to the earth. This arrangement will cause electrical current to flow on all the utilities connected to that ground point. The locator will then be unable to induce an EM 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, while the technician performs the location procedure. However, this solution is frequently impractical since the bond is often underground.
Locators 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. 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 locator to show breaks in the locator’s utility line, especially where it makes turns.
EM 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 line and the locator’s receiver will prevent the locator from detecting the line.
For Assistance Locating Underground Utilities
Underground Services, Inc. provides a full range of underground services, including our SoftDig® non-destructive excavation system for locating underground utilities. Call us at (610) 738-8762 or contact us online to learn more about how we can help your business.