What lies beneath?

February 2005 » Feature Articles
Advances in remote sensing technologies, namely groundpenetrating imaging radar (GPiR) and electromagnetometry (EM), have created invaluable solutions for engineering and construction. They are useful for sensing and mapping underground conditions, detecting underground objects, and differentiating between man-made and natural components and conditions. When underground investigation is needed, time, money, and danger top the list of why these new technologies may be more desirable than traditional investigative methods. Additionally, no area is too small or too large to take advantage of these technologies.

Modern technologies revamp underground investigations

BY JEAN CHILDERS, ATM-B

Advances in remote sensing technologies, namely groundpenetrating imaging radar (GPiR) and electromagnetometry (EM), have created invaluable solutions for engineering and construction. They are useful for sensing and mapping underground conditions, detecting underground objects, and differentiating between man-made and natural components and conditions. When underground investigation is needed, time, money, and danger top the list of why these new technologies may be more desirable than traditional investigative methods. Additionally, no area is too small or too large to take advantage of these technologies.

For example, the U.S. military is using GPiR and EM to find unexploded ordnance and munitions around the world. Digging in expansive range fields, where the locations of potential threats are unknown, could be deadly, so using remote sensing makes sense.

Additionally, military applications often require that retrieved data be stored, manipulated, and used with various software packages.

GPiR and EM address these needs with more sophisticated output than traditional approaches. (Learn about how AMEC is helping the military meet its needs with geophysical technologies in "On TARGET with the military," below) Remote sensing technologies are delivering modern solutions with modern flair for underground investigations. Among the various technologies within the realm of this developing arena, GPiR and EM provide substantial opportunities for civil engineers to learn more about "what lies beneath." Detecting buried objects with GPiR GPiR imaging is a non-invasive imaging technique used to map geologic structural and stratigraphic features, as well as man-made objects, in the near subsurface. It is used often when as-built information is unavailable.

GPiR can be used in real time to find out what is underground, under a slab, or under a floor. Also, when the accuracy of as-built information is questionable, or when as-built information is obsolete, GPiR is the clear choice for obtaining accurate information.

Furthermore, GPiR surveys routinely are used to locate metallic and nonmetallic underground utilities. The high-resolution capabilities of GPiR allow detection of closely spaced utilities, which are common in urban areas.

GPiR is based on the principle that electromagnetic energy (radio waves) will echo back to the ground surface after encountering stratigraphic changes or objects within the earth's subsurface. Mapping a series of echoes generated by the GPiR waves creates a continuous image of the subsurface.

During a GPiR survey, high frequency pulsed radio waves (generally ranging between 10 and 1,000 megahertz) are transmitted into the earth using an antenna. When the subsurface geology changes, or when a man-made object is encountered, some of the energy in the radio waves is reflected back to the surface, while the remaining energy penetrates deeper into the earth. A receiving antenna at the ground surface measures the changes in the returning radio waves.

Conductivity (the ability to conduct electrical current) is the primary physical property affecting the transmission of radio waves in the ground. Conductivity contrasts, which create GPiR echo boundaries, can be caused by changes in lithology, moisture content, void space, and contaminant distribution, as well as by buried objects.

Buried utilities are identified in GPiR records by the distinctive shape of an inverted U (hyperbola). Radio wave reflections from a buried pipe, for example, will take longer to return to the ground surface the farther the GPiR receiving antenna is away from the pipe. The reflection time is shortest when the receiver is directly over the pipe.

The hyperbolic responses of underground utilities in GPiR records are not just useful for target identification; they also are used to calculate buried depth. Radio waves travel at different speeds within different media. For GPiR signals, air is faster than rock, which is faster than soil, which is faster than water. The shape of hyperbolas is determined by the speed of the material in which the utilities are buried.

Software is used to model hyperbolic shapes on the GPiR data plots. By adjusting the model shape on buried utility signatures in GPiR records, an estimate of velocity is calculated.

As illustrated in Figure 4, the depth of an underground utility can be calculated by measuring the travel time of GPiR echoes. When the GPiR receiver antenna is located directly above an object (x=0), the following equation is used to calculate depth: D = (T / 2) * V where D is depth in feet, T is travel time in nanoseconds, and V is velocity in feet per nanosecond.

The travel time is divided by 2 in the depth equation because the GPiR system measures the total travel time of radio wave echoes from the ground surface to the buried object and back again.

