A PILOT STUDY FOR THE APPLICATION OF GROUND PENETRATING RADAR TO SUBSURFACE MAPPING OF LITHOSTRATIGRAPHIC FACIES IN THE LILESVILLE GRAVELS, NORTH AND SOUTH CAROLINA

Lilesville gravels occur on isolated terraces along the Pee Dee River and its tributaries near the Fall Line in North and South Carolina. Proposed ages for the Lilesville gravels range from Late Eocene to Pleistocene. Exposures in a gravel quarry reveal a 20 m vertical section through (oldest to youngest): (1) conglomerate with a lithic arenite matrix; (2) silty, medium-grained lithic arenite interbedded with conglomerate; (3) pebbly cross-bedded lithic arenite; and (4) gray, green, and pink mottled medium- to coarse-grained lithic arenite. The basal unconformity between this sequence and underlying late Paleozoic Lilesville granite is not exposed in the quarry, but a GPR profile placed along the toe of the cut face in a trench located that boundary at a depth of about 7.5 m.

A pilot GPR study was conducted to determine if this method could detect lithostratigraphic sequences for subsurface mapping. Both 100 MHz and 400 MHz antennas were used to collect duplicate profiles for a distance of 75 m approximately 1 m back from the top of the quarry cut face. Independent velocity analyses or relative permittivity have not yet been independently established; physical properties described below are based on a relative permittivity of 8 or a pulse velocity of 106 m/ns. The maximum depth of signal above noise for the 100 MHz antenna is about 10 m. Conglomeratic horizons appear as relatively lower frequency reflections possibly because of increased porosity and perhaps moisture content. Deconvolution of these horizons resolves lens-like structures, possibly packets of conglomerate within coarse arenite, about 0.3 m thick. On the cut face, distinctive hummocky and semi-continuous reflections correspond with conglomerate facies boundaries, and internal truncations of these boundaries mark cross-bedding or intercalated sand wedges. Within a more massive lithic arenite sequence above the conglomerate horizons, gently north-dipping (10-15° non-migrated; apparent dip) reflections extend over 20 m horizontally to indicate paleoslope.

The 400 MHz profiles are limited to a depth of about 2 m, so higher frequency profiles in this setting are not helpful in mapping lithofacies. Lower frequency profiles, on the other hand, provide good discrimination of conglomeratic facies and even useful resolution within finer grained facies.