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7 Observatory Superconducting Gravimeter Overview GWR Instruments, Inc. is the exclusive manufacturer of the Superconducting Gravimeter (SG). In the SG sensor, levitation of a spherical test mass in an ultra-stable magnetic field replaces the mechanical springs found in previous gravity meters. The field is generated by persistent currents in two niobium coils that are superconducting below a temperature of 9.3 K. The stability is derived from the zero resistance property of superconductors – after the currents are “trapped” no resistive (ohmic) losses exist that could cause them to ever decay in time. In addition, adjusting the ratio of currents in the magnet coils makes the magnetic force gradient (“spring constant”) very weak. As a result, small changes in gravity produce large displacements of the test mass that are easily detected in the capacitive displacement transducer that surrounds the mass. The ultra stable magnetic field, weak gradient and operation at cryogenic temperatures eliminate the sources of noise and drift commonly found in mechanical spring gravity meters. As a result, the SG is the world’s most sensitive and stable gravimeter.
Figure 1: GWR OSG Superconducting Gravimeter and Integrated Electronics To maintain the superconducting state, the Gravimeter Sensing Unit (GSU) is operated at 4 K inside a dewar filled with liquid helium. In 2003, GWR introduced the Observatory Superconducting gravimeter (OSG) which uses a 4 K refrigeration system and 35 Liter dewar. In this system, the refrigeration system operates below the boiling point of liquid helium thereby preventing any loss of the liquid during normal operation. Therefore, the OSG can operate indefinitely without the need for refilling with liquid helium. The refrigerator requires less power than previous models, consuming approximately 1.3 kW; however, it has enough excess capacity to liquefy helium gas when it is added to the dewar from a pressurized gas cylinder. This technique is used to replace any helium lost during maintenance or power failures thereby eliminating the requirement to transport and transfer liquid helium during normal operation. Liquid helium however is required when the SG is first setup. The OSG dewar is smaller and lighter than previous SGs. It weighs only 60 kg and is easily installed on any concrete pad 80 cm x 80 cm. The data acquisition system and control electronics, used for operating and monitoring the SG and for recording data are fully integrated in the OSG. The proof mass levitation, centering, leveling, and system monitoring are all accomplished through computer interface. This allows the operator to control and monitor the SG from his home or office. In addition to gravity and pressure, the data acquisition system (DDAS) records 30 status variables. Alarm thresholds can be set for all channels, to automatically generate warnings and alert the operator by email to initiate investigation and repair. After the operator enters the calibration factor, tidal parameters and barometric pressure admittance into the DDAS, it will calculate a theoretical tide and display the gravity residual signal in real time. This allows immediate visual examination of the gravity noise at sub-?Gal levels. Observations of small signals and changes in noise level are immediately observable and with some experience can be identified as of geophysical origin (atmosphere, ocean, or earthquakes) or due to equipment problems. In the latter case, GWR can examine the system on-line to analyze the problem with the user to provide a rapid solution. Remote access reduces the frequency of data gaps and ensures high quality of overall long-term data. As demonstrated by results from the Global Geodynamics Project (GGP)1, the SG provides a continuous record of changing gravity and provides data over wide period range from seconds (ocean noise) to several years (secular changes). It is common for the SG to measure small periodic tidal signals and long period seismic signals with a sensitivity of 1 nano-Gal and better. Therefore, one nano-Gal is generally referred to as the nominal precision, or sensitivity, of the SG. At quiet sites, typical noise levels at long period seismic frequencies are from 0.1 to 0.3 µGal Hz-1/2. For temporal studies, SG data averaged to 1 minute intervals achieves a precision of better the 0.04 µGal. For frequencies less than 1 mHz, SGs have achieved lower noise levels than attained by most long-period seismometers and are now being used to study low frequency normal modes.
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