SUBSIDENCE IN LOUISIANA

ELEVATIONS - A

MOVING TARGET

Cliff Mugnier

CHIEF OF GEODESY

CENTER FOR GEOINFORMATICS

LOUISIANA STATE UNIVERSITY

cjmce@LSU.eduC G

C e n t e r f o r

G e o I n f o r m a t i c s

4

Elevations versus heights

• Elevation benchmarks do not record

ellipsoid heights – a GPS-derivation

• Elevations are based on the tides

– Local mean sea level

Tides

• 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)

tidal benchmarks with a NAVD88 tie

tidal benchmarks without a NAVD88 tie

Effects on the Tides

• Chandler motion (1880) – migration of the

poles

• Great Venus term (+ Sun + Moon)

• Pertubations and nutations of the axes

Local Mean Sea Level

• 18.67-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

Types of Tidal Datums

• Mean Higher High Water (MHHW)

• Mean High Water (MHW)

• Mean Tide Level (MTL)

• Diurnal Tide Level (DTL)

• Mean Sea Level (MSL)

• Mean Low Water (MLW)

• Mean Lower Low Water (MLLW)

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

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)

GEOIDS

• EGM96 – 360 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

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 1929 - First continental

VERTICAL datum in the world

• 26 Tide Gauges for U.S.; Pensacola &

Galveston based on full Metonic Cycles

Year of

Adjustment

Kilometers of

Leveling

Number of

Tide Stations

1900

1903

1907

1912

1929

21,095

31,789

38,359

46,468

75,159 (U.S.)

31,565 (Canada)

5

8

8

9

21 (U.S.)

5 (Canada)

History of Levels in New Orleans

• 1935 – WPA local adjustment to SLD 1929

• 1951 - adjusted forward in time to 1955

• 1955 - tied to Morgan City & Mobile (‘29)

• 1963 - tied to Norco well (‘29 value)

• 1969 - tied to ‘63 lines

– 1973 Federal Register: SLD’29 changed to NGVD 1929

• 1976 - tied to Index, AR & Logtown, MS

1976-77 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 $1,500,000.00 later,

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.

• 1978 National Geodetic Survey changes

name from SLD 1929 to NGVD 1929

• Let’s do a “Regional Paper Adjustment.”

1982-83 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

Local Governments fund

Geodetic Surveys in 1986-88

• 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

North American Vertical Datum

of 1988

Actual published data available

starting in 1990

No data available for South Louisiana

(Crustal Motion Area)

FG-5 Absolute Gravity Meter

(±1µgal)

• 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.)

Absolute Gravity Observed in

New Orleans:

March, 1989 979,316,847.7 µgals

Sept., 1991 979,316,854.2 µgals

(-0.91 centimeters per year)

National Geodetic Survey returns

to New Orleans

• 1991-92 Field Observations in “support” of

Orleans Parish GeoCadastre

• Lines tied to:

– J-92 (Rigolets Bridge)

– Vets Blvd. @ Orleans Parish Line

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

Absolute Gravity Observed in

New Orleans:

Nov., 1993 979,316,856.3 µgals

Aug., 1994 979,316,860.6 µgals

(-0.91 centimeters per year)

Defense Mapping Agency awards

$1,000,000 contract to re-

compute the GEOID

GEOID03

• Released in 2003,

• Updated in 2005 for South Louisiana

• Accuracy is claimed to be ±1 centimeter !

Absolute Gravity Observations

• In 2002:– UNO (5th time)

– Stennis Space Center (2nd)

– Loyola University

– Southeastern Louisiana Univ.

– LSU

– McNeese State Univ.

– Venice-Boothville H.S.

– LUMCON @ 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 (6th time)

– Stennis Space Center (3rd)

– Loyola University (2nd)

– Southeastern Louisiana Univ. (2nd)

– LSU (2nd)

– McNeese State Univ. (2nd)

– Venice-Boothville H.S. (2nd)

– LUMCON @ Cocodrie (2nd)

– Oakdale H.S. (2nd)

– LSU Alexandria (2nd)

– Old River Aux. Control Structure (2nd)

– Nicholls State Univ. (2nd)

– Univ. of Louisiana in Lafayette (2nd)

– Grand Isle U.S.C.G. Station

– Lamar Univ. in Beaumont

– Univ. of Mississippi in Hattiesburg

Subsidence is caused by both natural and human-

induced 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

The Michoud Fault

Subsidence

Subsidence

at Cocodrie

The Baton Rouge Fault -5mm/yr

NAD83(CORS96) -to- NAD83(2011)

From GULFNet to C4GNet

Bursa-Wolf Model

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

-0.36 0.12 0.08 0.01 -0.01 -0.01 0.00

±0.16 ±0.11 ±0.15 ±0.01 ±0.00 ±0.00 ±0.01

Questions?

Cliff Mugnier

cjmce@LSU.edu

(225) 578-4578