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Optimizing the Performance of the SG during the GGP

Richard Warburton

GWR Instruments, Inc., San Diego CA, USA
First GGP Workshop on 21 July 1997 in Brussels

Summary

  • How to operate the SG to maximize the data quality
  • How to minimize data interruptions
  • Faster GWR/user response to identify, solve and repair problems as they occur during the GGP

Superconducting Gravimeter subsystems or subtopics:


Gravity Card - Revision 2 Features

  • New high precision reference oscillator
  • High current transformer driver
  • Improved low temperature dependence drive transformer
  • Magnetic modulator input for measuring frequency response
  • Electrostatic modulator input for measuring frequency response
  • GGP low pass filter
  • Improved layout with 4 layer PCB and shielded input stage
  • On-board temperature sensors
  • Improved components
  • GWR5 Low Pass Filter

GGP1 & GGP2 Low Pass Filters

  • GGP1 filter intended for 1 Hz sampling rate - (See Figure 1)
    • 8 pole Bessel filter
    • Corner frequency at 61.5 mHz (16.3 sec period)
    • Constant time delay of 8.2 seconds (Phase lag 0.034 deg/cpd)
    • 100 dB attenuation at 0.5 Hz (fnyq for 1 Hz sampling)
    • Attenuation < 1% (-.086dB) below 0.01 Hz (100 sec period)
    • Attenuation < 4% (-.341dB) below 0.02 Hz (50 sec period).
  • GGP2 filter intended for 0.5 Hz sampling rate (optional)
    • 8 pole Bessel filter
    • Corner frequency at 30.8 mHz (32.6 sec period)
    • Constant time delay of 16.4 seconds (Phase lag 0.068 deg/cpd)
    • 100 dB attenuation at 0.25 Hz (fnyq for 0.5 Hz sampling)
    • Attenuation < 1% (-.086dB) below 0.005 Hz (200 sec period)
    • Attenuation < 4% (-.341dB) below 0.01 Hz (100 sec period).

Characterizing Frequency Response

  • Magnetic modulator
    • Adder allows injection of current into the feedback circuit, to measure closed loop response.
    • Jumper selection removes adder eliminating unnecessary components.
    • Jumper selection allows both open and closed loop characterization.
  • Electrostatic modulator
    • Allows similar measurements by using electrostatic force
    • Response depends only on geometry of sphere & plates and is independent of magnetic levitation.
    • Allows measurement of charge.

General Noise Reduction on Gravity Card

  • Improved oscillator - (See Figure 3)
    • Improved temperature stability
    • Reduced harmonic distortion
  • Improved drive transformer - (See Figure 4)
    • Toroid design replaces bobbin design decreasing TC by order of magnitude.
  • High current transformer driver
    • Lowers distortion and impedance of drive circuit
  • 4 layer PCB, improved grounding and shielded input stage
    • Reduced cross-talk between drive and sense circuits - (See Figure 5)
    • Reduced overall broad band noise - (See Figure 6)
  • On-board temperature sensors
    • Simplifies monitoring of electronics temperature
  • Improved components
    • Hermetically sealed ultra stable passive components
    • Selected grade IC's for low noise, thermal drift, and long term stability
    • Conformal coating improves resistance to humidity and surface contamination

Decrease in Gravity Card Temperature Dependence

  • Old gravity card had a temperature dependence between 0.1 to 1 mgal/C. (See Figure 7)
  • New gravity cards have temperature dependence less than 0.01 mgal / C (See Figure 8)

GWR5 Low Pass Filter - (See Figure 2)

  • Intended for future acquisition system capable of sampling at approximately 1 plc (50Hz or 60Hz). Data system should implement real-time digital filter decimating output to 1 Hz.
  • 2 pole Bessel filter
  • Corner frequency at 200 mHz (5 sec. period).

