SUBSURFACE MAPPING. Presenters: Thomas Kevin Hanna, PLS. Paul J. Emilius Jr., PLS CP

Transcription

1 SUBSURFACE MAPPING Presenters: Thomas Kevin Hanna, PLS Paul J. Emilius Jr., PLS CP February 2 nd

2 Existing Underground Utilities Abound! Power, Telecommunications, Gas/Propane, Petroleum, Sanitary Sewer, Drainage, Steam, Water &Cable TV 2

3 WORKSHOP TOPICS 1.0 Utility Surveys Background! Current utility congestion and complexity.! Deregulation of telecommunication services! Increased excavation damage to utilities. 1.1 Subsurface Utility Engineering (SUE)! What is SUE?! Why use SUE?! How does SUE work? 1.2 Geophysical equipment categories:! M-Scope magnetic locators! Equipment components! How it works! Ground Penetrating Radar (GPR) units! Equipment components! How it works! GPR applications! GPR Advantages 1.3 Typical project guidelines 1.4 References! Useful websites! Glossary of terms 1.5 (APWA) Uniform color codes for temporary marking of underground utilities 3

4 1.0 Utility Surveys - Background Underground facilities have become increasingly complex and congested. Power and communication lines have joined water, sewer and gas distribution lines underground. Petroleum product and natural gas transmission lines have become more numerous and slurry product lines and cable television lines were added to the mix. A deregulation of telecommunication services added dozens of new underground lines for long distance carriers. Many new facilities were directly buried and fragile lines could be easily damaged by excavation or even by locating methods intended to prevent damage. Television cables of foam filled aluminum tubes could be easily dented with a resulting loss of use. Fiber optic telecommunication lines as small as a pencil may carry thousands of channels and could be cut with a shovel; usually these lines could not be readily detected with ordinary locating instruments. This made apparent the need for improved planning and design to minimize the potential for damage to facilities. As the number of installations increased, excavations increased and excavation damage to existing facilities began to soar. One-call systems were developed to reduce the number of telephone calls an excavator was required to make and to further encourage calling before digging. By 1970 the first one-call system began operating in Rochester, New York. Excavators were encouraged to contact facility owners/operators before excavating so that the locations of existing lines could be marked on the ground surface. Some states had adopted laws requiring various levels of excavation care. The emphasis was and still is to Call Before You Dig! The concept of designing excavations and facilities to avoid damage has developed slowly. Only a few states have included planning and design in their damage prevention laws. In many areas the use of the one-call center for planning and design is discouraged even prohibited as a one-call system service. However, planning and design must be recognized as an integral part of damage prevention and the one-call process. 4

5 1.1 Subsurface Utility Engineering (SUE) Equipment GPR UNIT M-SCOPE LOCATOR 5

6 WHAT IS SUE? Subsurface Utility Engineering DEFINITION: A branch of engineering practice that involves managing certain risks associated with...! Utility design! Utility coordination! Utility condition assessment! Utility relocation cost estimates! Utility relocation design and coordination! Utility mapping at appropriate quality levels! Implementation of utility accommodation policies! Communication of utility data to concerned parties [American Society of Civil Engineers, 2002] 6

7 ASCE PUBLICATION " Definitions of SUE " Quality Levels 7

8 WHY USE SUE?! EXISTING UTILITYS ABOUND! WE DON T KNOW WHERE MOST UTILITIES ARE!! EXISTING RECORDS OFTEN INACCURATE/INCOMPLETE! RISKS BECOME EXTREMELY DIFFICULT TO MANAGE Power Telecommunications Gas / Propane Petroleum Sanitary Sewer Drainage Steam Water Cable TV 8

9 WHY USE SUE? RISKS INVOLVED:! Project delays! Damage to utilities! Safety of workers! Safety of public! Redesign costs! Higher bids! Change orders! Extra work orders! Construction claims! Higher insurance costs! Higher financing costs! Detours! Bad publicity 9

10 WHY USE SUE? SUE BENEFITS PROJECT OWNERS AND UTILITIES IN THE FOLLOWING WAYS: " Unexpected conflicts can be avoided " The precise location of virtually all utilities is known and accurately shown on the construction plans. " Delays to the contractor during construction caused by unexpected encounters with utilities. " Delays caused by redesign when construction cannot follow the original design due to utility conflicts. " Contractor claims for delays resulting from unexpected encounters with utilities. " Accurate utility data is available to designers early enough in the development of a project to design around many potential conflicts. " Reduces costly relocations normally necessitated by construction projects. " Reduces delays to the project caused by waiting for utility work to be completed so construction can begin. " 10

