A microgravity survey measures extremely small differences in gravitational acceleration to investigate changes in the mass and density of materials below the ground. To interpret microgravity maps, Bouguer gravity anomalies, free-air and terrain corrections, or possible subsurface voids, first distinguish the gravimeter’s observed reading from a processed residual anomaly. A negative anomaly can be consistent with a low-density zone, but a blue patch on a contour map is not itself a verified cavity.
The essential physics is gravitational attraction: denser materials generally contribute more mass over a given volume, while voids or lower-density materials can reduce the expected signal. But the measured field also varies with station elevation, latitude, tides, instrument drift and surrounding terrain. Corrections are used to isolate the part of the signal that might reflect a target. Each correction carries assumptions, so a final Bouguer anomaly is a carefully processed quantity rather than direct observation of rock density.
This article explains how to read gravimeter records, units of mGal and microGal, correction steps, free-air versus simple and complete Bouguer anomalies, regional–residual separation, potential void signatures and uncertainty. Original arithmetic examples use declared fictional values and are checked for units. The purpose is to help learners and professionals interpret a gravity map without confusing data correction, geological inference and confirmed ground conditions.
Quick answer: what is a microgravity anomaly?
A microgravity anomaly is a relatively small variation in gravity recorded over a survey area after relevant corrections and referencing. Its size may be discussed in microGals, where one microGal equals one-millionth of a Gal and one-thousandth of a milliGal. Interpretation depends on survey goals, instrument sensitivity, station spacing and the density contrast between a target and surrounding materials.
A local gravity low may suggest a mass deficit, which can be compatible with a void, less dense sediment, altered material or geometry at depth. The signal is not unique to one feature type. A trustworthy report treats the anomaly as evidence for a potential density difference, then assesses alternative models and independent control such as boreholes, engineering records or another geophysical method.
Why gravity works for investigating unseen density
Every mass attracts other masses. A gravimeter records variations in gravitational acceleration at its location, which includes effects from the whole Earth and smaller local mass differences. A dense buried body can increase the measured field relative to a comparable background, while material lighter than the assumed background can contribute a relative deficit.
The method does not sample a sharply defined point directly below the sensor. Gravity integrates effects from masses distributed in three dimensions. A deep broad density contrast and a shallow compact source can sometimes produce similar surface observations. This nonuniqueness is central: gravity supports plausible subsurface models but seldom identifies an object’s shape or material uniquely without external information.
Begin with the question and target scale
A regional gravity map may investigate crustal structures across many kilometres, while a microgravity survey might seek shallow cavities or density contrasts at metre to tens-of-metres scales. Their sampling strategies, correction precision and appropriate interpretive statements differ. The adjective micro refers to the scale of gravity changes being investigated, not necessarily microscopic objects.
Before interpreting the colours, identify the target: a possible void, buried channel, contact between rock types or change in groundwater mass. Then check whether station spacing, positioning accuracy and survey extent are suitable to detect that target. A beautiful smoothed contour plot cannot recover a compact anomaly never adequately sampled.
The units Gal, mGal and microGal
The Gal is a unit of acceleration equal to one centimetre per second squared, or 0.01 metres per second squared. One milliGal is one-thousandth of a Gal, equal to 0.00001 metres per second squared. One microGal is one-millionth of a Gal, equal to 0.00000001 metres per second squared, or 0.001 milliGal.
A map labelled +30 microGal describes a much smaller difference than one labelled +30 mGal. Mixing them is a thousandfold error. Always read the unit printed by the instrument and used after processing. In local engineering work, small microGal-scale variations can matter, making careless unit conversion more damaging than it might appear on a page.
Observed gravity is dominated by the wider Earth
Most measured gravitational acceleration comes from Earth as a whole, not the buried feature a survey hopes to identify. Earth is not a perfect sphere; rotation and latitude affect the expected reference field. Station elevation also changes the distance to mass below and influences the observed acceleration.
Therefore, a gravimeter reading of approximately 9.8 metres per second squared is not yet an anomaly map. Scientists work with small differences relative to an appropriate reference field and after standard corrections. A field sheet that records only the raw sensor number cannot be interpreted as a void-detection result without the survey’s reduction and quality-control steps.
