Glossary#
Core terms used throughout the pyCSAMT documentation and codebase. Most
entries are cross-referenced elsewhere with :term: roles, so the
definitions here are the single source of truth.
- Agent#
In
pycsamt.agents, a small composable unit that performs one step of a workflow (routing, parsing, QC, forward modelling, inversion, reporting) and returns a standardisedAgentResult.- AMT#
- Audio-frequency magnetotellurics#
Magnetotellurics in the audio-frequency band (roughly 1 Hz to 10 kHz), giving shallow-to-intermediate depth resolution. The natural source is weak in the AMT “dead band” near 1–5 kHz.
- Apparent resistivity#
The resistivity of an equivalent uniform half-space that would produce the observed impedance at a given frequency, \(\rho_a = 0.2\,|Z|^2 / f\) (with \(Z\) in field units and \(f\) in Hz). It is apparent because the real earth is layered/heterogeneous.
- AVG file#
Zonge instrument-averaged CSAMT/AMT export format; pyCSAMT reads it and can transform it to EDI.
- CSAMT#
- Controlled-source audio-frequency magnetotellurics#
An active-source variant of AMT that uses a grounded electric dipole or magnetic loop transmitter to overcome the weak natural signal, at the cost of near-field and source-overprint corrections.
- CSEM#
Controlled-source electromagnetics — the broader family of active-source EM methods to which CSAMT belongs.
- Dimensionality#
A classification of the subsurface electrical structure sensed by a site as 1-D (layered), 2-D (strike-invariant), or 3-D, typically assessed from phase tensor skew and impedance invariants.
- EDI#
The Electrical Data Interchange file format — the SEG standard text format for storing MT/AMT impedances, tipper, and metadata per station.
- EMAP#
Electromagnetic array profiling — a spatially continuous acquisition and filtering approach used to suppress static shift.
- Impedance tensor#
- Z#
The frequency-dependent 2×2 complex tensor \(\mathbf{Z}\) relating the horizontal electric and magnetic fields, \(\mathbf{E} = \mathbf{Z}\,\mathbf{H}\). Its elements (\(Z_{xx}, Z_{xy}, Z_{yx}, Z_{yy}\)) are the primary MT observable, from which apparent resistivity and phase are derived.
- J-file#
The A.G. Jones (“BIRRP”) ASCII format for MT transfer functions, readable by pyCSAMT and convertible to EDI.
- MARE2DEM#
A 2-D/2.5-D goal-oriented adaptive finite-element inversion code for MT and CSEM; pyCSAMT can build its input files.
- ModEM#
A widely used 3-D MT inversion code; pyCSAMT can prepare its data files.
- MT#
- Magnetotellurics#
A passive electromagnetic method that images subsurface electrical resistivity from natural time variations of the Earth’s electric and magnetic fields. The ratio of horizontal electric to magnetic field components yields the impedance tensor.
- Occam2D#
A 2-D regularised (smooth) inversion scheme and file format for MT data; pyCSAMT can write its data, mesh, and startup inputs.
- Phase#
The phase angle of an impedance tensor element versus period; for a layered earth it tracks whether resistivity increases or decreases with depth. Reported in degrees.
- Phase tensor#
A real 2×2 tensor derived from the real and imaginary parts of \(\mathbf{Z}\) that is provably immune to galvanic static shift. Its ellipse and skew angle summarise dimensionality and strike without distortion.
- RMS#
- RMS misfit#
Root-mean-square misfit between observed and predicted responses, normalised by data error; the primary goodness-of-fit measure for an inversion.
- Skin depth#
The depth at which an EM field attenuates to \(1/e\) of its surface amplitude, \(\delta \approx 503\,\sqrt{\rho / f}\) metres. It sets the depth of investigation for a given period and resistivity.
- Static shift#
A frequency-independent vertical shift of the apparent resistivity curve caused by galvanic charges on small near-surface heterogeneities. Left uncorrected it biases inverted depths and resistivities.
- Strike#
- Geoelectric strike#
The azimuth of the principal geoelectric direction, estimated from the impedance or phase tensor. Rotating data to strike separates the TE and TM modes for 2-D interpretation.
- TE mode#
- TM mode#
The transverse-electric (electric field along strike) and transverse-magnetic (magnetic field along strike) polarisations into which 2-D MT data separate after rotation to strike.
- Tensor#
In magnetotellurics, a frequency-dependent 2×2 matrix relating two vector field quantities — most importantly the impedance tensor and the phase tensor.
- Tipper#
The complex vertical magnetic transfer function relating the vertical to the horizontal magnetic field. Its induction arrows point toward (or away from) lateral conductivity contrasts.