PRELIMINARY INTERPRETIONS OF THE STRUCTURE AND STRATIGRAPHY OF WESLEY CHAPEL GULF USING GROUND PENETRATING RADAR

A pulseEKKO ground penetrating radar (GPR) system with a 50 MHz antenna was used to collect data on subsurface profiles of Wesley Chapel Gulf, a karst feature on the Mitchell Plain in south-central Indiana. GPR data were collected along three profile lines on the alluvial floor of the gulf. Lines 1 and 2 were 31 meters in length along the long axis of the gulf and Line 3 ran perpendicular to the long axis. This research identifies the thickness of the alluvial floor, depth to bedrock, and nature of sedimentation and stratigraphy of the fluvial deposits. In addition, hypotheses about the factors controlling deposition of strata are postulated. GPR data were analyzed and then compared to previously collected sediment cores (Jones et al., 1999: Durbin et al., 2003). The GPR data support interpretations from core data that indicated limestone bedrock or possible collapse rubble at depths of -10.8 m. Core analysis identified two units, each with distinctive sediments. The upper unit (0 to -3.5 m) is dominated by silt and the lower unit (-3.5 to 10.8 m) possessed a basal sand that became finer-grained upward. GPR data compares well with the core stratigraphy. A transition zone in wave velocity change occurred between 3.0 to 3.5 m depth correlative to the boundary between upper and lower units. Distorted velocity changes at depths of 9.0 to 12.0 m in lines 1 and 2 and -7.0 to -12.0 m in line 3 are strong evidence for a probable varying bedrock depth. Wide parabola-like features in several profiles at –7 m are interpreted as either a sediment contact (silt/clay to sand) and/or subsurface reflective points. Close proximity to a large tree (roots), pockets of high velocity sediment and/or buried limestone boulders is hypothesized sources for the observed parabolic reflections.