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Drainage pipe detector: Ground penetrating radar shows promise in locating buried systems.

One of the more frustrating problems confronting farmers and land improvement contractors in the Midwestern United States involves locating buried agricultural drainage pipes. Conventional geophysical methods, particularly ground penetrating radar (GPR), presently being used for environmental and construction engineering applications can potentially provide a solution to this problem.

Scientists and engineers from the USDA-Agricultural Research Service, The Ohio State University (OSU) ElectroScience Laboratory and the OSU Department of Geological Sciences collaborated in an extensive study funded by the Ohio Agricultural Research and Development Center. They found that GPR was quite successful in detecting clay tile and corrugated plastic tubing drain pipe down to depths of around 3 feet (1 meter) in a variety of different soil materials.

GPR grid surveys were conducted in Ohio at 11 test plots containing subsurface drainage systems. The technology proved to be 81 percent effective in locating the total amount of pipe present.

Nature of the problem

According to a 1985 USDA-Economic Research Service survey, Midwestern states have about 31 million acres (12.5 million hectares) of mostly cropland containing subsurface drainage systems. This does not include an extensive amount of drainage pipe installed since 1985. The magnitude of acreage involved indicates how crucial subsurface drainage is to the Midwestern farm economy. Without it, excess soil water could not be removed, making current levels of crop production impossible to achieve.

Before the 1960s, agricultural drainage pipe was constructed primarily of clay tile, and to a lesser extent, concrete tile. Clay tile drainage pipes were fabricated typically in 12-inch (30-centimeter) long segments with diameters of either 4 or 6 inches (10 or 15 centimeters). Segments were laid down end-to-end in an excavated trench and buried at depths usually between 1.5 and 3 feet (0.5 and 1 meter).

In the 1960s, clay and concrete tile were superseded by corrugated plastic tubing. In the United States, high-density polyethylene tubing is extruded and then packaged in long coils, commonly 4 inches (10 centimeters) in diameter. The corrugations provide bearing strength along with pipe flexibility, allowing installation equipment to concurrently trench, place and backfill the pipe. Burial depth averages about 3 feet (1 meter).

Increasing the efficiency of soil water removal that already contains a functioning subsurface drainage system often requires reducing the average spacing distance between drain lines. This is typically accomplished by installing new drain lines between the older ones. By keeping the older drain lines intact, less new pipe is needed and costs are substantially reduced. However, the older drain lines need to be located.

Finding drainage pipe is not easy, especially with systems installed more than a generation ago. Often, records have been lost, and the only clue is a single pipe outlet extending into a water conveyance channel.

Without precise location records, finding a drain line with heavy trenching equipment causes pipe damage, often costly to repair. The alternative, using a hand-held tile probe rod, is tedious at best. Satellite or airborne remotesensing technologies have their various limitations.

The way GPR works

GPR directs an electromagnetic radio energy (radar) pulse into the subsurface, followed by measurement of the elapsed time taken by the signal as it is travels downward from the transmitting antenna, partially reflects off a buried feature and eventually returns to the surface, where it is picked up by a receiving antenna. Reflections at various depths from different features produce a signal trace, which is a function of radar wave amplitude versus time. Antenna frequency, soil moisture conditions, clay content, salinity, and the amount of iron oxide present all have a substantial influence on the depth beneath the surface to which the radar signal can penetrate.

Differences in the dielectric constant across a discontinuity govern the amount of reflected energy returning to the surface. For buried drainage pipe, these discontinuities may include the interface between the pipe and surrounding soil material, the pipe and the air/water within it, or the backfilled trench containing the pipe and the adjacent undisturbed soil.

The commercial GPR unit used predominantly in this research had a center antenna frequency of 250 megahertz. An integrated odometer on the unit measured distance along lines of traverse. From data acquisition, a subsurface profile image was generated for each transect line along which measurements were collected. The profile itself is comprised of side-by-side signal traces, each collected at points a set distance (station interval) of 2 inches (5 centimeters) apart. To reduce background noise, 32 signal traces were collected and then added together (stacked) to produce one signal trace at each point on the line. The horizontal scale on a GPR profile represents distance along the transect line, and the vertical scale is in two-way radar signal travel time, which can in turn be converted into depth values.

Data was typically collected in a grid comprised of two sets of parallel transects perpendicular to one another. GPR profile data from each transect line in the grid was then combined to produce amplitude maps representing the amount of reflected electromagnetic radar energy corresponding to a two-way travel time (or depth) interval.

Typical detection results

Line profiles illustrate the GPR response to buried drainage pipe. The first GPR profile was collected along a line perpendicular to the trend of the drainage pipes. The upside down U-shaped features depicted are referred to as "reflection hyperbolas" by geophysicists, and are a typical response to buried pipes that are oriented perpendicular to the line on which GPR data was collected. The apex of one of these upside-down U-shaped features denotes the actual position of the top of a buried drainage pipe.

The second GPR profile was collected directly over top and along the trend of a buried drainage pipe. The banded linear feature found on this line profile below a depth value of 2 feet (0.6 meter) represents the drainage pipe itself. If the subsurface radar wave velocity has been determined accurately, depth to the top of the banded feature shown on this line profile can be used as a good estimate for the actual depth to the top of the drainage pipe.

On the example of a GPR amplitude map from this study, lighter shades represent locations where a greater amount of reflected radar energy returned to the surface. Locations showing greater reflected radar energy represent dielectric constant discontinuities in the subsurface and often indicate the existence of buried objects. Where there are mapped linear trends of high radar amplitude (energy), subsurface drainage pipes may be present.

Application shows promise

The potential for using GPR in this particular agricultural application appears promising. The research team involved in this project continues to analyze the data to recommend the best field conditions for the successful detection of buried drainage pipe using GPR. This will help land improvement contractors determine if and when GPR should be used, however further testing throughout the Midwest and elsewhere is needed to better assess the capability of GPR.

ASAE members Barry J Alfred, agricultural engineer, and Norman R. Fausey, research leader, work for the USDA-ARS Soil Drainage Research Unit, 590 Woody Hayes Drive, Columbus, OH 43210, USA; 614-292-9806, fax 614-292-9448, allred. 13@osu.edu, fausey. 1@osu.edu. Leon Peters Jr., emeritus professor, and ChiChih Chen, senior research associate, are with the Ohio State University ElectroScience Laboratory Jeffrey J. Daniels is a professor in the OSU Department of Geological Sciences. Hyoung-Sun Youn is a graduate student in the OSU Electrical Engineering Department.
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Article Details
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Author:Allred, Barry J.; Fausey, Norman R.; Peters, Leon, Jr.; Chen, Chi-Chih; Daniels, Jeffrey J.; Youn, H
Publication:Resource: Engineering & Technology for a Sustainable World
Geographic Code:1USA
Date:Dec 1, 2002
Words:1233
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