Temperature control

  • Check feedback characteristics
    Uses should measure the response of gravity, temperature power and temperature balance to a change in temperature control null position. In
    Figure 9, an offset from ``off" to +000 (=0.21 reset) equals about .084 mK and produces a gravity offset of 1.25 mgal. When the feedback is working well, both the temperature power and balance should return to equilibrium with only 1 to 2 excursions.
  • Expected temperature balance noise (and what gravity noise level this corresponds to)
    Temperature balance noise is about 0.05
    mV => 1.5 mK => 15 ngal
  • What are the implications of spikes on heater power or temperature balance signals? Could these produce offsets in data?
    Spikes in the temperature power or balance could indicate that an offset in temperature has produced an offset in gravity. In concept, one could measure an offset in power associated with the temperature offset. However, noise limits measurement of power offsets to 10 mV. Therefore, since the power sensitivity is only 0.4 mV/
    mgal, one can only resolve offsets larger than 25 mgal.
  • Can the spikes be recorded to indicate a problem?
    Yes, one must correlate gravity offsets to power spikes to prove cause and effect. However, this requires at least a 2 second sampling interval since the power spikes are only 10-15 seconds wide.

Tilt control

  • Check feedback characteristics
    Users should measure the response of tile power and tilt balance to a change in both X and Y reset. In
    Figure 10 the X Reset has been changed from +521 to +526 which corresponds to about 30 mradian. Check that both the power and balance return to equilibrium with only 1 or 2 oscillations.
  • Expected tilt balance noise
    The tilt noise will depend on how quiet the user's site is. At GWR the tilt balance noise is about 10 mV which corresponds to a tilt noise of about
    0.1 mradian. Users are encourage to measure the relationship between micrometer ``mils", tilt balance volts (BD=7), reset units and mradian. In the instrument tested at GWR: 1 mil = 2.9 V = 5.8 reset = 32 mradian. These relationships will depend on the length of the tilt arms, the electronic gain, and the tiltmeter sensitivity.
  • What are the implications of spikes on X or Y tilt power or balance signals? Could these produce offsets in data?
    As with temperature, these spikes could indicate that changes in the tilt control position are producing offsets. However, such a conclusion cannot be reached without using secondary tiltmeters or by correlation of spikes with gravity offsets. The width of tilt balance spikes is 2 to 4 minutes and can easily be observed by sampling with a 20 second interval.
  • Tilt geometry & manual tilt desensitizing (FB & LR)
    All compact Dewars and many other instruments now use equilateral leveling frames versus the older isosceles triangular frames. The geometry of the two systems is illustrated in
    Figure 11. The advantage of the isosceles support frames is that the left and right axes are orthogonal and that the two tiltmeters can be aligned with these axes. In SET-UP, this means that reading of the X (or Y) micrometer does not affect the null position measured with the other micrometer. With the equilateral frame, the user must simultaneously use both left and right micrometers to define two new tilt axes labeled Left-Right (LR) and Forward-Back (FB). LR is defined by moving both X and Y micrometers in the same direction, e.g.: DELTA X = +5 mils and DELTA Y = +5 mils; while FB is defined by moving in opposite directions, e.g.: DX = +5 mils and DY = -5 mils.
  • Tilt desensitizing in feedback using X or Y Resets
    With the leveling platform operating in feedback (RUN), both equilateral and isosceles support frames can be tilt minimized in an orthogonal fashion using the Left (X) Reset and the Right (Y) Reset. The X & Y reset functions produce orthogonal tilts because they produce electronic offsets in the tiltmeters themselves. However, since the thermal levelers do not produce orthogonal tilts, they both must respond to either a X or Y Reset change. This is shown in
    Figure 12, where the X tilt axis is being tilt minimized using only the X-Reset. As shown, the X Power response is larger than the Y power. From the geometry of the equilateral frame and tiltmeter alignment, the ratio of the Y tilt power should be 0.27. In the example, the power ratio was 0.36 which is most likely due to errors in the electronic square root function in the feedback network and to different leveler response to heat.
  • Expected tilt noise close to minimum
    The slope of the tilt curve
    Dg/( D Reset)2 can be used to calculate the expected gravity noise produced by tilts. For example, in the data show, the gravity noise is about:

DgN @ 4.6 x 10-4 (mgal/(mrad)2)(Q-Q0) DQ                           (1) 

