Introduction Underneath today's city streets exists a labyrinth of underground water, sewer, gas, electric and telecommunication lines. As urban populations increase, 50% of a predicted 7-9 billion people are expected to be large town or city dwellers by the year 2010 (Duddeck, 1996), so the quantity and complexity of underground utilities rises to meet the demand for potable water, telecommunications, energy and sewerage services. The location of such underground utilities has, historically, been based on utility record information maintained by the utility companies supplemented by on-site trial hole investigations. Unfortunately, this information, if it exists, is often inaccurate, incomplete or out of date. Furthermore, such information very often locates the utility line in only two dimensions, usually in the form of a line on a map.
The third dimension, depth, is critically important for the construction of accurate three-dimensional plans of all buried utilities - a prerequisite for the installation of utility networks by modern trenchless techniques. Various non-destructive detection techniques exist for pipeline location. However, compounding the location problem is the fact that the conduits for utilities range from metallic materials, such as steel and cast iron, through traditional materials such as clay and concrete, to synthetic materials such as polyethylene and polyvinyl chloride. This section reviews current pipeline location technology and the associated technique of underground mapping. UTIL-LOCATE, INC. provides private underground utility-line locating and mapping services prior to excavation and for design-build specifications. Our clientele-base consists of: Surveyors, Architects/Engineers, General Contractors, Government Departments, School Districts, Private-facility owners and Residential Property-owners who find it a vital concern in knowing the locations of their underground-infrastructures. Damage prevention to the utilities, as well as, the safety to the community is the foremost concern prior to excavation. Prior knowledge of the underground utilities, is also an important and cost-effective way in assuring a safe and successful out-come of your project. UTIL-LOCATE, INC. uses the most advanced and state-of-the-art Locating Instruments and Techniques in a comprehensive search for underground utility-lines. Our Certified Technicians apply the “best effort” approach, utilizing their ability and knowledge to the up-most standards of “competence” in identifying each utility-line being mapped on the surface of the ground.     The Electromagnetic (EM) locating equipment we use to locate metallic pipes and or cables provides us with a range of frequencies necessary in a survey. Having the choice of frequencies at-hand, gives the Technician the ability to adjust to the un-foreseen soil conditions as well as pipe/cable conditions which can be encountered unexpectedly on the job-site. The equipment also provides continuous depth-estimates to help confirm target conductors in which, secondly, can be applied for excavation and design information. Measure of accuracy is 93% to 97% allowing a “hand-dig” tolerance of 28” on either-side of the utility-line. Depth-readings are limited up to 13-feet. The depth-readings maybe provided upon request

We also have Ground Penetrating Radar (GPR) equipment to locate non-metallic lines such as: Clay, PVC, ABS and AC (Asbestos Concrete). The equipment also can locate UST (Underground Storage Tanks) and any other abandoned utility-lines. GPR though, is limited in its ability to perform in every area. It is strictly governed by the soil-type of each site.

Finding Pipes and Other Objects: Metal Detection Magnetic detection can be used to find the following: Iron, steel, and copper water lines Metal gas lines Surveying pins (property markers) Copper tracer wire Copper and aluminum electrical wires Steel cables Telephone and TV cables Aluminum conduit Any continuous metal pipe or line

For magnetic detection to work, the target must contain some iron or steel or have an electrical current flowing through it so that a magnetic detector can find it. Nonferrous objects or objects not carrying an electrical current (such as polyvinyl chloride or high-density polyethylene pipe) must be detected by other means. The strongest signals come from the ends of an object, as this is where the magnetic fields of force tend to concentrate. Therefore, if an object is oriented vertically, it will produce a stronger signal than one oriented horizontally. Oddly enough, this can make a relatively small object, such as a steel drum, easier to find than a long, cast-iron water main.

The same object buried horizontally would give off two weaker signals, one directly above each end (one signal being positive and one being negative). Though there is seldom an "end" to a pipeline, the same principles apply to iron or steel pipe. Cast-iron or steel water pipe laid end to end will produce a strong signal at each joint, even if the pipes are welded together. Magnetic detection of metal pipes often results in a series of peak signals designating the locations of welded end joints.

Electric cables must be energized (have power flowing through them) to be detected magnetically. Indirect mode allows for the detection of the magnetic fields generated by the electrical power cable. Direct mode allows for a similar effect to be induced into metallic pipes that would not normally create their own magnetic fields. This is done by attaching a clip or a clamp (connected to a low-voltage power source) to the pipe or cable (target line) and inducing an electrical current. This transmits a strong signal directly to the target line, allowing you to trace it at a greater depth and for a longer distance.

If the target line is not accessible for attaching a clamp, place the transmitter over the target line and activate it. This is broadcast induction mode. Then use the receiver in the same manner to trace the signal emitted from the target line, regardless of the transmitter mode used.

