gravimeter, Oil & Gas Glossary

A gravimeter is an instrument that measures the acceleration due to gravity, or more usefully, the tiny variations in the gravitational field from one point to another caused by differences in the density of the rock below. Because gravity at any location depends on the mass distribution beneath it, a gravimeter responds to buried structures where dense rock such as basement, salt, or carbonate sits against less dense sediment, or where porous, fluid-filled reservoir rock replaces tight matrix. The readings are reported in milligals or microgals, where one gal equals one centimetre per second squared and the Earth's average surface gravity is about 980 gals, so an exploration target might show an anomaly of only a few milligals against that large background, demanding extraordinary instrument sensitivity. Most field gravimeters are relative instruments built around a delicate spring or a fused-quartz hinge that holds a small proof mass; as gravity changes, the mass deflects and the operator nulls it with a calibrated screw, reading the difference between stations rather than an absolute value. Modern superconducting and atom-interferometry gravimeters and MEMS chip-scale sensors push sensitivity into the microgal range. In petroleum work, gravimeters serve two distinct roles. At the surface, gravity surveys map regional structure cheaply over large areas, helping define basins, locate faults, image salt diapirs on the offshore East Coast, and high-grade leads before expensive seismic is shot. Downhole, the borehole gravimeter is run on wireline to read the bulk density of a large volume of formation surrounding the well, far beyond the shallow reach of a conventional gamma-gamma density log; from that bulk density an engineer computes porosity and, critically, can detect gas behind casing where resistivity and neutron logs are ambiguous. In the Western Canadian Sedimentary Basin, gravity methods support reconnaissance over the foothills and the deep basin, help resolve density contrasts in carbonate plays such as Leduc, Nisku, and Slave Point, and underpin time-lapse 4D gravity monitoring of reservoir fluid movement, including the steam chambers of SAGD heavy oil projects and the plumes of carbon capture and storage schemes where measuring mass change over time confirms where injected fluid has gone. The technique is corrected for latitude, elevation, terrain, and tidal effects before the residual anomaly that reflects subsurface geology can be interpreted, and it is almost always integrated with magnetics and seismic rather than used alone, because gravity data are inherently nonunique and many density models can fit the same observed field.

Key Takeaways

Measures density contrast, not depth directly: A gravimeter senses lateral and vertical changes in rock density, reported in milligals and microgals against the roughly 980-gal background field. Dense basement, salt, and carbonate produce positive anomalies; porous fluid-filled reservoir and thick low-density sediment produce negative ones, which is how the instrument images buried structure without drilling.
Surface versus borehole roles differ: Surface gravity surveys cover large WCSB areas cheaply to define basins, faults, and salt features ahead of seismic. The borehole gravimeter instead reads bulk formation density over a large rock volume around the well, delivering porosity and behind-casing gas detection that shallow-reading nuclear logs cannot match.
Relative spring and quartz designs dominate: Most field gravimeters are relative instruments using a quartz hinge or metal spring holding a proof mass, nulled by a calibrated screw, so they read differences between stations rather than absolute gravity. Superconducting, atom-interferometry, and MEMS chip-scale sensors now reach microgal sensitivity for time-lapse work.
Corrections are essential before interpretation: Raw readings must be corrected for latitude, free-air elevation, Bouguer slab and terrain mass, instrument drift, and Earth and ocean tides before the residual anomaly reflects geology. In the rugged WCSB foothills the terrain correction is large and a poor digital elevation model can swamp a genuine reservoir-scale signal.
4D gravity monitors fluid movement: Repeating high-precision surveys over time tracks mass changes from fluid migration, making time-lapse gravity a tool for monitoring SAGD steam chambers, gas-cap depletion, aquifer influx, and CO2 storage plumes. Because it measures mass directly, 4D gravity independently confirms where injected or produced fluids have actually moved underground.

Borehole Gravimetry and Behind-Casing Porosity

The borehole gravimeter measures gravity at successive depth stations inside a well, and the difference in gravity between two stations, combined with their vertical spacing, yields the average bulk density of the slab of rock between them. Because gravity integrates over a very large radius, the tool effectively sees tens of metres into the formation, far beyond the few centimetres a gamma-gamma density log samples, so it reads true formation density unaffected by near-wellbore invasion, washouts, or casing. When the pore fluid is known, that bulk density converts directly to porosity, giving reservoir engineers their most representative porosity value. Its standout application in WCSB carbonates is detecting low-density gas behind casing in old or cased wells, where electric logs are blind or contradictory.

Time-Lapse 4D Gravity for SAGD and CCS

Because a gravimeter measures mass, repeating a high-precision survey over the same stations months or years apart detects the gravity change caused by fluids moving in the reservoir. In a SAGD heavy oil project, replacing dense cold bitumen and water with a lower-density steam chamber produces a measurable negative gravity change that maps chamber growth between well pairs. In carbon capture and storage, injecting CO2 changes pore-fluid mass and shifts gravity, so 4D gravity becomes a monitoring, measurement, and verification tool that regulators value because it independently confirms containment. Microgal-class superconducting and absolute gravimeters make these subtle WCSB measurements feasible.

Fast Facts

The Hungarian physicist Roland von Eotvos built the torsion-balance gravity gradiometer that found the first commercial oil structure detected by gravity, the Nash salt dome in Texas, drilled in 1924. That single discovery launched applied gravity exploration across the Gulf Coast salt-dome province. A century later, the unit of gravity gradient, the eotvos, still carries his name, and gravity gradiometry first developed to hunt salt domes was later adapted for submarine navigation before returning to oil exploration.

Gravity surveying sits within the broader toolkit of potential-field and well-logging geophysics. A gravity survey is the field program a gravimeter executes, and its results are usually interpreted alongside a magnetic survey because the two potential-field methods constrain different rock properties over the same area. The borehole gravimeter competes with and complements the density log, which reads near-wellbore density at high resolution, and the porosity it derives is cross-checked against the neutron log, particularly for the gas detection where the two methods together resolve ambiguity.

WCSB Gravity Reconnaissance Over a Deep Basin Carbonate Lead

Ahead of committing to a multi-million-dollar 3D seismic program over a deep basin Slave Point carbonate lead in northwestern Alberta, an operator commissioned a high-resolution surface gravity survey at roughly CAD 250 to 400 per station across a grid of several hundred stations, a fraction of seismic cost. After latitude, free-air, Bouguer, and a careful foothills terrain correction, the residual Bouguer anomaly outlined a subtle density high consistent with a reefal buildup, high-grading two of five candidate blocks.

The operator focused its 3D seismic and a subsequent CAD 9 million test well on the gravity-defined high rather than shooting the whole trend. The well encountered porous dolomitized Slave Point reef, and the modest gravity spend paid for itself many times over by steering capital away from the three blocks the anomaly had ranked low.