Induced/remanent magnetization and a total-field anomaly
A weak-field linear isotropic rock model separates magnetizationM=M_r+χH into retained remanence and an induced response to localH.
Key knowledge
A weak-field linear isotropic rock model separates magnetizationM=M_r+χH into retained remanence and an induced response to localH. A scalar survey's anomaly relative to vector backgroundB₀ isΔF=|B₀+b|−|B₀|, approximatelyB̂₀·b when|b|≪|B₀|. The localized geological fieldb depends on magnetization, geometry and observation position; scalar anomalies alone do not uniquely recover those properties.
Values and equations
M=M_r+χH; ΔF=|B₀+b|−|B₀|≈B̂₀·b (|b|≪|B₀|)
- M,M_r,H
- total/remanent magnetization and local magnetic field strength A m⁻¹
- χ,B̂₀
- isotropic susceptibility and background unit direction 1
- B₀,b,ΔF
- background induction, geological induction and total-field anomaly T
Conditions and scope
- M andH are in A m⁻¹,χ dimensionless andB in tesla. A constantχ is a local weak-response model; using ambientH for internalH additionally neglects significant demagnetization/interactions. B₀ must be nonzero for the projection approximation.
- Account for main-field/time-dependent external variations, sensor calibration, position/height and cultural interference. Remanence need not align with today's field; anisotropy, nonlinear hysteresis and overprints need alternatives. A positive/negative anomaly is not a unique rock-type or fault indicator.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
- Investigation of Linear Magnetic Anomalies, Surveying Methods, printed4 and7, total-intensity, induced/remanent model and base correctionUS Geological Survey — Open-File Report03-12 (2003)Reference guide · PDF
- 12.002 Lecture19 finalpage, core/crustal field and induced/remanent componentsLeigh Royden and Benjamin Weiss — MIT OpenCourseWare (2008)Reference guide · PDF
Evidence and review
Directly fetched original USGS PDF after web403 and read extracted printed4/7 Surveying Methods: scalar intensity, main/external/geological components, M=M_r+kH and base-station correction. Read actual MIT19 lastpage components and thermoremanence. Total-field projection is a disclosed vector-norm expansion withO(|b|²/|B₀|) error. Excluded USGS broad dominant-rock/ideal-fault-detection claims and confused field-intensity units, MIT induced-as-remanent umbrella, spin-as-classical-current and crust-weakness explanation; separate shared physics supplies magnetization/dipole models.
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