Because travel time is measured directly in the GPiR data, and velocity is determined from hyperbolic fitting, an accurate calculation of depth for buried utilities can be made. Modern GPiR systems are integrated with GPS antennas, which give GPiR surveys the ability to map the exact X, Y, and Z positions of underground targets.

Electromagnetometry EM measures the response of an electromagnetic field induced into the earth. Low frequency (1 to 10 kilohertz) signals are transmitted by a small coil. The transmitter produces low-frequency, long-wavelength electromagnetic fields. Where there is electrically conductive media in the earth, these electromagnetic fields induce current flow.

This induced current flow then produces secondary electromagnetic fields that radiate back to the surface. A receiving coil detects the secondary field and measures its strength and phase relative to the transmitted signal.

The data are presented as the relative amplitude of the secondary signal, in parts per million.

The depth of penetration of the transmitted field is a function of the frequency of operation. Lower frequencies penetrate deeper, while higher frequencies are attenuated more rapidly. Because the penetration depth is frequency-dependent, it provides the opportunity to interpret multi-frequency EM data to evaluate the depth and size of targets.

EM is an important geophysical tool in the detection of unexploded ordnance. Other common applications include detection of buried metal objects, site investigation, archaeological investigation, and geotechnical and environmental site characterization.

Conclusion Geophysical technologies can be used in concert with each other to create more robust results. For example, using GPiR and EM technologies together generates complementary data that is of higher confidence. This is critical for applications such as military range clean-up.

Although the military has special concerns -- such as life safety and an urgent need for information -- which are not found in typical engineering projects, many non-military projects benefit from GPiR and EM. As knowledge and confidence in these technologies increase, new applications will be discovered and applied on military and non-military projects.

Jean L. Childers, ATM-B, is a freelance writer and the Account Operations Manager at AMEC, in Minneapolis. She can be reached at jean.childers@amec.com. For information on the TARGET Program, please contact Michael F. Warminsky, P.E., at mike.warminsky@amec.com.

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On TARGET with the military

The U.S. military has been tasked with assessing all of its ranges and determining appropriate cleanup approaches by 2008.

Range assessment, as well as subsequent clean up, require using GPiR and EM technologies to find unexploded ordnance, as well as to investigate and map cultural and natural resources.

Therefore, AMEC has developed a program specifically designed to work with Department of Defense (DoD) customers. Called the Total AMEC Range Group/Emerging Technologies (TARGET) Program, its goal is to provide a core technical team with range-specific expertise to all areas of the military and serve the DoD's diverse needs. The program includes range remediation, unexploded ordnance detection, and military munitions response.Other TARGET services include range design, range sustainment, impact analysis, range construction, and emerging technology evaluation/validation.

The TARGET program is serving the military well. For example, at one U.S. military base, AMEC staff applied GPiR and EM technologies to locate both metallic and non-metallic ordnance items,as well as to find underground man-made objects. An initial GPiR test survey in an ordnance disposal pit found underground objects, and these early results were mapped in 3-D.This data is being used to focus subsequent assessment on areas of likely contamination.

At another base,an archeological site investigation and Phase I Cultural Resource Survey were accomplished with GPiR. Additionally, AMEC's TARGET team expedited an emergency task order to locate an historic building foundation.

Its efforts were recognized by the base's Directorate of Public Works,who awarded AMEC its Golden Castle Award for the project.

Besides efforts to help keep military personnel safe in times of peace,AMEC aims to expand its reach to those in battle.

Consider the example of modern buried armament.It is made up of metallic artillery shells that are strung together and buried along roadsides.

The armament is detonated by pressureplate technology or by enemies using remote control. An RST survey conducted by remote control before troops enter an area could find buried armament and keep people safe.

AMEC is researching remote-controlled allterrain vehicles (ATVs) that could be equipped with GPiR and tow-behind EM carriages. The remote control stations could be located as far away as 2 to 3 miles from the survey site area.

The ATVs could have front- and rear-mounted cameras for real-time viewing of survey area conditions,as well as global positioning systems (GPS) for precise control. Troops in the combat zones,trained in advance on operating the ATVs and gathering data, would send the data to AMEC for processing, and results would be returned in a matter of hours.

Other ideas include using airborne GPiR systems to gather data across large areas. GPIR systems mounted on military aircraft could help locate large caches of metallic weaponry. GPS coordinate systems could be used to locate all anomalies precisely. Data could be gathered and processed in the same manner as described for the remote-controlled ATVs.

AMEC sees great promise in the use of GPiR and EM in many types of military applications.

GPiR, EM, and other remote sensing technologies reward users with timely, accurate information.

They also may save lives.


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