The use of hand-held magnetic susceptibility meters in the field: an invaluable tool in regional studies of dyke swarms
Bibliographic record
Abstract
Diabase dykes and sills, unmetamorphosed to very weakly metamorphosed, are typically magnetic, enough to cause minor compass deflections and to attract a small hand-held magnet in the field. On aeromagnetic maps, such intrusions reveal themselves by more or less well-defined magnetic anomalies that contrast with more variable and typically less magnetic bedrock signatures. With some experience, one may be able to tell different dyke swarms apart simply by gauging the deflection of a hand-held magnet in the field, particularly in cases where weakly metamorphosed dykes - with a greatly reduced magnetic susceptibility due to alteration and metamorphic reactions of Fe-Ti oxides to secondary minerals such as chlorite - are intermixed with younger, less altered dykes. Nevertheless, this simple observation remains somewhat subjective and fails to take full advantage of the wide range of magnetic susceptibilities intrinsic to intrusive rocks of differing compositions. Mafic dyke rocks show magnetic susceptibilities (MS) that may vary over two full -3 orders of magnitude, from ~2 to ~200 x 10-3 SI units (factor not repeated hereafter). Most fresh diabase dykes have MS >20, whereas dykes affected by weak alteration or incipient metamorphic recrystallization typically have reduced MS values <10. Fully hydrated metadiabases have MS values <1, typically converging on 0.7 ± 0.2. Here I present preliminary results on a systematic survey of dyke susceptibilities in various parts of Canada, and a comparison of the utility of two different hand-held susceptibility meters. It is concluded that hand-held susceptibility meters are of great use in field studies of dykes, with applications ranging from distinguishing between different dyke sets, and defining heterogeneities within individual dykes, to resolving dyke intersections.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.003 | 0.002 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".