SUBSIDENCE IN LOUISIANA

Transcription

1 SUBSIDENCE IN LOUISIANA ELEVATIONS - A MOVING TARGET 4 C G C e n t e r f o r Cliff Mugnier CHIEF OF GEODESY CENTER FOR GEOINFORMATICS LOUISIANA STATE UNIVERSITY cjmce@lsu.edu G e o I n f o r m a t i c s

2 Elevations versus heights Elevation benchmarks do not record ellipsoid heights a GPS-derivation Elevations are based on the tides Local mean sea level

3 Diurnal = Gulf of Mexico northern coast One high/low tide cycle per day Semidiurnal = East & West U.S. coasts Two high/lot tide cycles per day High tide is 11 minutes later each day Affected by storms, geology, variation of the Earth s density, wobble of Earth & Moon, and the planet Venus (and currents) Tides

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5 tidal benchmarks with a NAVD88 tie tidal benchmarks without a NAVD88 tie

6 Effects on the Tides Chandler motion (1880) migration of the poles Great Venus term (+ Sun + Moon) Pertubations and nutations of the axes

7 Local Mean Sea Level year Metonic cycle To determine local mean sea level, observe tides for at least one Metonic cycle. Every 5 years = new tidal Epoch (based on a running average). New epochs are published by the International Hydrographic Organization (IHO), Monaco

8 Types of Tidal Datums Mean Higher High Water Mean High Water Mean Tide Level Diurnal Tide Level Mean Sea Level Mean Low Water Mean Lower Low Water (MHHW) (MHW) (MTL) (DTL) (MSL) (MLW) (MLLW)

9 Elevations and height Mathematical equation which models the geoid Geoid an imaginary surface where no topography exists and the oceans are only subject to gravity Equipotential surface (gravity potential is constant) Not smooth because of composition of the Earth

10 Geoid models Spherical harmonics (polynomials) Models the relationship between geoidal and ellipsoidal heights H = geoid height (elevation) h = ellipsoid height (GPS vertical ) h H (Topography)

11 GEOIDS EGM degree/order, 15 minute grid GEOID96 meter level NGS, U.S. model GEOID99 decimeter level, 1-minute grid NGS, U.S. model GEOID03 decimeter level, 1-minute grid 10 cm absolute, local is closer to 1 cm relative NGS, U.S. model

12 Historical Leveling in the U. S. Corps of Engineers: General Survey of the Mississippi River in 1876 Coast & Geodetic Survey Tide Gauges for Atlantic, Pacific, & Gulf Coasts Sea Level Datum of First continental VERTICAL datum in the world 26 Tide Gauges for U.S.; Pensacola & Galveston based on full Metonic Cycles

13 Year of Adjustment Kilometers of Leveling Number of Tide Stations ,095 31,789 38,359 46,468 75,159 (U.S.) 31,565 (Canada) (U.S.) 5 (Canada)

14 History of Levels in New Orleans 1935 WPA local adjustment to SLD adjusted forward in time to tied to Morgan City & Mobile ( 29) tied to Norco well ( 29 value) tied to 63 lines 1973 Federal Register: SLD 29 changed to NGVD tied to Index, AR & Logtown, MS

15 NGS Leveling (funded by Corps of Engineers) Start at Index, Arkansas, through Simmsport, LA to: Morgan City & Baton Rouge, both then to: New Orleans, thence to: Venice, LA (spur) and Logtown, MS to close the line. and $1,500, later,

16 Surprise! There s subsidence way down yonder Allowable misclosure for 530 kilometers of levels was 92 mm, Actual misclosure was 86 mm, but Too much error manifested in Metro New Orleans to close locally National Geodetic Survey changes name from SLD 1929 to NGVD 1929 Let s do a Regional Paper Adjustment.

