Infrasense and Radar Solutions International Team to Map Utilities for 2 Liquid Natural Gas Facilities in Massachusetts

Infrasense and Radar Solutions International (RSI) have recently completed utility mapping at two liquid natural gas facilities in northeastern Massachusetts. The Infrasense/ RSI team used a combination of utility detection tools including ground penetrating radar (GPR), electromagnetic (EM-61)time-domain metal detection, and electromagnetic induction (EMI) to provide maximum accuracy and depth of penetration. The resulting CADD compatible utility maps will be used to facilitate future construction planning efforts.

The Infrasense/RSI team scanned the two sites, which have a combined area of approximately 250,000 square feet, in 7 days. A high spatial-resolution survey pattern was carried out with each of the three scanning technologies, with the utility location data referenced to project stationing and tied to differentially corrected GPS coordinates at well-defined site features. Following data collection, the data was analyzed and the detected utility locations, types, and depths were plotted on a plan-view site map. The maps were provided to the client in both a PDF and CADD compatible format.

The ground penetrating radar system used for these projects provides penetration of 5-8 feet This system collects the subsurface data on distance using a wheel mounted encoder. The surveys consisted of a series of GPR lines spaced at 2.5 feet in both directions to accurately locate all utilities within the prescribed investigation area. Each line of GPR data represents a cross sectional slice of the subsurface. GPR is capable of detecting the interface between different materials, such as the buried utility lines (metal, plastic, other) and the surrounding subgrade material. The contrast at the interface of the different materials provides an identifiable feature in the data which, when it appears in successive adjacent cross sections, can be identified as a utility. The features corresponding to utilities are selected in the data, plotted, and then integrated into the final planview utility map.

Two other electromagnetic methods were used to supplement the GPR results. EM Induction was used to detect any live (60-cycle) subsurface electrical conduits, and to track other non-electrical conductive utilities accessible above the ground with a clamp-on transmitter or through indirect induction when the subsurface position was known or could be inferred. EMI can achieve up to 7 feet of investigative depth. Additionally, a Geonics EM-61 time-domain electromagnetic instrument was used to detect buried metal. This technology measures the strength of the electromagnetic field, measured in milli-volts, induced within buried metal objects after the primary electromagnetic pulse has been switched off. The measurements are made at four separate times, where the decay of the induced voltages are measured, and at both bottom and top receiver coils. High induced voltages, especially observed at the later elapsed times, indicate the presence of below-ground metal.

One advantage of the EM-61 equipment is that it is capable of detecting subsurface utilities in areas which cannot be penetrated by GPR, such as soils with high clay content. EM-61 data was collected along lines using a line spacing of 2.5 feet. The EM-61 technology has the ability of being able to determine the horizontal position of buried metal within 1 to 2 feet, and is capable of reaching depths of up to 12 feet.

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