Environmental Geodesy Lecture 9 (March 22, 2011): observing gravity changes on the Earth's surface - Introduction - Relative gravimeters - Absolute gravimeters

Introduction Gravity field of the Earth is observed with in situ, airborne and spaceborne sensors: - Relative gravimetry surveys: mainly in order to improve the geoid locally at short wave-lengths but also for exploration purposes. - Highly accurate relative and absolute gravimeters: measure temporal variations of gravity locally and stationary at sites at the Earth surface - Gravimeters on ships and airplanes measure profiles along the track of the vessel. - Satellite orbit perturbations can be integrated to determine the static or time variable gravity field model with (low) spatial (and temporal) resolution. Focus on highly accurate relative and absolute gravimeters applied to the measurement of temporal variations of gravity stationary at sites at the Earth surface

Relative Gravimeter Askania and Lacoste Romberg Spring gravimeters: large drift, sensitive to environmental parameters From:

Relative Gravimeter Askania and Lacoste-Romberg spring gravimeters Many early spring gravimeters use a stable spring. Example: Askania Use of mechanical instability exaggerates small movement due to change in gravity. * Horizontal hinged beam supports mass at end. * Beam supported by spring connected above hinge. * Increase in gravity extends spring, but shortens d reducing increase in restoring force and allowing greater movement.

Relative Gravimeter Askania and Lacoste-Romberg spring gravimeters Lacoste-Romberg Earth Tide (LCR-ET) Gravimeter: low drift, high accuracy LCR-ET 25 (Earth Tide Observatory Zimmerwald): Electrostatic feedback system results in a precision of better than ms -2 for the one-minute averages; see _activities/earth_tide_observatory/index_eng.html.

Relative Gravimeter gPhone: derived from LCR-ET

Relative Gravimeter Comparison of six gPhone recordings (under test) at Lafayette, CO, USA. From erg.com/sichuanEarthq uake.htm gPhone: derived from LCR-ET

Relative Gravimeters Superconducting (cryogenic) gravimeters: very low drift, very high accuracy Superconducting Gravimeter (SG): - Levitation of a spherical test mass in an ultra-stable magnetic field replaces the mechanical springs. - The field is generated by persistent currents in two niobium coils that are superconducting below a temperature of 9.3 K. - 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. - Adjusting the ratio of currents in the magnet coils makes the magnetic force gradient ("spring constant") very weak. - Small changes in gravity produce large displacements of the test mass that are easily detected in the capacitive displacement transducer that surrounds the mass.

Relative Gravimeters Superconducting (cryogenic) gravimeters: very low drift, very high accuracy - 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. - The accuracy of the SG in the time domain is on the level of 1 nms −2 (= 10 −9 ms −2 = 0.1 microgal) or better, - Accuracy at high frequencies (< 1 d −1 ) at the level of 0.01 nms −2 (= 10 −11 ms −2 = 1 nanogal).

Relative Gravimeters Superconducting (cryogenic) gravimeters: very low drift, very high accuracy Long period normal modes from the Mw = 9.1 Sumatra-Andamen earthquake (2004/12/26) recorded by the SG at Canberra. The vertical lines are the theoretical multiplet peaks. The high signal-to-noise ratio is generally high.

Relative Gravimeters Superconducting (cryogenic) gravimeters: very low drift, very high accuracy Example of atmospheric mass transport during heavy rain. The signal at the top of the figure (at the start of the record) is gravity with tides removed, and the curve beside it is the pressure. After correcting with a frequency dependent admittance, the residual gravity is the lower curve (left). Note this residual gravity begins to decrease sharply just before the onset of the rain (lowest curve) due to a mass increase above the station that is not seen in the surface pressure.

Relative Gravimeters From: ing_signals_i Superconducting (cryogenic) gravimeters: very low drift, very high accuracy

Relative Gravimeters Global Network of Superconducting (cryogenic) gravimeters

Absolute Gravimeters Gravity changes at a point on the Earth’s surface are generally associated with displacements of the Earth surface or some other processes. The gravity anomaly measured by a gravimeter is therefore the sum of the effect due to the vertical motion of the gravimeter through the unperturbed gravity field and the contribution from mass changes in the vicinity of the gravimeter. In order to separate these two effects, gravimeters need to be co-located with geometric instruments such as a GNSS receiver.

