Fred J. Klopping

A new generation of absolute gravimeters

We describe the design improvements incorporated in a new generation of absolute gravimeters, the FG5. A vertically oriented (in-line) interferometer design is used to remove the influence of floor vibration and tilt on the optical path length. The interferometer uses an iodine-stabilized laser as a primary length standard, with circuitry for automatic peak detection and locking. The seismic isolation system is an active long-period seismometer (Super Spring). The new design has improved passive isolation and thermal drift characteristics over previous systems. Programming flexibility and control of the test mass trajectory have been improved. The computer system has also improved real-time analysis and system capability. The FG5 instrument has a higher level of robustness, reliability and ease of use. These design advances have led to an instrumental uncertainty estimate of 1,1 × 10-8 m s-2 (1,0 μGal). Instrument agreement among nine similar devices is 1,8 μGal and observations under optimal conditions exhibit standard deviations of 5 μGal to 8 μGal.

New constraints on Laurentide postglacial rebound from absolute gravity measurements

Repeated absolute gravity measurements have been made over a period of several years at six sites along a 3000 km-long, mid-continental, North American profile from the coast of Hudson Bay southward to Iowa. With the exception of the southern-most site, the observed rates of change of gravity are significantly higher than rates predicted by current models, such as ICE-3G and a laterally homogeneous, standard Earth. The observed gravity change rates suggest significant modifications, such as a 2 to 3-fold increase in lower mantle viscosity or a 50% increase in Laurentide ice sheet thickness west of Lake Superior.

Calibration of a superconducting gravimeter by comparison with an absolute gravimeter FG5 in Boulder

This paper reports the results of a calibration of a superconducting gravimeter using simultaneous measurements of Earth tides with an absolute gravimeter. Nine consecutive days of observations were made with both the C024 superconducting gravimeter and the FG5-202 absolute gravimeter at the NOAA Table Mountain Gravity Observatory near Boulder, Colorado. The precision of the calibration factor is better than 0.1%. The calibration factor obtained in this fashion agrees well with that obtained from a moving platform. This experiment provides a noise estimate for both the superconducting and absolute gravimeter. In addition, the local air pressure admittances of both instruments compare well with a value close to −0.35 µGal/mbar.

The 4D microgravity method for waterflood surveillance: Part 3 - 4D absolute microgravity surveys at Prudhoe Bay, Alaska

The 4D microgravity method is becoming a mature technology.Aprojecttodeveloppracticalmeasurementandinterpretation techniques was conducted at Prudhoe Bay,Alaska, from1994through2002.Beginningin2003thesetechniques have been systematically applied to monitor a waterflood in the gas cap of the Prudhoe Bay reservoir. Approximately 300 stations in a 150 km 2 area are reoccupied in each survey year with sub-5 Gal precision absolute gravity and centimeter precision Global Positioning System GPS geodetic measurements. The 4D gravity measured over epochs 2005‐2003, 2006‐2003, and 2007‐2003 has been successfullymodeledtotrackthemassofwaterinjectedsincelatein 2002.AnewandimprovedversionoftheA-10field-portable absolute gravity meter was developed in conjunction with this project and has proven to be a key element in the success of the 4D methodology.The use of an absolute gravity meter in a field survey of this magnitude is unprecedented. There aresubstantialdifferencesbetweena4Dabsolutemicrogravity survey and a conventional gravity survey in terms of station occupation procedures, GPS techniques, and the 4D elevation correction. We estimate that the overall precision of the4Dgravitysignalineachepochislessthan10 Gal.

Intracomparison tests of the FG5 absolute gravity meters

The FG5 series absolute gravimeters have an estimated instrumental accuracy of 1–2 µGal (1-2 × 10−8 ms−2). A number of instrument comparisons were conducted with six FG5 instruments over a period of 9 months; the predecessor series JILA4 instrument was also used. The standard deviation of mean g values (averaged over 24–48 h), as observed by different instruments, is 1.8 µGal. Observations taken within 48 h typically agree within 2 µGal, and the maximum observed disagreement is 6 µGal for two observations taken 37 days apart. Individual data sets of 100–250 measurements (16–42 min duration) show minimum standard deviations of 7–8 µGal, placing an upper bound on instrument noise. The data are not corrected for local water-table effects or regional atmospheric loading, and thus the observed repeatability is probably an underestimate of the instrument accuracy. The observed instrument agreement is consistent with the instrumental accuracy estimate. Ignoring deficiencies in modeling environmental gravity signals, this accuracy level should allow resolution of vertical motions at the sub-centimeter level.

Analysis of data from the 1994 International Comparison of Absolute Gravimeters with a single computational protocol

Data from the 1994 International Comparison of Absolute Gravimeters were processed with the National Oceanic and Atmospheric Administration (NOAA) Geosciences Lab protocols and processing algorithms. The rootmean-square discrepancy between results obtained from individual groups and the NOAA processing is 2,3 μGal; the average discrepancy is 1,5 μGal. The sources of the discrepancies are probably (i) differences in the vertical gradients used for datum level transfers, (ii) inconsistencies between processing algorithms, and (iii) differences in data editing criteria.

Short- and Long-Term Stability of the JILAG-4 Absolute Gravimeter

Variations of absolute gravity measured with the JILAG-4 absolute gravimeter at intervals ranging from 2 hours to 5 years are reviewed to ascertain short- and long-term instrument stability. We find that the standard deviation of the twenty-four 2-hourly drop set means taken during a given station occupation is 1-2/xGal when natural or man-induced microseismic conditions are low and 3-5/xGal when the microseismic activity is high. The standard deviations of the station gravity values obtained by repeated occupations weeks or years apart are within these same ranges, with lower standard deviations found again at bedrock sites where the microseismic noise is low. Based on the repeatability of observations since the beginning of the measurement program in 1987, there is no indicate!on for drift, gradual deterioration, or aging of the instrument. However, because of the degraded performance of the lasers used since 1990, the standard deviation of repeated station occupations increased from 2.27 to 2.87/xGal, and data had to be rejected at several sites. Individual station gravity Values in excess of +_3/xGal from the station mean are found mostly at those sites where density Variations between reoccupations are expected on the basis of geological conditions, usually due to groundwater table fluctuations and/or soil moisture changes.

A new fiber optic gradiometer for 4-D absolute differential gravity

We introduce the concept of absolute differential gravimetry using fiber optic-based laser interferometry. Absolute differential gravimetry provides an instantaneous determination of the difference in gravity between two or more points on or beneath the surface of the earth. Applications include enhancing the resolution of the gravity field, as well as subsurface fluid and structure monitoring. With a network of sensors, observation of time dependent variations of the gravity field is also possible. Results from a proof-of-concept prototype instrument using fiber-optic coupler technology represent the first time absolute laser interferometric differential gravity data have been collected. Implications of the new measurement method in terms of geophysical studies and future research plans are discussed.