11 WHY USE SUE? SUE BENEFITS PROJECT OWNERS AND UTILITIES IN THE FOLLOWING WAYS: " Safety is enhanced. " When excavation or grading work can be shifted away from existing utilities, there is less possibility of utility damage that might result in:!! Personal injury,! Property damage! Releases of product into the environment. 11

12 HOW DOES SUE WORK? SUE doesn t follow any set pattern. Rather it is tailored to individual projects. It essentially involves: systematically identifying the quality of utility information needed to design a project, and acquiring and managing that level of information. 12

13 HOW DOES SUE WORK? Four Quality Levels are defined in the ASCE Standard Quality Level-D Quality Level-C Quality Level-B Quality Level-A 13

14 HOW DOES SUE WORK? Quality Level-D! Records Research! Recollections 14

15 HOW DOES SUE WORK? Quality Level-C 15

16 HOW DOES SUE WORK? Quality Level-B Designating 16

17 HOW DOES SUE WORK? Quality Level-A! AS-BUILT/LOCATING! SOFT DIG 17

18 1.2 GEOPHYSICAL EQUIPMENT CATEGORIES: M-Scope magnetic locators 18

19 1.2 GEOPHYSICAL EQUIPMENT CATEGORIES: M-Scope magnetic locator components 19

20 M-SCOPE TECHNOLOGY HOW IT WORKS The science of cable and pipe locating is based on the principal that a current flowing along a conductor creates a magnetic field, and that magnetic field or signal, which is either passive or active in nature, can be detected via a receiver. A passive signal is one that is naturally occurring around a conductor, or in this case an underground utility. Some examples of passive signals include the following: 1. Current flowing in an electric supply cable. 2. Earth return current from power systems that use metal pipes or cable sheaths as a convenient conductor. 3. Radio frequency currents from very low frequency (VLF) radio transmissions that have penetrated the ground and flow along a buried utility. A passive sweep is performed to search for inaccessible, abandoned or unknown utilities using only a receiver. To perform a passive sweep, a survey grid is traversed in power mode, with the receiver blade in line with the direction of movement and at right angles to any utilities that may be crossed. When the receiver indicates the presence of a utility, it is pinpointed, traced and marked. The sweep is then continued until all detected utilities have been marked and the entire grid has been traversed in both directions. After completing the sweep, the entire process is repeated in radio mode to search for utilities that radiate VLF radio signals. Passive signals enable utilities to be located, but not identified, because the same signal may appear on multiple utilities within the grid. To solve this problem, an active signal must be applied to each individual utility line. An active signal is one that is intentionally generated by a transmitter. In this mode, the signal can be applied directly to the utility via direct connection or induction. This enables utilities to be identified, traced and their depth determined with a receiver. 20

21 M-SCOPE TECHNOLOGY HOW IT WORKS Direct connection involves plugging a connection cable into a transmitter output socket and connecting directly to the target line. This can be accomplished with connection leads or with a transmitter clamp. Connection leads are generally used to apply a signal to metallic conduits, sight lighting structures and metallic pipes. This is the preferred method for locating secondary electric, water and gas. Many electric, telephone and cable lines are housed within plastic conduits or buried into the ground without protection. In addition, directly connecting to these lines is usually too risky or forbidden. In such instances, a transmitter clamp is used to apply a signal to the cable without interrupting service to the line. The clamp is easy to apply, but the signal may not travel as far as it does with connection leads, and works best if the target line is grounded at each end.this is the method of choice for locating primary electric, telephone and cable lines. If an active signal cannot be applied to a line because it is inaccessible, an induction sweep must be performed. The transmitter contains an antenna, that when placed on the ground directly on top of a utility line, can induce a signal into it. The advantage of using induction is that a signal can be applied without access to the line and it is very quick and easy to use. The disadvantages are that induction efficiency is poor on deep targets, it is only useful at depths down to 6 feet and the signal can induce into lines other than the target. In addition, signal strength is often lost in the surrounding soil, the signal is shielded by reinforced concrete and a signal will not apply to a well-insulated line unless it is effectively grounded at each end. Despite its shortcomings, an induction sweep can sometimes successfully locate unknown or abandoned utilities when GPR results are inconclusive. An active signal cannot be applied to non-conductive (nonmetallic) utility lines. To combat this, a detectable duct rod or self contained transmitting sonde must be inserted into the line via a manhole, handhole, cleanout or catch basin. The disadvantages of this method are that some nonmetallic utility lines do not have access points or might be obstructed by detritus. Nonetheless, this is the best method for locating fiber optics, future use lines, sanitary sewer and storm sewer. 21

