C4 · Publication Volume 14

Exploration Geophysics

Connects rock-property contrast, survey design, processing and inversion to defensible geophysical interpretation.

Purpose and boundary of this book

Exploration geophysics uses measured physical fields to test subsurface hypotheses. A map of magnetic intensity, gravity, radiation, voltage, electromagnetic decay or seismic travel time is not a picture of geology. It is an observation produced by rock properties, body geometry, source–receiver configuration, distance, acquisition conditions, processing choices and noise. Many different subsurface arrangements can produce observations that agree within measurement uncertainty. The scientific task is therefore to connect physics to a decision while keeping that ambiguity visible.

This book teaches that complete chain. It covers property contrast, survey geometry, magnetic, gravity, radiometric, electrical, electromagnetic, seismic and borehole methods; field corrections; filtering and enhancement; forward modelling; inversion; multi-method integration; uncertainty; and reproducible records. It is designed for learners who must plan a survey, inspect acquired data, audit a processing history, compare plausible models and specify the next observation that would discriminate between them.

The tutorial does not prescribe a universal line spacing, station interval, filter, inversion parameter, target threshold or preferred instrument. Those choices depend on the question, expected dimensions and depth, property contrast, terrain, platform, cultural environment, noise spectrum, safety constraints and decision consequence. It does not provide legal, safety, environmental, procurement, investment, resource-estimation or operational approval. A geophysical anomaly is a measured departure requiring explanation; it is not proof of a material, geometry, deposit or economic outcome.

General and institution-neutral scope

This is a general, institution-neutral tutorial. It has no affiliation with, sponsorship by, endorsement from or curriculum relationship to any company or individual. It is not written for a named operator, contractor, consultancy, university, agency, software product, instrument brand, licence, property, mine or private dataset. Every unnamed survey, line, station, borehole, body, model and locality in an example is synthetic teaching material.

People, public bodies, standards organisations, journals and repositories named in source notes identify technical sources only. A citation does not make any person or organisation the author, publisher, sponsor, provider, partner, endorser, scientific authority or curriculum subject of this tutorial. The website that hosts these pages is only a delivery surface. It is not the tutorial's author, publisher, sponsor, provider, scientific authority or curriculum subject, and it asserts no ownership of the curriculum.

This separation is part of good interpretation. Reputation cannot repair an incorrect coordinate system, an unknown sensor height, an undocumented correction, an unstable inversion or a model that violates the measured physics. Conversely, a modest dataset can support a strong decision when its geometry, calibration, uncertainty, transformations and alternatives are explicit. Claims in this book stand or fall on traceable observations and reproducible reasoning.

The physics-to-decision chain

Use the following sequence throughout the book:

  1. State the geological question, decision and at least two viable subsurface hypotheses.
  2. Translate each hypothesis into distributions of density, magnetisation, radioelement abundance, electrical properties, elastic properties or another measurable contrast.
  3. Predict the field or waveform for a defined source, receiver, platform, orientation, height and sampling geometry.
  4. Design acquisition so the expected spatial or temporal bandwidth is sampled and the decisive contrast can exceed total uncertainty.
  5. Record navigation, geometry, timing, calibration, environmental state and quality controls with the observations.
  6. Apply corrections only when their physical meaning, parameters, order and provenance are declared.
  7. Inspect profiles and residuals before gridding, filtering, imaging or inversion can conceal acquisition defects.
  8. Compare forward responses and inverse models with data using an error model appropriate to the measurement.
  9. test geological, acquisition, cultural and processing explanations for every material feature.
  10. Communicate a decision, alternatives, uncertainty and the observation most likely to separate the remaining models.

The chain is reversible. A reviewer should be able to start from a coloured image, recover every transformation, locate the contributing observations and reconstruct the instrument geometry and field record. If that path is broken, visual plausibility is not evidence.

