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Understanding GPR reflections in sediment
Remke L. Van Dam and Elmer H. Van Den Berg
GPR in sediments: applications an interpretation. August 20-21, 2001, London, UK
In the current state of development of ground-penetrating radar (GPR), the role of several
important variables for the reflection process, such as varying moisture conditions and reflection
and interference of reflections from sedimentary structures, are unclear. To address these points,
we combined GPR profiles of an eolian sedimentary succession with textural, dielectric, and
moisture-retention characteristics from trenches (1) to illustrate the influence of moisture
concentrated in soils on radar-wave reflection, and (2) to show the process of GPR reflection and
interference over primary sedimentary structures.
Owing to the ability of organic matter to hold water, soils are excellent reflectors of GPR signals.
Organic matter in the studied succession varies from less than 0.15% for clean sand to 7% for the
most prominent soil. Moisture-retention curves show a complex relationship between suction
potential (pF) and volumetric water content, which depends on organic-matter content. With
increasing suction potential, clean sand experiences a sudden loss of water around field conditions,
whereas soils show a more gradual decrease in water content. It follows that the dielectric contrast
and radar-wave reflectivity between clean sand and soils varies strongly with suction potential.
GPR is a suitable technique to image sedimentary structures in the vadose zone because small
textural-property variations in sedimentary structures lead to changes in capillary pressure and
water content. We used binarized thin sections from sedimentary structures to quantify these
small-scale variations in textural characteristics in order to reconstruct moisture-retention curves.
This allowed for the construction of GPR synthetics to illustrate the combined process of
reflection and interference over sedimentary structures.
Vrije Universiteit, Faculty of Earth Sciences, Department of Sedimentology, De Boelelaan 1085, 1081
Amsterdam, The Netherlands. E-mail: [email protected]
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