FACIES ANALYSIS IN THE LOWER GREENSAND USING GROUND-PENETRATING RADAR

This paper presents the results of a ground-penetrating radar survey i n the Woburn Sands Formation, Lower Greensand Group near Leighton Buzz ard. A total of almost 4 km of survey has been acquired to investigate the internal structure of the sands. The radar survey achieved a reso lution on the order of tens of centimetres with penetration up to 10 m and revealed a variety of horizontal, dipping and curved reflectors. The reflections are interpreted using seismic interpretation methodolo gy. Two scales of unconformity-defined radar sequences are described. First-order radar sequence boundaries can be identified at the termina tion of primary reflectors. Second-order radar sequence boundaries can be identified at the terminations of both primary reflectors and firs t-order surfaces. First-order radar sequence boundaries are interprete d as reactivation surfaces within bedforms which are produced by chang es in bedform geometry in response to a change in hydraulic regime or superimposed bedforms. Second-order surfaces are interpreted as the pr oduct of bedform migration. Reflection character and geometry are desc ribed and form the basis of a ground-penetrating radar facies analysis . Nine different radar facies have been identified. These are interpre ted as large sets of planar cross-stratification sets of trough cross- stratification, cosets of cross-stratification tidal bundles, clay dra pes, diagenetic concretions and possibly bioturbated sands. A reconstr uction of the bedforms suggests that they were formed by very large cu rved crested dunes. These interpretations are checked against outcrop as the sands are excavated in working sand pits.