GWR INSTRUMENTS, INC.

SUPERCONDUCTING GRAVIMETERS

Figure 1: GWR Compact Tidal Gravimeter

The superconducting gravimeter (SG) is the world's most sensitive and stable gravimeter. With a sensitivity of one nanogal, or one part in 10¹² of surface gravity, precise measurements of earth tide parameters and the nearly diurnal free wobble of the earth can be made. This high sensitivity will enable the Global Geodynamics Project to search for internal gravity waves in the earth's liquid core and "slow or silent" earthquakes. The stability of the SG approaches a few m Gal per year which makes it invaluable for geodetic purposes, such as monitoring sea-level changes and tectonic deformations.

GWR Instruments, Inc. now manufactures the SG inside of a compact 125 liter Dewar, which is 1 m high, 0.7 m in diameter and weigh less than 100 Kg. Using 3 small granite blocks, these Dewars can be mounted on any level concrete floor in a 1 m x 1 m area. The Dewars' liquid helium hold-time is longer than 2 years when operated with an efficient cryogenic refrigerator, as shown in the photograph above.

The Compact Tidal SG's small size, easy installation, long hold-time, and simple maintenance make it ideal for operation at remote sites and in tunnels and mines. This extends its applications to monitoring gravity changes at existing seismic stations and volcano observatories. Other uses include high resolution gravity measurements at petroleum, geothermal and groundwater reservoirs.

6264 FERRIS SQUARE, SUITE D * SAN DIEGO, CA 92121 * USA FACSIMILE NO. 619 452 6965 * TELEPHONE NO 619452 7655 EMAIL: marketing@gwrinstruments.com

1. PRINCIPLES OF OPERATION

Before the invention of the SG, all gravimeters used mechanical springs to support a test mass. These springs produce creep, hysteresis and non-linearities which degrade precision and stability. As shown in Figure 2, the only moving part in the SG is its spherical niobium test mass. The 2.5 cm diameter sphere is suspended by a magnetic field which replaces the mechanical springs found in conventional gravimeters. The magnetic field is extremely stable since it is generated by persistent currents flowing in the superconducting coils. This stability virtually eliminates the drifts seen with mechanical spring type gravimeters.

The position of the sphere is measured using conventional AC phase sensitive detection electronics in conjunction with a capacitance bridge surrounding the sphere. The position of the sphere is held fixed using a small feedback coil in close proximity to the levitation field coils. The current through the feedback coil provides a measurement of changes in the gravitational force and is responsible for the instrument's high linearity. By design, the vertical magnetic gradient ("spring constant") can be made very weak by adjusting the ratio of currents in two field coils. In this way, small gravity changes produce large displacements of the test mass and the sensor acts as a "noise free" gravity amplifier. The extremely low-noise and low-drift of the SG results from three factors: the very weak magnetic gradient or "spring constant" produced by the coils; the inherent stability of persistent currents flowing in a superconductor; and the mechanical stability of materials at cryogenic temperatures. The entire system is surrounded by superconducting magnetic shielding and is temperature regulated to a few m ^OK inside a vacuum can. The gravimeter is operated at 4.2 ^OK, suspended in a liquid helium bath inside of a superinsulated Dewar.

2. THE COMPACT SUPERCONDUCTING GRAVIMETER

For the past 14 years, the size of the SG largely limited its operation to established geophysical observatories or to custom built structures. These instruments use large 200 liter helium storage Dewars which are 1.7 m high and 0.85 m in diameter. They are supported from 1.3 m high concrete piers and require an overhead clearance of at least 3 m. These SGs are operated around the world, including Antarctica, Belgium, Canada, China, France, Germany, Greece, Italy, Japan and the USA. Modified instruments have operated deep within the Asse salt mine in Germany and in a tunnel below Lake Brasimone in Italy.

