Time-domain induced polarization (TDIP) curves show how an electrical voltage remaining in the ground decays after a controlled current is switched off. If you need to read a soil or rock IP decay curve, chargeability in millivolts per volt, gate windows, primary voltage, electrode arrays or an inverted chargeability section, begin by identifying exactly when the transmitter current stopped and how the instrument normalised the residual voltage. A large late-time voltage is not automatically an ore body.
The basic physical idea is that subsurface materials can become electrically polarised while current passes through the ground. After current shutoff, that stored polarisation relaxes and produces a transient voltage across receiving electrodes. Instruments measure portions of the decay through time windows, sometimes called gates, and calculate an apparent chargeability. Its value depends on those selected windows and on the primary-voltage reference, not only on the subsurface material.
This article explains time-domain IP from electrodes and Ohm’s law through charging, shutoff, transient curves, gated averages, chargeability conventions, polarity reversal, quality control and inversion. It gives checked arithmetic examples with time, voltage and current units, distinguishes IP from direct-current resistivity and frequency-domain spectral IP, and explains why clay, metallic minerals, groundwater chemistry and coupling can complicate interpretation. Every numeric scenario is illustrative, not a reported survey outcome.
Quick answer: what does a TDIP decay measure?
A TDIP survey introduces current through a pair of ground electrodes and records a voltage response between another pair. During the current-on interval, the subsurface carries a primary electrical response and develops polarisation under suitable conditions. After shutoff, the potential does not always vanish immediately; a decaying secondary voltage may remain.
The decay is divided into measured time windows and compared with a defined primary voltage to produce an apparent chargeability or related parameter. That measurement is influenced by both ground properties and survey geometry. Inversion can estimate a spatially distributed chargeability model, but that model is not a direct map of uniquely identified minerals. First state which voltage, time window and normalisation were measured.
Current injection makes this an active electrical technique
Unlike self-potential surveys, which measure naturally existing voltage differences, TDIP deliberately applies electric current through galvanically coupled electrodes. The same array may also measure direct-current resistivity. The current passes through a complex volume of subsurface controlled by electrode positions, ground conductivity and possible three-dimensional structure.
This active-source character matters when comparing geophysical methods. The US EPA induced polarization and complex resistivity guide describes time-domain decay after current shutoff and frequency-domain alternatives. A TDIP instrument is not simply listening to an ambient electromagnetic wave; it sets up a known electrical forcing and measures the response.
Four electrodes and two distinct roles
A common surface electrical array uses two current electrodes and two potential electrodes. The current electrodes drive electrical current through the ground, while the potential pair records voltage difference caused by that current and subsequent polarisation. Array configurations include Wenner, Schlumberger and dipole–dipole arrangements, selected according to survey geometry and target.
Current and potential electrode positions influence measured apparent resistance, resistivity and chargeability. Therefore, a value from one electrode spacing cannot automatically be treated as a material property at a single point. Inspect the electrode sequence, spacing and contact conditions before reading a pseudosection or model image. Survey geometry is part of the measurement definition, not a drawing convention.
Primary voltage provides a normalising reference
During the current-on part of an IP cycle, the measured potential often includes an ohmic response and a polarisation-related contribution. A defined primary voltage, conventionally Vp in many descriptions, is used to normalise the later secondary decay. The choice and sign convention for Vp must be clear because the chargeability calculation depends on it.
If Vp is very small due to electrode geometry, poor coupling or a low signal, the normalised secondary voltage can become unstable even if the absolute decay is modest. Never interpret a large dimensionless ratio without inspecting both numerator and denominator. A technical table should preserve primary voltage, transmitted current, chargeability and the specific gating definition rather than reporting only one coloured cell.
Current-on and current-off periods form a cycle
A transmitter may apply current for a defined period, then turn it off and record the decay. In practice, current polarity may be reversed over successive cycles, and repeated measurements may be stacked to improve signal quality. Current-on duration and shutoff timing affect how much polarisation develops and which relaxation processes are observable.
A curve labelled ‘one-second gate’ cannot be fully interpreted without knowing the current cycle. A short charging interval may not reach a comparable polarised state to a much longer one. Similarly, gating that starts immediately after shutoff may include transmitter or electromagnetic coupling artefacts. Always read the complete acquisition timing rather than interpreting gate numbers in isolation.
