NGS’ Plans for a Geoid Monitoring Service...NGS’ Plans for a Geoid Monitoring Service Dr....

NGS’ Plans for a Geoid Monitoring Service

Dr. Theresa Damiani NOAA- National Geodetic Survey

Day 5: Time-Varying Gravity and the Geoid

Friday, May 27, 2016

The Purpose of Monitoring Geoid Change

Static, except for model updates

Deforms, subsides, uplifts at all length scales

Changes along with long-wavelength gravity change (caused by mass movement)

Beginning in 2009

Develop a long-term geoid change monitoring service

– What models of change exist and what measurements will be needed?

– Thought to include: • A tracking/validation

network – CORS, gravimeters,

GPS on Benchmarks? • Geophysical Models-

GIA, subsidence, etc.

Working group established - Boulder conference in 2009 - Started exploratory work toward a plan in 2017

Need to Know for Planning

• For all gravity change signals, need to know: – Amplitude – Spatial extent – Time scale

• How long does it take for these signals exceed the threshold for gravity change that affects the geoid at the 1 cm level?

• Also, need to know these for both the trend and variability (e.g. seasonality) in the gravity change signals, to evaluate accumulation of error over time.

• Are the monitoring methods reliable, replaceable (if there’s a failure), and cost effective?

Signal Magnitudes for Monitoring

4000 km

Equation courtesy of Simon Holmes, Pers. Comm.

GRACE Resolution for Monthly Solutions (200 km)

To the right of black line: Creates 1 cm or more of geoid change Current airborne gravimetry technique (1 mGal) not very useful for monitoring change. Satellite and terrestrial gravity measurements are two of the primary techniques.

Approximate equation to produce a 1 cm geoid change: Δg = 30.73/d + 0.00154

where Δg is the amplitude in mGal of the gravity change signal necessary and d is the diameter in km of the spatial feature.

North American Geoid Height Change Question: How often will updates need to be made to maintain 1-2 cm accuracy in the North American geoid model over the next 100 years?

Conclusions from Jacob, et al. 2012

• 6 important sources on 3 distinct timescales: – Episodic changes:

• Cataclysmic seismic and volcanic events • Magnitude 9 megathrust earthquakes (ignore smaller thrust earthquakes

unless repeated in same location and strike-slip events) • Mount St. Helens explosive eruption (Yellowstone inflation is negligible)

– High rate changes • < 10 years to 1 cm geoid change • Glacial isostatic adjustment (GIA) and ice mass loss

– Lower rate changes • 30 – 100 years to 1 cm geoid change • Groundwater withdrawal, continental hydrology, climate variability

• Sea level rise will create a 1 cm geoid change in 150 - 400 years (negligible for at least next 75 years).

Conclusions from Jacob, et al. 2012

• They considered the next 100 years of updates.

• Strategy for maintaining 1 cm accuracy: – Update the vertical datum sooner than every 10 years

for North America. – For the next 30 years, only GIA and ice mass loss matter. – Local adjustments will be needed after disasters

• Strategy for maintaining 2 cm accuracy: – Don’t need to consider local adjustments for cataclysmic

events – Don’t need to consider continental hydrology or climate

variability in next 100 years

Needs of Gravimetry Community

• Comprehensive lists of sources of gravity change, time scales, and spatial extents

• Concurs with the results of Jacob, et al. 2012 – Same signals are important

How will error accumulate spatially?

• Must take into account known errors that we will not track (e.g. seasonal, drought/monsoon).

• Add to that the errors that accumulate with incomplete modeling/measurement of the 6 important sources of change.

• How much sooner than 10 years do we need to update?

Geoid Change Based on GRACE Data 2002 Through 2014

Annual Change Amplitude in mm (NGS plans to ignore, for now)

Trend in cm Accumulated Change Over 1 Decade

Images courtesy of Ryan A. Hardy, pers. comm.