Relative gravity and absolute gravity are different
Many field gravimeters measure relative gravity changes between positions using a sensitive spring or another sensing principle. To compare stations, measurements must be tied to a network or base value. Absolute instruments can measure gravity in a more direct reference sense using a different technology, but they bring their own setup and processing requirements.
A local microgravity project may not need an absolute gravity value at every station if differences are reliably referenced. What matters is a transparent datum and control of temporal drift. A report should say what reference was used and which sensor, field schedule and base ties produced the final values. Unspecified ‘gravity units’ are insufficient for reproducibility.
Instrument drift can resemble a site trend
A relative gravimeter may change its indicated value gradually even if the physical station gravity stays constant. Temperature sensitivity, mechanical response and other instrument behaviour can contribute drift. A survey that moves monotonically along a line while the instrument drifts may create an artificial spatial gradient.
Repeated readings at a base station help estimate drift through time. When the instrument returns to the base, a difference from the earlier reading can guide correction. The correction itself assumes something about drift between observations, such as approximate smoothness. Document base reoccupations and their timing rather than assuming a software button has removed every source of instability.
Earth tides matter at small signal levels
The Moon, Sun and Earth produce time-varying gravitational influences. Relative field surveys intended to resolve small anomalies may need to account for tidal gravity changes and other temporal variations. A station read in the morning and reoccupied later can differ even when the buried geology did not change.
A valid workflow either applies an appropriate tidal correction or justifies why its design and differencing control the effect. It is not enough to attribute every repeated-station difference to the instrument. Field chronology is part of gravity data, not an administrative detail to discard after constructing a map.
Station elevation is unusually important
A gravimeter positioned higher above the Earth’s reference surface generally records a different gravity value partly because of greater distance from Earth’s mass. Elevation must be measured accurately to apply a defensible correction. In shallow microgravity work, centimetre-level errors can be consequential because the expected anomalies themselves may be small.
The US EPA gravity-method guide emphasises precise station elevation for microgravity investigations. When a map claims to detect a small cavity but survey elevations came from an unverified low-precision source, readers should inspect whether height-related uncertainty could rival the reported anomaly.
The free-air correction isolates a geometric effect
Under a commonly used near-surface approximation, the free-air gravity correction has a magnitude near 0.3086 milliGal per metre of elevation difference. The correction addresses the reduction in gravity with height above a reference level before considering the mass of rock between observation and datum.
If a station is five metres above the chosen datum, the illustrative free-air addition is approximately 0.3086 multiplied by five, or 1.543 milliGal. This calculation does not mean a five-metre hill contains zero rock; it isolates the geometric height effect in one step. Actual regional work may use more precise formulas appropriate to location and survey standards.
The Bouguer slab correction accounts for intervening mass
After the free-air step, a simplified Bouguer correction considers the gravitational attraction of a uniform slab of material between the measurement level and the reference datum. A conventional infinite-slab approximation has magnitude about 0.04193 times density in grams per cubic centimetre times elevation in metres, producing milliGals.
For density 2.67 grams per cubic centimetre and height five metres, the slab magnitude is approximately 0.04193 multiplied by 2.67 multiplied by five, equal to about 0.560 milliGal. In a typical simple-Bouguer reduction above the datum, this slab effect is subtracted from the free-air-corrected value. The density is an assumption requiring disclosure.
A worked simple-Bouguer reduction
Imagine a fictional station where observed gravity minus the theoretical reference gravity is minus 1.000 milliGal before height and slab corrections. With a five-metre elevation, the illustrative free-air addition is +1.543 milliGal. Subtracting the simple Bouguer slab effect of approximately 0.560 milliGal gives a simple-Bouguer anomaly of about minus 0.017 milliGal.
The arithmetic is minus 1.000 plus 1.543 minus 0.560. This example omits latitude refinements, terrain corrections and other project-specific terms. It demonstrates how the order and signs of reductions matter. Without describing the datum, density and applied corrections, the final small value looks more direct than it really is.