The first milliseconds may be troublesome
Immediately after current shutoff, inductive or capacitive coupling in cables, electrodes and the environment can produce transients unrelated to the desired subsurface polarisation response. Instrument settling and switching behaviour may also affect early samples. Many surveys deliberately use a delay before the first trusted IP gate, and that delay should be documented.
Do not take a large first sample as automatically indicating a strongly polarizable rock. Inspect the waveform near shutoff, instrument specification and quality flags. Rejecting contaminated early samples can be appropriate when rules are applied consistently, but deleting inconvenient points merely to produce a preferred curve is not sound analysis. The retained time range determines what relaxations the method can constrain.
Why the voltage decays instead of vanishing instantly
Some soil and rock systems can accumulate charge separation at mineral-fluid interfaces or around conductive grains when an electric field is applied. After the external forcing changes, the redistributed charge relaxes over time. This produces a measurable voltage response that can be represented by a decay curve and, in some cases, fitted with a phenomenological relaxation model.
The response depends on mineralogy, pore-fluid chemistry, surface properties, grain size and the distribution of relaxation times. The term chargeability describes a measurement of polarisation-related response, not necessarily a literal large store of recoverable energy like a battery. A strong decay can have several environmental causes and must be interpreted with geological context.
Resistivity and chargeability are not the same property
Direct-current resistivity relates applied current and observed primary voltage, with an electrode-array geometric factor accounting for survey geometry. Time-domain IP chargeability relates the secondary transient to the primary reference over specified time windows. Both can be collected during one survey but they respond differently to material properties.
A low-resistivity zone is relatively conductive, whereas a high-chargeability anomaly indicates a substantial measured polarisation response under the instrument’s normalisation. Some clays and disseminated metallic minerals can influence both. Do not call chargeability ‘resistivity after shutoff.’ The US EPA electrical resistivity guide explains how apparent resistivity depends on electrode geometry and ground current paths.
Ohm’s law begins the resistivity calculation
The measured resistance for a current-on observation can be described by R equals voltage divided by current, with ohms as the unit. Apparent resistivity uses an additional geometric factor K, giving rho-a equals K times measured voltage divided by injected current. K has length units under standard surface-array conventions, so the result has units of ohm-metres.
If a table lists 20 millivolts of primary voltage at 100 milliamperes of transmitted current, the corresponding measured resistance is 0.20 ohms. Applying an illustrative geometric factor of 50 metres gives apparent resistivity of 10 ohm-metres. This is a geometry-normalised response, not necessarily the true resistivity of one rectangular cell immediately beneath the electrodes.
The word apparent warns about mixed ground volumes
An apparent resistivity or apparent chargeability is produced by a spatially distributed subsurface electrical response. Electrode placement, current paths and varying materials all contribute. An inverse model attempts to estimate intrinsic properties that could explain the observations under a selected forward model and regularisation.
A pseudosection is a plotting arrangement for the measured or calculated apparent values, not a literal cross-section of measured material at each plotted pseudo-depth. Readers should not directly label a bright pseudosection patch as a rock body at that geometric coordinate. The plot is useful for pattern recognition and quality control, but inversion and geological interpretation require additional steps.
Gates integrate different portions of the transient
After shutoff, the residual voltage is sampled over a sequence of time windows, each with specified start and end times. Instruments may report an average voltage within each gate, an integrated voltage, or a normalised chargeability based on those quantities. The earliest and latest gates can show different sensitivity to relaxation processes and to noise.
A curve with ten gates is not ten independent soil samples. It is a time sequence from one electrical observation, often repeated for stability. To compare surveys, use physical times in seconds or milliseconds rather than assuming gate number 5 represents an identical interval across instruments. Gate boundaries are an essential part of the measurement definition.
A common normalised chargeability formula
One definition of apparent integral chargeability is the average secondary voltage over a selected interval, divided by the magnitude of the primary voltage. In symbols, M equals one divided by delta-t times Vp, multiplied by the integral of secondary voltage over the window. Under consistent polarity conventions it is dimensionless and often reported in millivolts per volt.
