nderground utility strikes are dangerous and expensive, and the job of a utility locator is to reduce the likelihood of a strike to as close to zero as possible. The key to safely locating utilities before excavating is to combine multiple locating tools in the proper way. Using a combination of Electromagnetic (EM) Locators, Ground Penetrating Radar (GPR) and Magnetic Locators provides a more complete locate and minimizes utility strikes.
EM Locators are the most common locating tool. Technically, these locators do not locate cables or pipes directly; they locate the EM field around the utility. This can be performed passively, or actively.
In Passive mode, the utility line already has a locatable signal, such as AC power. In Active mode, a transmitter applies a locatable signal to the utility line. This principle is known as EM induction (Oersted’s Law).
Active mode is the most widely used EM search method. The EM transmitter, located on the surface, applies a signal on the utility in one of three methods: (Direct Connect, Induction Ring Clamp or Broadcast Induction). Each of these methods causes electrical current to flow on the utility, creating a cone shaped magnetic field. This signal is detectable by the EM receiver (Faraday’s law) when the receiver is moved side-to-side over the utility by the operator (Figure 1). However, if you have non-metallic objects without tracer wires, or no way to place a locatable object into the pipe (like a sonde), EM locators are ineffective. In these cases, GPR is the best tool for locating the utility.
EM Locators range from units with limited functionality such as the Radiodetection Cable Avoidance Tool (C.A.T.), to the top-of-the-line Radiodetection Precision Locators that have multiple features, functions, and frequencies.
While EM requires the object to be metallic and carrying a current, GPR works on a completely different principle, sending radio waves into the ground and measuring their reflections (Figure 2). Since currents are not required for detection, GPR excels at finding all types of objects, regardless of what material they are made of. This key strength of GPR can also be a drawback, as it will find all types of objects, even ones that you are not concerned with, for example, tree roots and rocks. However, this is easily overcome for utility locating by tracking linear objects in the survey area.
GPR accurately locates the position of the subsurface object, including depth, typically within 10% of the actual depth, but it cannot identify the type of utility. GPR is dependent on soil conditions, so it can see very deep in some soils, such as sand, but may have limited penetration depth in others, such as clay. Due to the versatility of GPR, it is used in many different applications and not limited to utility detection alone.
Magnetic locators detect a subsurface, ferrous (iron-rich) object by detecting the difference in the Earth’s magnetic field created by the presence of the object (Figure 3). Objects that are iron or steel in composition will be affected by this magnetic field and, in turn, generate their own magnetic field. Examples of these objects include manhole covers, underground storage tanks and valve boxes. Like GPR, magnetic locators have the potential to see very deep, but, unlike GPR, getting an accurate depth measurement can be challenging.
The table below summarizes the key strengths and challenges with using each technology:
| Strengths | Challenges | |
|---|---|---|
| EM Locators (Precision Locator) | • Different ways to locate (direct connect, clamp, induction) • Portable, fast • Can induce different frequencies to locate different types of utilities |
• Utility must be a conductor (or have a tracer wire) • Signal can sometimes jump to another metal utility |
| EM Locators (CAT) | • Inexpensive • Simple to operate |
• Utility must be a conductor (or have a tracer wire) • Single frequency only – cannot vary induced signal frequency |
| GPR | • Find metallic and non-metallic utilities, and other features (e.g., vaults) • Accurate depths |
• Performance varies with soil conditions • Doesn’t tell you what the object is |
| Magnetic Locators | • Potential to see deep • Inexpensive |
• Can pick up many other objects • Object must be made of iron/steel |