Why terrain correction is separate from the slab
Real landscapes are not infinite flat rock slabs. Hills above a station, valleys nearby and irregular topography can change gravitational attraction relative to the simple Bouguer model. Terrain corrections account for these deviations using surveyed elevation information, a digital terrain model and computational assumptions.
In the common reduction convention the net terrain correction is added to the simple Bouguer value, but actual software may express intermediate terms differently. Always inspect the adopted sign convention. A complete Bouguer anomaly is not obtained by simply subtracting every visible correction column regardless of definitions. Processing documentation is necessary to interpret the final map.
Simple Bouguer and complete Bouguer are not synonyms
A simple Bouguer anomaly includes a slab approximation but not the full modelled effect of surrounding terrain irregularities. A complete Bouguer anomaly generally incorporates terrain correction in addition to the other reductions. Both remain dependent on assumed density and the reference field. The choice may matter substantially in rugged terrain or where small near-surface anomalies are the goal.
The USGS gravity-grid explanation distinguishes free-air, simple and complete Bouguer anomalies. The terms describe processed products with different included corrections. A figure labelled ‘Bouguer’ should therefore state whether it means simple or complete, particularly before two datasets are compared.
Choosing reduction density is a scientific decision
The conventional reference density 2.67 grams per cubic centimetre is often used in regional work, but actual subsurface materials can differ. Sediments, weathered rock, cavities and compact crystalline rock create contrasts. If the reduction density is poorly matched to the terrain, topography-correlated artefacts can remain in the anomaly map.
A good study explains why its reduction density was chosen, whether rock samples or other geophysical evidence support it, and how sensitive the outcome is to plausible alternatives. Changing the density does not change the raw gravimeter observations, but it can change the processed anomaly. That distinction is central to any honest comparison of different gravity products.
A sensitivity test for the slab assumption
Use a fictional five-metre station again. If the reduction density is 2.40 instead of 2.67 grams per cubic centimetre, the slab magnitude becomes about 0.503 milliGal rather than 0.560 milliGal. The difference is approximately 0.057 milliGal, or 57 microGal. That can be much larger than the target signal in some shallow surveys.
This arithmetic shows why a small density assumption deserves attention. The correction model can influence the amplitude and spatial trend of a map even when instrument precision is excellent. A sensitivity table varying reduction density may be more informative than publishing one overconfident contour interpretation.
Microgravity station spacing and buried targets
A compact target can produce a short-wavelength gravity anomaly that is missed when sampling points are too far apart. Conversely, dense sampling can improve definition but cannot remove ambiguity between possible three-dimensional density distributions. The spacing should reflect the expected target depth, size and required confidence, balanced against terrain and practical access.
Contour interpolation can create a smooth closed low between widely separated measurements. That attractive ellipse is not independent data. Always inspect original station locations and measured values. If an anomaly was inferred mainly by gridding between sparse stations, state the limitation and consider targeted additional readings.
A gravity low does not prove an empty cavity
A cavity replaces surrounding material with air or water and may create a relative mass deficit, depending on density contrast and geometry. But low-density sediment, altered rock, a buried channel or an incorrectly corrected topographic feature may also produce a gravity low. The field response is integrated over distance, so distinct sources can produce similar surface patterns.
A safe interpretation describes a localised negative residual consistent with a possible mass deficit. It then lists plausible sources and explains which geological or engineering evidence makes one interpretation more likely. Excavation or geotechnical decisions should not be based solely on contour colours. Gravity mapping is evidence for investigation, not a guarantee of subsurface geometry.
The depth–mass ambiguity in gravity interpretation
Consider two fictional models. One contains a small but very low-density cavity close to the surface; another contains a larger region with a modest density reduction farther down. Both can produce a negative surface anomaly with comparable amplitude and shape over a limited survey interval. Gravity alone may not uniquely distinguish their mass distributions.
This is the inverse problem: many subsurface arrangements can predict similar measurements. Model fitting can narrow possibilities when assumptions about depth, geometry and density are justified. It cannot manufacture uniqueness. A responsible report tests alternative models and states which parameters are constrained by data and which depend strongly on prior geological information.