Other instruments report a chargeability as an integral normalised by Vp without dividing by the window duration. That form has time units, such as milliseconds. Two values labelled chargeability are not directly comparable unless their definitions and timing windows match. The formula used by the processing software should be reproduced in the methods section.
A checked gate example in millivolts per volt
Assume a primary voltage of 250 millivolts and a secondary-voltage average of 5 millivolts across a one-second gate. The normalised average chargeability is 5 divided by 250, or 0.020. Expressed as millivolts per volt, it is 20 mV/V. The calculation uses the ratio of like voltage units, so the result is dimensionless before expressing it in the chosen display convention.
This is not the same as a statement that the ground contains 20 millivolts of stored voltage everywhere. It is a normalised apparent response associated with a particular electrode configuration, current cycle and gate timing. If the primary voltage changed while the absolute secondary voltage did not, the ratio would also change. Read the denominator before interpreting the colour.
Gate averages can hide different decay shapes
Two observations can have identical average secondary voltage across one broad gate yet have very different waveforms. One may decay rapidly at the start and approach baseline early, while the other has a slower tail. A single chargeability value collapses that timing information and may be insufficient to distinguish relaxation behaviour.
Plot the gate-resolved decay when available, preferably with physical time coordinates and uncertainties. An anomaly whose broad-gate mean is high may arise from a brief initial transient or a sustained late response. Those situations may have different geophysical and instrumental explanations. Avoid inferring a universal grain size or mineral type from one integrated number.
Logarithmic time axes change visual judgement
IP decays often span milliseconds to seconds. A logarithmic time axis can show early and late behaviour in one graph, but equal horizontal spacing then represents a ratio rather than a constant time increment. A rapidly falling curve may look deceptively gentle when plotted on a log axis.
Check whether the vertical axis is voltage, absolute voltage, normalised voltage or signed gate chargeability. Some authors plot absolute values to accommodate changing polarity, which can hide sign reversals or early switching artefacts. An accurate caption names both scales and transformations. Before fitting a decay slope, reconstruct the actual units and gating intervals.
A late-time signal is not automatically more trustworthy
Early gates are susceptible to switching transients, but late gates can approach the noise floor, natural self-potential drift or electrode instability. A late-time point that appears dramatically different after normalisation may be dominated by a very small absolute voltage. Increasing time does not necessarily increase the accuracy of the IP signal.
Inspect absolute voltage amplitudes, repeated cycles and instrument noise estimates. A good survey may exclude both early coupling-dominated gates and late noise-dominated gates, with a documented acceptable window. Interpreting the usable range is a balance between physical sensitivity and observational reliability, not a rule to retain the last available point.
Current polarity reversal helps with persistent offsets
Many IP acquisition sequences reverse applied current direction. This can help identify or reduce constant electrical offsets such as naturally occurring self-potential, electrode potentials and certain drifts when cycles are combined appropriately. It is not a guarantee that all interference disappears because some offsets change over time or respond asymmetrically.
A report should describe stacking or polarity averaging and whether positive and negative current cycles produced consistent transients. Strong asymmetry may signal contact problems or nonlinearity requiring investigation. Do not automatically take an absolute value of every cycle and average; doing so can preserve artefacts or erase sign information that quality control needs.
Electrode contact and cable geometry matter
Poor electrical contact can make current injection unstable and measured potentials noisy. Dry ground, frozen surface layers, loose cables or highly resistive contact interfaces can affect survey quality. Cable layout may contribute electromagnetic coupling, especially near current shutoff. Appropriate field design and calibration reduce but do not remove these risks.
A high apparent chargeability value should therefore be evaluated alongside contact resistance, current stability and waveform quality. Changing electrode spacing or a cable route can alter the response even if subsurface material remains unchanged. Reproducibility at repeated configurations is useful evidence that an anomaly is not merely a field setup artefact.
Clay and metallic-mineral effects can overlap
Disseminated metallic minerals can create strong polarisation responses, but clay minerals and interfaces with high surface area can also contribute. Pore-fluid ionic strength, pH and texture influence the measured signal. Therefore, chargeability is not a one-to-one identifier of metallic ore, contamination or a specific clay species.
