TDEM and FDEM#

Electromagnetic induction, in the time domain and in the frequency domain. Both respond to electrical conductivity, the same property ERT responds to, but they cover ground far faster and without electrodes, at the cost of resolving a layered model rather than a full section.

What you can do#

Forward modelling of layered-earth soundings, time domain and frequency domain.
Single-sounding inversion.
Laterally constrained inversion along a line, so neighbouring soundings are tied to one another.
Low-moment and high-moment TDEM fitted together to a single layered model.
Joint FDEM and TDEM inversion.
Predict soundings directly from hydrological states.

Required inputs#

A sounding or a line of soundings with the system geometry, waveform and gate times. The readers below cover what the common instruments write.

Format	Notes
TEM2Go project, `.tiw` or `.db`	Station stacks, gate flags, errors, geometry and saved inversion settings. Both project layouts are detected automatically.
TEM2Go acquisition folder, `.stb`	A folder copied straight off the instrument, with no project and no export. Gate times, calibration and loop geometry come from the raw stream.
`_StationData.xyz`, `_RawData.xyz`	Self-describing exports. Station stacks are preferred for inversion.
tTEM raw, `.skb` with `.sps`	Uses the system GEX for geometry, waveform, gates and calibration, and the TFI import filter when both are present.
Column tables, `.csv`	Airborne and synthetic datasets with a geometry file alongside.

Gate-level quality control applies the acquisition protocol’s own thresholds, so a survey is filtered on the terms it was recorded under.

Typical outputs#

A layered conductivity model per sounding, with the fit to the observed decay or the observed in-phase and quadrature response. Along a line, the laterally constrained result is a stitched conductivity section.

Choose a workflow#

Your goal	Start here
Predict TDEM or FDEM data from a hydrological model	Hydrology to EM (TDEM and FDEM)
Invert a line of airborne soundings	Ex. Airborne EM Line Inversion
Fit low and high moment together	Ex. Synthetic LM+HM Line Inversion with Lateral Constraints
Run a frequency-domain workflow end to end	Ex. FDEM Forward + Inversion Workflow
Compare EM against ERT over the same subsurface	Ex. Hydrology to Multi-Geophysics Responses

Desktop support#

Desktop Studio has an EM processing module covering import, gate quality control, inversion settings and the recovered section.

Python examples#

Ex. TDEM Workflow: From Hydrological Models to EM Responses and Inversion: hydrology to a layered model to a simulated sounding, then back by inversion.
Ex. FDEM Forward + Inversion Workflow: the frequency-domain equivalent.
Ex. Airborne EM Line Inversion: calibrate and invert a line of airborne soundings.
Ex. Synthetic LM+HM Line Inversion with Lateral Constraints: both moments with line constraints, using the bundled nine-station project.

A self-contained frequency-domain round trip, forward then inverse, with no data file involved:

import numpy as np
from PyHydroGeophysX.forward import FDEMForwardModeling, FDEMSurveyConfig
from PyHydroGeophysX.inversion import FDEMInversion

thicknesses = np.array([5.0, 10.0, 20.0])      # 4 layers -> 3 thicknesses
sigma_true = np.array([0.01, 0.02, 0.05, 0.08])
cfg = FDEMSurveyConfig(frequencies=np.logspace(2, 4, 12))

fwd = FDEMForwardModeling(thicknesses=thicknesses, survey_config=cfg)
dobs = fwd.forward(sigma_true)
uncert = 0.05 * np.maximum(np.abs(dobs), 1e-12)

inv = FDEMInversion(
    frequencies=cfg.frequencies,
    dobs=dobs,
    uncertainties=uncert,
    thicknesses=thicknesses,
    receiver_component="secondary",
)
result = inv.run()
print("FDEM chi2:", result.chi2)

Relevant API#

Forward Module for TDEMForwardModeling, FDEMForwardModeling, the survey configuration classes and hydro_to_tdem.
Inversion Module for TDEMInversion, FDEMInversion, tdem_joint_invert, JointFDEMTDEMInversion and the laterally constrained routines.

Limitations and optional dependencies#

The EM forward and inverse solvers are SimPEG, installed by the geophysics extra:

pip install "pyhydrogeophysx[geophysics]"

The models are 1D layered earths, stitched laterally rather than solved as a single 2D or 3D system. Where the subsurface varies strongly across a sounding footprint, ERT resolves it and EM does not.