Imaging soil moisture using GPR tomography and reflection field experiments
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ORIGINAL PAPER
Imaging soil moisture using GPR tomography and reflection field experiments Mohamed G. El-Behiry & Sherif M. Hanafy
Received: 18 December 2011 / Accepted: 3 April 2012 / Published online: 15 May 2012 # Saudi Society for Geosciences 2012
Abstract We performed GPR tomography and GPR reflection field experiments using a 500-MHz antenna to image relative soil moisture distribution around a poplar tree at the botanic garden of Kiel University, Kiel, Germany. The GPR tomography field experiment is carried out in two consecutive phases in order to obtain ray paths traveling from all directions and intensively covering the target. The radar tomographic data are inverted using the authors’ developed software code SeismoRad based on the finite difference technique. The attained Root-Mean-Square (RMS) errors after 200 iterations between the measured and calculated times range between 1.066 and 5.7 % in the two tomography experiments. The estimated GPR velocities range between 5.3 and 15.1 cm/ns. Two low-GPR velocity zones could be delineated coinciding with the locations of the tree root zone and a previously excavated sector. The high water saturation zone around the tree root system is found to be the main reason for such a decrease in GPR velocity. Interpretation of the two phases proved that the coverage of ray paths from all directions is important to delineate the effect of the poplar tree root system and hence to obtain accurate tomographic results. Furthermore, four GPR reflection lines are performed along the sides of the four trenches such that the antenna is moved longitudinally in M. G. El-Behiry (*) : S. M. Hanafy Geophysics Department, Faculty of Science, Cairo University, Giza, Egypt e-mail: [email protected] M. G. El-Behiry Geophysics Department, Faculty of Earth Sciences, King Abdulaziz University, Jeddah, Saudi Arabia S. M. Hanafy King Abdullah University for Science and Technology (KAUST), Jeddah, Saudi Arabia
the trenches and the radargrams are recorded along the horizontal xy-plane parallel to the ground surface. On the processed GPR reflection radargrams, relatively high-amplitude GPR anomalies could be outlined and are attributed to the boundary between the saturated and wet zones where different water contents affect the GPR velocity. Comparable results are obtained between the tomogram and the radar reflection results with respect to zones of increase in water content. Keywords GPR . Tomography . Moisture content . Poplar tree
Introduction Ground penetrating radar (GPR) is applied to a wide range of scientific fields as non-invasive tool for mapping sedimentary structures (Neal 2004), depth of reflecting surfaces such as ground water table (van Overmeeren et al. 1997; Roth et al. 2004; Annan 2005), or permafrost tables (Hinkle et al. 2001; Moorman et al. 2003), as well as for estimating volumetric soil-water content and soil moisture (Huisman et al. 2003; Wollschlaeger et al. 2004; Wollschlaeger and Roth 2005; Hanafy and Hagrey 2006; Gerald et al. 2008). For hydrogeophysical studie
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