|Instrument: SUPERCONDUCTING GRAVIMETER|
In Situ/Laboratory Instruments
Instrument Class: Magnetic/Motion Sensors
Instrument Type: Gravimeters
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GWR Instruments, Inc. is the exclusive manufacturer of the Superconducting Gravimeter (SG). The SG uses persistent supercurrents, which are trapped in superconducting magnets, to produce an ultra stable magnetic field which levitates a superconducting test mass (sphere). The SG consists of two basic components: 1) the Gravimeter Sensing Unit (GSU) which includes: the superconducting magnets, the sphere, circuitry for energizing the coils, temperature control circuitry and magnetic shielding; and 2) the liquid helium tank (Dewar) and refrigeration system which keeps the GSU close to 4.2OK to maintain the superconducting state. In 1993, GWR introduced the CT Compact Tidal Gravimeter which dramatically decreased the overall size of the SG and simplified field installation. The standard support equipment for the Compact Tidal Gravimeter includes a pair of cryogenic tilt meters, an automatic tilt compensating system, a gravimeter electronics package, a current supply/heater pulser, a helium level sensor and a helium transfer kit. The Compact SG is routinely equipped with the CD-125 Standard holdtime Dewar refrigeration system. This allows the gravimeter to operate for approximately 500 days between liquid helium refills.
Some users of the SG have found that it is necessary to remove small infrequent offsets from the gravity data to preserve the baseline (zero) on a continuous record. However, geophysical signal and noise sources often mask the offsets and make them difficult to measure and remove precisely. In response to this need, GWR now manufactures a new Dual Sphere SG GSU that incorporates two levitated test masses into a single sensor (See section I.A.2). Since it is unlikely that offsets occur on both spheres simultaneously, the differential signal gives a precise measurement of any offsets that may occur in the record. This makes offset identification and removal a straightforward procedure during data processing.
In response to customers' requests, GWR has also developed a second high performance refrigeration system. This Ultra Long Holdtime (ULHD) Dewar (See section II.B) allows indefinite operation after a single filling of liquid helium. (citation from: http://www.gwrinstruments.com/GWR_ctspec.html)