22 M-SCOPE TECHNOLOGY HOW IT WORKS 22

23 M-SCOPE TECHNOLOGY HOW IT WORKS M-SCOPE TECHNOLOGY HOW IT WORKS 23

24 M-SCOPE TECHNOLOGY HOW IT WORKS 24

25 M-SCOPE TECHNOLOGY HOW IT WORKS Passive signals Active signals 25

26 M-SCOPE TECHNOLOGY HOW IT WORKS 26

27 M-SCOPE TECHNOLOGY HOW IT WORKS 27

28 1.2 GEOPHYSICAL EQUIPMENT CATEGORIES: GROUND PENETRATING RADAR (GPR) COMPONANTS CONTROL UNIT & POWER SUPPLY ANTENNA 28

29 1.2 GPR TECHNOLOGY HOW IT WORKS Quite often, non-metallic, inaccessible, unknown or abandoned utilities cannot be located with traditional cable and pipe locators. When this occurs, Ground Penetrating Radar (GPR) must be used in conjunction. GPR is a non-invasive, nondestructive geophysical surveying technique that is used to produce a cross-sectional view of objects embedded within the subsurface. The GSSI SIR-3000 is the industry s number one choice for data accuracy and versatility to perform GPR surveys. All GPR units consist of three main components: a power supply, control unit and antenna. To understand how GPR works, we must first understand the performance of a scan. A scan is performed by moving the antenna across the surface linearly to create a series of electromagnetic pulses over a given area. During a scan, the control unit produces and regulates a pulse of radar energy, which is amplified and transmitted into the subsurface at a specific frequency by the antenna. Antenna frequency is inversely proportional to penetration depth, which makes antenna selection the most important step in the survey design process. Typical scan profile 29

30 1.2 GPR TECHNOLOGY HOW IT WORKS Antenna frequencies, their application and maximum penetration depth. 30

31 1.2 GPR TECHNOLOGY HOW IT WORKS During a scan, the control unit records the strength and time required for the return of any reflected energy. Reflections are produced in the data screen profile (on the control unit) whenever the energy pulse enters and exits contrasting subsurface materials. The way it responds to each material is determined by two physical properties: dielectric constant and electrical conductivity. The dielectric constant is a descriptive number that indicates how fast electromagnetic energy travels through a material. Energy always moves through a material as quickly as possible, but certain materials slow down the energy more than others. The higher the dielectric, the slower the energy will move through the material, and vice versa. Below is a list of some common materials with their corresponding dielectric constants and velocity values. 31

32 GPR TECHNOLOGY HOW IT WORKS To determine the location of a subsurface target in the data screen profile, there must be a contrast between the dielectric values of the material one is scanning through and the target one is searching for. For example, a pulse moving from dry sand (dielectric of 5) to wet sand (dielectric of 30) will produce a strong, highly visible reflection, while moving from dry sand (5) to limestone (8) will produce a weak one. In addition, if one knows the dielectric value of the subsurface material one is scanning through, the control unit can measure the amount of time required to receive the reflected signal and convert this to depth. Since the GPR emits electromagnetic energy, it is subject to attenuation (natural absorption) as it moves through a material. Energy moving through resistive (less conductive) materials such as dry sand, ice or dry concrete will penetrate much further than energy moving through absorptive (more conductive) materials such as salt water or wet concrete. As a result, the greater the contrast in electrical conductivity between the material one is scanning through and the target one is searching for, the brighter the reflection; high conductive materials such as metals produce the brightest reflections. To understand how dielectric and electrical conductivity differences translate into visual data requires an understanding of how the antenna emits energy. Imagine the antenna scanning perpendicular to a pipe. Energy emits from the antenna in a 3-dimensional cone shape, not in a straight line as one might think. The two-way travel time for energy at the leading edge of the cone is longer than for energy directly below the antenna. 32

33 1.2 GPR TECHNOLOGY HOW IT WORKS Because it will take longer for energy at the leading edge to be captured, when the antenna first approaches the pipe, it will appear low in the data screen profile. As the antenna moves closer to the pipe and the distance between them decreases, the reflections will appear higher in the profile. At the point where the antenna is located directly above the pipe, the minimum distance of separation is reached and the reflections reach their zenith. As the antenna moves away from the pipe and the distance between them increases, the reflections appear lower in the profile once again. After the scan is completed, the center of the pipe will appear in the data screen profile as an upside down U, which is referred to as a hyperbola. HYPERBOLA 33

