There are several potential disasters waiting to occur at each excavation site. High on the list of things to avoid is the accidental rupturing or breaking of underground utilities such as electrical power cables, communication wires, and pipelines. Though accurate information concerning their location and depth is usually available for most of these utilities, contractors must ensure prior to breaking ground that unrecorded utilities are not present. Though many of these unrecorded utilities might have been deactivated and abandoned in place, there remains the serious possibility that an excavation crew might stumble upon an undocumented, active utility-with disastrous results. Though it would be nice to have x-ray vision to allow us to see through the ground surface or pavement to determine what lies beneath, the next best thing can be obtained from a range of subsurface detection technologies on the market. This article will examine basic subsurface investigation devices, their operational techniques, the results they provide, the effects of soil characteristics on their performance, and other limitations. Utility Maps and Agency Information The first source for information concerning underground utilities is the local utility offices (telephone, electrical power, cable television, gas, and so on) and the county engineer’s office (storm sewers, sanitary sewers, water lines, and so on). Plan drawings showing the location, alignment, and depth of known utilities will be available at least in hard-copy format. Only occasionally will a utility or engineer’s office have an electronic file drawing in CAD format, which is unfortunate since these files can be easily integrated into a global positioning system (GPS) for field surveying. As more resources are made available for recordkeeping, more hard-copy records will be translated into electronic format.Especially in older urban industrial environments, utility records are going to be sparse or nonexistent. The buildings that once comprised old factory complexes (some dating back to the 1800s) may contain myriad steam lines, gas pipes, and other utilities. Often the only records (such as they are) might be extracted from the archives of the industrial company-which might or might not still be in existence. Even if a gas line has been turned off and isolated for many years, there is no guarantee that potentially dangerous amounts of gas are not trapped in the abandoned pipeline segment. Old sanitary sewers could also have accumulated methane at isolated high points. These older commercial and industrial sites might also contain deposits of inert metallic waste and debris that can be confused by the detector with utilities. At no time should it be assumed that just because a utility is old and abandoned, it is no longer dangerous.Finding Pipes and Other Objects: Metal DetectionMagnetic detection can be used to find the following:iron, steel, and copper water linesmetal gas linessurveying pins (property markers)copper tracer wirecopper and aluminum electrical wiressteel cablestelephone and TV cablesaluminum conduitany continuous metal pipe or lineFor 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.Voltage leakage can be detected from damaged electrical cables by measuring and locating voltage differences in the earth. This is a secondary method to direct magnetic determination of cable locations. Voltage leakage detection requires a detector with a large-enough frame to straddle the electrical power line. The location of the voltage leak is determined by peak voltage differential readings across the power line. Typically a readout flashes in the direction of the fault; these flashes reverse direction when the detector has moved past the fault in the line.Ground-Penetrating RadarGround-penetrating radar (GPR) uses an electromagnetic (radio wave) antenna tuned to a frequency that can penetrate soils, rock, concrete, ice, and other common natural and manmade materials. Such capabilities make radar a prime technique in obtaining geotechnical information and evaluating hazardous-waste sites. As such it is more suited for use outside of urban areas where natural conditions predominate. Furthermore, a GPR unit requires considerably more time to process than do simpler magnetic detection techniques. This is also the most expensive of the underground survey techniques.GPR functions in a manner similar to standard aerial radar. A GPR determines subsurface conditions by sending pulses of high-frequency radio waves into the ground from a transmitter antenna located on the surface. Subsurface structures cause some of the wave energy to be reflected back to the surface, while the rest of the energy continues to penetrate deeper. The result is a series of radio “echoes” that delineate underground interfaces such as bedding, cementation, changes in moisture and clay content, voids, fractures, and intrusions as well as manmade objects. The radio wave reflections are essentially no different than airplane blips on a radar screen. Resolution of radar reflections