ABSTRACT: Geophysics in hydrogeological exploration: comparison between surface resistivity soundings and resistivity cone penetration tests

CGRG Bibliography of Canadian Geomorphology
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A research project has been undertaken by QGC (Québec Geoscience Center) to delineate and characterize the granular aquifers of the Laurentian piedmont (Québec) in the perspective of groundwater resources protection and conservation. An integrated approach in hydrogeological exploration has been developed using resistivity sounding and resistivity piezocone penetration test. Surface resistivity soundings have been extensively used in the past as a rapid, non-invasive way to determine the apparent electrical resistivity of near-surface materials. One obvious advantage of this method is that the electrodes are located on the ground surface, thus eliminating the need for costly boreholes for site investigation. However, the interpretation of resistivity soundings in terms of depths and true electrical resistivities of a horizontally layered units requires the use of forward filters or sounding inversion. Consequently, more than one depth-resistivity multilayer model may fit on the same resistivity sounding. The final interpretation depends on the expertise and judgement of the geophysicist, while field information from boreholes reduce the number of solutions. Piezocone penetration test, or CPTU, is an in-situ method for site investigation. As a soil logging tool, the piezocone can delineate the stratigraphy of deposits and yield information on soil behavior from measurements of tip resistance, sleeve friction and dynamic pore pressure. In recent years, additional measurements capabilities have been added to the piezocone. These additions include measurement of downhole seismic waves with geophones (SCPTU) and electrical conductivity along a resistivity module (RCPTU). The inverse of conductivity can be considered as the true electrical resistivity of the penetrated formation. Comparison between the results of resistivity soundings and RCPTU has been made for three sites in the Laurentian piedmont. At two of the sites, the comparison is fairly good and the stratigraphic profiles in terms of layered units of different resistivities are identical for both methods. At the third site which is in a deltaic sand deposit, the interpretation of two perpendicular resistivity soundings with the same mid point yield two resistivity layer models which differ from the RCPTU profile. At this site, the assumptions of a horizontally and laterally homogeneous layered units are not satisfied. The dip of the foreset beddings from the deltaic sand deposit creates anisotropy, thus affecting the surface resistivity measurements. The range of characteristic resistivities of sediments has been also established for the Laurentian piedmont. This information can then be used for the interpretation of resistivity soundings elsewhere.

Bibliography of Canadian Geomorphology