Locating Technology

Acoustic Pipe Locating - A Future Trend for Challenging Situations?

o matter the technology used to locate buried pipes and cables today, there is no single tool that can do it all. Just as a carpenter’s tool box con-

sists of application-based tools like the hammer, chisel and screwdriver, the role of the utility locator also calls for more than one tool to be effective in the field. To fully un-derstand what sets these different tools apart, and when a specific tool should be selected over another, one must first understand the applications and limi-tations of currently avail-able locating equipment.

Today, the most commonly used methods to locate bur-ied infrastructure are elec-tro-magnetic (EM), ground penetrating radar (GPR), and acoustic locators. This article will highlight the technical limitations of each technol-ogy and in the process, show what sets the Sensit Ultra-Trac Acoustic Pipe Locator apart as a truly essential tool for the locator’s toolbox.

Electromagnetic Utility LocatorsIn 1931, Gerhard Fischer in-vented the first handheld utility locator for commercial use, to primarily locate buried metal-lic pipes and cables. Electro-magnetic (EM) pipe and cable locators use electricity to create a magnetic field to trace the path of buried metallic pipes and ca-bles. EM locators can only detect metallic pipes, cables and wire. They are commonly called conventional locators, due to their widespread use. Conventional EM utility loca-tors have two main parts: the transmitter and the receiver. The transmitter functions as a miniature power plant and is used to transmit alternating current to energize a metallic pipe, cable or wire. The byproduct of alternating current flowing on a metallic conductor is a magnetic field that may be detected by the receiver.

Limitations of EM Locators – Other metal present near the target line can cause

distortion, causing error in the reading. This could be anything metallic, such as a parallel utility line, a metal building or a vehicle.

– EM locating equipment can only be used to locate metallic utili-ties. It cannot be used to find non-metallic lines unless a tracer wire is present (for plastic or con-crete-asbestos pipe, for example) and is metallically continuous.

– EM locating equipment cannot tell what type of utility is be-ing located. The operator must verify the utility type by either potholing or tracing the utility line structure to structure.

Ground Penetrating RadarThe first large-scale applica-tion for Radio Detection and Ranging (RADAR) was used during World War II by the British and American mili-tary, to detect electromag-netic pulses reflected by aircraft. Ground Penetrating Radar (GPR) was first used to determine the depth of a glacier in Austria in 1929. Today, despite its common

limitations due to soil condi-tions, modern use of GPR to lo-

cate buried utilities has increased in popularity, due to the ability to find both metallic and non-metallic lines.

Environmental FactorsThe dielectric constant of the media or substrate being scanned determines the amount of signal that is absorbed by the substrate through attenuation. Because soil type determines this factor, soil conditions must be optimal for GPR to work. Soil moisture content greatly affects

the GPR signal. In general, dry soil is better than wet. Also known as permittiv-ity, the dielectric constant is frequency dependent for GPR. The higher the fre-quency, the better the reso-lution with shallower depth penetration. Conversely, the lower the frequency, the bet-ter the depth penetration with lower resolution. The size and material of the target line will also impact the ability to be seen using GPR.

The soil type and moisture content definitely matter for GPR to successfully locate bur-ied pipes. Ranked from best to worst: air, solid ice, rock, sand, silt, and finally clay. As shown in Fig. 2, most areas of the United States do not have optimal soil conditions for the effective de-ployment of GPR.

GPR Limitations– Soil type plays a major role in the ability to find pipes

and cables using GPR.

– The greater the depth, the greater the target utility size needs to be. Smaller pipes and cables may not be found using GPR.

– Certain types of pipe materials simply cannot be seen by GPR in any soil type, at any depth.

– GPR cannot tell the type or material of the buried utility line. It must be verified by potholing or by tracing the utility line from structure to structure.

Acoustic Pipe Locator (APL)Originally invented by the Gas Technology Institute (GTI), SENSIT Technologies acquired the rights for commer-cialization and production in 2011. Since the first ULTRA-TRAC® Acoustic Pipe Locator (APL) rolled off SENSIT’s production line, many advancements have been made based on input from end users. The latest advancement is the incorporation of a Windows-based tablet into the unit to improve user friendliness and reduce false read-ings. The APL can be used to find metallic and non-metal-lic pipes and conduit, in any soil type, to depths of 15-30 feet. The APL provides an alternative and supplemental method of locating buried pipes. The APL transmits and receives acoustic sound waves and then looks for differ-

ences in acoustic impedance in the soil caused by pipes, cables, ducts and other buried infrastructure using a pro-cess known as impedance mismatch. The APL is able to locate buried utilities, regardless of material type, broken tracer wire and soil conditions.

