gravity units, Oil & Gas Glossary

A gravity unit, abbreviated g.u. and sometimes written gu, is a measure of gravitational acceleration equal to one-tenth of a milliGal, so that 10 g.u. equals 1.0 mGal, and it appears mainly on older gravity maps and in some European literature as an alternative to the milliGal that dominates North American practice. To place it on the physical scale, the Gal, named for Galileo, equals one centimetre per second squared; a milliGal (mGal) is one thousandth of a Gal, or 10 to the minus five metres per second squared; and a gravity unit is one thousandth of that again times ten, precisely 10 to the minus six metres per second squared, which is why some texts define the g.u. as the micrometre-per-second-squared unit. The reason such tiny units exist is that the anomalies exploration geophysicists chase are minute fractions of Earth's roughly 9.8 metre-per-second-squared field. A buried salt dome, a Devonian reef, a granite intrusion, or a sedimentary basin produces a gravity anomaly of only a few tens of milliGals at most, and the subtle density contrasts that matter for petroleum exploration may express as anomalies of just a few g.u. Gravity surveying works because rock density varies: dense carbonates and basement rocks pull a gravimeter reading slightly higher, while low-density salt, porous sandstone, and thick shale pull it slightly lower. In the Western Canadian Sedimentary Basin, gravity data historically helped map the shape of the basement, locate Devonian reef trends whose tight carbonate stands out against surrounding shale, and outline salt dissolution edges in the Prairie Evaporite that control younger structure. A land gravity survey can readily achieve an accuracy near 0.1 g.u., which is why the g.u. is a natural unit for expressing survey precision, whereas marine gravity, complicated by ship motion, is far less precise, closer to plus or minus 10 g.u. Modern instruments and processing quote results in milliGals or microGals almost universally, but any geoscientist working with legacy datasets, older government gravity compilations, or reprocessed vintage surveys will encounter values in g.u. and must convert correctly, dividing g.u. by ten to obtain milliGals, before merging them with newer data. Getting that factor of ten wrong is a classic and costly error, since it silently scales every anomaly on the map. Gravity units live alongside the raw measurement corrections, latitude, elevation, terrain, and the Bouguer correction, that turn an observed reading into an interpretable Bouguer anomaly, and they belong to the same potential-field toolkit as magnetic surveying used to reconnaissance-map large basins before the expense of seismic.

Key Takeaways

Ten g.u. Equals One milliGal: A gravity unit is one-tenth of a milliGal, so 10 g.u. = 1.0 mGal. On the physical scale the Gal is 1 cm/s squared, the milliGal is 10 to the minus five m/s squared, and the g.u. is 10 to the minus six m/s squared, equivalent to one micrometre per second squared. Converting old data means dividing g.u. by ten to get milliGals.
Built For Tiny Anomalies: Exploration targets perturb Earth's roughly 9.8 m/s squared field by only tens of milliGals or a few g.u. Such small units are necessary because the density contrasts that reveal reefs, salt, basins, and basement are a minute fraction of the total field, far too small to express usefully in whole Gals.
Density Contrast Is The Signal: Gravity surveying detects lateral changes in rock density. Dense carbonates and crystalline basement raise the reading; low-density salt, porous sandstone, and thick shale lower it. In the WCSB this let early crews map basement relief, Devonian reef trends, and Prairie Evaporite salt-dissolution edges that control overlying structure.
Precision Naturally Quoted In g.u.: A careful land gravity survey achieves accuracy near 0.1 g.u., making the g.u. a convenient precision unit. Marine gravity, degraded by vessel motion and Eotvos effects, is far coarser at roughly plus or minus 10 g.u., which is why offshore reconnaissance gravity resolves only large features.
A Legacy-Data Conversion Trap: Vintage gravity maps, older government compilations, and reprocessed historical surveys frequently report values in g.u. Merging them with modern milliGal or microGal data without applying the factor-of-ten conversion silently rescales every anomaly, a subtle error that can misplace or erase an interpreted structure.

From Raw Reading To Bouguer Anomaly

A gravimeter reading in g.u. or mGal is meaningless until corrected. Geophysicists subtract predictable effects: the latitude correction for Earth's shape and rotation, the free-air correction for station elevation, the Bouguer correction for the mass of rock between the station and sea level, and a terrain correction for nearby hills and valleys. What remains is the Bouguer anomaly, the residual that reflects subsurface density variation. In WCSB reconnaissance, mapping the Bouguer field in g.u. or mGal outlined basement highs and lows that guided where to spend seismic dollars, since gravity covers wide areas cheaply and points explorers toward the structures worth imaging in detail.

Gravity In The Potential-Field Toolkit

Gravity rarely works alone. It is paired with magnetic surveying, which senses susceptibility contrasts in basement and igneous bodies, to build a two-method picture of a frontier basin before seismic acquisition. Gravity excels at delineating broad density features such as salt basins and sedimentary thickness, while magnetics maps basement composition and faulting. Together they are inexpensive per square kilometre and ideal for regional screening. Values expressed in g.u. on older magnetic-and-gravity compilations remain useful today once converted and re-datumed, feeding modern basin models that integrate potential fields with seismic and well control.

Fast Facts

The Gal honours Galileo Galilei, whose experiments on falling bodies founded the study of acceleration, yet the unit he inspired is so large for exploration that geophysicists work three to six decimal places down, in milliGals and micrometres per second squared. A single g.u. of one micrometre per second squared is roughly one ten-millionth of Earth's surface gravity, a sensitivity so fine that a modern gravimeter can detect the pull of a water table dropping a few metres or a body of ore buried hundreds of metres below the survey line.

Gravity units sit within potential-field geophysics. The Bouguer anomaly is the corrected gravity value, expressed in g.u. or mGal, that geologists actually interpret for density structure. Density contrast is the rock property gravity measures, since heavier rock deflects a gravimeter more. Magnetic survey is the companion potential-field method run alongside gravity for cheap basin reconnaissance. And the gravimeter is the instrument whose readings, once corrected, are reported in these very units.

Real-World WCSB Scenario: Reprocessing A Vintage Gravity Compilation

A junior explorer evaluating a Devonian reef play in central Alberta acquired a 1970s government gravity compilation whose values were tabulated in g.u. rather than milliGals. Before merging it with a modern microGal survey the geophysics team divided every legacy value by ten to convert to milliGals and re-referenced the datum, then gridded the combined dataset. The reprocessing cost only a few weeks of a contractor's time, on the order of 15,000 to 25,000 CAD, far cheaper than shooting new gravity.

The corrected grid revealed a subtle Bouguer low consistent with a porous reef margin, a feature the mis-scaled raw data had buried. That anomaly high-graded a lease block and focused a follow-up seismic program on the most prospective trend, saving the company from imaging the entire play area at full seismic cost.