can be increased by increasing the frequency of the radar waves transmitted into the ground. GPR is best suited for determining a region’s hydrogeology, which is its main function. Smaller manmade objects such as individual drums and utilities are harder for a GPR to delineate, though a sensitive unit can reveal their locations as part of a general survey.Finding Leaks: Passive Sound DetectionSometimes a contractor needs to find a buried utility for the purposes of repair. This is most commonly done for broken water mains when the precise location of the leak must be determined. Leak noise detectors enable the user to listen for the noise of a leak through a highly sensitive sensor, referred to as a ground microphone. The sensitive microphone detects the noise of a water leak transmitted along pipes, valves, and hydrants and through the ground. Sound vibrations tend to carry farther and more distinctly along the pipelines themselves. The closer you are to the leak, the louder and clearer the leak noise, and the higher the signal displayed on the device’s meter. Once a reading has peaked and starts to diminish as the microphone continues to travel along the pipe, the leak’s location has been determined with a high degree of accuracy. Typically most leak detectors can detect the sound from most any leak that is pressurized at 10 psi or more.As can be seen from the statement above, when conducting a leak survey, it is both helpful and efficient to know where the water pipes are located under the ground. This is not absolutely essential but will greatly improve the efficiency of the search, as it allows you to move along over the pipeline on top of the ground or pavement while listening for the leak. Water-main alignments can be reasonably determined from the locations of valve boxes on the surface. If necessary, you can initially use magnetic locators to assist you in finding the pipes. The following is another technique that can be employed during a leak survey: Clip a pair of spring-loaded snap lock-type pliers to the water valve. Screw on the strong magnet to the sensor. Attach the magnet to the pliers and listen for the leak. The water-leak noise will transmit through the valve, pliers, and magnet, into the sensor. You might hear a leak up to 6 or 8 ft. down and 50 ft. away or more. Use this method to isolate two valves on which the leak noise is the strongest. Search the ground or pavement along the route of the pipe between these two valves. Using the sensor, look for the strongest water-leak noise signal between the two valves. When the signal is the strongest, you have found the area of your leak.Computerized Databases
This GPR image illustrates the location of steel pipes buried several feet underground.Recording the locations of the buried utilities can be done with something as simple as a chalk mark or as sophisticated as a GPS computerized data entry. Auxiliary field computers and GPS survey equipment can be used to create a computerized plan of the project, including changes in terrain elevation, existing utilities, property lines, roads, railroads, and rivers. Typically, once a buried utility has been detected, a surface mark (e.g., chalk, orange paint) is made to show the utility’s type, location, alignment, and approximate depth. These surface marks guide the actual excavation effort.Should the detector be connected to a GPS unit or a similar automated surveying device, the information will be entered into the computer’s database. If the computer also provides a graphic image (liquid crystal display, or LCD, screens being the most popular), real-time CAD updates of site drawings can be provided. Each newly discovered utility is added to the project’s design drawings, showing the relationship of the utilities to other structures and lines associated with the site work. If the utilities in question were to be avoided during excavation and are not to be uncovered, this would complete the data entry. If excavation is to purposefully or incidentally expose the buried utility, however, then a second direct survey of its location, depth, and so on is performed.Equipment ModelsThe following are descriptions of utility detection products currently available. The information is taken from manufacturer and supplier literature:The Ditch Witch Subsite 75R/75T is a multifrequency receiver that locates underground cable and pipe. It places a traceable signal onto selected cable and pipe. The product is lightweight, designed for rugged use, and sealed against normal moisture. It uses standard consumer batteries. It can be operated in either direct-connect mode, with an optional induction clamp, or in broadcast induction mode.The Subsite AF1/FT12 Fault Finder System detects leakage from cable by measuring and locating voltage differences in the earth. It uses an A-frame detector (AF1) and transmitter (FT12). Readout “flashes” in the direction of the fault; these flashes reverse direction when the Fault Finder has moved past the fault in the line.The Subsite 750 Tracker provides vital tracking