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Methods: Ground Penetrating Radar (GPR)

    GPR methods are commonly employed for site investigations where the following information is sought:
    WHAT IT MEASURES

    Ground penetrating radar (GPR) is a surface method that uses the properties of electromagnetic waves to create a detailed image of subsurface layers. The GPR signal responds to a contrast in the electrical properties of different materials: where two materials with different electrical properties are juxtaposed, a reflection is usually observed on the GPR profile. Examples of large contrasts in electrical properties between two materials are: air vs. rock or soil, and soil vs. metallic material. More subtle features can also be detected with GPR, such as dry soil vs. saturated soil, shallow stratigraphy, and shallow contamination, among others. Because this method uses travel time to record reflections from different layers it is highly precise and may be used to measure the thickness of surface materials (such as concrete) or the depth to a given interface. The GPR method is ideal for the vertical and lateral delineation of features that have widths and depths as small as a few inches (such as voids, or fractures) to as large as many feet (such as USTs, utilities, tunnels or stratigraphic layers), depending on soil conditions. In addition, the GPR method can be used to delineate shallow LNAPLs. Spectrum uses the Sensors & Software Noggin Smart Cart coupled to a 250-MHz, 500-MHz or 1000-MHz antenna to collect GPR data.

    HOW IT WORKS

    During a GPR survey, an antenna containing both a transmitter and a receiver is pulled along the ground surface. The transmitter radiates short pulses of high-frequency (center frequencies of 250 MHz up to 1 GHz) electromagnetic energy into the ground. This electromagnetic wave propagates into the subsurface at a velocity that is dependent on the relative dielectric constant of the medium through which the wave travels. When it encounters the interface of two materials having different electrical properties, a portion of the energy is reflected back to the surface. The contrast in velocity between the two media can be quantified by a reflection coefficient at the media interface, where the magnitude of the reflection coefficient increases as the contrast in velocity increases. When the target of a GPR survey is metallic, the characteristic response is readily identified because the electromagnetic wave is completely reflected upon reaching the metallic object (much the way a mirror works).


    Once the reflected signal is detected at the receiver antenna, it is transmitted to a control unit (the Smart Cart) and displayed as a vertical GPR profile on the digital video logger in real time. During typical GPR data acquisition, semicontinuous GPR profiles are collected either along individual transects or along a regularly spaced grid. GPR data are usually collected in “wheel mode”, where a digital odometer connected to the wheel of the Smart Cart is used to trigger the transmission of radar impulses. Each reflected impulse is received at the antenna and is digitized into 150 and 220- 16 bit samples. The two way travel time of these signals is 5 to 200 billionths of a second (ns). Digital data are viewed by the operator in real time on a screen, and saved for later processing. Spectrum uses GPR-Slice 7.0® for 3D processing and/or Echo View Deluxe software for 2D processing of GPR data.


    GPR-Slice® creates time-slice plan view maps of GPR data for small time intervals – which can be converted to depth intervals if the dielectric constant of the materials is known. Essentially, each slice map represents the amplitude variations in the GPR signal within a specified depth range (such as 1.0 to 1.5 feet).


    The processing flow in GPR-Slice® is as follows:
    • Enter the information parameters for the GPR data set, such as name of project, time range for the data, GPR system used, navigation method.
    • Reverse the data, if necessary
    • Convert the data if 16 bit data were acquired
    • Apply appropriate gains and/or filters to the data
    • Slice and resample
    • Grid the GPR slices
    • Display and save data

    The end product of GPR-Slice® are several (generally 5 or more) time-slice plan-view maps representing GPR signal amplitudes within a specified time window. Final printouts using GPR-Slice® consist of a color scheme and transform scheme whereby each GPR reflection amplitude is assigned a certain color. Once the slices are generated the data are reviewed carefully for both shapes and amplitudes that could be associated with features of interest.
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