Most detectors provide an audio signal to the operator that indicates the detector is over a buried utility. As you walk along without encountering any iron or steel items, the receiver’s two magnetic-field sensors will balance out the Earth’s magnetic field and the frequency of the audible indication will remain at a low level (approximately 40 Hz). However, as you approach a buried vertical piece of iron pipe, for example, the frequency of the audio indication will begin to increase as the strength of the magnetic field becomes stronger at one of the sensors. When the tip of the receiver is directly over the pipe, the strength of the magnetic field at the first sensor is maximized, which causes the frequency of the audio signal to peak. After you’ve outlined the target area, reduce the sensitivity level and slowly move the receiver back and forth in an X pattern over the area. Very quickly the well-defined peak of the audio signal will pinpoint the target.

Electromagnetic and Radiofrequency Line Locators

The basic principle of underground pipe location is based on a singular phenomenon of electronics, which gave birth to the induction balance and has not changed since the early models at the start of the last century. Like so many inventions it was stumbled upon accidentally. The radio direction finder developed for the U.S. Navy by Dr. Gerhard
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Fisher in the 1920's allowed pilots to determine their position by homing in on radio signals broadcast from major cities. Pilots, however, reported errors when flying over metallic buildings or large underground ore deposits. This unexpected phenomenon led Fisher to think that underground pipes and cables could be located by reading these distorted signals, thus he set out to build a handheld pipe locator that used radio signals and a loop antenna (Chernekoff and Toussaint, 1994).

Electromagnetic and Radiofrequency line locators operate either in "passive" mode by locating a background signal or in "active" mode by locating a signal introduced into the utility line using a transmitter. Three sources of background signals exist as follows (Twohig, 1998):

• Background signals due to the flow of electrons in a conductor acting like an antenna. These oscillate producing fields which can be detected by an electromagnetic receiver (Radiofrequency method);
• Current carrying conductors producing a 60Hz signal can be detected by an electromagnetic receiver (Electromagnetic method); and
• Utilities in close proximity to power lines or used as grounds may also be picked up with a receiver.
Signals introduced into a utility line can be either indirectly or directly induced. Introducing a signal indirectly consists of generating and transmitting a magnetic field from a transmitter above ground. For optimum results the transmitter should have the same general orientation as the utility line which can be obtained through trial and error. Alternatively, a signal can be introduced directly using an induction clamp, a circular clamp, which induces a signal only in the particular conductor that it is clamped around. 14

The best possible tracing signals are those which are generated using the direct connect method. By virtue of the closed current loop, there is very little chance of the resulting signals being distorted. This is the preferred method of tracing a utility when and where possible (Twohig, 1998).

The main weakness of this method is its inability to trace non-metallic objects, although this can be overcome by using an "in-pipe transmitter" sonde or a trace wire. When access can be gained to a pipeline a flexible insulated trace wire can be fed into the conduit. The transmitter is connected to the trace wire and the signal in the wire can be traced as before. The need to insert a trace wire into the pipeline could be, at least partially, overcome if all new non-metallic pipes incorporated tracer wires (Vickridge and Leontidis, 1997). Alternatively, an "in-pipe transmitter" sonde can be used in the same manner. Sondes are small, self-contained radio transmitters, which create an electromagnetic field of their own. In both cases the receiver is passed over the approximate location of the pipeline and the location with the highest signal strength is marked as the horizontal location of the pipeline. By rotating the receiver until the highest signal strength is received the approximate orientation of the pipeline is determined. Finally, the depth of the pipeline can be estimated, but this measurement is prone to error. A particular area of error is the situation where a pipe, situated near the target pipe, possesses a strong magnetic field causing extraneous currents to be induced thus leading to misinterpretation (Fedde and Patterson, 1988). This would indicate a single utility with greater depth than the actual depth due to the combined field. Conversely, the presence of a conductor above the pipeline being traced could lead to a shallower than the true depth being recorded.

Electromagnetic Locators

Used to locate buried pipes, cables, and sewers, this type of device detects the alternating magnetic fields that surround a conducting metallic line. As a result, it can't locate nonmetallic lines, such as plastic pipe, unless it is installed with tracer wires. This technology works in all soil conditions, even under water. In addition to locating buried lines and blockages and collapses in ducts or pipes, it can locate cable faults, monitor rust and corrosion of pipeline surfaces, and identify the position of joints in iron gas pipe.

There are several major manufacturers of electromagnetic locators. The company's line of products features a self-contained transmitter, which is placed in a drain or a duct, and a transmitter that generates an AC signal that creates an electromagnetic field around a buried conductor. These signals are picked up by a handheld receiver.

By setting a switch, an operator can use these locators to detect both passive and active signals. Passive signals include those emitted by power cables, telephone lines, power-system return currents, and radio-frequency currents from long-wave radio transmissions that penetrate the ground and flow along buried pipes and cables. Locating a line using passive signals requires only a receiver. Passive signals can't identify a specific line, however, if multiple lines are present. That's where the ability to detect active signals produced by the transmitter pays off. It allows operators to identify lines more precisely in terms of depth and signal strength. By varying the transmitter frequencies, operators can use this type of electromagnetic locator to positively identify and trace a single line in a congested area, such as below-street services in a city.