How much tTEM coverage is enough to trust a geological interpretation? Evidence from mixed road/field-based data acquisition across the Ordovician–Silurian boundary and Devonian cover in Estonia

Geological mapping of the 25 × 25 km Torma 1:50,000 map sheet is challenged by:

• the crossing of the Ordovician–Silurian carbonates boundary,
• Devonian siliciclastic rocks overlapping parts of the area,
• alternating Quaternary cover of primarily glacial origin.

The bedrock geology is further complicated by a north–south oriented facies transition within the Ordovician succession, from relatively shallow carbonate facies towards more deep facies. Drilling-based constraints are limited: historical borehole information is sparse, descriptions too general, and locally conflicting, while available cores are of insufficient quality for reliable stratigraphic control. To improve geological understanding within restricted budgets, we selected towed time-domain electromagnetics (tTEM) as a rapid data acquisition method for regional-scale mapping.

We report results from over 100 km of tTEM profiling, acquired predominantly with a 3 × 3 m 1-turn transmitter configuration. Data were collected primarily along unpaved roads, smaller roads, and paths, complemented by targeted measurements on selected fields. This mixed acquisition strategy produces strongly variable lateral sampling density and enables an assessment of how survey geometry and data coverage influence interpretational confidence. Road-based acquisition enables rapid spatial coverage but with lower effective lateral resolution compared to field grids, and introduces additional noise and artefacts related to infrastructure. While mapped utilities can be considered during planning, abandoned cables and scattered ferrous objects (e.g., signs, posts, culverts) create intermittent interference that must be identified and mitigated during processing and interpretation.

Preliminary results do not support the presence of a large buried valley previously inferred from multiple older (now lost) drill cores; this is consistent with nearby seismic lines at the reported locations. Across most of the area, tTEM provides the most continuous constraint on Quaternary thickness, and field-based segments resolve internal variability sufficiently to discriminate between different Quaternary units with higher resistivity contrasts, providing a new tool for Quaternary mapping in Estonia as well. Bedrock-related contrasts are detectable in parts of the survey area, but not consistently across all geological situations. Thickness estimates of the uppermost bedrock units correlate well with drill-core control where available, yet indicate substantially higher spatial variability elsewhere than expected from existing conceptual models.

The dataset highlights the areas where drilling remains necessary to resolve key ambiguities, while providing a markedly improved basis for defining regional trends and constructing geological models and updated maps in a complex carbonate–siliciclastic setting.