The US EPA induced polarization overview describes these mechanisms and applications. A geological interpretation should combine IP with resistivity, sampling, mineralogy or other independent evidence. ‘High chargeability consistent with polarizable material’ is appropriately bounded language; ‘IP proves sulphide ore’ is much stronger than the measurement alone usually warrants.
Check electrode layout before comparing profiles
A dipole–dipole array and a Wenner array can have different sensitivity patterns and signal levels even at apparently similar electrode spacing. Survey topography, line orientation and target geometry may further affect measured response. Apparent chargeability plots from different layouts should not be compared as though each pixel directly sampled the same ground volume.
Record current electrodes, potential electrodes, spacing, line direction and positioning uncertainties. If inversion is used, check whether the correct geometry and topography were included. A systematic array mismatch can create a plausible but false electrical section. The best way to prevent overinterpretation is to treat acquisition geometry as a measured input throughout analysis.
An initial diagnostic from a single decay plot
Suppose a decay decreases smoothly across the first several trusted gates but then begins alternating positive and negative at late time. One possibility is that the later voltage has approached the instrument or environmental noise floor. It would be premature to infer a new material relaxation mechanism from the sign changes without checking amplitudes and repeatability.
Now suppose the earliest gates show a large spike tied to transmitter shutoff. That pattern suggests examining electromagnetic coupling and instrument settling before calling it geological polarisation. A useful quality-control narrative describes alternative artefacts before selecting the physically attractive explanation.
A chargeability unit of milliseconds needs explanation
Some time-domain IP conventions normalise the area under the secondary-voltage curve by the primary voltage but do not divide by the integration interval. Their result consequently has the dimensions of time, often displayed in milliseconds. Other systems report average normalised voltage as a ratio, often in millivolts per volt. Both may be called apparent chargeability.
For example, if the normalised average response is 0.020 over one second, its time-integrated normalised value is 0.020 seconds, or 20 milliseconds, under the simplified definition. The numerical coincidence with 20 mV/V occurs here because the chosen window is exactly one second. Change the window duration and the two reported numerical values will generally diverge. Never compare unit labels by number alone.
Two windows can yield different chargeability values
Imagine a response whose average secondary voltage is 10 millivolts from 0.1 to 0.3 seconds but only 2 millivolts from 1.0 to 1.5 seconds, with primary voltage 200 millivolts. The early window’s average normalised chargeability is 50 mV/V; the later window’s is 10 mV/V. Both are valid arithmetic summaries of the invented curve.
They do not contradict one another because they sample different parts of the decay. A report listing ‘chargeability equals 50’ without units or gate timing is incomplete. To compare two sites or two papers, align time intervals, normalisation and cycle duration. A change in an instrument default could otherwise be mistaken for a large geological difference.
Integrated chargeability can reward a longer window
Suppose a residual voltage remains small but positive over a long interval. An unnormalised time integral of the voltage divided by primary voltage will grow with window length even when average response is modest. This is why chargeability reported as an integrated time parameter cannot be compared across incompatible time windows by simply treating every value as a material constant.
If two papers use different integration limits, report them. A method may specify a standardised on/off duty cycle and window, but other implementations may differ. Respect the actual protocol rather than assuming all modern instruments reproduce a historical standard. A technically precise comparison either recalculates from common gate data or acknowledges the remaining incompatibility.
Repeat cycles are useful for a confidence test
A clean TDIP anomaly should be assessed through repeated transmitter cycles, not only one waveform. Stacking helps reduce some random noise, while comparing individual cycles can reveal unstable contact, intermittent cultural interference or drift. A high average chargeability built from inconsistent cycles deserves scrutiny.
If cycles differ systematically by current polarity, examine electrode potentials and circuit asymmetries. If later cycles show progressively stronger response, the charging time or polarisation state may have changed. These patterns may be technically or physically meaningful, but they should not be erased blindly. The method should document how outliers were flagged and why a stacked curve was accepted.
Small primary voltage amplifies normalisation errors
Chargeability uses primary voltage as a denominator. If Vp is near the instrument’s noise floor, even a small secondary voltage can produce an extreme ratio. For a hypothetical secondary average of 2 millivolts, a primary of 200 millivolts yields 10 mV/V, while a primary of only 20 millivolts yields 100 mV/V. The latter is ten times larger despite identical residual voltage.