information, including location, depth, roll angle, pitch, beacon temperature, and battery status. It is capable of tracking bore depths from 0.75 to 100 ft. and uses fore/aft arrow indicators. It has three primary methods of operation: walkover beacon tracking, remote guidance, and line-locating mode. A 750 Display used with the 750 Tracker and the Trac Management System (TMS) is integrated into the control panels of many Ditch Witch Jet Trac directional drilling systems. It receives data up to 2,000 ft. from the 750 Tracker, and it allows information to be downloaded to the TMS to produce as-drilled maps.The Subsite 86B and 86BH Beacons rely on solid-state electronics to provide shock and vibration endurance. They also rapidly respond to temperature changes. Used with the wide-gain Subsite 750 Tracker, the 86B beacon can be tracked to a depth of 40 ft., and the 86BH beacon to more than 50 ft. Data are presented on LCD readouts.The TMS creates a computerized plan of the project, including changes in terrain elevation, existing utilities, roads, railroads, and rivers, and punch-in and punch-out points when modeled into the plan. Real-time data are available with a single press of a button. Information is downloaded from the 750 display to the TMS, so the operator can visually monitor progress of the bore. The TMS can create a map of the completed pilot bore when the user simply clicks on “Print.”SubSurface Instruments Magnetic Locators, ML-1 and ML-1M, can be used to find the following items and any target containing at least a small amount of iron or steel: corner markers (iron or steel), PK nails, valve and curb boxes, iron and steel pipes, well casings, manhole covers, steel drums, energized electric cables, and marker magnets. The unit is lightweight, weighing only 2 lb., including batteries. A simple keypad is provided for operational controls, with a microprocessor that remembers the volume and gain settings from the last time the unit was operated.SubSurface Instruments offers three water-leak detectors, the LD-7, LD-10, and LC-2100. The LD-7 and LD-10 leak noise detectors enable the user to listen for the noise of a leak through highly sensitive sensors. The patented sensitive microphone detects the noise of a water leak transmitted along pipes, valves, and hydrants and through the ground. The closer you are to the leak, the louder and clearer the leak noise and the higher the signal displayed on the LD-10’s LED meter. The Trimble AG132 GPS is used in conjunction with buried-utility detectors. Trimble modified (demagnetized) one of its standard products (AG132) for operation with the Geometrics G-858 magnetometer. The package includes a 12-channel GPS receiver as well as an L-band differential satellite correction receiver. The Trimble system combined with Fugro’s Omnistar differentially corrected data provides real-time, submeter location/positioning.The Geonics EM38 ground conductivity meter provides depths of exploration of 1.5 and 0.75 m in the vertical and horizontal dipole modes, respectively. Very lightweight and only 1 m long, the EM38 generates rapid surveys. The EM38 can be used in measuring soil salinity, for many geotechnical applications, and for locating utility lines.Geometrics’ OhmMapper TR1 is a capacitively coupled resistivity system designed to measure subsurface resistivity in areas with high surface resistivity where exploration using traditional galvanically coupled (DC) resistivity systems is impractical, slow, and expensive. The OhmMapper consists of an ungrounded dipole transmitter and receiver and a data logger. An alternating current is induced in the earth by the transmitter and measured at the receiver. The measured voltage is proportional to the resistivity of the earth between the two dipoles. Apparent resistivity is calculated using the appropriate geometric factor for the capacitively coupled antenna array of dipole lengths. The OhmMapper is designed to be pulled along the ground as a streamer, providing a nearly continuous apparent resistivity profile. This design increases the resolving power and productivity of the system relative to traditional DC resistivity systems. Data are logged using the Geometrics DataMapper Console. The OhmMapper receiver is connected to one of the console’s serial ports for data acquisition via a fiber-optic interface. Data are graphically displayed in real time on the console screen. You can view the last five profiles or scroll a window through the entire data set-right in the field. At a sampling rate of two times per second, the OhmMapper TR1 has a total storage of approximately 24 hours of data acquisition. Schonstedt Instrument Company’s Mac-51Bx magnetic locator provides two active frequencies for pipe, cable, and line tracing. Passive operation is performed for locating iron and steel targets and energized 50-/60-Hz power lines. Inductive and conductive signal-coupling detection is also available. An MT-2 (Mole) mini-transmitter is used in conjunction with this unit to trace nonmetallic pipes, pinpoint obstructions, and