Utility Locating: Electromagnetic Locating
GPRS is the nation’s largest private subsurface locating company. GPRS is one of the company that provides concrete scanning, utility locating, leak detection, and video pipe inspection services. Our dedication to safety has helped us achieve an over 99.8% subsurface damage prevention scanning rate on hundreds of thousands of scanning and location projects. GPRS has unparalleled accuracy, and as a company, we want to raise the industry standard. To help push the industry forward, GPRS is releasing articles about our training methodology. This article will review our training documentation and methods concerning our electromagnetic locating.
How to locate Electromagnetic Fields & Circuits
Locators do not locate buried pipes or cables - locators detect the electromagnetic signals (EMfields) radiating from metallic pipes and cables. If there is noAC current flowing, there will be no locate signal.
This signal can be created by the locator’s transmitter applying current to the pipe, or the signal can result from current flow in alive electrical cable. It can also result from a conductive pipe acting as an antenna and re-radiating signals from stray electrical fields (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 the ground to complete the current.The 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, only by other EM fields.
How to Identify Distorted Fields
Use Peak & Null modes to identify distorted fields:
• As shown here, the horizontal antennas in the locator receiver detects the field in peak mode. The signal will be the strongest when centered on the field because more of the field passes through the antenna.
• The vertical antenna detects the field in null mode. The field is not passing through the antenna when the vertical antenna is positioned over the center of the field.
• If the field is round (no distortion), then peak and null will agree and provide the same location.
Use Peak & Null modes to identify distorted fields:
• There are other ways to detect distortion, but this is the simplest way
• On a clean, undistorted field, the Peak and Null locate response will line up
• If distortion is present, the peak and null locate response will no longer line up
Typically, the greater the distortion, the further apart these locate responses.
Note: the Peak should always be more accurate than the Null. Always use the Peak for locating and the Null only for assisting in identifying distortion. If there is distortion, the Peak can still be inaccurate so add a buffer to your markings to the side opposite the Null position.
If you want to get technical with it, the true position of the line should be measured off of the peak position to the opposite side of the Null position by half of the distance between the peak and the Null. I know that probably doesn’t make sense; look at the picture below for an example.
Depth Measurements
Do not rely on depth measurements made 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 & current readings.
You should get accurate depth measurements if:
- Reasonable: The value should be a reasonable number. If it says 90’ deep, it can be ignored regardless of the rest of the considerations in this list.
- Repeatable: The depth reading should be fairly consistent when multiple readings are taken over an area of roughly 10’-15’.
- No congestion: Do not take depth readings when a 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. \
1. Note: Steps 1-3 happen intuitively once they are learned
- Peak/Null Agree: Peak and null should give the same location to indicate that the field is not distorted. On the Vivax, the bar should be at least blue or green ideally; red would indicate a high level of distortion.
- Move receiver up and down: More on this on the next page.
1.Note: Steps 4-5 need to be applied physically and performed each time.
Do rely on depth and current measurements made if:
• Once a depth reading is obtained, move the receiver vertically to help verify the accuracy of the depth.
• 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 field. Therefore, depths are measured to the center of a pipe, not the top of a pipe.
Direct Connection
There are three ways to apply the signal:
- Direct connection – red lead to the target line, black lead to 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 Modes
• 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
Always remind the contractor to Call 811 Before They Dig – it’s the law!
Always follow local, state, or national regulations and GPRS’s work and safety practices.
Signal Application – Direct Connection
- Direct connection is suitable when there is safe access to the target line
- Remove any rust or paint to ensure a good electrical connection
- Must have a metal-to-metal connection.
1.Can be direct 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.
4. Place the ground stake at 90° to the cable and as far away as practical.
Active Signal – Frequency Range
When direct connecting, different frequencies can be used depending on the situation. Our commonly used frequencies are 512 Hz, 8kHz, 33 kHz, 65 kHz, 200 kHz (some models)
- Low frequency - 100Hz – 1 kHz
• Best for cables
• Direct connection only
• Long-distance
• Low distortion
- Medium frequency -8 kHz – 33 kHz
• Good for pipes & Cables
• Direct connection and signal clamp
• Reasonable distance
- High frequency - 65 kHz – 200 kHz
• Direct connect, signal clamp, induction
• Short distance
• Prone to distortion
Use the minimum output power needed to locate the target line successfully.