The width of an anomaly is not a direct cavity diameter
A gravity source influences measurements at positions beyond its physical footprint. The response becomes broader with depth, and irregular geometry may distort its shape. Thus, an anomaly extending ten metres across a map does not necessarily correspond to a void exactly ten metres wide. The observed wavelength includes the effects of source depth and integration.
Readers should check whether an author’s source-size estimate came from a quantitative forward model, inversion or merely visual inspection of a contour ellipse. If no model or calibration is described, use cautious words such as ‘anomaly zone’ instead of ‘cavity outline’. This distinction prevents an interpretive boundary from being copied as an engineering excavation boundary.
Forward modelling tests a proposed subsurface structure
In gravity forward modelling, researchers specify a hypothetical density distribution and calculate the gravitational response that it would produce at survey stations. They compare the predicted values with observed and corrected anomalies. If the fit is poor, the hypothesis may need to change; if it is good, the model becomes plausible but not uniquely proven.
Different parameters can compensate for one another. Greater depth, increased source size or stronger density contrast may produce similar curve amplitudes. A transparent study reports the geometry assumptions, fixed parameters and error criterion used in the fit. The model supports a constrained interpretation, not a detailed photograph of every underground boundary.
Inversion estimates a model from observations
Gravity inversion begins with field observations and estimates a density distribution satisfying a chosen set of constraints. Because the problem is underdetermined, inversion often uses regularisation, smoothness, compactness assumptions, bounds or geological prior information. The output is one solution favoured by those choices, not a uniquely measured density image.
A smooth inversion might spread a compact void into a broad diffuse low-density zone, while a compactness-promoting inversion may focus the anomaly differently. Both may fit measured data. Readers should compare data residuals, model assumptions, resolution and sensitivity before treating the coloured inversion cells as literal geology.
Regional and residual fields answer different questions
A processed gravity map often contains broad trends related to deeper or regional structures and shorter-wavelength components possibly associated with local features. Analysts may estimate a regional field and subtract it to form a residual map. A residual anomaly can reveal a near-surface target more clearly, but the separation is not unique.
A high-order polynomial or aggressive high-pass filter can create artefacts, erase real broad features or distort amplitudes near map edges. The reader should identify the method and scale of separation before comparing residual peaks. A small negative residual might be meaningful locally while having no direct correspondence to a similarly sized feature on a regional Bouguer map.
Upward continuation smooths shallow contributions
Upward continuation mathematically estimates what a gravity field would look like if observed on a higher surface under appropriate potential-field assumptions. Because nearby small-scale sources attenuate more strongly with upward distance, the transformed map tends to emphasise broader, deeper contributions. It is a processing technique rather than an additional measurement at altitude.
Subtracting an upward-continued field can help isolate shorter-wavelength structure, but the result depends on chosen continuation height. Too large a height may remove features that matter for the survey objective. A report should show the original corrected map and identify the continuation parameters. A residual produced by filtering cannot be interpreted without knowing what was removed.
Terrain errors can masquerade as subsurface structure
Suppose a row of gravity stations climbs a slope. If station elevations or nearby terrain corrections are inaccurate, the remaining height-related signal may resemble a geologic anomaly aligned with the hillside. Such systematic error can be particularly severe when the target is only tens of microGal in amplitude.
The 2025 USGS study on lidar-derived elevation models for gravity reduction investigated how elevation data quality affects processed gravity. Its findings address specific regional-survey accuracy conditions and do not prove that any unverified elevation source is suitable for centimetre-sensitive microgravity. The practical lesson is to match elevation accuracy to the anomaly scale and correction gradients.
Estimate a height-error contribution
Using a simple combined free-air-minus-slab vertical gradient at reduction density 2.67 grams per cubic centimetre, the approximate coefficient is 0.3086 minus 0.1119, or 0.1967 milliGal per metre. A height error of two centimetres, or 0.02 metres, can then produce about 0.00393 milliGal, which equals roughly 3.93 microGal in this simplified reduction.