1 to 1.5

0.5 to 1

0 to 0.5 -0.5 to 0

-1 to -0.5

AK: Up to .45 cm annual change becomes part of accepted error in geoid model

AK: Up to 1 cm per decade Canada: 1.5 cm per decade

Update every 3-5 years?

Using GRACE mascons to measure modern ice mass loss

“If these computed geoid trends remain more or less constant into the future, one might expect a geoid change of 10 mm every 3.5–6.5 years immediately over the glaciated regions of North America. Some hundreds of kilometers away from these regions, a 10-mm geoid change is likely to occur at decadal time scales.”

Jacob, et al., 2012

How Accurate are GIA models?

GRACE and GIA Models are very similar in error. Using current GIA models (not GRACE) would introduce only a 10 mm geoid offset over 18 years.

ICE-5G GIA Model Geoid Height Trend

GRACE GIA Geoid Height Trend, corrected for modern ice loss

Difference between (a) and (b)

Jacob, et al., 2012

Improvements in Future Satellite Gravity Missions

~ factor of 5 improvement over GRACE

Pail, et al., 2015

~ factor of 10 improvement over GRACE

Potential Plan

Primary Operations: Estimate gravity change from mass change models and GRACE-FO. Validate with surface gravity measurements. (75% modeling, 25% new campaigns) Secondary Operations For episodic events without available models, measure with surface gravity and GNSS instruments. (100% new campaigns) Backup Operations (if failure of GRACE, GRACE-FO, and so on): Estimate gravity change from 1. mass change models and 2. co-located vertical velocity measurements (GNSS) and surface gravity measurements. (33% existing data, 33% modeling, 33% new campaigns)

Caveat: With Current Technology!

U.S. Absolute Gravity Station Network

Image and Google Earth file courtesy of Dan Winester, Pers. Comm.

Gravity/Uplift ratios

Create a gravity/GNSS GIA ratio. Demonstrates that carefully-selected absolute gravity measurements with co-located continuous GNSS measurements can be used to track change in central North America. Need a larger data set to be widely-applied.

GNSS vertical velocity (mm/yr)

Abso

lute

Grv

Cha

nge

(µG

al/y

r)

Current CORS Distribution

Choose the subset of most stable, with least local variability in height change.

CORS velocities for North America

Actual Motion as detected by the NGS CORS Network (Continuously Operating Reference Station)

Sella, et al.

Canada as an example

• (Veronneau, 2009) Presented at Boulder Workshop

• GRACE + a regular cycle of absolute gravity measurements at the CBN network of marks.

Canada’s Federally active and passive stations

CACS: • 50 stations • Concrete pillar anchored to bedrock • GNSS receivers (9 stations) • 3 stations at St-Johns, ARO, Yellowknife, and Penticton (all former VLBI sites) • Real-time (1 sec.) CHAIN (New): • 9 stations (some co-located with CACS) CBN: • 151 stations • 5-year obs. cycle Not shown: active • Provincial active stations

• NB, QC, BC • Private RTK networks Not shown: passive • High Precision Network (HPN)

• Provincial

(Veronneau, 2009)

Next Steps • Create a research plan that will yeild recommendations for NGS on

implementing the geoid monitoring service. • Re-convene the 2009 Working Group in 2017 • Recruit a postdoc (for this or airborne gravity or aircraft

positioning) via the U.S. National Research Council: – This cycle of applications starts June 1!

http://nrc58.nas.edu/RAPLab10/Opportunity/Opportunity.aspx?LabCode=26&ROPCD=260913&RONum=B8362

Conclusions

• Advances in airborne gravimetry to below the 1 mGal accuracy level are needed before it can be useful in monitoring gravity change.

• Terrestrial and satellite gravity are the primary instruments, in addition to physical models.

• Models of GIA and measurements of modern ice loss in Greenland and Alaska/Canada are critical to maintaining the North American vertical datum.

• The NGS monitoring plan will be tailored to the 3 different timescales of the critical signals affecting the geoid at the 1 cm level.

Thank You!

Dr. Theresa Damiani U.S. National Geodetic Survey, NOAA

theresa.damiani@noaa.gov