A shallow subsurface investigation using seismic refraction near Pennfield, New Brunswick

Abstract

An analysis was performed on data obtained from a seismic refraction survey conducted by Gemtec Limited of Fredericton, New Brunswick. The survey consisted of three separate lines (A, B, and C), each 550 meters in length, and was undertaken east of Pennfield, N.B. as part of a groundwater exploration program. The surficial geology of Pennfield consists of glacial outwash with abundant sand and gravel deposits. This geological environment is ideal for the application of seismic refraction techniques in the delineation of undulating refractors. The VIEWSEIS™ interactive seismic refraction analysis system was used to interpret the data. This software package provides a complete set of functions for seismic refraction analysis based on the Generalized Reciprocal Method (GRM) and is distributed through VIEWLOG Systems of Toronto, Ontario. The average depth-to-bedrock for lines A and B were found to be approximately 25 meters and 28 meters, respectively. Line C was excluded from the analysis owing to excessive background noise. The interpretation of Line A revealed an undulating slope, shallowly dipping towards the north; whereas, Line B traversed a shallow bedrock channel. By comparing a projected seismic interpretation with borehole information, it was determined that seismic refraction accurately represented the bedrock surface topography, but had a depth discrepancy equal to 20% of the total depth to bedrock. Three depth migration techniques are available within VIEWSEIS™; the Time-Intercept Method, the Average Velocity Method, and the Approximate Velocity Method. Consistent subsurface geometries were obtained when various combinations of these techniques were used and compared, thus lending credibility to the initial interpretation.