GEOPHYSICAL EXPLORATION FOR BURIED VALLEYS IN AN AREA NORTH OF TWO HILLS, ALBERTA

Abstract

Geophysical techniques may be used in groundwater exploration for the detection of groundwater itself, for the detection of potential aquifers, and for the detection of geologic situations favorable for the occurrence of aquifers. In Alberta, these buried preglacial valleys commonly contain deposits of permeable sands and gravels which, if of sufficient thickness and extent, can constitute important aquifers. Thus, location of the valleys by geophysical means leads to the identification of areas in which groundwater prospecting stands an improved chance of success. If, in addition, the geophysical methods can be used to indicate whether adequate thicknesses of permeable deposits exist in the valleys—that is, to detect the aquifers—the areas suitable for exploration can be further restricted, possibly to one or more locations with particularly encouraging prospects of a good groundwater supply. Resistivity was useful for the detection of both near‐surface and deeper‐lying permeable deposits and, hence, has some potential for the tracing of buried valley courses wherever these buried valleys contain significant sand and gravel deposits. The method had limited application, however, to the measurement of depths to bedrock because of a lack of resistivity contrast between drift and bedrock materials for a great part of the study area. The seismic refraction method was reasonably successful in the determination of bedrock depths and, thus, in the location of buried bedrock valleys. Success, however, depended on advance knowledge of velocity conditions in the surficial materials and in the uppermost bedrock layers, in order to understand and deal with the interpretative problem presented by a lack of velocity contrast between drift and bedrock materials in regions of elevated bedrock. Anomalously large calculated bedrock depths for regions of elevated bedrock were summarily rejected. Another interpretative problem was posed by the presence of a blind zone within the surficial materials. Blind‐zone velocity segments were introduced on all time‐distance plots on which this characteristic velocity did not originally appear, a method successful in about two thirds of the cases. There was no indication, other than a spurious one discounted by independent evidence, of correlation between the gravity results and the distribution of buried valleys or of any near‐surface materials. The Two Hills investigation effectively demonstrated the importance of adequate control—geophysical as well as geologic—for the successful interpretation of shallow geophysical exploration results in the Alberta plains.