Seismic techniques in exploration of quaternary deposits

Abstract

The shallow depth of most Quaternary deposits, the wide range in their seismic velocity and the small size of lithological units are major factors controlling the use of seismic exploration techniques. Reasonable success has been achieved with the seismic refraction method in mapping the total thickness of unconsolidated Quaternary deposits overlying bedrock of an earlier age. The presence of sand, gravel, clay and till can often be detected by correlation with ranges of seismic velocities established for these units in a particular area. However, interfaces between the various lithological units in the Quaternary section are rarely located with any confidence because of their indistinct nature and lack of continuity of a well-defined velocity contrast over a sufficient distance. This paper describes the techniques normally used for the seismic exploration of Quaternary deposits on land. Uphole wavefront investigations are strongly advocated as an integral part of the exploration program. A computer program is described that calculates uphole wavefront times for a multilayer earth and also provides information for plotting the associated ray paths. Examples are presented for typical problems that might occur in Quaternary deposits. Results of actual uphole wavefront investigations in surficial materials in Western Canada are also discussed. These field results demonstrate the lack of velocity contrast between some lithological units and the disappearance of head waves with horizontal distance in a thin till unit. Finally, recent developments in seismic exploration techniques that hold promise for possible application to Quaternary deposits are reviewed.