Therefore, the tilt noise depends on the tilt slope, how close the instrument is adjusted to the tilt null Q0 and the noise level DQ at the site. In the example shown, if (Q-Q0 < 1 Reset (=5mrad) and the noise DQ = 0.1mrad, then the tilt induced gravity noise DgN @ 2.5 ngal. At a site such as Membach, the tilt noise is about 5 times quieter than at GWR. Therefore, the tilt induced gravity noise will be DgN (Membach) @ 0.5 ngal

  • Need for an annual tilt check
    GWR recommends that users perform annually tilt checks for both X-Reset and Y-Reset. When the system is operating properly, neither the X or Y Reset values will change in time. This means that the tilt minimum of the gravity meter and the null points of the tiltmeters have remained stationary with time.

Data Acquisition and Analysis

- (Membach, Belgium data as an example)

  • Instrumental channels to record gravity and for SG maintenance
    Data channels can be classified into three categories. Recording the main signals of gravity and air pressure is already fully discussed in previous GGP meetings. The auxiliary signals are used to check and monitor the subsystems of the gravimeter to make sure they are operating correctly. It is important to establish a baseline of operation on the subsystems for regular comparison to make sure performance does not degrade in time. This will be especially helpful when problems arise that must be discussed with GWR. Comparison of data with the system working correctly versus improper operating is essential for diagnosing failures rapidly. The geophysical signals are of prime importance for correlating with observed gravity changes. This correlation allows further reduction of secular or short term signals from the data. For example, groundwater will produce long term secular signals; while rain fall may produce 1 to 3 day spikes in the data. Finally, regular absolute gravity measurements allow either confirmation of long term trends in superconducting data or correction of instrumental drift.
  • Main signals:
    Gravity
    Air pressure
  • Temperature balance
    Heater power
    X & Y Tilt balance
    X & Y Tilt power
    Electronics temperature
    Vault temperature
    Neck temperatures
    Helium flow
    (Compressor water coolant temperatures)
    (mode data?)
  • Geophysical signals:
    Permanent GGP measurement of elevation changes
    Ground water variations
    Soil moisture
    Rain and snowfall
    Other??
  • Periodic Absolute Gravity measurements
  • Notes of caution:
    1) User's data systems should use differential and isolated inputs. Each signal from the GEP-2 electronics has a corresponding return (common) which is referenced to a specific location on the board where the signal is generated. Connecting commons together at the data system will disrupt the electronic design and will produce ground currents and noise. It is for this reason that GJ1 through GJ4 and the front panel connector are isolated BNC connectors. The commons of these BNCs should not be grounded.
    2) Use caution on recording Heater Power. We have observed that connecting a long cable to the heater power (GJ5 pins 7/15) can cause noise on the gravity signal. Pin 7 connects directly to the output of the temperature feedback integrator (U7). The increased capacitance to ground present in the cable connected to pin 7 may cause U7 to oscillate at a high frequency. This oscillation will shift the DC level of the output and produce a shift in the temperature control position. If this happens rapidly the result looks like noise. If it happens infrequently it will look like random offsets on the gravity meter. Users with short leads between GEP-2 and their data systems are probably safe from oscillations. However, for users with long leads we recommend that they discontinue recording the heater power by disconnecting pins 7/15 in the cable connected to GJ5.
  • Daily & weekly analysis of data

    Daily
    - It is important to analyze and monitor the main and auxiliary signals frequently in order to minimize data interruptions and gaps. The best way to guarantee the highest quality gravity data is to generate a tidal residual signal by subtracting a tide model based on the analysis of previous data. Ideally this can be done on a daily basis as in the data shown in Figure 13 (air pressure, tidal residual, & theory from Membach). Once the residual is generated, the user can determine:
    1) Has the instrumental noise level remained low?
    2) Are there any offsets or spikes (besides earthquakes)?
    3) Are there any abrupt changes in slope or drift?
    4) Is the data system, timing and data storage working properly?