In 1994, GWR started producing SGs built into a compact 125 liter Dewar, as shown in producing SGs built into a compact 125 liter Dewar, as shown in Figure 3. There were three design goals for the Compact SG. The first was to increase its efficiency so that it would use less liquid helium and would operate for longer time intervals between helium refills. The second was to support the Dewar from its bottom so that it could operate on a concrete floor without requiring an expensive and complicated concrete support pier. The third was to reduce its physical size and weight so that it could be moved easily into existing geophysical observatories and seismic stations. The Compact SG successfully meets these goals.

Dewar Efficiency - The Dewar's neck and electrical leads to the instrument are designed to minimize heat loss and to maximize the cooling power of the APD DE-202 coldhead. Using only 125 liters of liquid helium, the Dewar will operate more than 2 years between helium refills. This is almost 4 times more efficient than the previous 200 liter Dewar. The coldhead is supported by a 3-legged frame which surrounds the Dewar. The length of the 3 legs are adjusted to center and to mechanically isolate the coldhead from the Dewar neck. The coldhead is designed for easy removal and installation using only 4 screws.

Bottom Mount - A thick Al ring, which is welded around the circumference of the Dewar, provides attachment points for three support points configured as an equilateral triangle. Two of the support points have micrometers in series with thermal levelers. These allow either manual or thermal tilting of the system with respect to the third fixed point. The micrometer/thermal levelers and the rear screw are supported by three granite blocks (19 cm high). The Compact SG fits easily on a floor space of only 1 m x 1 m area.

Mt. Aso, Japan & Membach - In December 1994, a Compact SG was successfully installed in the Observation Tunnel of Mt. Aso, Japan to monitor volcanic activity. It is operating in the tunnel 30 m below ground alongside extensometers, water-tube tiltmeters and long-period seismometers. Copper extension hoses connect the refrigeration coldhead to its compressor which operates with its cooling system in a room near ground level. This separation serves two purposes. It isolates other instruments from the vibrations of the compressor and it removes this large heat source so that it does not affect the stable tunnel temperature.

In August 1995, a Compact SG will be installed in the seismic station at Membach, Belgium in a room next to long period seismometers permanently stationed there. The compressor and cooling system will be placed at the tunnel entrance and extension hoses will route the compressed helium gas to the coldhead. Initial measurements made at Membach indicate that the coldhead vibrations only affect the nearby seismometers minimally with the coldhead frame resting on the floor. If required, vibration adsorbing materials can be placed between the frame and the floor, or the coldhead can be pneumatically supported above the coldhead frame.

3. PERFORMANCE AT TIDAL FREQUENCIES

Measurements of the tidal spectrum using the SG are described in detail in various published papers. Recently, Hinderer, Crossley and Xu (1994)¹ compared two-year long SC records from France and Canada over a common time interval. Figure 4 is derived from the Canadian data and shows a power spectral density of the gravity signal (in m Gal² per cpd) compared to the atmospheric pressure (in mbar² per cpd). The precision of SG gravity measurements is not limited by instrumental noise but is limited by geophysical "noise" from gravity variations caused by local incoherent fluctuations in the atmospheric density. This is easily verified by comparison of the two spectra in Figure 4. Strong correlations between gravity and pressure appear below 0.5 cycles per day and at the daily pressure harmonics of 3, 4, 5, 6, 7 and 9 cycles per day.

Merriam (1992)² recently modeled the effects of the atmosphere on gravity and refers to several papers published earlier. His model suggests a division of the globe into local, regional, and global zones for measuring and computing corrections. About 90% of the signal is from the local zone within 50 km of the gravity station and can be modeled by hourly values of pressure and temperature at the gravity site only. Neumeyer (1995)³ derives a method using cross spectral analysis to determine a frequency dependent atmospheric pressure correction to improve detection of wave groups of small amplitudes.

