Seismic facies analysis using chirp sub-bottom profile data from large sandy-bed rivers, lower São Francisco River, Northeast Brazil

Abstract

A chirp sub-bottom profiler (SBP) system (shallow high-resolution seismic) was used to characterize the internal architecture of submerged bedforms and fluvial bars of the lower São Francisco River, Northeast Brazil. From upstream to downstream, the area was subdivided into two geomorphic channel reaches: anabranching-braided; and anabranching-stable. The uppermost sedimentary layers of the riverbed were analysed. A processing flow (bandpass filtering, automatic gain control, time-varying gain and stacking) to attenuate the effect of noise, equalize the gain and highlight the internal structures that were not clear in the primary recorded SBP section (envelope mode) was applied. Detailed seismic analysis and description of the SBP seismic profiles allow the interpretation of seven seismic facies. An idealized stage of development for each channel landform is based on the seismic characteristics of the sandy sets of the anabranching-braided reach. In the anabranching-stable islands reach, the bedforms have a low degree of preservation. Our result demonstrates that the SBP chirp data is a potential tool that allows the identification and definition of sedimentary facies in large sandy-bed rivers.