34 1.2 GPR TECHNOLOGY HOW IT WORKS To gather, organize and present the data, a series of scans are performed within an orthogonal survey grid. At the end of each scan, the data screen profile is reviewed for the presence of hyperbolic targets. If present, the antenna is moved backward to place a cursor (which depicts the center of the antenna) on the center of the targets. The location and depth of the targets are then marked on the surface with chalk, paint and/or flags. Once the entire survey grid has been scanned, the marks are reviewed to search for patterns similar to that of the desired targets, in this case a pipe. Any marks that run in straight line and whose hyperbolas appear to be highly conductive metal targets are then connected, thereby displaying the location and depth of the pipe. 34

35 1.2 GPR TECHNOLOGY HOW IT WORKS LIMITATIONS: The depth range of GPR is limited by the electrical conductivity of the ground, the transmitted center frequency and the radiated power. As conductivity increases, the penetration depth also decreases. This is because the electromagnetic energy is more quickly dissipated into heat, causing a loss in signal strength at depth. Higher frequencies do not penetrate as far as lower frequencies, but give better resolution. Optimal depth penetration is achieved in ice where the depth of penetration can achieve several hundred meters. Good penetration is also achieved in dry sandy soils or massive dry materials such as granite, limestone, and concrete where the depth of penetration could be up to 15 m. In moist and/or clay-laden soils and soils with high electrical conductivity, penetration is sometimes only a few centimeters. Performance is also limited by signal scattering in heterogeneous conditions (e.g. rocky soils). Other disadvantages of currently available GPR systems include: Interpretation of radargrams is generally non-intuitive to the novice. Considerable expertise is necessary to effectively design, conduct, and interpret GPR surveys.ground-penetrating radar antennas are generally in contact with the ground for the strongest signal strength; however, GPR air launched antennas can be used above the ground. 35

36 1.2 GPR TECHNOLOGY HOW IT WORKS APPLICATIONS & ADVANTAGES: " It is a safe and harmless method for accurately detecting hidden elements in concrete. " GPR is safer and unequivocal compared to X-Raying, mostly because GPR equipment is not dangerous to use around people without any safety constraints (radiation) or setup requirements. " GPR can locate the position of buried utilities, tension cables, rebar, and electrical conduits embedded in concrete superstructure and/or foundations. " GPR eliminates dangers associated with cutting or drilling and the high expenses required for their repair from exploring through excavation and cutting. " Accurate target location within a concrete slab on-grade, wall, or supported slab can be achieved more quickly, safely, and economically with GPR instead of other existing techniques. " GPR is the most accurate system on the market for the non-destructive location of all subsurface utilities. " Utility detection metallic and non-metallic " Concrete inspection locate metallic and non-metallic targets in walls, floors Structure inspection bridges, monuments, walls, towers, tunnels, balconies, garages, decks. " Geological investigation " Void Location " Archaeology " Forensics " Road inspection " Measure slab thickness 36

37 TYPICAL PROJECT GUIDELINES # Interview the client or individual familiar with the site prior to mobilization. # Obtain any available utility record information from client. # Review aerial photography online prior to visiting the site. # Gather information on soil types at project site. (sandy soils, clays) # # Inconsistencies in pavement surfaces may be evidence of previous trenching or utility repair work. # Conduct a visual inspection on site to search for utility poles, manholes, handholes catch basins, conduits, cleanouts, water valves and gas valves within the survey area and beyond the immediate limits of the project site. 37

38 Useful websites 1. Geophysical Survey Systems, Inc.: 2. North American Database of Archaeological Geophysics: 3. United States Department of Agriculture Soils Website: 4. USDA-Natural Resources Conservation Service, Ground Penetrating Radar Program: New Jersey One Call 811 or Equipment Rental Companies 1. Geophysical Applications East\Northeast Holliston, MA SAIC/Environmental Equipment & Supply East /Northeast Harrisburg, PA

39 1.5 The American Public Works Association (APWA) Uniform Color Codes for temporary marking of underground utilities Utility color codes are used for identifying existing underground utilities in construction areas with the intent of protecting them from damage during excavation. Public utility systems are often run underground; some by the very nature of their function, others for convenience or aesthetics. Before digging, local governments often require that the underground systems' locations be denoted and approved, if it is to be in the public right-of-way. Colored lines at and/or flags are used to mark the location and denote the type of underground utility. A special type of spray paint, which works when the can is upside-down, is used to mark lines, often in a fluorescent color. On flags, a logo often identifies the company or municipal utility which the lines belong to, or an advertisement for a company which has installed an irrigation system for lawns or gardens. In this case, each sprinkler head is usually marked, so that landscaping crews will not cover or bury them with soil or sod, or damage them with tractors or other construction equipment while digging holes for trees, shrubs, or other large plants or fenceposts. This is also important because a vehicle (tractor, truck, or otherwise) can break a sprinkler or the hard-pvc pipe or joint it is mounted on simply by driving over it, particularly on newly-moved soil which is uncompacted and therefore unsupportive of such weight. 39

