Time-Variable Gravity Signals and Their Uncertainties: An Assessment of the Current State of Knowledge

Technologies for gravimetry and positioning are evolving, with major changes projected within the decade. These new technologies are anticipated to improve measurement accuracies so that: kinematic relative gravimeters would be accurate to < 1 milliGal; static relative gravimeters would be accurate to < 1 microGal; and static absolute gravimeters would be accurate to < 10 nanoGal. For instance, 1 nGal precision is expected to become available from cold atom gravimeters (a.k.a matter wave or atom interferometers) currently in development in the U.S. and Europe. Expressing these accuracies as rough height changes by using the linear gravity gradient approximation (0.3086 mGal/m), we see that a 1 cm of height change is equivalent to a 3.1 µGal gravity change and a 1 mm of height change is equivalent to a 3.1 nGal gravity change. So, detecting 1 mm height changes is theoretically possible with a 1 nGal-accuracy instrument.

Looking ahead to the availability of gravity instruments that are sensitive to signals several magnitudes smaller than currently recorded, results in several questions. Which dynamic (i.e. time-variable, or ġ) gravity signals will be measureable with a 1 nGal accuracy instrument? Which of these signals are well-characterized and which are not? Which signals will be targets for research in future gravity measurements?

This study compiles currently-known ġ signals that are in the sensitivity range of a 1 nGal gravity instrument, identifying the current knowledge about their physical sources, magnitudes, spatial scales, periodic/episodic characteristics, and uncertainties. There are 34 distinct sources of ġ identified in this study, grouped by source types of “Earth mass movements,” “Planetary,” and “Instrumentation.” The Earth mass movements type is the largest grouping and further divided into the sub-categories of atmospheric, hydrologic, erosional, volcanic, cryospheric, non-tidal ocean loading, and other (including human activities).

Results of the study show that there are 16 ġ signals whose uncertainties are not well-understood. Of those, the mass movement signals with the largest uncertainties (up to 100s of µGal per instance) include landslides and avalanches, coastal erosion, and surface water body changes. All of these can be measured with current instrumentation but are not well-studied because their gravity changes are mostly episodic, making research on them more difficult. Instrument drift  also has a large uncertainty and is unique to each instrument and often each setup of that instrument. Together, these present current research challenges, as well as barriers to fully-utilizing more precise instrumentation because they are not easily removed from current measurements.

There are several sub-µGal magnitude ġ signals that are not easily measured with today’s gravity instruments, but are known in seismology and other fields. These will be targets for gravity study with a 1 nGal instrument. They include: ambient temperature effects, Earth “noise” (hum and microseisms), sea level rise, subduction zone lithospheric processes, and variation in Earth’s length of day.

Currently, the ġ signal with the smallest uncertainty is Earth tides, at least when considering the 3 Earth tides of largest magnitude (diurnal, semidiurnal, and annual). These three Earth tides are known to the sub-nGal accuracy. Their time series are often used to calibrate superconducting gravimeters and will continue to provide the same benefit for cold atom gravimeters.