locate concrete septic tanks. When attached to a plumber’s snake, the Mole emits a signal detectable at depths of 18 ft. using the MAC-51Bx Receiver. An inductive signal clamp provides a method of selectively applying the trace signal to conductors covered with nonmetallic insulation. The MAC-51Bx simultaneously transmits 82.5-kHz (HF) and 571-Hz (LF) signals. This feature lets you select and compare received audio signals from both frequencies along with magnetic information without having to return to the transmitter. Setting the MAC-51Bx mode switch to HI allows you to trace the 82.5-kHz signal applied to a continuous metal conductor. The HF signal also jumps gaskets between pipe sections, bad telephone cable bonds, and small breaks in a cable’s sheath.Schonstedt’s underground magnetic locators, the GA-52Cx and GA-72Cd Locators, detect the magnetic field of iron and steel objects and energized power lines. Both provide audio detection signals that peak in frequency when the locator’s tip is held directly over the target.The GA-52Cx provides five individual sensitivity settings and does not respond to aluminum, brass, or copper. With experience, a user can distinguish between small pieces of scrap iron and actual targets. The device has 40 hours of operation (intermittent usage) on two 9-volt batteries. Signal output is approximately 40-Hz audio tone on speaker. Tone frequency increases or decreases with gradient-field intensity. The GA-72Cd has easy-to-read digital and bar graph displays of signal strength and polarity. Once you have located a target using the audio signal, the digital readout and popularity indicators will help you visually pinpoint the target and, in some instances, even determine its orientation. The GA-72Cd’s audio signal and three-digit LCD readout will peak, and the (+) or (-) bar graph will expand to full scale when the tip is directly over a vertical iron pipe. One end of a horizontal target will be (+) and the other will be (-). This difference is very significant because it helps you distinguish between vertical and horizontal pipes. Designed for one-hand operation, the on-/off-sensitivity and volume controls are located on the underside of the cover on both locators. This provides easy access, and it protects the knobs and contributes to their overall ruggedness and dependability. You can select any one of four sensitivity settings. The GA-72Cd’s LCD bar graphs show the selected gain and the battery status. The GA-72Cd’s headset jack can be connected to your data logger using a standard stereo audio plug. The GA-72Cd is configured initially to provide an audio signal that is always present (audio output switch set to “B”). If you do not want to hear a signal until the locator is within the detection range of a target, you can set the audio output switch to “A.” The Rycom 8875 Portable Locator is capable of locating long or short range, inductive or conductive, active or passive. By operating the receiver at multiple frequencies, performance can be optimized for the specific need of the user. Low frequency of 815 Hz provides longer range and fewer errors from adjacent cables. High frequency of 82 kHz will locate a path past bad telephone bonds, locate underground stubs, and permit inductive locating with either the optional Flexicoupler or direct soil induction. Two frequencies can be compared simultaneously without having to return to the transmitter. Excellent passive 60-Hz locating will pinpoint active power lines, and other utilities where AC is present, without the use of the transmitter. High power at low frequency with the 8875 Locator solves the difficult problem of locating multipoint grounded utilities. High power at high frequency provides inductive coupling and direct soil induction that will bridge or jump across an open or damaged cable. No direct connection is needed. Depths up to 15 ft. are quickly displayed in inches or centimeters. The AVO International Biddle Split-box Pipe and Cable Locator is a hand-held instrument consisting of a transmitter and a receiver for tracing underground conductive networks, such as water and gas mains and telephone, cable TV, and electric power cables. It determines buried line depth and locates underground metallic masses, such as valve caps and manhole covers. A fault/cable analysis system, the DART 5000 from AVO, allows users to build a custom fault-locating system that utilizes multiple technologies and techniques to accommodate application and budget needs. The basic system comes with time domain reflectometry (TDR) and digital arc reflection modes as standard. Buyers can then select up to six additional optional fault-locating modes. The DART 5000 is range-selectable from 100 ft. to 50,000 ft. TDR pulse widths are from 50 µs through 10 µs. The system automatically sets the optimal pulse width for the selected range. To simplify operations, the system’s instrument characteristics are configured only for the cable fault-locating methods chosen by the user. New techniques can be added to the instrument at