40 Existing Underground Utilities Abound! 40

41 Glossary Antenna: a paired transmitter and receiver that sends electromagnetic energy into a material and receives any reflections of that energy from materials in the ground. Also called a transducer. Antennae are commonly referred to by their center frequency value (i.e. 400MHz, 1.5Ghz). This frequency determines the depth of penetration and the size of the objects or layers visible. Attenuation: the weakening of a radar pulse as it travels through different materials. Center Frequency: the median transmit frequency of an antenna. The antenna will also transmit energy at a frequency range of times its center value. For example, a 400 MHz antenna may actually transmit at a range from MHz. Clipping: occurs when the amplitude of a reflection is greater than the maximum recordable value. The system disregards the true value of the reflection and writes in the maximum allowable value. Clipping appears in the O-Scope as signal that goes off the scale at the sides of the window. Dielectric permittivity: the capacity of a material to hold and pass an electromagnetic charge. Varies with a material s composition, moisture, physical properties, porosity, and temperature. Used to calculate depth in GPR work. EM: Acronym for electro-magnetic. FCC: Acronym for Federal Communications Commission. The United States governmental body that oversees the UWB industry of which GPR is a part. Gain: amplifying the signal to certain section of a radar pulse in order to counteract the effects of attenuation and make features more visible. GHz: Acronym for Gigahertz. A measurement of frequency equal to one billion cycles per second. GPR: Acronym for Ground Penetrating Radar. Ground-coupling: the initial entry of a radar pulse into the ground. Hyperbola: an inverted U. The image produced in a vertical linescan profile as the antenna is moved over a discrete target. The top of the target is at the peak of the first positive (white in a grayscale color table) wavelet. Interface: the surface separating materials with differing dielectric constants or conductivity values. KHz: Acronym for Kilohertz. A measurement of frequency equal to one thousand cycles per second. Linescan: commonly used method of depicting a radar profile. Linescans are produced by placing adjacent scans next to each other and assigning a color scheme to their amplitude values. 41

42 Glossary Macro: a preset list of processing options that may be applied to perform repetitive functions on an entire dataset. Macros may be created and edited to include different functions (see RADAN manual for addition information). Mark: point inserted along a survey line manually by the operator or at preset intervals. MHz: Acronym for Megahertz. A measurement of frequency equal to one million cycles per second. Migration: mathematical calculation used to remove outlying tails of a hyperbola and to accurately fix the position of a target. Nano-second: unit of measurement for recording the time delay between transmission of a radar pulse and reception of that pulse s reflections. Equal to one one-billionth of a second. Noise: unwanted background interference that can obscure true data. Noise floor: time depth at which the noise makes target identification impossible. ns: see Nano-Second. Oscilloscope: device used to view and measure the strength and shape of energy waves. Common term in GPR industry for a method of data display showing actual radar wave anatomy. Passband: the frequency range at which the antenna is emitting energy. It is roughly equivalent to times the center frequency. RAM: Acronym for random access memory. Temporary memory in which a computer stores information used with a running program, or temporarily stored data before it is written to a hard drive. Range: the total length of time (in nanoseconds) for which the control unit will record reflections. Note: indicates two-way travel time. RF: Acronym for radio frequency. Sample: a radar data point with two attributes: time and reflection amplitude. A third attribute, position, is assigned by the user. Undersampling will produce a scan wave that does not contain enough information to draw a smooth curve. It may miss features. Over-sampling will produce a larger data file. 42

43 Glossary Samples/Scan: the number of samples recorded from an individual radar scan. Commonly set to 512. Scan: one complete reflected wave from transmission to reception, sometimes called a trace. Survey wheel: wheel attached to an antenna and calibrated to record precise distances. Necessary for accurate data collection. Time-slice: a horizontal planview of amplitude values drawn from adjacent vertical profiles. The time-slice is produced for a particular timedepth and is vital for understanding the horizontal positions of features in a survey area. Time window: the amount of time, in nano-seconds, that the control unit will count reflections from a particular pulse. Set by the operator. Transect: a line of survey data. An area is systematically surveyed by recording transects of data at a constant interval. The transects are then placed in their correct position relative to each other in a computer and horizontal time-slices are produced. UWB: Acronym for Ultra-Wide Band. Refers to the wide frequency band of emissions put out by a GPR device. Wiggle trace: method of GPR data display showing oscilloscope trace scans placed next to each other to form a profile view. Commonly used method in seismic studies. 43

44 NOTES 44