Moving superconducting gravimeters to the field: first experiences with an iGrav
in a field enclosure
Güntner, Andreas1; Mikolaj, Michal1; Reich, Marvin1; Schröder, Stephan1; Wziontek, Hartmut2
1 Helmholtz-Zentrum Potsdam - Deutsches GeoForschungsZentrum GFZ 2 Bundesamt für Kartographie und Geodäsie (BKG)
A. Güntner | Hydrogravimetry 2
Monitoring continental water storage variations
Deployment of superconducting gravimeters (SG) for monitoring hydrological dynamics
– State: Water storage variations
– Fluxes: Groundwater recharge, Evapotranspiration
– Products: Derivation of drought or flood indices
A. Güntner | Hydrogravimetry 3
Objective
• Overcoming limitations of observatory gravimeters
– Disturbed/unknown local hydrology in the near-field
– Umbrella effect of observatory building
• Requirements on gravimeter:
– Field-deployable: water-/dust-proof, high temperature range
– Small footprint: minimal umbrella effect
– High precision: resolve small gravity changes
– Long-term stability: low drift and no/minimum steps
A. Güntner | Hydrogravimetry 4
Instrument
• Latest generation of superconducting gravimeters of GWR (iGrav)
– Height (with coldhead): 1m
– Diameter (base with thermal levelers): 55cm
– Weight: 40kg
A. Güntner | Hydrogravimetry 5
Installation site
• iGrav-006 installed in Wettzell (Germany) in February 2015
A. Güntner | Hydrogravimetry 6
Technical setup: Field enclosure
• Temperature controlled enclosure:
– iGrav-006, PC, heater, water cooling grill, temperature sensors
A. Güntner | Hydrogravimetry 7
Technical setup: Peripheral hardware
• Field box with peripheral hardware
– Compressor, water chiller, power supply + UPS, PC, controller, He-Gas bottle
A. Güntner | Hydrogravimetry 8
Overall layout
iGrav-006 Field enclosure iBox: peripheral hardware
A. Güntner | Hydrogravimetry 9
Overall layout
• iGrav on concrete pillar (diameter 1m, height 80cm)
• Sensor height above terrain surface: 1.05m
A. Güntner | Hydrogravimetry 10
• Impact of sensor height on the signal amplitude
– Assuming a soil moisture variation of 10 Vol% in the top 1m
– Integration radius: 10 km
Sensitivity of sensor height
1.0 m
variable
iGrav
A. Güntner | Hydrogravimetry 11
Calibration and removal of steps
• No co-located measurement with FG-5 currently possible
– iGrav-006 calibrated against SG030
• Distance between iGrav-006 and SG030: ~ 40 m
• SG030 calibrated using absolute gravity measurements
A. Güntner | Hydrogravimetry 12
Calibration and removal of steps
• No co-located measurement with FG-5 currently possible
– iGrav-006 calibrated against SG030
• Distance between iGrav-006 and SG030: ~ 40 m
• SG030 calibrated using absolute gravity measurements
• Steps removed by visual inspection and comparison to local hydrological effect (09/06/2015)
Gravity residuals iGrav006 (tides, polar motion, atmosphere effects removed)
A. Güntner | Hydrogravimetry 13
Emerging technical issue: spurious temperature effects in gravity residuals
1) Steps in gravity time series if board temperature exceeds threshold
2) Marked diurnal variations, related to ambient temperature
A. Güntner | Hydrogravimetry 14
Gravity effect: Forward modelling based on hydrological observations
• Soil moisture (SM):
– 0 – 10 cm: TOMST sensors
– 10 – 225 cm: TDR soil moisture vertical profiles
• Vadose zone (VZ):
– 225 cm – Groundwater (~6m depth): extrapolation of TDR profile
• Groundwater (GW):
– Groundwater variation: GW table observation well
• Umbrella effect:
– 1 m below iGrav pillar
– 2 m below SG030 building
• Global hydrological effect (GHE):
– GLDAS/NOAH 0.25°
A. Güntner | Hydrogravimetry 15
Gravity effect: Forward modelling based on hydrological observations compared to gravity residuals of iGrav006 (tides, polar motion, atmosphere effects removed)
A. Güntner | Hydrogravimetry 16
Gravimeter drift
• Minimal apparent drift in iGrav time series (03/2015 – 08/2015)
– iGrav006: - 3.0 nm.s-2/month
• Linear drift estimations:
– Manufacturer (GWR): +5 nm.s-2/month
– Soil moisture effect: - 19.7 nm.s-2/month
– Vadose zone effect: - 8.5 nm.s-2/month
– Groundwater effect: - 0.8 nm.s-2/month
– Global hydrological effect: - 6.1 nm.s-2/month
A. Güntner | Hydrogravimetry 17
Gravimeter drift
• Minimal apparent drift in iGrav time series (03/2015 – 08/2015)
– iGrav006: - 3.0 nm.s-2/month
• Linear drift estimations:
– Manufacturer (GWR): +5 nm.s-2/month
– Soil moisture effect: - 19.7 nm.s-2/month
– Vadose zone effect: - 8.5 nm.s-2/month
– Groundwater effect: - 0.8 nm.s-2/month
– Global hydrological effect: - 6.1 nm.s-2/month
• Estimated drift to match forward modelled hydrological effect
– iGrav006: +32.1 nm.s-2/month
A. Güntner | Hydrogravimetry 18
• Footprint difference (field enclosure vs. building): 1 vs 88 m2
– Theoretical umbrella effect: 1 m deep, 10% soil moisture difference: 1 vs 22 nm.s-2
Comparison field deployment versus observatory SG (iGrav-006 vs. SG030)
1.0 m
1.2 m
1.05 m
0.25 m
1.0 m
4.1 m SG030
iGrav-006
umbrella effect
A. Güntner | Hydrogravimetry 19
• Footprint difference (field enclosure vs. building): 1.6 vs 88 m2
– Theoretical umbrella effect: 1 m deep, 10% soil moisture difference: 1 vs 22 nm.s-2
• Topographic effect: height difference 4.1 m
– Theoretical effect of soil layer (10%, radius 100 m,-umbrella effect): 0 – 1 m = 39 vs 6 nm.s-2
1.0 m
1.2 m
1.05 m
0.25 m
1.0 m
4.1 m SG030
iGrav-006
umbrella effect
Comparison field deployment versus observatory SG (iGrav-006 vs. SG030)
A. Güntner | Hydrogravimetry 20
Wettzell time series for the iGrav period 03-08/2015
A. Güntner | Hydrogravimetry 21
Acknowledgments
Ilona Nowak (BKG), Thomas Klügel (BKG)
BKG staff at the Geodetic Observatory in Wettzell
Eric Brinton (GWR)
A. Güntner | Hydrogravimetry 22
Gravity effect: Forward modelling based on hydrological observations
Gravity residuals iGrav006 (tides, polar motion, atmosphere effects removed)
A. Güntner | Hydrogravimetry 23
• Topographic effect as a function of integration radius
– iGrav sensor height: 1.05 m
– SG030 (lower sphere) sensor height: 0.25 m
– Altitude difference: 4.1 m
iGrav-006 vs SG030
A. Güntner | Hydrogravimetry 24
iGrav-006 vs SG030
• Correlation analysis:
– Residuals iGrav-006 vs SG030: 0.87
– De-trended residuals iGrav-006 vs SG030: 0.93
A. Güntner | Hydrogravimetry 25
Explanation
• Vadose Zone extrapolation – Average amplitude decrease 1.0 -> 1.5 -> 2 m = 3% / meter
– Extrapolate SM variation in depth 2 m up to groundwater