17 NGS Regional Adjustment of South Louisiana Catastrophic Floods of : May 3, 1979; April 12-13, 1980 Orleans, Jefferson, and Plaquemines Parishes funded NGS to re-observe BMs. Corps of Engineers concerned with the NGS FREE ADJUSTMENT Deep casement marks introduced

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20 Local Governments fund Geodetic Surveys in Jefferson Parish Benchmark System Entire East Bank Metro West Bank and south to Lafitte Relative gravity observed at ~350 benchmarks St. Bernard Parish Benchmark System IHNC to Reggio Relative gravity observed at ~100 benchmarks

21 North American Vertical Datum of 1988 Actual published data available starting in 1990 No data available for South Louisiana (Crustal Motion Area)

22 FG-5 Absolute Gravity Meter (±1µgal)

23 The acceleration due to gravity at the Earth's surface is 976 to 983 gal, depending on the latitude and the ellipsoid height A µgal is one-millionth of a gal! (That s nine significant figures.)

24 Absolute Gravity Observed in New Orleans: March, ,316,847.7 µgals Sept., ,316,854.2 µgals (-0.91 centimeters per year)

25 National Geodetic Survey returns to New Orleans Field Observations in support of Orleans Parish GeoCadastre Lines tied to: J-92 (Rigolets Bridge) Vets Orleans Parish Line

26 1993 Adjustment by NGS for Subsidence Zone Elevations Last visit to New Orleans for the century National Geodetic Survey loses funding for Long Line Leveling Crew GPS Constellation continues to grow Defense Mapping Agency downgrades security classification on the GEOID

27 Absolute Gravity Observed in New Orleans: Nov., ,316,856.3 µgals Aug., ,316,860.6 µgals (-0.91 centimeters per year)

28 Defense Mapping Agency awards $1,000,000 contract to recompute the GEOID

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30 GEOID03 Released in 2003, Updated in 2005 for South Louisiana Accuracy is claimed to be ±1 centimeter!

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34 Absolute Gravity Observations In 2002: UNO (5 th time) Stennis Space Center (2 nd ) Loyola University Southeastern Louisiana Univ. LSU McNeese State Univ. Venice-Boothville H.S. Cocodrie Oakdale H.S. LSU Alexandria Old River Aux. Control Structure Nicholls State Univ. Univ. of Louisiana in Lafayette Northwestern State Univ. Sicily Island H.S. LSU Shreveport Louisiana Tech Univ. In 2006: UNO (6 th time) Stennis Space Center (3 rd ) Loyola University (2 nd ) Southeastern Louisiana Univ. (2 nd ) LSU (2 nd ) McNeese State Univ. (2 nd ) Venice-Boothville H.S. (2 nd ) Cocodrie (2 nd ) Oakdale H.S. (2 nd ) LSU Alexandria (2 nd ) Old River Aux. Control Structure (2 nd ) Nicholls State Univ. (2 nd ) Univ. of Louisiana in Lafayette (2 nd ) Grand Isle U.S.C.G. Station Lamar Univ. in Beaumont Univ. of Mississippi in Hattiesburg

35 Subsidence is caused by both natural and humaninduced processes: Natural processes Sediment compaction Sediment consolidation Compaction of semi-lithified rock Major, regional faulting Sediment load-induced down-warping Salt evacuation Human-induced processes Organic sediment decomposition due to drainage projects Groundwater extraction-compaction of shallow aquitards (clays) Groundwater extraction-compaction of shallow aquifers (sands) Oil/gas extraction related-compaction of aquitards (clays) Area of subsidence restricted to ONLY the area of the oil/gas field Oil/gas extraction related-compaction of aquifers (sands) Area of subsidence restricted to ONLY the area of the oil/gas field Fault motion induced by shallow groundwater withdrawal

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37 The Michoud Fault

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40 Subsidence

41 Subsidence at Cocodrie

42 The Baton Rouge Fault -5mm/yr

43 NAD83(CORS96) -to- NAD83(2011) Bursa-Wolf Model From GULFNet to C4GNet Solution for 3 Translation, 1 Scale and 3 Rotation Parameters DX DY DZ Scale-1 Omega_z Psi_y Epsilon_x Meters Meters Meters (x10^6) Seconds Seconds Seconds ±0.16 ±0.11 ±0.15 ±0.01 ±0.00 ±0.00 ±0.01

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45 Questions? Cliff Mugnier (225)