Facies analysis in the Lower Greensand using ground-penetrating radar

摘要

This paper presents the results of a ground-penetrating radar survey in the Woburn Sands Formation, Lower Greensand Group near Leighton Buzzard. A total of almost 4 km of survey has been acquired to investigate the internal structure of the sands. The radar survey achieved a resolution 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 methodology. Two scales of unconformity-defined radar sequences are described. First-order radar sequence boundaries can be identified at the termination of primary reflectors. Second-order radar sequence boundaries can be identified at the terminations of both primary reflectors and first-order surfaces. First-order radar sequence boundaries are interpreted as reactivation surfaces within bedforms which are produced by changes in bedform geometry in response to a change in hydraulic regime or superimposed bedforms. Second-order surfaces are interpreted as the product of bedform migration. Reflection character and geometry are described and form the basis of a ground-penetrating radar facies analysis. Nine different radar facies have been identified. These are interpreted as large sets of planar cross-stratification, sets of trough cross-stratification, cosets of cross-stratification, tidal bundles, clay drapes, diagenetic concretions and possibly bioturbated sands. A reconstruction of the bedforms suggests that they were formed by very large curved crested dunes. These interpretations are checked against outcrop as the sands are excavated in working sand pits.