This does not automatically mean the subsurface at the second station is ten times more polarizable. Different current paths, electrode contacts, injected current or geometry can alter Vp. Check primary signal strength and accepted error limits before treating a high normalised chargeability as exceptional geology.
Inverse models differ from apparent pseudosections
A pseudosection positions apparent chargeability observations according to an array-dependent plotting scheme. A two- or three-dimensional inversion searches for a distribution of model properties that reproduces the observed voltages or derived chargeability data under specified physics. The inversion image is not a direct section cut from the ground.
A bright chargeability body in an inversion model may be distributed or smeared by sensitivity and regularisation. Sharp contours on a colour map do not demonstrate an equally sharp mineral boundary. Compare the model with original gate data and resistivity observations. A trustworthy interpretation names whether a value came from measured apparent response or an inferred intrinsic property.
Inversion requires an appropriate resistivity background
The electrical current distribution depends on conductivity structure. Consequently a chargeability inversion often uses or jointly estimates a resistivity model to calculate sensitivity and forward responses. A poor resistivity description can influence where an IP anomaly appears or how its amplitude is recovered.
If a paper presents only the chargeability model without the resistivity model or data fit, the reader may be unable to evaluate an important source of uncertainty. Examine which resistivity inversion was used, whether it included topography and how strongly the final chargeability distribution depends on those assumptions. Imaging algorithms do not remove the physics of current flow through heterogeneous materials.
Smooth inversion may hide a narrow polarizable body
Regularisation often favours smoothly varying properties so an underdetermined inverse problem does not fit every noise fluctuation. This can spread a relatively compact polarizable target across adjacent model cells. Alternative regularisation settings may produce a sharper or more diffuse anomaly even when both models fit the same data.
A study should distinguish features stable under reasonable modelling choices from boundaries heavily influenced by smoothing. Sensitivity tests can explore this issue. A model cell’s precise chargeability is not automatically the material’s laboratory-measured property at that location; it belongs to a selected mathematical representation of the survey.
High apparent chargeability can be caused by several materials
Metallic mineral grains, clay minerals and electrochemical interfaces can all contribute to polarisation. Their effects depend on pore-fluid chemistry, grain size, surface area and rock texture. A high IP anomaly is useful for directing further investigation, but it does not independently prove a particular metal concentration or pollution source.
The US EPA discussion of induced polarization explicitly addresses metallic minerals and clays. If a report calls an anomaly an ore zone solely because chargeability is elevated, ask which geological evidence supports that identification. Sampling, mapped structure and complementary resistivity or geochemical data can reduce ambiguity.
The influence of pore-fluid salinity
Electrical current in many soils and rocks is carried partly by ions in pore fluids and along mineral surfaces. Increasing salinity can change bulk conductivity and the relative contribution of polarisation processes. An apparent chargeability response can therefore vary even when the rock framework remains similar.
When a time-lapse TDIP survey reports changing chargeability, inspect whether temperature, water saturation and pore-water conductivity also changed. A new fluid could alter the electrical response without new minerals appearing. A responsible environmental interpretation identifies physical and chemical alternatives to the preferred explanation and tests them when possible.
Time-lapse surveys add a new uncertainty category
Repeated surveys can monitor changes associated with groundwater, remediation or other processes, but repeatability depends on electrode positions, contact resistance, temperature and the same current-cycle configuration. If a field crew changes electrode spacing or gate timing between visits, an apparent difference may arise from acquisition rather than subsurface change.
A useful time-lapse report documents baseline and follow-up settings, performs reciprocal or repeat checks where relevant, and discusses whether any inversion constraints enforce similarities between surveys. Do not call every coloured difference in a model a measured physical change. Separate response changes from model changes and show their relationship.
Resistivity–chargeability pairing is informative, not unique
Consider a zone with low resistivity and high chargeability. It might be consistent with certain clay-rich or metallic-mineral-bearing materials, but other combinations of fluids and interfaces could also generate the response. A high-resistivity, high-chargeability zone likewise requires interpretation in its specific setting. Resistivity and IP are complementary observables.
