Effect of porosity and permeability on seismic reflections from aquifers

Abstract

The variations in synthetic seismic reflection shot records due to different porosities and permeabilities of aquifers imbedded in non-porous rocks are sufficient for seismic techniques to be used to map these parameters using seismic reflection surveys, and thus increase the probability of drilling into productive portions of aquifers. The critical frequency of a water-filled Biot porous solid with the high permeability and porosity associated with gravel aquifers is within the frequency range commonly used in reflection seismic prospecting. As permeability and porosity vary, the character of the reflections also changes because of changes in the partitioning of energy amongst the reflected and transmitted waves. As the critical frequency is reduced (by decreased porosity or increased permeability) the attenuation of the waves is reduced and reverberation at short offsets is increased. At offsets less than the depth of the aquifer the amplitudes of the reflections are more affected by the permeability than the porosity of the aquifer. Differences in shot records from explosive sources and hammer or vibrator sources (modelled as vertical tractions) are due to the generation of both shear and compressional waves by the latter, whereas in general an explosive source generates only compressional waves, although shear waves do result from the interaction of the compressional waves with changes in properties of the media and the free surface. This difference in source manifests itself by the presence or absence of shear-wave reflections on the records. If the offset range is great enough (say more than the depth of interest) then these shear-wave reflections will be recorded on vertical as well as horizontal geophones.