Utility Locating: Electromagnetic Locating
Ground penetrating radar (GPR) is the primary technology used by GPRS for locating utilities.
There are times, however, when GPR is not the best tool for the job. Electromagnetic (EM) locating compliments GPR in these instances, detecting the electromagnetic signals radiating from metallic pipes and cables.
How Do EM Locators Work?
It’s important to note that EM locators do not locate buried pipes or cables – they detect the electromagnetic signals radiating from metallic pipes and cables.
These signals can be created by the locator’s transmitter applying current to the pipe, or from current flow in a live electrical cable. They can also result from a conductive pipe acting as an antenna and re-radiating signals from stray electrical fields (detected by the EM locator functioning in Power Mode) and communications transmissions (Radio Mode).
Signals are created by the current flowing from the transmitter which travels along the conductor (line/cable/pipe) and back to the transmitter. The current typically uses a ground to complete the current. A ground stake is used to complete the circuit through the ground.
How to Find Distorted Fields
The magnetic field (the signal) radiating from buried lines can be distorted by the presence of adjacent metallic conductors or other signals.
This is caused by:
- Signals induced from the target line to other lines
- Commonly bonded structures
- Poorly positioned ground (at the transmitter)
The result is that the locator detects signals from more than one source. Distortion can cause inaccurate depths and inaccurate locations.
Note: The field is not affected by the shape of a pipe, pipe insulation, or soil types.
EM locators can function in Peak & Null modes to identify distorted electromagnetic fields.
In Peak Mode, the horizontal antennas in the locator receiver detects the field. The signal will be strongest when centered on the field because more of it passes through the antenna.
In Null Mode, the vertical antenna detects the field. The field is not passing through the antenna when it is positioned over its center.
If the field is round (no distortion), then Peak and Null modes will agree and provide the same location.
Note: Peak Mode should always be more accurate than Null Mode. As such, Peak Mode should always be used for locating and Null Mode for identifying distortion.
Depth Measurements
- Depth measurements should not be relied upon if:
- Close to bends in the line
- Close to a “T” in the line
- Close to the transmitter
- Where the line is changing depth
- Where field distortion has been identified
- Using Power Mode
All these factors can result in inaccurate depth and current readings.
A GPRS Project Manager can obtain accurate depth measurements if:
- Reasonable: The value should be a reasonable number.
- Repeatable: The depth reading should be consistent when multiple readings are taken over an area of roughly 10’-15’.
- No congestion: Do not take depth readings when an electromagnetic field could be interfered with or distorted, such as near the transmitter, near a T, near a bend in the pipe, or when other utilities are nearby.
- Peak/Null Agree: Peak and Null modes should give the same location to indicate that the field is not distorted.
Once a depth reading is obtained, the Project Manager will move the receiver vertically to help verify the accuracy of the depth.
The Project Manager will move upward by approximately 1’. Depth readings are based on the receiver’s end placed on the ground. For example, if the depth says 2’ when the receiver is on the ground, it should say 3’ when the receiver is 1’ above the ground.
Depth is measured to the center of the electromagnetic field. Therefore, depths are measured to the center of a pipe, not the top of a pipe.
Direct Connection
When an EM locator’s transmitter needs to apply current to a pipe to create a signal to detect, there are three ways in which that signal can be applied:
- Direct connection – Red lead to the target line, black lead to the ground.
- Clamp – Induces a signal into a pipe or cable without making a direct connection.
- Induction – Induces a signal into a cable or pipe by placing the transmitter on the surface over the target line.
When to Use Active Mode
An EM locator’s Active Mode is used:
- When locating a specific line in congested areas
- When tracing a particular line for any distance
- When pinpointing a buried line
- When a depth measurement is required
Signal Application – Direct Connection
Direct Connection Mode is utilized when there is safe access to the target line.
There must be a metal-to-metal connection for this mode to function properly. Any rust or paint must be removed from the connection point to ensure a good electrical connection.
The EM locator can connect direction to the utility or metal objects in contact with the utility, such as a bolt-in contact with a light pole in contact with a ground wire. The ground stake should be placed perpendicular to the cable and as far away as practically possible.
Active Signal – Frequency Range
When direct connecting, different frequencies can be used depending on the situation. Commonly used frequencies include 512 Hz, 8kHz, 33 kHz, and 200 kHz.
