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name: near-surface-geophysics description: >- Plan and validate near-surface interpretation from GPR, electrical resistivity tomography, or magnetotelluric data. Use when selecting acquisition-dependent processing and inversion branches, comparing survey results, or relating geophysical anomalies to independently supported geological hypotheses. license: MIT metadata: version: "1.0.1" author: Geoscience Skills skill_type: workflow tags: '["Near Surface", "GPR", "ERT", "Magnetotellurics"]' dependencies: '["gprpy", "pygimli", "mtpy-v2", "numpy", "pandas", "scipy", "matplotlib"]' complements: '["gprpy", "pygimli", "mtpy", "simpeg", "pyvista"]' workflow_role: analysis

Near-Surface Geophysics

Select only branches supported by the supplied data. This is a single-agent workflow; companion skills are optional references, and a missing GPR or MT dataset does not prevent an ERT-only interpretation.

Input-dependent routes

Input	Start with	Required checks before interpretation
GPR traces and acquisition geometry	gprpy	Trace order/spacing, sample-time units, time zero, antenna offsets and an independently supported velocity model
ERT electrode geometry and measurements	pygimli	Electrode indexing, measured quantities, geometric factors, uncertainty/reciprocals and topography
MT transfer functions or EDI files	mtpy	Frequency coverage, impedance units and convention, rotations, tensor components and uncertainty
Multiple methods with compatible survey support	Process each branch independently, then compare	Common CRS/vertical reference, resolution, uncertainty and acquisition timing

If only an image is available, state which quantitative operations require raw data or survey metadata. Do not infer a complete inversion dataset from colours or an unlabelled axis. Check package version and file-reader support before choosing a library-specific implementation.

Shared preparation

Retain raw files and acquisition metadata with checksums. Establish horizontal CRS, elevation datum, profile direction, measurement units and sign conventions. Keep instrument flags and a record of excluded readings. Describe the physical question and independent borehole, geological or hydrological constraints before tuning an image or model to resemble an expected structure.

GPR branch

Inspect unprocessed radargrams first. Apply time-zero correction only when a justified reference is available, and select documented signal processing; compare each step to the input. Record filter bands relative to sample interval and antenna bandwidth. Gain changes amplitude interpretation, and background removal can suppress genuine laterally continuous reflectors. Preserve original amplitudes when an amplitude comparison is required.

Keep two-way travel time as the native vertical coordinate until velocity is supported by survey measurements or a stated scenario. Report the assumed velocity, units and uncertainty for depth conversion or migration; do not use a generic dielectric constant as a measured site property. Separate an observed reflector from a geological boundary hypothesis.

The checked GPR route is raw SEG-Y via segyio or native paired MALA via GPRPy, selected sample/trace operations, then NPZ plus JSON export and readback. The gprpy helper records encoded headers and leaves unresolved time/position units as indices. GPRPy's dewow window is a sample count; it has no native SEG-Y export API. Use the companion helper when installed, or implement the same auditable steps with the available libraries and their current documentation.

ERT branch

Distinguish measured resistance/voltage/current from apparent resistivity. Verify electrode indices and geometry before recomputing geometric factors, including topography where required. Estimate errors from repeat/reciprocal observations when available; document any assumed error floor and filtering criteria. See the pyGIMLi topography example when selecting the matching data and geometric-factor route.

Check forward response and data units before inversion. Compare observed and predicted data, normalised residuals, convergence and sensitivity to mesh, regularisation and starting model. A pseudosection is a data display, not a resolved depth model. A small misfit alone does not establish uniqueness or depth of investigation; mark weakly constrained regions using justified sensitivity/resolution analysis and alternative plausible models.

For an explicitly flat two-layer diagnostic, the bundled ERT helper provides an executed route from quadrupole measurements and nominal electrode positions to inversion, a fixed reciprocal-group holdout, and CSV/JSON response/residual readback. It checks local convergence and reports when the combined model/error assumptions fail. This lightweight route is not a topographic 2D/3D ERT reconstruction; use an appropriate mesh/solver for those tasks. See the input schema and branch limits.

MT branch

Inspect tensor components, uncertainty, frequency sampling and phase behaviour. Confirm units, time convention and rotations before plotting apparent resistivity/phase or exporting inversion inputs. Use the mtpy skill and the installed-version documentation for EDI and transfer-function details. Choose 1D, 2D or 3D modelling only after examining dimensionality, survey geometry and available computational tools; a plotting library does not guarantee an installed inversion solver.

The checked MT route uses the mtpy-v2 distribution: read direct-impedance EDI, inspect source masks/variances, apply an explicitly requested tensor rotation, then export QC CSV/JSON and optional response plots with readback. Complete tensors also undergo actual EDI write/read. A field-derived transfer-function sample supports this route; raw time-series estimation and MT inversion remain separate tasks. Preserve signed component phases and unknown uncertainties.

Comparison, outputs and limits

Deliver QC tables, processing/model configuration, observed-versus-predicted checks, georeferenced products and uncertainty notes. Preserve each method's native sampling/resolution when overlaying results; resampling onto one grid does not create equal resolution or independent evidence.

Test interpretation against withheld lines, repeat surveys or independent site information when available. Shared preprocessing, priors and reused boreholes must not enter both model fitting and a supposedly independent validation set. Electrical properties also depend on fluids, salinity, saturation, temperature and clay; avoid converting a resistivity anomaly directly into a unique lithology, water table or hydraulic conductivity without calibrated supporting evidence.

The three branches now have separate executed import-to-export checks: observed GPR sample-index processing plus separately generated physical-unit data, observed ERT fit/holdout plus separate controlled model recovery, and an upstream field-derived MT EDI plus independent tensor/impedance checks. They are different surveys and must not be combined into a fictitious common-site interpretation. Joint inversion, general field accuracy and every vendor format are outside these checks; report the branch and physical assumptions actually exercised.