Incised Valley in the Taim Swamp Area
The research data presented here allow us to describe the trajectory of river paleo-channels that corroborate the hypothesis of The Taim Swamp as being the remnant of an Incised Valley. The database consists of: a grid of 2D seismic lines acquired in the north cell of The Mirim Lagoon, a 2.5D grid of GPR (Ground Penetrating Radar) acquired over the area of the Holocene coastal barrier, and finally drill holes collected on the Holocene coastal barrier. The seismic data in SGY format has a total length of 510 km and was obtained by a subbottom profiler system (GeoAcoustics) at a frequency of 3.5 kHz, and comprising a GeoPulse 5430A transmitter, a GeoPulse5210A receiver, a 132B 4-mount transducer array, a GeoPro processor system, and a SonarWiz seismic data acquisition software. These data were acquired using a DGPS Trimble® ProXRS (datum: WGS84) and analyzed in a Geographic Information System (GIS). The three-dimensional (2.5D) high-resolution stratigraphic GPR survey was performed in the north part of The Taim swamp area, in a total length of 72,4 km. The method employed consisted of a 2.5D grid (Radan™ format), forming two united blocks. The utilized GPR system was composed of a GSSI™ (Geophysical Survey Systems, Inc.) SIR-3000 data collector for bistatic aerial antennas 80 MHz (Subecho SE-70 – Radarteam Sweden AB). According to Barboza et al. (2014a), the GPR profiles were collected using the Common Offset method. A high pass (150 MHz), low pass (20 MHz), stacking (32), and gain filters were applied during the time of data acquisition. The drill holes were made using a percussion drilling system (SPT – Standard Penetrating Test). The SPT integrates percussion and water circulation; however, this method does not preserve sedimentary structures. After penetrating 55 cm by water circulation, a percussion sampler was used to recover a cylindrical sample (45 cm length and a diameter of 3.8 cm) every 1 m of drilling penetration. The percussion enabled an evaluation of the compaction of all the sampled lithologies. The total of two drill holes were positioned using a DGPS Trimble® ProXRS (datum: WGS84) and analyzed in a Geographic Information System (GIS). For the seismic and GPR sections' interpretation is used the seismostratigraphy method (Payton, 1977) adapted for GPR (Neal, 2004). The method is based on reflections termination (onlap, downlap, toplap, and truncations), geometry, and pattern of reflections (Mitchum and Vail, 1977; Abreu et al., 2010; Barboza et al., 2014b; Neal et al., 2016).
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For the post-processing flow applied to the acquired seismic data (SEG-Y format; ~10 cm of vertical resolution) was proposed by Gomes et al. (2011), an acoustic velocity of 1,650 m/s was used to calculate the thickness of the sedimentary units (Jones, 1999). The GPR data were post-processed with the Radan™, Reflex-Win®, and Prism2® software packages after background removal and the application of bandpass frequency filters, Ormsby bandpass filter, gain equalization, zero-point, topographic corrections, and time-to-depth conversion. The trace analysis was done according to Leandro et al. (2019), and a dielectric constant of 6 for wet sand was used to convert travel-time to depth, which represents a velocity of 0.12 m/ns (Daniels et al., 1995). This constant was validated using lithological data obtained from SPT drill holes (Dillenburg et al., 2017). The GPR profile was topographically corrected using GNSS post-processed elevation data points collected along the profile lines at interval times of 1 second. The samples were classified according to Munsell Rock Color Chart. Grain size analysis was performed using a LASER CILAS® particle analyzer system (model 1180) on aliquots of 5 to 10 grams. The granulometric parameters (grain size and average selection) were calculated according to Folk and Ward (1957).