Reconstructing migration phases of meandering channels by means of ground-penetrating radar (GPR): the case of the Obra River, Poland

Abstract

Purpose The main goal of the study was to compare the potential of conventional and geophysical methods (ground-penetrating radar; GPR) to reconstruct the migration phases of a meandering lowland riverbed: the Obra River in western Poland. The purpose was to verify that the migration phases can be distinguished using the GPR measurement providing near-continuous imaging of alluvial structures and to discuss differences between the spatial extent of the migration phases distinguished using geologic data and GPR surveys. Materials and methods Historical maps from the eighteenth and the nineteenth century were analyzed to study transformations of the Obra River bed pattern. One hundred twenty tube sample borings were undertaken along five profiles to recognize the floodplain lithology. Granulation coefficients were determined using the method of moments to distinguish lithofacies of the alluvial deposits. The GPR measurement was performed using a georadar MALA ProEx equipped with shielded 100 and 250 MHz antennae. The GPR images were compared to geologic profiles to determine the depth scale and accuracy of the measurements. The depth scale of the GPR images was determined on the basis of the correlation of the radar reflections with the geologic profiles. Ten organic sediment samples were collected to determine the age of the migration phases.Results and discussion Point bars with layering of sand deposits, organic sediment fills, erosion surfaces, and gravel beds were distinguished using the 250 MHz antenna. However, some of sedimentary structures (e.g., inserts of fine sands in the organic deposits) were beyond the detection range of the antennae. The glacial/alluvial sediment interface was detected using the 100 MHz antenna which enabled the determination of the thickness of the alluvial fill. Traces of a period of early development of the Obra valley and three migration phases of the Obra River bed were distinguished. The research indicated that the Obra was a meandering river at least 7,000 years before present and that during the last 3,000 years, channel islands were formed within the studied section of the valley. Conclusions The GPR method allowed for the analysis of the spatial variability of alluvial deposits and helped to describe how meander bends were formed. The presented GPR images illustrate the spatial extent of the migration phases in a more detailed way than using only sedimentary information. However, geologic data are necessary for interpretation of the GPR images. It was shown that the GPR method gives useful results in spite of rough land surface and dense vegetation.

Imaging and locating paleo-channels using geophysical data from meandering system of the Mun River, Khorat Plateau, Northeastern Thailand

“…The reflection records by a receiver antenna. Depth of GPR is dependent on the antenna frequency [15]. A higher frequency of the antennas resulted in a higher resolution, but the penetration depth decreased.…”

“…In contrast, the low frequency decreases the resolution, and the survey depth is deeper [16]. GPR has been used to analyze fluvial facies [15,[17][18][19][20][21][22][23]. However, silt and clay depositions cause attenuation of the radar signal which limits the depth range of measurement but can also be useful to determine the thickness of alluvial fill [24].…”

“…The apparent attenuation of radar signal increased as the content of claysize particles in the subsurface increased [29]. This is explained by the organic and clay deposits, ground water level, roughly land surface and dense vegetation may attenuate a GPR signal concluded that fine-coarse interfaces cannot be detected using GPR method [15].…”

Transformation of a lowland river from a meandering and multi-channel pattern into an artificial canal: retracing a path of river channel changes (the Middle Obra River, W Poland)

“…4, 6b, c). Migration phases of meander bends were recognized by Słowik (2011b) in the lower course of the Obra River. The preserved traces might have been the effect of a period featured with long-lasting high water stages and/or frequent flood events.…”

“…The preserved traces might have been the effect of a period featured with long-lasting high water stages and/or frequent flood events. A lateral, stagnant water channel might have been formed in such conditions, in the vicinity of an active river bed (Słowik et al 2010;Słowik 2011b). …”