    Weekly
    - The same type of analysis can be done on a weekly basis as in the data shown in Figure 14 (air pressure, tidal residual & theory from Membach). This residual data should be compared to the X & Y Tilt balance and power (Figure 15), Heater power and temperature balance (Figure 16), electronics temperature, and vault temperature. For example on the X tilt power there is a large spike. However, by comparison of this event to the tidal residual one can determine that this event did not produce a corresponding spike or noise on the residual data. What caused this spike then? If it was a user entering the vault, it should cause a change in vault temperature or produce an entry into the log book.

    Clearly, the more often the tidal residual is generated and checked the shorter gaps in the data will be. However, weekly checks of the temperature, tiltmeters, and refrigeration systems are most likely adequate. Monitoring the neck thermometers to observe an increase in temperature is the most rapid way to determine when the cold head is beginning to age. This allows plenty of time to replace the cold head since they degrade slowly.
  • Is the Mode filter useful anymore?
    Recording the mode bandpass filter on a strip chart recorder (See Figure 17) is a quick and simple way to examine the high frequency noise present on the gravity signal. Therefore, it may be useful for users who are not generating and examining daily tide residuals. However, it can only be used for checking for changes in instrumental noise and for offsets and spikes. It cannot be used for changes in drift (since DC signals are filtered out) or for operation of the data system or storage medium.

    The mode filter could be recorded as auxiliary data at 20 second intervals and be used to scan the data for offsets. As shown in Figure 17, a 40 mV step function into the mode filter produces a spike response of about 1.22 V (peak to peak). Therefore, the magnitude of offsets producing such spikes can be (practically) read of the mode filter data if the noise level is low. For high noise regions it is more difficult to remove such offsets. Possibly, the mode filter data would be useful to other GGP participants to quickly determine data quality before they commit to the process of ``cleaning" the data for further analysis.
  • Importance of Absolute Gravity Measurements
    Figure 18 shows the gravity residuals of the superconducting gravimeter (SG) compared to the measurements from the absolute gravimeter (AG) at Membach, Belgium. In this case there was a data gap and offset that occurred in the SG near the end of May-96. This offset was estimated and corrected by comparison of SG to AG data. From these data sets it appears that the SG at Membach has very little instrumental drift. Generally, however, some drift may be present on the SG at other sites. Such drifts are always monotonic and usually decrease in time. These drifts can be measured by comparison of the SG residual data to AG data if AG is taken at regular periodic intervals. The SG can also be used to check proper operation of the AG. As can be seen from the data, there are two AG data points at 18000 hours that disagree significantly with the rest of the data.
  • Importance of measuring other geophysical data
    The agreement of SG and AG data markedly increases confidence in both data sets and proves that the observed gravity variations are of geophysical origin and not of instrumental origin. The geophysicist's job is now to identify the cause of such variations. One common method of doing this is by establishing a correlation between the gravity residual and other geophysical signals. However, this powerful technique can only be used if the geophysical signals have been recorded over the same time period as the gravity data itself! Therefore, users must establish a list of geophysical signals that are most likely to influence data at their sites and implement methods to measure and record them as soon as possible.

    Acknowledgment: The author sincerely thanks Olivier Francis and Marc Hendrickx for supplying the data from Membach, Station, Belgium which is used as examples in this section.

Barometric Measurements

  • GOP recommends accuracy better than 0.1 hPa.
  • Admittance of air pressure to gravity is about 3 nms2 / hPa.
    • GOP recommends measuring to accuracy of 0.3nms2
  • Best transducers are stable to no better than 0.1 bPa / year.
  • To maintain GOP specification, calibration on yearly basis is required.
  • Calibration by factory with dead weight tester.
  • Calibration in the field with transfer meter.

Common Pressure Transducer Operating Principles

  • Quartz Bourdon Tube

    • Mainly for Laboratory standard
    • Very Expensive.
  • Vibrating Cylindrical oscillator (Weston or Schlumberger. sensor)
    • Long history of stable measurements
    • Sensitive to changes in media density (humidity).
  • Silicone Resonant Pressure Transducer (e.g. RPT by Druck)
    • Newest Technology
    • Stable, accurate, insensitive to media
  • Capacitance bridge (e.g. MKS Baratron)
    • Best suited for low pressure (vacuum)

Digital Sensors

  • Vibrating Cylindrical Oscillator and RPT sensors are intrinsically digital
    • Changes in Pressure effect the frequency of a resonating structure
    • Conversion to an analog signal is an unnecessary step and degrades the system stability
    • Data should be collected digitally which may require modification of data acquisition software.

