Abstract

The purpose of this paper is to evaluate the effectiveness of large loop, ground-based, time-domain electromagnetic surveys for modeling of paleovalley geometry and valley fills as a basis for determining groundwater potential. The electromagnetic (EM) survey was successful, despite limited resistivity contrast between adjacent valley-fill units, as well as the limited contrast between the valley-fill and bedrock. Thin, coarse-grained units and low contrast in the resistivity present challenges for constraining 1-D inversions. Thin units cannot be resolved and are merged, resulting in simplified models that reflect averaged apparent resistivity. Supplementary data (e.g., borehole resistivity logs) are useful for constraining complex multi-layer inversions, but where absent, complex models are not realistic. As the ratio of the resistivity of two layers decreases, there is more uncertainty in the placement of layer boundaries even though the root mean squared standard error of fit for the model may remain small (<6%). Nonetheless, when 1-D inversions are combined into resistivity-depth sections we interpret four generalized lithologies: 1) bedrock (∼10–30 ohm-m); 2) fine-grained sediment (clay, silt, fine sand, and diamict; 30–50 ohm-m); 3) medium-grained sediment (silt and sand; 40–80 ohm-m); and 4) coarse-grained sediment (sand and possibly gravel; 90 to >350 ohm-m). When the 2-D resistivity-depth sections are combined, there is 3-D continuity and the geometry of the Groundbirch Paleovalley can be traced. The valley is approximately 3–4 km wide with a maximum depth of about 120 m. The data suggest a local basal aquifer (medium- to coarse-grained unit) in the central survey area and possibly an unconfined aquifer towards the northwest region of the survey area. The EM-based model is consistent with models based on water well logs and field observations. Although the EM data remain exploratory, the resulting EM models provide clear guidance for groundwater resources management strategies.

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