Hinderer, Crossley and Xu (1994)¹ also examined residual gravity signals which are generated after subtracting a least squares fit tidal model (505 waves), a parabolic function for drift, and a single coefficient for atmospheric pressure admittance. For periods shorter than one day, the data was further processed using a high pass filter. This process reduces the effect of various low frequency contributions, such as long period atmospheric loading which cannot be well accounted for. Figure 5 (Figure 16 of reference 1) shows the amplitude spectrum for the French (J9) and Canadian (CSGI) stations in the range 0.10 - 0.30 cph. Tidal residual signals are left with amplitudes less than 10 nanogal in the terdiurnal band (.0.125 cph) and 5 nanogal in the quarter-diurnal band (.0.167 cph). These spectra show that, outside of these tidal bands, a 1 nanogal noise level is obtainable for periods between 3 and 6 hours. This is of special interest in the current search for periodic geophysical signals from the Earth's deep interior in this frequency band (See Hinderer and Crossley (1993)⁴.

4. PERFORMANCE AT SUB-TIDAL FREQUENCIES

In order to observe secular variations in gravity, the Earth tides, effects of ocean loading, and the influence of the atmosphere must be removed from the data. Figure 6 shows data from a stable site in various stages of analysis. After removing the earth tides and ocean loading signals, one observes a signal which highly correlates with the local atmospheric pressure. This effect is due to the gravitational attraction of the atmosphere and is not an instrumental artifact.The pressure effect is usually removed using a single admittance parameter of about -0.32 m Gal/mbar. The remaining variations in the gravity "residual signal" may result from instrumental drift, long-period tides, or from density or elevation changes in the earth's crust.

Klopping et al. (1994)⁵ and Peter et al. (1994)⁶ provide detailed analysis of two 525 day long data sets obtained with two side-by-side SGs recording at Richmond, Florida. The authors provide detailed analysis of instrumental drift, tares, polar motion, and water table and soil moisture effects. They find that the maximum long-period differences between the two instrument were +\- 1 m Gal, which could result from errors in drift removal. Comparison of optimal instrument performance indicated the nominal noise level for the systems is below 0.1 m Gal. In addition, two absolute gravimeter readings were made -300 days apart, which agreed with the SG data within < 2 m Gal.

As reported by Warburton and Brinton (1995)⁷, GWR Instruments identified several sources of drift in the early SGs. The SG is now made with a solid copper gravimeter body which eliminates an indium O-ring between the magnet support coils and the capacitance bridge. Mechanical creep in this O-ring was identified as the main source of drift in previous instruments. All superconducting components of the SG are now made with niobium. The sphere is welded and annealed to high temperature to insure it has uniform superconducting properties. Improved helium leak checking procedures insure that no helium gas leaks are present in either the sphere cavity or vacuum can. This eliminates changes of sphere buoyancy and temperature control point as sources of drift.

These improvements were made in serial numbers GWR T016 through T020 and all Compact SGs starting with serial number C021. Long-term drift results have been reported from GWR T016 and T018. Dittfeld (1994)⁸ reports that the drift rate of GWR T018 is about 3 m Gal/year. Sato et al. (1994)⁹ reports the long term drift of GWR T016 to be well approximated by an exponential with amplitude of -12 m Gal and a time constant of 78 days. With this drift function, GWR T016 will stabilize with a drift rate much less than 1 m Gal/year within one year.

5. MEASUREMENT OF TECTONIC UPLIFT VERSUS DRIFT

To measure gravity changes over decades, both a SG and an absolute gravity meter (AG) must be used to achieve m Gal precision. A continuous SG record is required to characterize the site and its response to oceans, atmosphere, groundwater and other environmental effects. The continuous record allows one to identify and model the environmental influences by their correlations with the gravity record. When absolute gravity (AG) measurements are made without such characterization, the scatter in AG data will greatly exceed its accuracy at most sites, and the data sampling will be too infrequent to correlate with other influences.

At some AG stations, a one to two year SG record may be enough to allow precise modeling of all geophysical signals at that station. If so, the SG can be moved to another station. At many AG stations, a continuous SG record will be required indefinitely to measure m Gal variations over periods of years. Examples include: coastal or island stations where ocean signals are large and less predictable than inland stations; sites where rainfall is frequent and groundwater is difficult to model; and sites where annual tides are several m Gal in amplitude and out of phase with predicted models. Using short term SG records to model a site may also be problematic when weather conditions change. Since weather may cycle over periods of several years to decades, its unlikely that a model generated during a dry cycle will accurately predict gravity during a wet cycle.

