The application of the shallow seismic reflection method and AVO analysis to identify the water table reflection

A simple mathematical model of a sandstone aquifer has been constructed based on a local example, the Sherwood Sandstone of the East Midlands, UK. Simple seismic reflectivity calculations show that the air-water interface should theoretically produce a detectable seismic reflected wave for sandstone porosities as low as 10%. A synthetic seismic reflection dataset was constructed for a typical field survey geometry, and processed using the Promax system to produce a stacked section. The final section clearly shows the water table reflector. A field dataset from a subsequent survey has also been processed using the same sequence which also imaged a clear reflector at 30m depth. This is important evidence that the method has uses in identifying water table as a part of progress in shallow seismic reflection survey. The methods currently employed are (1) to define the optimum field, and (2) to define the optimum processing sequence, so that water table reflection can be imaged in a variety of geological situations.;The application of Amplitude versus Offset (AVO) analysis to CMP gathers from the field data shows a characteristic increase of amplitude with increasing angle of incidence for super-critical reflection. In this way the water table reflector is clearly identified with the amplitude increasing by 30% over the range of incident angle from 28° to 34°. AVO analysis has also been applied to other field data that has a similar geological setting, but with a lithological reflector over the same super-critical angle. The resulting AVO curve shows a decrease in amplitude of over 90% with increasing offset, clearly differentiating from the water table reflection. Both water table and lithological results closely agree with theoretical predictions..