Practical Considerations for GRM Refraction Surveys in Glacial Terrains

Abstract

Generalized Reciprocal Method (GRM) refraction surveys in glacial terrains frequently encounter complex geologic and hydrogeologic conditions. The complexity frequently centres around the shallow water table and low velocity of the unsaturated zone. Inaccurate determination of the velocity and thickness of this low velocity layer can have a significant effect on the depth estimate of lower layers. This paper discusses methods to optimize field data collection and processing to ensure proper analysis in these situations. A general rule of GRM surveys is that the geophone spacing should be less than one third of the XY distance of the shallowest layer. Since the XY of a shallow water table layer is often less than 2 metres, geophone spacings of less than 1 metre may be necessary. These spacings are frequently not cost effective, and alternative approaches may be required. Alternatives include; 1. using the GRM average velocity method and an estimate of the XY based on modelling, 2. combining the upper two layers, and 3. using time intercept methods to interpret short spreads collected at various points along the line. The sensitivity of the GRM is evaluated using a simple model. Random pick errors are introduced into the model, and the merit of arrival averaging is studied. Similarly, layer velocity errors, XY analysis errors and hidden layer problems are reviewed. Practical suggestions for GRM surveys in glacial terrains are offered. An approach for performing cost effective GRM surveys is presented. The approach is based on high redundancy data collection (multiple mid shots) for water table velocity analysis and duplicate arrival averaging. Finally, the importance of XY analysis for detecting hidden layers and bedrock surface features is discussed.

Geometry of Quaternary sediments along the north Norfolk coast, UK: a shallow seismic study

“…Complete coverage could not always be obtained, partly because of obstacles such as creeks, but there were also sections where data quality was very poor and could not be used. There can be great difficulties in obtaining reliable models in glacial and post-glacial deposits (Kassenaar & Luttinger, 1993), and there can be particular difficulties with modelling via the GRM where a low velocity contrast is coupled with topography either at the surface or on interfaces, and also where there are uncertain travel-time picks. The data from the extra offset shots were invaluable in enhancing confidence in a model.…”