Most new SGs have initial drift curves which decay exponentially in time. For this reason, SGs should be checked periodically using an AG. In this way, the AG data can be used to determine the drift function of the SGs to m Gal precision. Since the SG drift function is monotonic and decreasing, it can be measured, modeled and removed. Even after a SG reaches a "drift free" state, the AG measurements should be continued. The agreement between two gravimeters, based on different principles, will markedly increase confidence in the scientific conclusions resulting from the work.

At quiet sites, the precision with which gravity measurements can distinguish changes in elevation from underground density depends on the accuracy of the environmental models. For example, inexact models of groundwater and/or soil moisture could mask vertical crustal movements. For this reason, precision stations will require GPS or VLBI measurements for further confirmation of tectonic motion. On the other hand, at active tectonic sites, a combination of SG, AG and GPS are required to differentiate crustal motion from the underground mass movement of interest. Examples are regions of volcanic activity, geothermal sites, or regions of subsidence due to groundwater withdrawal.

6. GLOBAL GEODYNAMICS PROJECT - GGP

Over the last two decades, the SG has been the most sensitive and stable instrument for the measurement of the vertical component of the Earth's gravity field. No other geophysical instrument can span the period range of one second to 10 years with its high performance. For short periods, it compares favorably with seismometers in the measurement of the Earth's normal modes. For tidal periods, the SG is significantly better than spring gravimeters in determining the elastic response of the Earth. For long periods of months to years, the new low drift Compact SGs will complement absolute gravimeters in providing reference gravity at the m Gal level.

In recognition of the high quality data attained with the SG, the Global Geodynamics Project (GGP) was formed. The GGP has been endorsed by the IUGG Inter-Union Project SEDI (Study of the Earth's Deep Interior) and its organization and goals are outlined in the last Global Geodynamics Status Report - GGP: Status Report 1994¹⁰. The GGP was formed to further scientific goals through global analysis and "stacking" of high quality SG records. This is required because many signals of interest are at the level of a few nanogal and are hidden by geophysical noise. Therefore, to be considered reliable, a global signal identified on the record of an individual instrument must be confirmed with similar signals on other instruments. In addition, a wide distribution of instruments around the Earth is required because global gravimetric signals have theoretically predictable spatial and temporal global variations.

THE GOALS OF THE GGP ARE TO MEASURE:

• Earth tides and the nearly diurnal free wobble: Through global tidal analysis of SG records, refined estimates of the nearly diurnal free wobble (NDFW) can be obtained along with better models of oceanic loading on the solid Earth. See Defraigne, Dehant & Hinderer (1994, 1995)^11,12 or Sato et al. (1994)¹³ for recent results on NDFW.

• Core modes: The detection of these waves will give direct information on the mechanical equilibrium of the fluid in the core, and thus information on the operation of the geodynamo. See Hinderer and Crossley (1993)⁴ for a summary.

• Atmospheric interactions: Stacking global gravity and pressure data is essential for evaluating the effects of global atmospheric surface pressure and mass redistribution on the Earth's gravity field.

• Earth rotation and polar motion: With global coverage, it should be possible to continuously monitor the location of the rotation pole on the time scale of minutes.

• Earthquakes: A SG, with a bandwidth of 1 second to several years, is the only instrument capable of monitoring both earthquake activity and tectonic motions. At intermediate time scales the SG is ideal for detecting slow and silent earthquakes.

• Seismic normal modes: The SGs have excellent noise characteristics for the observations of the Earth's normal mode spectrum following a moderate to large earthquake.

• Gravity changes due to tectonic motions: In combination with absolute gravity, the SG can monitor the long-term changes due to tectonic motions, sea-level changes affecting the survival of coastal cities, post-glacial uplift and the deformation associated with active tectonic events.

• Enhancing absolute gravity measurements: SG's will be required at many absolute gravity sites to achieve 1 m Gal accuracy.