Comparison of various types of pressure transducers

  Temp. Controlled Capacitive Sensor (MKS Baratron) Silicone Resonant Pressure Transducer (RPT Druck) Vibrating Cylinder (Weston or Schiumberger sensor)
Accuracy 1.5 hPa 0.1 hPa 0.1 hPa
Drift not specified 0.1 hPa / year 0.1 hPa / year
Operating temp. range +15C / +40C -20C / +60C -40C / +70C
Temperature induced errors 0.3 hPa / C 0.2 hPa over full temperature range better than stated accuracy
Humidity sensitive NO NO YES
output format analog (DCV) digital (R5232/R5422 selectable) digital (R5232 OR R5422)
DC! Gain calibration Evacuate! dead weight test dead weight test dead weight test
Typical usage low pressure general barometric barometric
History   Recently developed Long History, industry standard

Barometer Recommendations

  • All GGP participants should upgrade pressure transducers to either the RPT or Vibrating Cylinder type
  • Data should be collected digitally via a serial port rather than through an analog channel
  • GWR now recommends a Silicone Resonant Pressure Transducer manufactured by Druck Instruments
  • GWR (or other?) could maintain a calibrated transfer meter for use by GGP participants if the community desires it

Maintaining Cryogenic Refrigeration System

  • Refrigeration system is the most maintenance intensive part of the SG system!
  • Compressor adsorber replacement every 10,000 hours (13 months)
  • Annual coldhead clearance check
  • Coldhead maintenance approximately every 20,000 hours
  • Compressor cooling must be maintained within specified limits
    • Evaluate heat management criteria carefUlly when setting up the system
    • Choose proper cooling system for ambient conditions
    • Monitor cooling water temperatures

Helium Compressor Cooling Requirements

  • Failure to provide adequate cooling will eventually result in premature compressor failure
  • In some cases inadequate cooling can result in oil contamination to the entire cooling system including the gas hoses and coldhead!
  • APD HC2 Compressor specifies cooling water is maintained within certain limits!
    • 2.3 liters / minute minimum flow rate
    • 27 "C maximum inlet temperature
    • Approximately 1.8 kW of heat rejection is required.

Different Types of Water Coolers Have Different Limits!

  • Dry type coolers
    • Operate by passing water to be cooled through tubes attached to cooling fins. Fans force ambient air over fins to remove heat
    • These coolers will never cool to the ambient air temperature, cooling to within 5 "C to 10 "C of ambient is typical
    • Inexpensive, simple, reliable
  • Compressor driven chillers
    • Operate with freon or other compressed gas (like a home refrigerator or air conditioner)
    • Require additional electrical power
    • Can maintain cooling water at temperature far below ambient air temperature

Possible Cooling Options

  • APD Cool-Pack 4
    • Dry type cooler
    • Maintains cooling water inlet to HC2 approximately 5C above ambient
    • Acceptable when ambient air temperature does not exceed 22"C
  • GWR AW75 Cooler
    • Dry type cooler
    • Maintains cooling water inlet to HC2 approximately 4 C above ambient
    • Acceptable when ambient air temperature does not exceed 23C
    • Increased pump capacity decreases compressor outlet temperature maintaining increased safety margin
    • Heavy duty pump, fan, and heat exchanger for increased reliability
  • Schreiber 1 Ton Compressor Driven Water Chiller
    • Compressed freon refrigerator maintains low temperatures in extreme ambient conditions (up to 45C)
    • Rated for outdoor use
    • Low ambient controls allows operation in sub zero conditions
    • Large water reservoir reduces refrigeration cycling improving reliability
  • RECOMMENDATION - Purchase suitable water chiller from your local refrigeration expert!

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