Abstract

Abstract

In NE England, dissolution of Permian gypsum beds has caused subsidence at many localities. We report results from trial 2D profiles at five sites to evaluate the seismic reflection method as a tool for site investigation. At all five sites, the seismic profiles provided potentially useful information. Gypsum beds are imaged at depths in the range 30–70 m, and show evidence of rugose upper surfaces caused by contortion of the strata during rehydration and/or dissolution. The migrated data also reveal structures on the metre scale within the strata overlying the gypsum beds. These structures are interpreted as evidence of foundering as a result of the dissolution of gypsum. Many of them have no expression in the topography at the ground surface. Where gypsum is present at rockhead, the seismic results are generally poor, probably because of the weak contrast in acoustic impedance between hydrated gypsum and unconsolidated superficial deposits. Even this negative result is of value because it indicates that the subsurface may be unstable. As a general strategy, 2D profiles should be acquired at an early stage in site investigation and, if the results are satisfactory, a 3D seismic reflection survey can then be commissioned to provide a geological model of the site.

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