• Geodesy using space techniques: At the proposed sub-centimeter level of accuracy, the current Satellite Laser Ranging (SLR) and the proposed Geodynamics Laser Ranging System (GLRS) mission require precise knowledge of the Earth's dynamics, including resonances in the liquid core. A global net of SGs will give this information.

• Sea level changes: Defining the origins of sea-level changes is a scientific program of international concern and requires input from several different sources. Elevation changes due to post glacial rebound or plate tectonics must be differentiated from sea level changes which result from global warming. This necessitates the establishment of a global geodetic/geophysical observatory such as FLINN (Fiducial Laboratories for an International Natural Science Network), an IUGG-sponsored project initiated at the Coolfront Workshop in 1989. A central feature of such a network is the monitoring of the gravity field at a group of fiducial stations equipped with SGs as well as precise positioning instruments (e.g. SLR, VLBI or GPS) and having accurate connections to reference ocean tide gauges.

8. REFERENCES

1. Hinderer, J., Crossley, D. & Xu, H., 1994. A two-year comparison between the French and Canadian superconducting gravimeter data, Geophys. J. Int. 116, 252-266.
2. Merriam, J. B., 1992. Atmospheric pressure and gravity, Geophys. J. Int., 109, 488-500.
3. Neumeyer, J., 1995. Frequency dependent atmospheric pressure correction on gravity variations by means of cross spectral analysis, Bull. Inf. Mar. Terr., 122, 9212-9220.
4. Hinderer, J. & Crossley, D., 1993. Core dynamics and surface gravity changes, in Dynamics of Earth's Deep Interior and Earth Rotation, Geophys. Monograph Ser., 47, 1-16, Am. Geophys. Un., Washington DC.
5. Klopping, F. J., Peter, G., Berstis, K. A., Carter, W. E., Goodkind, J. M. & Richter, B. D., Analysis of two 525 day long data sets obtained with two side-by-side, simultaneously recording superconducting gravimeters at Richmond, Florida, U.S.A., submitted to the Second IAG Workshop on Non-Tidal Gravity Changes "Intercomparison between absolute and superconducting gravimeters", Walferdange, Luxembourg, Sept. 6-8, 1994.
6. Peter, G., Klopping, F. J. & Berstis, K. A., Observing and Modeling gravity changes caused by soil moisture and groundwater table variations with superconducting gravimeters in Richmond, Florida, U.S.A., Ibidem.
7. Warburton, R. J. & Brinton, E. W., Recent developments in GWR Instruments' superconducting gravimeters, Ibidem.
8. Dittfeld, H-J., Non-tidal features in the SG-record at Potsdam, Ibidem.
9. Sato, T., Shibuya, K., Ooe, M., Tamura, K., Kaminuma, K., Kanoa, M., & Fukuda, Y., Long term stability of the superconducting gravimeter installed at Syowa Station, Antarctica, Ibidem.
10. Crossley, D. & Hinderer, J., Global Geodynamics Project - GGP: Status Report 1994, Ibidem.
11. Defraigne, P., Dehant, V., & Hinderer, J., 1994. Stacking gravity tide measurements and nutation observations in order to determine the complex eigenfrequency of the nearly diurnal free wobble, J. geophys. Res., 99, B5, 9203-9213.
12. Defraigne, P., Dehant, V., & Hinderer, J., 1995. Correction to "Stacking gravity tide measurements and nutation observations in order to determine the complex eigenfrequency of the nearly diurnal free wobble", J. geophys. Res., 100, B2, 2041-2042.
13. Sato, T., Tamura, Y., Higashi, T., Takemoto, S., Nakagawa, I., Morimoto, N. Fukuda, Y., Segawa, J., & Seama, N., Resonance parameters of the free core nutation measured from three superconducting gravimeters in Japan, J. Geomag. Geoelectr., 46, 571-586.

GWR INSTRUMENTS

6264 FERRIS SQUARE, SUITE D * SAN DIEGO, CA 92121 * USA FACSIMILE NO. 619 452 6965 * TELEPHONE NO 619452 7655 EMAIL: marketing@gwrinstruments.com