The chassis, or foot of the unit, houses the battery, elec-tronic components and send-and-receive sensors. Locat-ed near the front of the chassis, the actuator sends a series of sound waves, or ‘pings’ into the ground. To the rear of the chassis, dual matched accelerometers receive the sound waves once they have been reflected from a buried pipe, cable, or duct.

How APL Works(See Fig. 1) With the push of the APL’s scan button, sound pressure waves are sent into the ground. A series of pings are delivered at a single location, known as a slice. A series of slices is known as a scan. A scan must consist of at least five slices, to allow internal software to calculate the location of a buried utility line. A minimum of three rows, spaced 5-25 feet apart, must be used to conduct a utility survey grid. A pattern will emerge in the survey, identifying the possible location of a buried pipe. The tablet displays this information to the operator on-the-fly and is used to mark locations on the ground. Survey image data can also be stored for use as a client deliver-able, emailed to a supervisor for verification, or stored for record keeping purposes.

N

C - 36%

M - 29%

Y - 98%

K - 3%

Hex #2c5c64

C - 84%

M - 51%

Y - 50%

K - 25%

Fig. 2 - Ground Penetrating Radar Suitability Map

PineBluff

LittleRock

FortSmith

Fayetteville

Yuma

Tucson

Phoenix

San Jose

SanFrancisco

SanDiego

Sacramento

LosAngeles

Fresno

Pueblo

Longmont

Greeley

FortCollins

Denver

ColoradoSprings

Boulder

WaterburyNewHaven

Danbury

Bristol

Bridgeport

Wilmington

Dover

Tampa

Orlando

Miami

Jacksonville

Fort

Lauderdale

Macon

Atlanta

Albany

WaterlooSiouxCity

Omaha

IowaCity

Dubuque

DesMoines Davenport

CedarRapids

Pocatello

IdahoFalls

BoiseCity

Rockford

Chicago

TerreHaute

Muncie

Indianapolis

FortWayne

Evansville

Bloomington

Anderson

Wichita

Topeka

St.Joseph

Lawrence

Owensboro

LouisvilleLexington

Huntington

Shreveport

NewOrleans

Monroe

LakeCharles

Lafayette

BatonRouge

Alexandria

Worcester

Springfield

New Bedford

LowellLawrence

BrocktonBoston

Baltimore

Portland

Lewiston

Bangor

ToledoSouth Bend

LansingGrandRapids

Flint

Detroit

AnnArbor

St.Cloud

Rochester

Minneapolis

La Crosse

GrandForks

FargoDuluth

Springfield

St.Louis

KansasCity

Columbia

Pascagoula

Jackson

Hattiesburg

Biloxi

Missoula

GreatFalls

Billings

Winston-

Salem Raleigh

High Point

Fayetteville

Durham

Charlotte

Bismarck

Portsmouth

Manchester

Trenton

Santa Fe

LasCruces

Albuquerque

Reno

LasVegas

Rochester

NewYork

Buffalo

Wheeling

ParkersburgDayton

Columbus

Cleveland

Cincinnati

Akron

Tulsa

OklahomaCity

Lawton

Salem

Medford

Eugene

PittsburghPhiladelphia

Providence

Newport

Sumter

Spartanburg RockHill

Greenville

Columbia

Charleston

SiouxFalls

RapidCity

Nashville

Memphis

Knoxville

JohnsonCity

Jackson

Clarksville

Chattanooga

SanAntonio

Lubbock

Houston

El Paso

Dallas

CorpusChristi

Austin

SaltLakeCityProvo

Ogden

Logan

Washington

DC

Roanoke

Richmond

NorfolkLynchburg

Burlington

Yakima

Tacoma

Spokane

Seattle

Portland

Bellingham

Sheboygan

Racine

Oshkosh

MilwaukeeMadison

KenoshaJanesville

GreenBay

EauClaire

Appleton

Charleston

Cheyenne

Casper

Tuscaloosa

Montgomery

Mobile

Huntsville

Gadsden

Dothan

Decatur

Columbus

Birmingham

Lincoln

WY

WI

WV

WA

VA

VT

UT

TX

TN

SD

SC

RI

PA

OR

OK

OH

ND

NC

NY

NM

NJ

NH

NV NE

MT

MO

MS

MN

MI

MA

MD

ME

LA

KY

KS

IA

IN

IL

ID

GA

FL

DE

CT

CO

CA

ARAZ

AL

Ground-Penetrating Radar Suitability Map

HAWAII

PUERTO RICO &U.S. VIRGIN ISLANDS0 50 100 150 200 25025

Kilometers0 50 100 150 20025

Kilometers

0 10 20 30 405

Kilometers

