The Optimum Offset Shallow Seismic Reflection Technique

The development in the early 1980's of digital enhancement<br>engineering seismographs with high-pass filtering capabilities, and the<br>wide availability of increasingly powerful microcomputers, have allowed<br>the application of seismic reflection methods to groundwater and<br>engineering problems. The simple "optimum offset" shallow reflection<br>technique can be implemented with a minimum investment in equipment and<br>computing capabilities and, therefore, can be cost-effective when<br>compared to drilling, especially where detailed information on the<br>bedrock or overburden structure is desired. The data are recorded on a<br>12-channel engineering seismograph, using a single high-frequency<br>geophone per channel, and a hammer or in-hole shotgun as the seismic<br>source. Processing and display of the "optimum offset" records require a<br>small microcomputer with a dot-matrix printer. A preliminary section can<br>be produced within hours of collecting the data.<br>The user must be aware of the limitations on the use of the<br>"optimum offset" technique imposed by site conditions and other factors<br>affecting resolution. The technique is most suited to mapping the<br>overburden-bedrock interface because this often produces a largeamplitude<br>reflection. The best results are obtained in areas that are<br>most favorable for the transmission of high frequency energy; that is,<br>where the surface materials are fine-grained and water saturated.<br>Under favorable site conditions, the "optimum offset" shallow<br>seismic reflection technique can be an effective tool for mapping bedrock<br>topography beneath a minimum of 15-20 m of overburden, and for<br>delineating structure within the overburden. Thus, it can be a useful<br>technique for engineering, environmental or groundwater studies, or for<br>Quaternary mapping projects.<br>A case history is presented from Dryden, Ontario, where buried<br>valleys were successfully delineated in support of Geological Survey of<br>Canada overburden drilling for stratigraphic studies.