1. Low Frequency: 100Hz – 1 kHz
- Best for cables
- Direct connection only
- Long-distance
- Low distortion
2. Medium Frequency: 8 kHz-33 kHz
- Good for pipes and cables
- Direct connection and signal clamp
- Reasonable distance
3. High Frequency: 65 kHz-200 kHz
- Direct connect, signal clamp, induction
- Short distance
- Prone to distortion
GPRS Project Managers will use the minimum output power needed to successfully locate the target line. This is because excess power may increase the risk of coupling to other lines. More power also reduces battery life.
When positioning the ground stake, there are some steps that can be taken by our Project Managers to minimize coupling to other lines:
- Do not place it close to other lines
- Do not place it on the other side of adjacent lines
- Do not place it close to metallic fences or barriers
Signal Clamp
A signal clamp is used when it is not possible to connect directly to a utility, insulated sheath, or cable identification. The clamp is placed around the utility and below the grounding point to ensure the signal has a signal path between near and far ground points.
The signal clamp can be the best method to isolate conduits that are bonded together. Anytime multiple conduits are entering the ground together, the clamp should be used around each one to identify them individually.
Induction
Induction involves placing a transmitter on the surface over a target line to induce a signal into that cable or pipe.
The Project Manager will move at least 30 feet away from the transmitter, which must be in line with the utility when locating it using induction. They will use frequencies of 65 kHz or higher. The signal travels from the transmitter down into the ground in a cone shape, and as such it will “jump” onto other pipes in the area beyond the target line. The signal will also travel through the air, so the target line can’t be accurately located within about 30’ of the transmitter because there will always be a false reading in line with the transmitter through the air.
There are three methods GPRS Project Managers use to determine whether they are air-couple as opposed to having located the actual pipe.
- They can move the transmitter a few feet to the side and see if the supposed pipe “moves” as well or remains in its original location.
- They can angle the transmitter slightly while keeping it mostly oriented with the pipe. The false reading will follow the same angle, but the true pipe will not move.
- They can point their receiving wand at the transmitter. If the signal is coming from the transmitter, then the meter response will get stronger. If the pipe has been located, then pointing the want into the air will weaken the signal.
When using induction, the signal travels out of the top and bottom of the transmitter. It travels downward in a cone shape, which will “jump” onto other pipes in the area. A shallower pipe or better conductor in the area may steal all the EM locator’s signals, making the target line impossible to trace.
To minimize a signal emanating from an unwanted line, a GPRS Project Manager is trained to either move the transmitter off the target line away from the unwanted line to minimize the signal from the latter. They can also lay the transmitter on its side using the kick stand. The signal will come out of the top and bottom but will cancel out what is directly below the transmitter.
Passive Sweeps
Passive sweeps are used to mark the location of unidentified, buried lines prior to breaking ground. This mode is not used to identify or trace specific lines or provide depths of buried utilities.
Active Location
Active Location Mode is used to trace, identify, and pinpoint a buried line. It also can be used to measure the depth estimation of the buried line, or the signal current on that line.
Passive Location
Passive signals can originate from a variety of sources:
Power
- Current flowing through a live electrical wire
- Cathodic protection for pipelines
- Stray currents from power transmission systems can use any conductive pipe as a return path
Note: Finding a power reading does not necessarily mean that an electrical line has been located. The line should be identified by tracing it to its source or an electrical structure.
Radio
- Any conductive pipe or utility can act as an antenna for radio wave transmissions in the atmosphere and enter the ground
- Some active phone lines
- Stray currents from power transmission systems can use any conductive pipe as a return path
Note: Radio Mode cannot calculate depth readings. Once a radio reading has been found, induction could be used to assist with tracing and depths.
Rebar
- Reinforcing such as rebar and mesh will often re-radiate these signals and can provide false positive readings
- Raising the receiver and adjusting the gain to eliminate the weaker readings from the rebar may allow the stronger reading from the utility to be traced
- After adjusting the height, a GPRS Project Manager will continue sweeping and tracing at that height
GPRS’ team of Project Managers receive industry-leading training so that they can tailor our suite of infrastructure visualization services to your needs. Whether you need utility locating, precision concrete scanning and imaging, video pipe inspection, leak detection, or one of our other services, we have the tools and the training to help you Intelligently Visualize The Built World™ and keep your project on time, on budget, and safe.
Learn More
GPRS’ team of Project Managers receive industry-leading training so that they can tailor our suite of infrastructure visualization services to your needs. Whether you need utility locating, precision concrete scanning and imaging, video pipe inspection, leak detection, or one of our other services, we have the tools and the training to help you Intelligently Visualize The Built World™ and keep your project on time, on budget, and safe.
What can we help you visualize?
