Imaging Buried Culverts Using Ground Penetrating Radar: Comparing 100 MHz Through 1 GHz Antennae
A. A. Aziz, R. R. Stewart, S. L. Green, 2014. "Imaging Buried Culverts Using Ground Penetrating Radar: Comparing 100 MHz Through 1 GHz Antennae", Sedimentary Basins: Origin, Depositional Histories, and Petroleum Systems, James Pindell, Brian Horn, Norman Rosen, Paul Weimer, Menno Dinkleman, Allen Lowrie, Richard Fillon, James Granath, Lorcan Kennan
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A 3D ground penetrating radar (GPR) survey, using three different frequency antennae, was undertaken to image buried steel culverts at the University of Houston’s La Marque Geophysical Observatory 30 miles south of Houston, Texas. The four culverts, under study, support a road crossing one of the area’s bayous. A 32 m by 4.5 m survey grid was designed on the road above the culverts and data were collected with 100 MHz, 250 MHz, and 1 GHz antennae. We used an orthogonal acquisition geometry for the three surveys. Inline sampling was from 1.0 cm to 10 cm (from 1 GHz to 100 MHz antenna) with inline and crossline spacings ranging from 0.2 m to 0.5 m. We used an initial velocity of 0.1 m/ns (from previous CMP work at the site) for the display purposes.
The main objective of the study was to analyze the effect of different frequency antennae on the resultant GPR images. We are also interested in the accuracy and resolution of the various images, in addition to developing an optimal processing flow. The data were initially processed with standard steps that included gain enhancement, dewow and temporal-filtering, background suppression, and 2D migration. Various radar velocities were tried in the 2D migration and ultimately 0.12 m/ns was used.
The data are complicated by multipathing from the surface and between culverts (from modeling). Some of this is ameliorated via deconvolution. The top of each of the four culverts was evident in the GPR images acquired with the 250 MHz and 100 MHz antennas. For 1 GHz, the top of the culvert was not clear due to the signal’s attenuation. The 250 MHz shielded antenna provides a vertical resolution of about 0.1 m and is the choice to image the culverts. The 100 MHz antenna provided an increment in depth of penetration, but at the expense of a substantially diminished resolution (0.25 m).