A two-property plot can be useful for clustering responses, but it cannot turn broad electrical classes directly into rock taxonomy without calibration. Report the data ranges, environmental conditions and possible overlapping material signatures. The aim is to narrow hypotheses, not to assign a material identity from a lookup table.
A worked resistance and chargeability comparison
Imagine two electrode configurations. At station A, primary voltage is 100 millivolts, injected current is 50 milliamperes and averaged secondary voltage in a stated gate is 2 millivolts. Measured primary resistance is 2 ohms, and normalised chargeability is 20 mV/V. At station B, Vp is 50 millivolts, current 50 milliamperes and secondary voltage 1 millivolt.
Station B has primary resistance of 1 ohm but the same 20 mV/V chargeability. That is not a contradiction: resistance and normalised IP response describe different relationships. To calculate apparent resistivities, one must also know the geometry factors. A student should never infer equal resistivity merely because normalised chargeability happens to match.
A decay plot is a time series, not a depth profile
Time after transmitter shutoff describes relaxation of polarisation and instrumental processes, not the depth of a target. A gate at 0.5 seconds does not mean the response came from 0.5 metres underground. Spatial sensitivity still depends on electrode geometry and subsurface electrical structure.
This is especially important because transient electromagnetic sounding also uses time gates, but measures a different physical response. The TEM transient-decay guide explains induced eddy-current decays under electromagnetic induction. Similar-looking graphs should not lead a reader to copy a TEM depth interpretation into a TDIP voltage relaxation analysis.
TDIP versus spectral IP asks a different question
Time-domain IP describes response after controlled current shutoff, often through integrated decay gates. Spectral induced polarization measures complex electrical response across multiple sinusoidal frequencies, including phase shifts and dispersion. The methods are related by linear-system theory under appropriate assumptions, but practical acquisition, noise and parameterisations can differ.
Do not call a TDIP chargeability value the same thing as the phase in milliradians of a spectral IP survey. A model fitted to both may use a common relaxation family, but its assumptions should be stated. Where a site shows different results, check measurement bandwidth and sensitivity before inventing conflicting geological explanations.
Cole–Cole relaxation fits are models, not direct clocks
Some time-domain or spectral IP analyses fit relaxation parameters through a model such as the Cole–Cole family. Such models can summarise time constants and a distribution of relaxation behaviour. They do not directly identify a single physical grain size or unique mineral without further evidence.
Different parameterisations may define characteristic time and exponent differently. A report that quotes a fitted relaxation time should provide the model equation and accepted frequency or time band. Otherwise, transferring a formula from one paper to another may produce inconsistent interpretations. A good reader asks whether parameters are well constrained or strongly correlated in the fitting process.
Data weighting can change an IP inversion
Early gates may contain strong amplitudes but more coupling; late gates may contain weak signals and higher relative noise. Inversion weights decide which observations dominate the fit. If the error model is unrealistic, a recovered chargeability section can overfit certain gates or neglect useful ones.
Inspect which gates were included, whether errors were assigned by gate and how outliers were handled. A visually smooth inverted section is not proof that individual data windows were fit accurately. The residual plot may reveal systematic time-dependent misfit even when a single aggregate statistic appears acceptable.
RMS misfit depends on the error definition
Inversion software often reports a root-mean-square data misfit. That statistic may be normalised by the assigned uncertainties. A low RMS obtained with large assumed errors does not necessarily mean the predicted voltages closely match the observations in absolute units. Conversely, unrealistically small errors can cause overfitting.
Compare observed and predicted waveforms or apparent chargeabilities as well as the scalar misfit. A model that fits early gates but systematically misses late-time behaviour may be physically incomplete. The safest conclusion identifies which features the data genuinely require and which remain dependent on regularisation, error weights or starting models.
An end-to-end diagnostic example
Suppose an inverted TDIP section shows a narrow high-chargeability body at a location where field electrodes had persistently poor contact. The first response should not be to label it a metal-rich vein. Inspect the raw decay curves, current stability, reciprocal measurements and whether the anomaly persists across electrode configurations and repeat cycles.
If the feature survives quality control and modelling alternatives, geological sampling can test its material origin. This staged approach separates an interesting signal from an independently verified geological cause. The same logic applies when the anomaly looks attractive because it aligns with a mapped boundary; visual coincidence is not sufficient confirmation.
