Tests of seismic methods for imaging stratigraphy of the Fredericton aquifer

Abstract

Seismic test methods for imaging stratigraphy of the Fredericton aquifer were conducted in Wilmot park, Fredericton, New Brunswick. The primary project objective was to investigate the possibility of using seismic refraction techniques to determine the location of clay "windows", in the clay aquitard, of the Fredericton aquifer. The secondary project objectives included recording seismic reflections from bedrock at a depth of approximately 60 m, and measuring shear wave velocities in the clay layer. The engineering properties of clay, such as shear modulus and Poisson's ratio, can be obtained from the velocities. Three seismic methods were investigated, at three locations in Wilmot park, Fredericton, New Brunswick. These methods were p-wave seismic refraction, p-wave seismic reflection, and shear wave refraction tests. These tests were carried out over a three day period in November of 1999. The seismic p-wave refraction data have been interpreted by the generalized reciprocal method (GRM) to yield two depth vs. distance plots. The seismic p-wave reflection data have been interpreted, assuming a single layer constant velocity unit. This yielded a single velocity and depth estimate to bedrock. Data quality obtained for the shear wave refraction was adversely affected by the collection equipment. From this data shear wave velocities were obtained with the hope of estimating engineering properties of the clay, however; due to the poor data quality, this was not possible. Interpretation of the seismic refraction data yielded depths to near surface refractors along these profiles. The depth to clay was estimated at 4.5-5.0 m, which agrees with radar data (Paasche, 2000), and borehole logs in the area. In an area where no clay was believed to be seen the depth to saturated sediments was estimated at 12.0-14.0 m also in correspondence with radar and borehole information. The seismic p-wave reflection initially yielded depths to bedrock of approximately 80m. The actual depth to bedrock as measured in borehole logs is approximately 60m. A possible explanation for this fallacy is the presence of unsaturated sands and gravels of low seismic velocity in the top 5.0 m of the stratigraphy. To correct for this problem, the travel times through the 5 m sand and gravel layer were subtracted from the total travel times, yielding an approximate depth to bedrock of 66m. The shear wave refraction tests yielded shear wave velocities of approximately 154 m/s and 259 m/s respectively, however it is not certain as to weather these velocity are for the clay or the overlying sands.