The internal structure of scrolled floodplain deposits based on ground-penetrating radar, North Thompson River, British Columbia

A study of floodplain deposits using ground-penetrating radar (GPR) was conducted on a confined meander of the North Thompson River near Kamloops, British Columbia. A survey grid consisting of 20 individual GPR profiles from 200 m to 900 m in length was constructed on a 0.9 km2 section of the floodplain. All GPR profiles were collected using a pulse EKKOTM IV GPR system with 100 or 200 MHz antennae, dependent on the thickness of floodplain sediments. Floodplain sediments consist of sand and gravel deposits approximately 7 to 14 m thick that overlie silts and clays interpreted to be glaciolacustrine in origin. Vibracore and auger data obtained at nine locations in the GPR survey grid show that changes from medium sand to gravel do not produce distinct radar reflections in GPR profiles but that the water table may locally produce a distinct, high-amplitude radar reflection. Area well-log data show that rapid signal attenuation at depth in all GPR profiles coincides with underlying silt and clay sediments. GPR profiles were, therefore, useful in mapping 3-D variability in the thickness of floodplain deposits. All GPR profiles were divided into macroscale (>150 m2 in 2-D cross-section) radar facies, delineated on the basis of the nature and orientation of reflections relative to surface scroll topography. Radar stratigraphy suggests that the floodplain is composed of a single, complex lateral accretion deposit with periodic channel fills located between surface scroll ridges. Dip direction profiles, normal to surface scroll ridges, exhibit mainly inclined stratification (IS) and/or preserved ridge-and-swale (RAS) deposits dipping in the direction of floodplain accretion. Well-developed RAS architecture in 200 MHz GPR profiles displays mainly wavy reflections conforming with surface topography and little inclined stratification. These reflectors are interpreted to be the result of processes of vertical accretion associated with floodplain formation. Preserved scroll ridge deposits exhibit asymmetry at depth, dipping more steeply in the direction of floodplain accretion. Profiles of strike direction, parallel to surface scroll ridges, show either parallel or slightly inclined (1–2°) reflections dipping in the downstream direction.