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A publication for those interested in topics relating to above-ground and subsurface utility locating, and Geographic Information System (GIS) and Glo
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A publication for those interested in topics relating to above-ground and subsurface utility locating, and Geographic Information System (GIS) and Global Positioning System (GPS) mapping. Join us on the 3rd Wednesday of every month to explore how cutting edge technology and innovation, combined with a desire to educate and protect the public, will serve you well as a professional in the land use industry. |
 "Underground Utilities in NYC" Photo Credit: Balkan Plumbing: NYC Sewer and Water Main Specialists |
Under the crowded streets, where does one begin?
Take a look at the photo to the right. What you see is the complex network of underground facilities below the streets of New York City. Notice the various types of pipes and cables, ranging in diameter, depth, and direction, the different materials used, and the quality of the pipes themselves. Now imagine how one might have located all of these prior to the street being excavated.
The chosen method of utility location is based on all those factors, as well as the condition of the ground, so it's important to gather as much data as possible prior to locating to get a sense of what you might find. This includes contacting Dig Safe to have member utility companies first locate and mark their own facilities, contacting the local Public Works department, other utility companies, etc., acquiring any maps and records that may have been kept for a particular area.
To be an efficient locator, thorough research and knowledge of diverse locating equipment is key. When underground facilities become more complex and maps become outdated, the real art of locating comes into play. One must determine what data is useful, and what data needs to be recollected. It is important to distinguish what type of instrument and method will be best suited for locating specific types of facilities. The goal, of course, is to capture the most current and accurate picture of those utilities to ensure public safety prior to excavation. |
Utility Locating in a Nutshell
The most common methods for locating underground utilities are electromagnetic, magnetic, optical, infrared (thermal), and elastic wave (audible). For most purposes, a combination of electromagnetic, magnetic, and optical will provide an accurate image of the facilities below our feet.
Often times, a combination of both active and passive locating, using a variety of instruments, is used.
Active locating involves using either a direct connection (conductive) or an inductive method to locate a specific line.
Passive locating involves simply "sweeping" an area with a receiver, picking up frequencies that radiate naturally from the pipe or cable. In this case no transmitter is used. This method is used mostly when checking areas for unknown lines, or in areas where there isn't a highly complex network of facilities.
A range of frequencies are needed to find pipes made from different materials at different depths. Multi-frequency locators try to optimize efficiency by having a single instrument with a broad range, but this can sometimes lead to signal interference. At AAB, we use a range of different types of locators to try and minimize that interference and get the most accurate locate. |
Electromagnetic Induction
In general, metallic utilities are located through “electromagnetic induction”, using a transmitter to apply an electromagnetic field that wraps around the conducting metal of the cable.
When possible, the transmitter is connected to the utility using a signal clamp, forming an inductive connection (see photo). The receiver, or wand, from the paired equipment is then tuned to the oscillation frequency of the electromagnetic field and swept across the area, tracing the position of the field as it travels down the path of the conducting metal of the utility. When a direct connection is not possible, the transmitter induces the signal into the ground and the receiver picks it up as it re-radiates from the utility. |
Ground Penetrating Radar
The electromagnetic induction method is typically used first, because many wires and cables that are metallic are too small in diameter to be found reliably with a Ground Penetrating Radar. But for larger subsurface utilities, and non-metallic utilities that are unable to carry an electromagnetic field, we use the MALA Ground Penetrating Radar (GPR). The GPR uses radar pulses in the microwave band of the radio spectrum to create subsurface images.
The GPR is the ideal instrument for finding facilities made of concrete, clay, or plastic, and non-conductive objects. This may include septic tanks, leach fields, unmarked grave sites,and storm water drains. Soil type and condition will influence the depth that GPR can scan. |
We want our clients and the public to understand why it is so important to locate underground utilities properly and efficiently. There are thousands of cases where property damage and personal harm could have been avoided had locating practices been done differently. When it comes down to it, this profession is all about damage prevention. |
What does it mean?
Ever wonder about all those markings on the ground after a utility has been located?
Standardized color codes indicate the type of line and initials will describe who did the locating, When possible, the width and depth of the facility will be indicated. Other markings may include length, changes in direction, and tolerance zone.
It is all part of helping others get a better picture of what's going on below our feet. |
The establishment of a tolerance zone is another way to increase safety. Even if the accurate location of a facility is known, this tolerance zone decreases the chances that an excavator will hit the line. The width of the zone depends on the width of the facility, and whether or not the locator or "operator" is a member of Dig Safe or not. (A greater tolerance distance is given to those who are not.)
You can learn more about the particulars of marking in Section B of The Public Utilities Commission's Underground Facility Damage Prevention Requirements. |
Thank You
We would like to thank you for your interest in Above And Below. We welcome your feedback as we launch this new publication, and hope that you find it both interesting and informative. |
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