The exact error depends on local corrections and reference conventions, but the example shows why centimetres matter. A survey searching for a twenty-microGal signal cannot ignore systematic errors of several microGal. Writing height precision in metres, then converting the resulting gravity error into microGal, reveals the relationship more clearly than saying only ‘GPS accuracy was acceptable’.
Reading drift-correction graphs
A field report may display base-station gravity readings against time with an interpolated drift correction. Compare raw and corrected base differences, instrument warm-up behaviour, time between reoccupations and whether a smooth trend reasonably describes the observed changes. Large jumps may indicate an instrument disturbance, not a process best repaired by a straight line.
Do not infer high accuracy simply because the correction made the first and last base readings match. A loop closure can hide nonlinear drift between those times. Good quality control includes intermediate checks and repeat observations at survey stations where feasible. The report should show how much uncertainty remains, not just the fact that a drift procedure was executed.
Gravimeter repeatability and survey repeatability differ
A gravimeter may produce similar values over repeated readings at one stable point, indicating instrumental repeatability. A survey that returns days later may differ because of positioning, height, environmental conditions, reference ties or genuine mass changes. These are distinct levels of uncertainty. Instrument specifications do not alone establish the total survey error.
When a paper claims a microgravity change through time, ask whether observations were made on exactly the same stable marks and corrected using consistent methods. If station location moved by a small amount near a steep anomaly gradient, the difference may reflect spatial offset rather than changing subsurface mass. Good monitoring requires both temporal and spatial control.
Time-lapse gravity can track mass redistribution
Repeated gravity measurements may reveal changes in subsurface mass related to water extraction, infiltration or other processes, provided the signal exceeds survey uncertainty and other effects are controlled. The method’s appeal lies in measuring a physical consequence of mass movement without directly sampling every point. Yet the inferred water volume remains model-dependent.
An observed decrease near a pumping location, for instance, may be consistent with a local reduction in stored water mass. But translating that signal into a unique aquifer storage parameter requires geometry, boundary conditions and additional hydrological evidence. The EPA gravity-method overview discusses groundwater and void-related applications with their method limitations.
Field examples help, but do not remove ambiguity
A documented karst study combined microgravity with multichannel surface-wave analysis and borehole observations to investigate underground conduits. The complementary methods provided evidence about mass deficits and mechanical properties from different physical perspectives. The field setting and geology mattered for interpreting the low-gravity anomalies.
A reader should not generalise a cavity detection threshold or contour shape from that site to every landfill, urban street or natural rock setting. Instead, borrow the validation logic: use gravity to target plausible contrasts, use independent methods to test the hypothesis and retain uncertainty where no direct confirmation exists. Successful published case studies demonstrate possibilities, not universal guarantees.
Combine gravity with seismic information carefully
Surface-wave or seismic methods respond to mechanical wave propagation and elastic properties, while gravity responds to integrated mass distribution. A void, low-density layer or fractured zone can affect both, but not in identical ways. Agreement can increase confidence when physical models are consistent and acquisition limitations differ.
The existing MASW dispersion and shear-wave velocity guide explains how surface-wave data are transformed into velocity models. A lower shear-wave velocity does not mathematically imply a particular gravity deficit. Interpret each signal on its own terms before combining the conclusions.
Combine gravity with radar where conditions allow
Ground-penetrating radar can resolve electromagnetic reflectors and diffraction patterns at appropriate depths, while microgravity may respond to a mass deficit even if the object has weak radar contrast. In conductive soils, however, radar penetration may be poor. A combined programme can reduce uncertainty only if both surveys actually have adequate sensitivity to the target.
The GPR radargram and dielectric-depth guide discusses hyperbolas, velocity calibration and the limits of object identification. A gravity low near a GPR reflection is useful cross-evidence, but location coincidence is not conclusive proof that both methods detected one cavity. Consider separate sources and survey resolution.
Negative residuals can emerge from filtering choices
Imagine a broad regional field sloping across a study area. Fitting and subtracting a polynomial can leave a local negative patch. If a different polynomial order or continuation height changes the patch greatly, the residual may be sensitive to the processing choice rather than a stable physical feature.
