Glacial Sediment 2D ERI
Glacial Till Mapping Using ERI, Calibrated to Local Geology, Penetrating to 50 m Depth.
Fig. 1. Geological cross-section along a 3.6 km transect based on coring. Location in Fig.3. The Coteau Ridge escarpment resulted from faulting. The postulated stratigraphy on the uphill side is clay-rich till over a gravelly sand over bedrock. On the downhill side, the stratigraphy is a lacustrine clay over bedrock. The lateral extent of and textural variations within all glacial units are unknown. 1D resistivity soundings were collected at 11 locations (blue triangles). Lithology data from cores LL3 and LL6 were used to calibrate the 1D resistivity profiles (Fig. 4) which provide average resistivities (circled numbers within profile). Red bar at base shows location of 2D resistivity profile in Fig. 2.*
Fig. 2. Low-resolution 2D resistivity section for 850 m along transect in Fig.1 covering escarpment. This is profile P2 in Fig. 3. Resistivity technique used penetrates to ~50 m depth. High resistivities (reds) on uphill side are confirmed off the profile to represent sand and gravel. Profile shows gravel extent southward to the fault. Overlying intermediate resistivities (greens) are confirmed to reflect clay-rich till. Resistivity anomalies in till indicate locations of till heterogeneities. Evidence for fault zone (red dashed line) is provided by significant, vertically oriented resistivity contrast. Resistivity on downhill side provides evidence for two horizontal units (till over lacustrine mud) whereas previous interpretation lumps all into one lacustrine unit. *
Fig. 3. Location map for 3.6 km long resistivity transect crossing the Coteau Ridge, a fault structure, just north of Luck Lake, southern Saskatchewan, Canada. Red lines show locations of 2D resistivity profiles such as P2 in Fig.2.. Two cores, LL3 and LL6, were used to calibrate the 1D resistivity soundings collected at the points 1-13. *
Fig. 4. Calibration of 1D electrical resistivity sounding at location of core LL3 (blue line with square-wave pattern) using core lithostratigraphy. SP and resistance logs are also shown for the core. Inversion of the 1D apparent resistivity data was constrained by lithologic contacts in the core. The 1D sounding penetrates 2x-3x deeper than the 2D resistivity profile. Note that the interpreted resistivity profile (result of inversion of apparent resistivity) is not sufficiently resolved to include the basal contact of the intertill gravels at 47 m. *
* Gemail, K. S., 2015, Application of 2D resistivity profiling for mapping and interpretation of geology in a till aquitard near Luck Lake, Southern Saskatchewan, Canada. Environmental Earth Sciences, v. 73, no.3, p. 923-935.