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GRAVITY SURVEY CORRECTIONS

NAME REASON CORRECTION

Observed Gravity(gobs)

Gravity readings observed at eachgravity station after correctionshave been app lied for ins trumentdrift and earth tides .

Worden measurements are indivisions. There is a dialcons tant of 0.0999 mgals/d iv.Convert div. measu rements tomgals.

Repeat measurments at a basestation. each hour. Remove anytrend in the bas e station removing a linear trend is theeasiest.

Lati tude Correction(g n )

Correction from gobs thataccounts for Earth's ellipticalshape and rotation. The gravityvalue that would be obs erved if Earth were a perfect (no geologicor topographic complexities),rotating ellipsoid is referred to asthe normal gravity . GravityINCREASES with increasinglatitude. Correction is ADDED aswe move toward the equator.Correction is applied ONLY for relitive movement in the N-Sdirection. Correction for a givenlatitude is linear over about 1 km.

g n = 978031.85 (1.0 +

0.005278895 sin 2(lat) +

0.000023462 sin 4(lat)) (mgal)

where lat is latitude in degrees

Taking the derivative w.r.t. lat and

converting to (m) from (rad) gives acorrection of:

g l = 0.000812 * sin( 2 * lat)mgal/m (N-S)

Free Air Corrected Gravity (g fa )

The free-air correction accountsfor gravity variations caused byelevation differences in theobservation locations. (does NOTinclude the effect for mass

between observed point and thedatum)

g fa = 0.3086h (mgal)

where h is the elevation (in meters)at which the gravity station isabove/below the datum

Correction is ADDED for stationsABOVE the datum

Correction is SUBTRACTED for stations BELOW the datum

Bouguer SlabCorrected Gravity

The Bouguer correction is a first -order correction to account for the

g b = - 0.04193 h (mgal)

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(g b ) excess mass underlyingobservation points located atelevations higher than th eelevation datu m (sea level or thegeoid). Conversely, it accountsfor a mass deficiency atobservation points located belowthe elevation datum.

is the average density of the rocks underlying thesurvey area in gm/cc

Correction is SUBTRACTED for stations ABOVE the datum

Correction is ADDED for stations BELOW the datum

Terrain Corrected Bouguer Gravity ( gt)

The Terrain correction accountsfor variations in the observedgravitational acceleration caused

by variations in topography near each obs ervation point. Becauseof the assumptions made duringthe Bouguer Slab correction, theterrain correction is p ositiveregardless of whether the localtopography consists of amountain or a valley.

g tc is computed with tablesor templates o r by

computer and is ADDED togobs.

Bouguer gravity is: g obs +/- g l +/- g fa -/+ g b + g tc