DENSITY-CONTRAST INVESTIGATION
Microgravity Survey for Subsurface Screening
Use carefully corrected gravity measurements to investigate subtle subsurface mass contrasts and prioritise verification of suspected voids or geological changes.
ENGINEERING OVERVIEW
Reading the gravity signal
A microgravity survey measures small spatial changes in gravitational acceleration that may reflect variations in underground mass and density. After careful correction for elevation, drift, tides and surrounding terrain, residual anomalies can inform a model of buried features. A negative anomaly may be consistent with a density deficit, but the cause, size and depth are rarely unique.
Potential cavities, buried channels and irregular rockhead conditions can create serious uncertainty for infrastructure planning. Microgravity offers a low-disturbance screening method where a density contrast is expected, particularly when intrusive coverage is limited. The challenge is distinguishing genuine subsurface mass differences from topography, building effects and measurement artefacts.
MEASUREMENT & INTERPRETATION
What the method actually tells you
Physical principle
Gravity observations respond to the combined mass distribution beneath and around a station. Denser bodies commonly produce positive relative anomalies; lower-density features can produce negative ones.
Survey elevation matters
Small elevation errors can be significant relative to a subtle anomaly. Precise station control is therefore fundamental.
Corrections are part of the result
Instrument drift, Earth tides, latitude, free-air, Bouguer and terrain effects are considered as appropriate to the survey design.
No unique inversion
Different combinations of anomaly size, depth and density contrast can produce similar gravity signatures. Geological constraints are necessary.
PRACTICAL SURVEY FLOW
From site question to defensible interpretation
Assess expected contrast
Establish plausible density models and target dimensions, with access and surrounding structures mapped.
Design a station network
Choose spacing, base station, levelling strategy and repeat observations to suit the target and terrain.
Measure and correct
Acquire gravity and station-elevation data; document repeatability, drift and required corrections.
Interpret alternatives
Evaluate regional trends, residual anomalies and density models against borehole logs, surface geology and other geophysical evidence.
WHERE IT ADDS VALUE
Engineering applications and use cases
Candidate void or karst screening
Investigate anomalously low-density zones, with targeted confirmation before design or remediation decisions.
Rockhead and buried feature mapping
Explore density contrasts that may reflect infilled valleys, intrusive bodies or structural variation.
Time-lapse mass change studies
Under carefully controlled conditions, repeat gravity observations can examine changes in stored groundwater or other mass distributions.
MEASUREMENT BOUNDARIES
Limitations and conditions of use
Interpretation quality depends as much on survey suitability and independent evidence as on processing software.
- Urban structures, underground services, moving heavy equipment and topography complicate microgravity correction and interpretation.
- A shallow small anomaly and a deeper larger anomaly may be difficult to distinguish from gravity data alone.
- Density contrast may be too weak or masked by background variation for the intended target.
- Negative gravity anomalies are not evidence of a cavity without supporting ground investigation.
METHOD SELECTION
How to choose the right approach
Microgravity versus electrical resistivity
Gravity responds to mass contrast; resistivity responds to electrical-property contrast. Both may screen void-related anomalies but neither uniquely confirms a cavity.
Microgravity versus seismic methods
Seismic surveys constrain wave velocity and interfaces, whereas gravity integrates the mass effects of surrounding ground.
One-time versus repeat surveys
Baseline mapping seeks spatial anomalies; repeat surveys seek changes over time and require highly consistent stations and corrections.
Choose microgravity where the primary question is sensitive to density differences and survey control can be achieved. For high-consequence suspected voids, plan corroborating resistivity, seismic or drilling investigations before claiming a defined cavity geometry.
ENGINEERING ASSURANCE
Field quality, interpretation and review
A traceable project record should retain the following information where applicable:
- Gravimeter calibration, base loops and drift records
- Station location and precise relative elevation control
- Terrain, tidal and other required corrections with audit trail
- Repeat readings, residual checks and uncertainty bounds for anomaly interpretation
PROJECT DELIVERABLES
What a useful technical report should contain
01 · Project output
Survey station map, elevations and base-loop records
02 · Project output
Raw gravity readings and documented correction chain
03 · Project output
Residual anomaly maps with alternative density-based interpretations
04 · Project output
Target-priority recommendations and stated detection limits
COMMON QUESTIONS
Frequently asked questions
Does a negative microgravity anomaly prove a void?
Can microgravity work near buildings?
What controls the accuracy?
TECHNICAL REFERENCES
Further technical reading
References describe the underlying methods and recommended data-quality principles; interpretation for any particular site must address its own evidence and constraints.
PROJECT ENQUIRIES
Discuss your ground investigation challenge.
GEOOE · GEOORIGIN ENGINEERING LIMITED
Share the site location, investigation objective, expected depth range, available boreholes and decisions the results need to support. We can discuss appropriate technical methods, limitations and project-specific collaboration.