Source and measurement uncertainty should travel into the summary
A report might state that a survey revealed ’30 mV/V chargeability at 20 metres.’ A more precise sentence identifies whether 30 mV/V is an apparent value from a specific gate or an intrinsic value assigned to an inversion cell, and whether 20 metres is a modelled depth rather than an electrode pseudo-depth.
The precision of the number should reflect the error and resolution. A modelled depth range may be more honest than one exact contour boundary. Technical translation helps readers by turning shorthand into statements whose method, unit and uncertainty can be checked. It should not exaggerate confidence simply to sound decisive.
Common mistakes and targeted repairs
One mistake is comparing chargeabilities in milliseconds and mV/V without understanding the integration window. Repair it by reconstructing the formula. Another is calling a late-time voltage a direct rock potential measured at one depth. Repair it by tracing electrode geometry and response physics. A third is treating a high IP anomaly as definitive sulphide ore. Repair it with material ambiguity and independent tests.
A fourth mistake is including electromagnetic switching spikes in the accepted decay without quality control. A fifth is treating gate index as elapsed time. A sixth is confusing a measured pseudosection with an inversion model. Each error can be repaired by checking original units, time boundaries, data quality and the model’s role.
Practice: compute before interpreting
An invented TDIP observation reports primary voltage 400 millivolts and an average secondary gate voltage 8 millivolts. What is normalised chargeability? The ratio equals 8 divided by 400, or 0.020, which is 20 mV/V. If the next gate averages 4 millivolts, its value is 10 mV/V under the same Vp. The difference indicates decay under the instrument’s operational definition.
Does that pair of numbers identify grain size or ore mineral? No. It describes two time-window averages and their normalisation. To infer geological cause, check the full decay, resistivity, electrode conditions, repeatability and independent information. Arithmetic establishes a trustworthy measurement statement, not a unique material identification.
More practice: dimensional checking
A student has 25 millivolts of primary potential and 50 milliamperes of injected current. The measured primary resistance is 0.5 ohms. With an assumed geometry factor of 100 metres, apparent resistivity is 50 ohm-metres. The student should show the conversion from milli-units cancels correctly in the voltage/current ratio.
If the same student reports this number as chargeability, the answer is conceptually wrong. It describes apparent resistivity. Chargeability requires a secondary decay and a separate defined normalisation. This distinction between adjacent columns is among the simplest and most useful checks in electrical geophysical interpretation.
Frequently asked questions
Does TDIP measure naturally occurring self-potential alone? No, it applies a current and measures polarisation response. Is chargeability always dimensionless? No, some reporting conventions use time units. Does a high mV/V value prove metallic ore? No. Are the first gates always trustworthy? No, switching artefacts can intrude. Is the last gate always best? No, late-time noise can dominate.
Does one gate correspond to one depth? No. Is apparent chargeability a direct rock sample? No. Can pore-fluid chemistry affect IP response? Yes. Can time-domain and spectral IP be compared without conversion? Not reliably. Does a low inversion RMS prove a unique subsurface model? No. These answers should be carried into a reading of every TDIP figure.
Authoritative sources and contextual learning
The US EPA induced polarization and complex resistivity method explains voltage decay, primary normalisation, gate windows and polarisation mechanisms. The companion US EPA electrical resistivity method explains the four-electrode geometry and apparent-resistivity definition. The US EPA borehole IP method shows how IP conventions can also apply in a different acquisition context.
For neighbouring curves, compare TEM induced-current decay and magnetotelluric impedance phase. To preserve meaning across technical language, explore cross-linguistic transfer, translation and meaning across languages and context lost in translation.
The lasting lesson: follow the current cycle and the denominator
Time-domain IP starts with controlled current injection, develops a polarisation response and observes voltage decay after shutoff. A gate extracts part of that curve; chargeability normalises it under a chosen definition; inversion may then estimate a distribution of polarizable properties. Those stages are useful only when the reader keeps them separate.
If you can name the current cycle, primary reference, accepted gates, chargeability units, electrode geometry and model assumptions, the graph becomes understandable without guessing its geological source. That is the foundation of accurate TDIP interpretation and a better way to communicate any technical measurement.