a later date through software upgrades. The AVO AccuTrace Cable Route Tracer consists of a transmitter, which energizes the line with a traceable signal, and a portable receiver, which detects the signal. This cable route tracer can trace and determine the depth of any conductive line and trace energized or de-energized lines through inductive or conductive coupling. The receiver picks up only the distinctive transmitted signal by filtering out electric noise and static. The extremely lightweight receiver increases operator comfort when tracing long cables or when used for extended periods. Pipe Hawk is a ground-probing radar system. It provides an underground mapping system, offering knowledge of what is underground before construction works are commenced. It can provide the information needed by displaying a full-size CAD drawing showing direction and depth of the different utilities. A two-man team without surveying or particular computing experience can operate the system.The Magna-Trak 100 identifies the target object, especially in noisy congested areas. It features simple, waterproof, membrane-pad push-button operation with an easy grip design. It features quick-action, two panel-control knob operation to adjust sensitivity and volume. The Magna-Trak 102 has the new push-button erase feature. Fisher Research Laboratory offers the TW-770, a depth-reading line tracer that uses digital signal processing (DSP) to provide readouts of the direction and depth of underground pipes and cables. When a button is pushed, a DSP chip in the TW-770 control housing computes line depth, which is displayed in inches or centimeters on a large LCD. Fisher’s product line also includes the M-95 valve, pedestal, and box locator. The M-95 locates buried metal objects, such as pedestals for telephone companies and manholes and reinforced concrete walls for utility companies and public works departments. Any metallic item can be located, including survey markers, conduit, and gas meters. Two other valve and box locators, the M-65 and M-55, are also available. The TW-6, a “two-box” line tracer, is similar to the TW-770 but does not use digital components. The Fisher XLT-20 is a leak detector that uses electronic sound filters, sensitive transducers, and a powerful amplifier to locate water leaks, even below concrete, asphalt, and paving. The sound of the leak guides the operator to its source. Another instrument from Fisher is the XLG-90, an ultrasonic sound detector that troubleshoots leaks in pressurized telephone cable. It also detects corona discharge, bearing wear, steam traps, valve noise, and some gaseous and water leaks. Geophysical Survey Systems Inc.’s (GSSI) Subsurface Interface Radar (SIR) Systems use GPR technology for utility mapping, infrastructure evaluation (roads, bridge decks, and rail beds), environmental site assessment (buried tanks and drums), and geology (mapping of bedrock, water table, or mine shafts), as well as for archaeology and forensics. All SIR Systems are compatible with the full range of antennas (16 MHz to 2 GHz); higher frequency provides greater resolution, while lower frequency penetrates deeper (down to 30 m). RADAN is the postprocessing software developed by GSSI to process and interpret GPR data files.The GEM-300 is a multifrequency, electromagnetic profiling device that can be configured to simultaneously measure up to 16 user-defined frequencies between 330 and 20,000 Hz with fixed coil separation. By acquiring multiple frequencies, the user can select the frequency(ies) with the best results for a specific application. Both in-phase and quadrature measurements are recorded. The GeoRadar Inc. Model 1000B GPR can locate plastic, ceramic, and metallic objects and voids. Proprietary stepped-FM technology provides clearer images than conventional pulse systems, making interpretation easy. Closely spaced objects can be resolved. Nearby surface objects do not interfere, and the system can be used around buildings, power lines, and vehicles. Features include a large LCD and a plotter for interactive real-time viewing and for producing paper records. Data can be stored on DOS media for later processing. An RS-232 or network interface provides data transfer to real-time mapping and imaging systems. The Model 1000B has an “unambiguous range” of 10 m. Heath’s LS-300 pinpoints ferromagnetic objects. This company also offers the LS-500 with its sensitive transmitter, integrally molded loop antennas, and rugged construction. The receiver’s unique filter circuitry greatly reduces background noise and interference. A narrow signal band creates a sharp, intensified signal when the receiver is above the line being traced. Heath’s product line also includes the LS-1000, a computerized sheath fault locator, cable locator, and fiber-optic locator. Also included is the computerized LS-990, which offers LCD readout and microprocessor technology.This TDR cable fault locator quickly finds faults on live, low-voltage power distribution cable.
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