Learning outcomes

After completing the book, a learner should be able to:

  • formulate property-based predictions for competing geological hypotheses;
  • select method and survey geometry from target scale, depth, contrast, noise and decision requirements;
  • distinguish sampling interval, footprint, detectability, resolution and depth of investigation;
  • audit magnetic, gravity and radiometric corrections without confusing them with geological signal;
  • explain electrical and electromagnetic sensitivity to conductivity, chargeability, geometry, coupling and time or frequency;
  • relate seismic velocity and density to travel time, impedance and reflected or refracted energy;
  • reconcile borehole logs by tool response, orientation, environment and depth registration;
  • identify filtering, gridding, derivative and continuation artefacts;
  • construct a forward model and a stated data-error model before interpreting an inverse result;
  • demonstrate non-uniqueness with at least two materially different models that fit the observations;
  • integrate multiple methods through shared hypotheses rather than visual coincidence alone; and
  • release observations, processing steps, models and decisions as an auditable, reproducible record.

Prerequisites and notation

The concept map places this book after quantitative foundations in A1, solid-Earth physics in A3, structural reasoning in B3 and staged mineral exploration in C2. Familiarity with C1 and C3 is helpful when geological-process and geochemical evidence are integrated. Learners should already be comfortable with vectors, units, logarithms, simple differential relationships, probability, coordinates, maps, matrices and competing hypotheses.

Let a property model be m(\mathbf r) and an observation be d_i. A forward operator F_i predicts d_i^{\mathrm{pred}}=F_i[m;g] for acquisition geometry g. The residual is r_i=d_i-d_i^{\mathrm{pred}}. An uncertainty or covariance model C_d states how residuals are weighted. Distance is in metres, time in seconds, frequency in hertz, density in kilograms per cubic metre, velocity in metres per second, magnetic flux density in tesla, electric potential in volts and resistivity in ohm metres unless a record explicitly declares another unit.

Vectors and signed quantities require a coordinate frame. State whether vertical is positive upward or downward, how azimuth and inclination are measured, which coordinate reference is used and whether a value is an absolute field, a component, a gradient, an anomaly or a transformed product. Never infer a unit, sign or datum from a filename or colour palette.

How to use the diagrams and synthetic cases

Each numbered lesson contains one purpose-built diagram. Arrows show physical influence, processing dependency or audit sequence. They do not assert that every survey has the same workflow. A blurred footprint represents sensitivity, not a sharp depth boundary. A model section is one hypothesis, not a recovered truth. Where two models are drawn, both should be treated as viable until an independent observation separates them.

All unreferenced numerical cases are synthetic. Values are selected to expose calculations, scale effects and failure modes; they are not expected property ranges, instrument specifications, survey prices, target grades or success rates. A worked answer must state assumptions, units, geometry, calculation, sensitivity to uncertainty, alternative explanation and decision implication. Reproducing a number without those elements is incomplete.

Reproducible geophysical records

Treat raw observations as immutable. Corrections, resampling, gridding, filters, transforms, picks, inversions and interpretations create versioned derivatives rather than replacements. A reproducible package links survey and line identifiers; platform and sensor configuration; coordinate reference; navigation and elevation sources; clocks and synchronisation; calibration and functional checks; raw fields and units; environmental and base-station records; correction parameters; rejected intervals and reasons; processing software-independent equations; grid origins and kernels; model meshes, bounds and regularisation; random seeds where relevant; residuals; uncertainty; and released products.

Every processing operation needs an input version, output identifier, ordered parameters, operator role, timestamp and rationale. If a manual edit is unavoidable, preserve the pre-edit value and a machine-readable edit mask. If access restrictions apply to exact locations, publish the permitted view without breaking the lineage available to an authorised reviewer. A screenshot is not a data archive, and a final grid is not a substitute for line data.

Assessment and completion standard

Completion requires a synthetic, method-neutral interpretation package containing:

  1. a decision statement and at least two geological/property hypotheses;
  2. predicted signals, decisive wavelengths or time ranges and expected confounders;
  3. a survey geometry justified by sampling, footprint and uncertainty;
  4. a field quality plan with calibration, repeats, control lines and failure actions;
  5. an immutable observation table and ordered correction ledger;
  6. profile-level checks before gridding and enhancement;
  7. a forward model with units, coordinates, mesh and boundary assumptions;
  8. an inversion or model comparison with an explicit error model and regularisation;
  9. at least two viable subsurface models plus a discriminating next test; and
  10. an integrated conclusion that separates observation, inference, uncertainty and decision.

The package passes when another reviewer can reproduce the principal products, explain why each correction and parameter exists, locate evidence for every interpretation, see where the data have little sensitivity and identify what result would reverse the proposed action. A polished image without recoverable observations and alternatives does not meet the standard.

Core sources