A robust interpretation compares several reasonable reductions and examines original measurements. If the feature persists with consistent position and amplitude under justified variants, confidence grows. Do not choose the one filtered map with the most visually compelling cavity-shaped low and discard the others silently. Such selective presentation can create an apparent target that the data do not reliably support.
Map interpolation can exaggerate certainty
Gravity measurements are made at finite stations, but final reports often display continuous coloured surfaces. Software interpolates between measured points. The style may include smooth contours, shaded relief or a dense grid whose pixel count is much greater than the number of independent observations.
Always overlay station positions when making detailed interpretations. A closed low defined by two or three stations should not be treated as a precise boundary. Interpolation cannot uniquely determine how the field behaves between widely spaced observations. If an area is critical, additional station measurements are a stronger way to reduce uncertainty than merely changing contour style.
The significance of a density contrast
Gravity responds to differences between the proposed source density and the surrounding density, not simply to whether a material is called rock or soil. A cavity filled with air creates a different contrast from one filled with water, and both differ from low-density sediment. The geometry and depth interact with the contrast to determine anomaly amplitude.
A model report should state source and host densities or their difference, with units such as kilograms per cubic metre or grams per cubic centimetre. If only an assumed density contrast was used, the resulting source-volume estimate remains conditional. Readers should not translate ‘gravity model volume’ into a direct measured excavation volume without geological verification.
A caution about void-depth estimates
Some interpretation methods use the width and amplitude of a gravity low to estimate how deep and large a mass deficit could be under simple shapes. These equations are teaching tools tied to specific geometric assumptions. Real cavities may have irregular roofs, multiple openings, water fill and nearby density changes that violate idealised shapes.
When a paper states that a void is ‘at four metres depth’, ask whether that value came from direct drilling, an assumed geometric model or inversion with depth constraints. Each carries a different evidential meaning. The most useful report gives a range of plausible models rather than one unjustifiably exact depth derived from a surface anomaly.
Gravity gradients are not the same as anomaly amplitude
Anomaly amplitude is the difference in processed gravity relative to a reference or surrounding field. A spatial gradient describes how rapidly gravity changes with distance. A steep gradient may identify the edge of a localised response, but its size depends on source geometry, depth, station spacing and processing.
A strong spatial gradient is not a direct measurement of a rock-wall slope. Likewise, a broad low need not be a large empty chamber. Preserve the units: microGal for anomaly values and microGal per metre for a lateral derivative. If software displays a derivative map, label it explicitly rather than comparing its colour scale directly with the original gravity values.
Detectability depends on signal relative to total uncertainty
A model might predict a fifteen-microGal anomaly from a target. Whether that is detectable depends on instrumental repeatability, station heights, drift, tides, neighbouring mass changes, survey geometry and reduction uncertainty. A prediction cannot be treated as a confirmed signal merely because fifteen is larger than one quoted instrument precision.
A proper detection argument compares the full estimated effect with the uncertainty of the actual corrected data. Repeat surveys and independent stations help assess reliability. If the predicted amplitude lies near the survey’s uncertainty, report it as an unresolved potential target rather than a positive identification. This protects both scientific accuracy and field decision-making.
A short worked map-reading exercise
Imagine three corrected stations along a line with residual anomalies of +8, -18 and +7 microGal after a clearly documented regional correction. The centre shows a relative low of roughly 25 to 26 microGal compared with its neighbours. This observation can justify investigating a local mass deficit, but it cannot by itself determine a void diameter, fill material or excavation depth.
If the centre’s elevation is uncertain or the station layout is sparse, additional readings may change the interpretation. The correct summary is a local negative residual of a stated magnitude, with a specified survey basis. It should not become ‘a 26-microGal underground cavern’ because microGal measures acceleration, not cavity volume.
Common false interpretations and how to repair them
One mistake is to call every gravity low a cavity. Repair it by considering alternative density contrasts and independent confirmation. Another is to compare raw gravity values from stations of different elevation without the required corrections. Repair it by checking reference field, free-air, Bouguer and terrain reductions.
