T. M. Niebauer

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.

Frequency stability measurements on polarization-stabilized He--Ne lasers

We present detailed stability measurements on six He-Ne lasers which have been stabilized by matching the intensity of the two orthogonal polarization modes. The frequencies of five different lasers were closely monitored for 1 month. Another laser was studied for 2 yr. All the lasers exhibited a stability of 1 part in 10(10) over the periods of about an hour and better than 1 part in 10(8) over 1 yr. An absolute accuracy of ~1 part in 10(9) can be attained by interpolating the linear drift between calibrations performed 6 months to 1 yr apart. These 1-mW lasers are rugged and simple to operate.

The diffraction correction for absolute gravimeters

The inherent curvature of the wavefronts in the laser beams used in most absolute gravimeters introduces a systematic reduction in the measured gravity value. This error depends strongly on the diameter of the laser beam, but is typically of the order of 1–5 µGal. This paper describes the origin of this reduction, and provides a correction based on the laser beam diameter as well as a practical method to measure the beams diameter.

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.

The self-attraction correction for the FG5X absolute gravity meter

The gravitational attraction of the body of a gravity meter upon its own proof mass is sometimes called the self-attraction. The self-attraction is a source of systematic error for absolute measurements of g, the acceleration of an object due to Earths gravity. While the effect is typically small—of the order of one part per billion of the Earths gravitational attraction—it is significant at the current level of accuracy of absolute gravity meters. In the past, a self-attraction correction for the FG5 gravity meter has been estimated by considering a rather coarse description of the instrument using simple geometrical shapes (spheres and cylinders). This paper describes a more complete calculation using a CAD-based digitized model of the newest FG5X instrument. We have also included the attraction of the co-moving drag-free chamber as well as the self-attraction of the counterweights used in the FG5X to reduce recoil. The results are also applicable to older style FG5 instruments with a fibre-optic interferometer base. The correction found with this new approach agrees with previous estimates but is now based upon a more complete and accurate model.

Simultaneous gravity and gradient measurements from a recoil-compensated absolute gravimeter

This paper discusses simultaneous gravity and vertical gravity gradient measurements obtained with a newly designed recoil-compensated dropping chamber adapted to an FG5 absolute gravimeter. The new dropping chamber incorporates counterweights to compensate recoil effects. It has the same physical length as the standard FG5 dropping chamber but the free-fall distance was increased from 20 cm to 25 cm. The new drive train pulls on the centre of the system to reduce unwanted horizontal velocity and rotation of the free-falling test mass. The test-mass material was chosen to reduce possible magnetic eddy-current damping caused by external magnetic field gradients. External lead masses were used to change the gravity and vertical gravity gradient. The measurements agree well with the theoretical gravity field changes derived from the position of the external weights. The experiment clearly demonstrates the efficacy of using an absolute gravity meter to measure both the gravity and the gravity gradient signals caused by variations in the external gravity field. This technique shows promise for passive gravity-monitoring applications.

Complex heterodyne for undersampled chirped sinusoidal signals

We describe a method for analyzing frequency-chirped sinusoidal signals using a complex heterodyne, sometimes also known as complex demodulation on the digitized waveform. This method allows one to use prior knowledge of the signal to reduce the effective bandwidth of the signal. The method can be used to extract a frequency-chirped signal even when it is sampled well below the Nyquist criterion. Accordingly, the method facilitates the use of less-expensive data acquisition and signal processing hardware than has traditionally been used for these applications. This technique is particularly useful for high-precision (parts in 10(9)) interferometer applications in which there exists a differential acceleration between the two arms (commonly found in absolute gravity meters or gradiometers).

Continuous gravity observations using Joint Institute for Laboratory Astrophysics Absolute Gravimeters

We discuss two different 1-month gravity records taken with two different Joint Institute for Laboratory Astrophysics (JILA) gravimeters. These records were corrected for the effects of Earth tides, ocean loading, local and global barometric effects, and polar motion. These data allowed precise determinations of the local air pressure admittance. We also obtained a value for the second-order gravimetric factor which agreed with the theoretical value to about 0.3% where the predominant uncertainty is the ocean load modeling. The corrected noise spectra were white above 1 cycle/d with a spectral noise density of about 80μGal/Hz. We note that absolute gravimeters have a sensitivity to time varying gravity that is comparable to that obtained with superconducting relative gravimeters. The sensitivity of these data emphasizes the need for better modeling of ocean loads, air pressure effects, and even man-made environmental noise.

Preliminary Absolute Gravity Survey Results From Water Injection Monitoring Program At Prudhoe Bay

A recent gravity survey conducted in Prudhoe Bay, Alaska using a newly developed A-10 portable absolute field gravity meter is described. The survey was performed for BP Exploration Alaska (BPXA) to obtain a regional baseline gravity map prior to the initiation of a water injection program scheduled for Fall 2002. Absolute gravity measurements will be used to monitor subsurface density changes over time, and in this current survey, absolute gravity was measured at over 200 sites located on land and ice. The measurements showed repeatability of 3.5 Gal, and production rates were as high as 10 new points per day.

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.