Gal: CGS Unit of Acceleration, MilliGal and MicroGal in Gravity Surveying, and Borehole Gravimetry

The Gal, sometimes written gal and named for Galileo Galilei, is the unit of acceleration used almost universally in gravity surveying and exploration geophysics, defined in the centimetre gram second system as one centimetre per second per second, so that 1 Gal equals 1 cm per second squared, which is 0.01 metres per second squared in SI units. Because the acceleration of gravity at the Earth's surface is roughly 9.8 metres per second squared, or about 980 Gal, and because the gravity anomalies that interest explorationists are minute variations superimposed on that large background field, the Gal itself is far too coarse for practical work and geophysicists instead report in its submultiples. One Gal contains one thousand milliGal, written mGal, and one million microGal, written microGal or with the Greek letter mu, so 1 mGal is one thousandth of a Gal and 1 microGal is one millionth. Regional gravity anomalies caused by large geological structures such as salt domes, basement highs, or sedimentary basin fill are typically a few to a few hundred milliGal, while the subtle signals sought in reservoir monitoring, microgravity site surveys, and groundwater studies fall into the microGal range and demand instruments of extraordinary sensitivity. In petroleum exploration the Gal and its submultiples quantify the output of gravimeters used in several modes: land gravity surveys tie readings to base stations and correct for latitude, elevation, terrain, and tidal effects to isolate the Bouguer anomaly that maps subsurface density contrasts; airborne and shipborne gravity and gravity gradiometry trade some precision for rapid regional coverage over frontier basins where large block displacements produce anomalies big enough to survive the noise of a moving platform; and borehole gravimetry, run inside a cased well, measures the vertical gravity gradient between depth stations to derive formation bulk density over a large radius of investigation, a measurement prized for detecting gas behind pipe and for evaluating reservoirs where conventional resistivity and density logs are ambiguous, such as fresh water sands, shaly sands, and zones behind multiple casing strings. Every one of these measurements is reported and interpreted in mGal or microGal, and the reduction workflow that turns a raw instrument reading into an interpretable anomaly is essentially a sequence of corrections each expressed in the same unit. In the Western Canadian Sedimentary Basin, gravity methods complement seismic by mapping basement structure, salt and carbonate density contrasts, and regional basin architecture, and by providing an independent constraint that helps de risk structural interpretations before committing to expensive drilling. The Gal thus functions as the common currency of a whole family of density based geophysical measurements, from broad reconnaissance to precise in well formation evaluation.

From Raw Reading to Bouguer Anomaly

A land gravity crew mapping a suspected basement high east of Red Deer occupies stations on a grid, looping back to a base station every few hours to track instrument drift measured in microGal per hour. Each reading is corrected for the latitude gradient, for free air elevation at about 0.3086 mGal per metre, for the Bouguer slab at roughly 0.0419 times density times height, and for terrain using a digital elevation model. The residual Bouguer anomaly, perhaps a 3 mGal high over the target, is contoured and forward modelled to estimate the depth and density contrast of the structure, giving the interpreter an independent check on the seismic picture before a well is proposed.

Borehole Gravimetry for Gas Behind Pipe

In a cased Nisku or Leduc carbonate well where a resistivity log is ambiguous over a possible bypassed gas zone, a borehole gravimeter is run to measure the vertical gravity gradient between closely spaced depth stations. The gradient converts to formation bulk density over a radius of several metres, so a low density interval betrays gas that a shallow reading log missed. Because the measurement, reported in microGal, is nearly unaffected by casing, cement, and near wellbore invasion, it resolves whether a zone behind pipe holds gas worth a completion, a decision that can be worth hundreds of thousands of CAD in a single well.

Related Terms

The Gal is the reporting unit for gravity surveying, the geophysical method that maps subsurface density contrasts and complements seismic survey data in structural interpretation. It underpins the Bouguer anomaly, the corrected gravity field that isolates geological signal from elevation and terrain effects, and it is the unit of borehole gravimetry, a formation evaluation technique that relates to bulk density logging by deriving density over a much larger radius of investigation than a conventional pad tool. Together these connect the Gal to the broader density and porosity workflow used to characterize reservoirs.

Real-World WCSB Scenario: Gravity Recon Over a Frontier Play

An operator evaluating a deep Devonian carbonate trend in northeastern British Columbia commissions an airborne gravity gradiometry survey over a 2,000 square kilometre block before committing to a costly 3D seismic program. The survey, flown at low elevation with line spacing tuned to the expected anomaly wavelength, resolves density contrasts to a few tenths of a milliGal and maps a broad basement controlled high that focuses the subsequent seismic acquisition on a 300 square kilometre area. Spending roughly 1.5 million CAD on gravity first avoids acquiring 3D seismic over ground with no structural merit.

The targeted seismic confirms a Slave Point reef buildup coincident with the gravity high, and the operator drills its first well on structure, saving several million CAD that a blanket seismic program would have consumed mapping barren acreage.