• Excess power may increase the risk of coupling to other lines.
• More power reduces battery life.
• The transmitter display will confirm how much current (mA) is being applied to the line indicating a good or bad connection.
• A change in speaker tone also confirms a good or bad connection
• If the display shows no current or there is no change of speaker tone, check the connection to the target line.
Note: some line scan be located with a small number (even 0) for mA. If it seems that a good connection has been made, then attempt to locate the line; it may trace out easily.
When positioning the ground stake – 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.
Ideally, the ground should be placed at a 90° angle from the target line and as far away as possible. However, 90° away may not be the best position if it means crossing another utility, coupling to an unwanted target, or is not good soil for grounding.
Advanced direct connection tips:double-ended connection:
Use a spool of wire similar to this to extend the ground lead to another connection point on the same pipe. This is a 14-gauge braided wire. A clamp can be added to one end, oran extra clamp can be used to secure the end of the wire to a pipe.
Examples:
1. Connecting between two fire hydrants to improve the signal.
2. Connecting to a water shutoff at the street and a domestic water line inside a building to isolate the line, reduce bleed-off, and positively identify the correct line.
Advanced direct connection tips: double-ended connection:
Apply the red lead to a pipe-like normal. Connect the blacklead to the bare end of the extension wire. Connect the other end of the wire to another connection point on the same utility shown below.
How to apply Signal Clamp
There are three ways to apply the signal:
- Direct connection – red lead to the target line, black lead to 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
Using a Signal Clamp
• Use when you cannot directly connect to a utility, insulated sheath, or cable identification.
• Place the clamp around the utility.
• Connect below the grounding point (to ensure the signal has a signal path between near and far ground points).
• For best results, the utility line must be grounded at each end.
Using a Signal Clamp
• As shown in the image to the right, 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 each.
Induction
There are three ways to apply the signal:
- Direct connection – red lead to the target line, black lead to 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
Move at least 30 ft away from the transmitter (the signal from the transmitter has an airborne element which you will locate)
Do not place it on top of a manhole cover or metal plate (the signal will not penetrate to the line)
The accuracy of depth readings may be influenced if taken close to a transmitter on induction.
High frequencies must be used, such as 65 kHz or higher.
The transmitter must be in line with the utility when using induction, as shown in the image below.
When using this method, you need to be aware that the signal travels downward in a cone shape and will jump onto other pipes in the area, not just the pipe you are on top of. The signal will also travel through the air, so you can never locate the target line within about 30’ of the transmitter because there will always be a false reading in line with the transmitter through the air. To know whether you are air-coupled as opposed to finding the actual pipe, you can use 3 methods:
- Move the transmitter a few feet to the side and see if the supposed pipe moves also or stays in the original locations.
- 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.
- Point your receiver wand at the transmitter.If the signal is coming from the transmitter, then your meter response will get stronger. If you find the pipe, then pointing 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 of your signals, making the desired line impossible to trace. Here are two ways to minimize that unwanted line:
1. Move the transmitter off of the waterline away from the electric line to minimize the signal on the electric line.
2. Lay the transmitter on its side using the kick-stand. The signal will still come out of the top and the bottom but will cancel what is directly below the transmitter.
Passive Sweeps
Use to mark the location of unidentified buried lines before digging (avoidance)
• Do not use it to identify or trace specific lines
• Do not use it to provide depths
Active Location
• Use to trace, identify & pinpoint a buried line
• Use to measure the depth estimation of the buried line
• Use to measure the signal current on the buried 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 is being 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 positives.
• 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, continuing sweeping and tracing at that height, as shown in the picture.
Learn More
GPRS specializes in ground penetrating radar, video pipe inspection, and mapping and modeling services. Our Project Managers have the equipment and expertise to handle all subsurface challenges presented. GPRS does this by utilizing various equipment paired with their industry-leading SIM process.
Visit our website to learn more, or click here to schedule a project with GPRS and allow our highly trained Project Managers to keep your projects on time, on budget, and safe.