USDA-NRCS. 2009. Ground Penetrating Radar Suitability - US (map). Using ArcGIS, Version 9.2 (Environmental Systems Research Institute, Inc., Redlands, Ca.). National Soil Survey Center, Lincoln, Nebraska. Scale 1:1,500,000. Map projection for continental U.S. using Albers Equal Area, North American Datum 1983 (NAD83). Map projection for Hawaii using Hawaii State Plane NAD83. Map projection for Puerto Rico and U.S. Virgin Islands NAD83. USDA-NRCS. 2008. Digital General Soils Map (GSM) version 2. Continental United States, Hawaii, Puerto Rico and U.S. Virgin Islands. Soil Data Mart Source (http://soildatamart.nrcs.usda.gov). December 2008 edition. Soil Survey Staff. 2009. NSSC DATA – Ground Penetrating Radar Suitability Index (GPRSI) [data file] - National Soil Information System (Evaluation Draft - 02/2009). USDA Natural Resource Conservation Service, National Soil Survey Center, Lincoln, Nebraska. (http://soils.usda.gov). Current State and Equivalent, TIGER/Line 2008 (cartographic boundary file, tl_2008_us_state.zip). 2008. U.S. Census Bureau. Available FTP: ftp://ftp2.census.gov/geo/tiger/TIGER2008/. [Accessed on February 20, 2009] Urban Areas (generalized cartographic boundary file, ua99_d00_shp.zip ). 2000. U.S. Census Bureau. Available FTP: http://www.census.gov/geo/cob/bdy/ua/ua00shp/. [Accessed on February 20, 2009] USGS. Analytical Hillshade computed from 1 kilometer National Elevation Dataset (NEDS) using the following parameters: 315 degrees altitude, 45 degrees azimuth, and z factor 1x. Prepared by USDA-NRCS-NSSC, Lincoln, NE.

Water

Not Digitized

International Border

State Line

Interstate Highway

1 Very High

Not Rated

6 Unsuited

5 Very Low

4 Low

3 Moderate

2 High

GPR Index

Urban Areas

Large Water Body

Fig. 1 - How Acoustic Pipe Locators Work

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APL Limitations– As with GPR and EM, the op-

erator should start a locate at a known facility location or surface structure, to verify readings on the APL display.

– As with EM and GPR, the APL cannot tell what type of facility is being located. The operator must verify the utility type by potholing or tracing the utility line structure to structure.

ConclusionGiven their inherent limitations, a toolbox consisting solely of EM and GPR locating equipment simply cannot find everything on their own, unless the target utility is metallic, or has an unbroken tracer wire for EM, or soil conditions are optimal for the use of GPR. As the USGS soil map shows, in the contigu-ous United States, optimal soil conditions for effective use of GPR may not be present in all areas. Pairing EM equipment with the APL enables the end user to find both metallic and non-metallic utilities, in a broad range of soil types.

The Sensit APL (Fig. 4) provides the operator with an essential tool to locate metallic and non-metallic pipes, cables and ducts, regardless of broken tracer wire and soil type. Its ease of operation, coupled with the ability to display and store scan data, makes the APL an excel-lent choice for anyone trying to find buried pipes, cables and ducts. Utility locators, engineers, surveyors, energy companies and municipalities alike, are currently de-ploying acoustic locating technology worldwide with the Sensit APL.

Find out more at GasLeakSensors.com or call us at 1-219-465-2700 to schedule a demo.

Fig: 3 - This contour map, known as ScanView, can be viewed after every scan to better understand the ground profile. Though a single scan is not an adequate search, it can provide important insight based on the strength of the reflected sound waves.

Fig: 4 - The SENSIT ULTRA-TRAC® Acoustic Pipe Locator (APL). The APL provides an alternative and supplemental method of locating buried pipes.

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