Absolute Gravimeters Geophysical Research * Detection of vertical crustal motion * Complementary verification of displacements measured with GPS and VLBI * Determination of the geoid * Volcanic magma flow monitoring (i.e. Mammoth Lakes) * Postglacial rebound studies * Uplift of subduction studies * Earthquake research * Long period tidal monitoring and modeling of earth anelasticity Environmental Monitoring * Water table monitoring in deep and/or multiple aquifers * Nuclear waste management and cleanup * Global sea level studies for global warming Exploration and Resource Management * Oil exploration * Mineral exploration Precision Measurements and Calibrations * Pressure transducer and load cell calibration * Redefintiton of the kilogram in the SI system of units * Big G determinations and the equivalence principle * Calibration of superconducting of other high precision relative gravity mete rs Inertial Navigation * Gravity reference station determinations * Relative gravity network control points * Establishing geodetic tie points for gravity networks * Defining the geoid

Absolute Gravimeters The low-frequency variations in gravity at a site are usually determined by episodic observations with an Absolute gravimeter (AG). Today, AGs are almost invariably of the free-fall type. The FG5 manufactured by “microG” (now LaCoste-Romberg) is the most accurate absolute gravimeter. The FG5 operates by using the free-fall method. An object is dropped inside a vacuum chamber (called the dropping chamber). The descent of the freely-falling object is monitored very accurately using a laser interferometer. The free-fall trajectory of the dropped object is referenced to a very stable active- spring system called a Superspring. The Superspring provides seismic-isolation for the reference optic to improve the noise performance of the FG5 The optical fringes generated in the interferometer provide a very accurate distance measurement system that can be traced to absolute wavelength standards. Very accurate and precise timing of the occurrence of these optical fringes is done using an atomic rubidium clock that is also referenced to absolute standards.

Absolute Gravimeters The FG5 operates by using the free-fall method. An object is dropped inside a vacuum chamber. The descent of the freely-falling object is monitored very accurately using a laser interferometer. The free-fall trajectory of the dropped object is referenced to a very stable active-spring system called a Superspring. The Superspring provides seismic-isolation for the reference optic to improve the noise performance of the FG5 The optical fringes generated in the interferometer provide a very accurate distance measurement system that can be traced to absolute wavelength standards. Very accurate and precise timing of the occurrence of these optical fringes is done using an atomic rubidium clock that is also referenced to absolute standards.

Absolute Gravimeters

Typical FG% measurement sessions take a few days. Single drops carried out every 20 seconds or so have a high scatter, but an accuracy of 1-3 μgal is achieved from the mean of a large number of drops that are done over typical campaign durations of several hours to days. In order to extract the secular signal from these observations, high-frequency variations caused by solid-Earth and ocean loading tides, polar motion and atmospheric loading have to be modeled and corrected for. Hydrological loading is usually not included in these corrections.

Absolute Gravimeters A plot of absolute gravity observations at Table Mountain. The absolute gravity measurements have not been corrected for polar motion; error bars indicate 1 sigma limits. The Polar motion signal computed from VLBI observations is plotted as a solid line. From

Absolute Gravimeters Absolute gravimeters are used to monitor drift in SGs.

Absolute Gravimeters Calibration of absolute gravimeter is a problem.

Absolute Gravimeters Calibration of absolute gravimeter is a problem.

Absolute Gravimeters The FG5-X is a follow on instrument to the FG5 freefall gravimeter. The FG5-X features an improved dropping chamber and an improved electronic interface.

Absolute Gravimeters The FG-L series absolute gravimeter is a simplified version of the FG5 absolute gravimeter optimized for small size, speed of data acquisition, ease of use, portability and price. Many of the subcomponents of the FG5-L are compatible with the FG5 in order to allow an upgrade path to the FG5. This gravimeter offers an attractive trade-off between low price and precision in an instrument that is easy to use in the field.

Absolute Gravimeters The A-10 is an absolute gravimeter optimized for fast data acquisition, ease of use, and portability in outdoor applications. The instrument allows true field operation in harsh field conditions on open outdoor sites in the sun, snow, and wind.

Absolute Gravimeters

Absolute Gravimetry and Reference Frame Problem: Connection of reference frame origin to center of mass of the Earth system

Absolute Gravimetry and Reference Frame Problem: Connection of reference frame origin to center of mass of the Earth system

Absolute Gravimetry and Reference Frame Problem: Connection of reference frame origin to center of mass of the Earth system

Exclude |delta| > 5.00 Remaining stations: 26 Regression results: * Correlation coefficient: < < * Regression line: g = -( )-( )u This gives: - bias in Z Component: 0.57 mm/yr - alpha: 0.64 mm/(nms -2 ) (compared to 0.65 mm/(nms -2 ) from numerical PGR studies)‏ Absolute Gravimetry and Reference Frame

Regression results support the claim that collocated absolute gravity measurements constrain the tie between RFO and CM. Recommendation: collocated absolute gravity measurements with the goal to constrain RFO to CM tie: - Collocate permanent absolute gravimeters with core geometric sites - Number of core stations: about 40 (GGOS 2020)‏ - Global distribution Condition: - Sites with small on-going mass changes Required: - Coordination of geometric and gravimetric networks Absolute Gravimetry and Reference Frame

Absolute Gravimeters and Definition of Kilogram In S.I., the kilogram is the only remaining base unit that still relies on a material artifact. This is unsatisfactory, especially because the stability of the kilogram prototype is not well constrained (changes could be 5 10(-9) per year). The most promising approach is the Watt balance experiment that allows expressing the kilogram in terms of the meter, the second and the Planck’s constant h, by equating mechanical and electrical power.