A third mistake is to read microGal as milliGal, creating a thousandfold unit error. A fourth is to accept a filtered residual without examining what the filter removed. A fifth is to treat a smooth interpolated contour as direct measurement between stations. A sixth is to assume a good inversion fit proves one unique geometry. Each error can be corrected by describing the measurement and model separately.
How to read a completed gravity-survey report
Locate the survey objective, target scale and field layout first. Next check instrument type, reference station, drift and tide controls, station elevation and positioning accuracy. Then inspect the latitude or theoretical gravity reference, free-air correction, Bouguer slab density and terrain reduction. Confirm whether the displayed product is raw, free-air, simple Bouguer, complete Bouguer or regional residual.
Only then study anomaly sign, magnitude, spatial extent, line profiles and model fits. Ask whether independent control supports the inferred rock type or cavity. The best interpretation is not the one with the most confident label; it is the one whose conclusions can be traced back through measurements and correction assumptions.
Transfer exercise: two reduction densities
A team computes a Bouguer anomaly map using density 2.67 grams per cubic centimetre, then repeats it at 2.40. A negative feature shifts in amplitude. Did the gravimeter measure new gravity? No. The original station readings are unchanged. What changed was the assumed slab-mass correction and therefore the processed anomaly.
The correct next step is to compare both maps against plausible geology, terrain and external density evidence. If the choice strongly affects the target inference, report that sensitivity. This exercise transfers to any inversion method: a model output can change when assumptions change, even when the underlying physical measurements do not.
Independent practice about units and height errors
A student reads a processed anomaly of -0.045 milliGal and writes -45 microGal. Is the conversion correct? Yes, because one milliGal equals one thousand microGal. Another student assumes that a station elevation error of 0.1 metre is trivial. Under a simplified combined correction of about 0.197 milliGal per metre, it corresponds to nearly 0.020 milliGal, or 20 microGal.
The error may be comparable with a target anomaly. Therefore, ‘trivial’ cannot be judged by the height alone; it must be compared with the survey’s sensitivity needs. The correct explanation includes a unit calculation and considers the full correction procedure rather than automatically treating a GPS value as exact.
Questions readers frequently ask
Does a negative Bouguer anomaly always indicate an empty hole? No; many density distributions can cause a low. Is a microGal the same as a milliGal? No; one thousand microGal equal one milliGal. Is a simple Bouguer anomaly the same as a complete Bouguer anomaly? No; terrain correction distinguishes them. Can microgravity measure every shallow pipe? No; detectability depends on mass contrast and survey resolution.
Does a strong contour pattern prove a unique subsurface model? No. Why revisit the base station? To help estimate drift and temporal effects. Can an elevation error distort a small anomaly? Yes. Does upward continuation add field measurements? No; it is a mathematical transformation. Can time-lapse gravity help study changing water mass? Yes, with suitable controls and modelling.
Sources and related reading
The US EPA Gravity Method guide explains density contrasts, microgravity applications, corrections and interpretation ambiguity. The USGS description of corrected gravity grids distinguishes free-air and Bouguer products. The 2025 USGS study of elevation models in gravity reduction illustrates why elevation quality affects the processed result. Each should be applied within its actual survey setting.
For complementary evidence, review GPR radargrams, MASW shear-wave inversion, soil ERT interpretation and cross-linguistic transfer. These measure different properties, so agreement and disagreement should be interpreted physically rather than by matching colours.
The lasting distinction is measurement versus density model
Microgravity provides a sensitive way to observe small differences in the gravitational field and investigate plausible mass distributions below the surface. Its strength lies in careful acquisition and correction. Its limit is that different buried structures can produce similar gravity responses, especially when elevation, density and sampling uncertainties are unresolved.
To translate a microgravity result accurately, begin with measured acceleration and its units, follow the reference and correction chain, describe the remaining anomaly and then state which models are consistent with it. This makes a gravity map both easier to understand and more useful for responsible subsurface decisions.