Why use GPR?

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        Traditionally, sites have been characterized by invasive methods such as drilling and boring. The problem with drilling and borings is that they will never represent the entire site. The information retrieved from invasive procedures such as these is extremely important and irreplaceable; however, they only indicate detailed information at a given point. Engineering and environmental firms have traditionally drilled holes at given intervals and interpolated between points to map a given area allowing for error in interpretation in that no data exists between points.

GPR surveys can often be completed more quickly & accurately than other types of investigations; and is non-disruptive and non-invasive making this technology completely safe. Ground penetrating radar (GPR) is a remote sensing geophysical technique that implements EMF energy in the form of radar pulses to detect subsurface objects based on the dielectric constants of the materials being investigated. Comparisons between objects buried in an otherwise homogeneous environment can now be quantified and interpreted. The ability of the Digital Video Logger (DVL) to display real time imaging while collecting data allows for the distinct competitive advantage over other invasive and destructive alternatives.

During GPR data acquisition a virtual cross section is created along the traverse line(s) to the effective depth of energy penetration. GPR data collection is best when performed over a fixed grid of closely spaced transects. Transect line spacing is dependent on antenna frequency; higher frequencies require a closer line spacing and lower frequencies allow for an increase in line spacing. During geophysical processing a series of virtual cross sections are now imported with a known geometry in the two dimensions (depth and distance traveled along transect). The advantage of this is that now the geophysicist has the ability to make horizontal depth slices through the "cube" over the area investigated creating a third dimension that will accurately locate subsurface targets while displaying spatial relationships of subsurface targets in all three dimensions.

"One of the best features of GPR is that it provides a real time, interpretable cross sectional view of the subsurface. Despite many mapping and 3-D visualization developments and advances, some professionals think that it is the only way to display GPR data. Going the next step of generating plan map images from data collected in grids greatly enhances the ability to understand and interpret GPR survey data. Displaying GPR data as a series of plan maps slicing deeper into the subsurface allows users to see spatial correlation of targets. Important targets can be differentiated from targets of non interest. For example, responses from utilities will produce linear targets while rocks will produce point targets." Sensors & Software Subsurface Views July 2008.

The principal difference between seismic reflection surveys and GPR surveys is that, GPR discharges EMF into the subsurface on fixed antenna geometries and maps reflections of boundaries with differing dielectric constants verses time; and seismology uses acoustic energy measuring differing acoustic impedances to detect boundaries with respect to time. GPR reflection surveys measure two-way times for determination of objects and interfaces in the substrate. This fact necessitates velocity tests for each site to obtain the travel times for the radar pulses due to variability between sites and applications. This allows the geophysicist the ability to convert time intervals to depth to targets based on the velocities of the reflected signal. GPR measures time extremely precisely from discharge by the transmitter until the reflected signal is captured by the receiver. GPR data are incremented in nanoseconds, one billionth of a second, allowing for accurate depth determinations. GPR is very cost effective and accurate as compared to other invasive procedures without the risk of encountering subsurface hazards that are expensive to remediate.

Many times, real estate transactions create the necessity of an environmental site assessment that usually requires contracting with a geotechnical firm. The verification of the presence or absence of underground storage tanks (UST's) is a determining factor in actual contract closing. A GPR reflection survey conducted on a fixed grid over the suspect area is most commonly implemented for detection of UST's.