Use of GPR in developing a facies model for a large sandy braided river, Brahmaputra River, Bangladesh

A large (2 km × 1 km) sand bar has been surveyed using ground penetrating radar (GPR), augmented by vibracoring and trenching, to investigate the sedimentary structures and three-dimensional alluvial architecture. Around 8km of GPR profiles were collected with nine survey lines running transversely across the bar at 250 m intervals with a profile along the bar. Profiles image structures beneath the water table to depths of up to 15m. Reflectors from GPR profiles are interpreted as sets of cross-stratification, channel erosion surfaces, bar slipface accretion, mud-drapes, upstream accretion surfaces and the water table. Elements of upstream, downstream, lateral and cross-channel accretion are identified. Apart from the preservation of a range of dunes, the dominant style of deposition within the bar is oblique downstream and cross-channel bar margin accretion with large low angle and angle-of-repose foresets. These GPR facies characteristics are used to construct a three-dimensional model of facies architecture which may be applied in the interpretation of ancient braided alluvium. Bounding surfaces on the GPR can be tied to earlier bathymetric profiles, yielding a unique insight into the chronology of sedimentation and bar evolution. Trenching and coring of the bar top confirms that dunes and bar-margin slipfaces are the predominant facies units.