GRAVITY SURVEY (ii)GRAVITY SURVEY (ii) --Gravity Data Correction-Gravity Data Correction-

JUNE, 2004

Gravity corrections

Measure gravity variations at stations around loopCorrect for driftObservations still subject to extraneous effects unrelated to subsurface geology

Must make corrections•Instrument drift•Tidal correction•Latitude correction•Free-air correction•Bouguer correction•Terrain correction

Drift - A gradual and unintentional change in the reference value with respect to which measurements are made

Tidal Effect - Variations in gravity observations resulting from the attraction of the moon and sun and the distortion of the earth so produced*.

3085.0=e

FA

dR

dg mgal/m

φ2sin305.1=ds

dg L φ2sin811.0= mgal/mile

mgal/km

LATITUDE CORRECTION

)φ2sin000023462.0φsin005278895.01( 420 ++=gg

hdR

dg

e

B ρ04192.0=

mgal/m 112.0=

e

B

dR

dg mgal/m

Variations in Gravity due to Nearby Topography

Index map of the survey areaIndex map of the survey area

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We have now described the host of corrections that We have now described the host of corrections that must be applied to our observations of gravitational must be applied to our observations of gravitational acceleration to isolate the effects caused by geologic acceleration to isolate the effects caused by geologic structure. structure.

The wide variety of corrections applied can be a bit The wide variety of corrections applied can be a bit intimidating at first and has led to a wide variety of intimidating at first and has led to a wide variety of names used in conjunction with gravity observations names used in conjunction with gravity observations corrected to various degrees. Let's recap all of the corrected to various degrees. Let's recap all of the corrections commonly applied to gravity observations corrections commonly applied to gravity observations collected for exploration geophysical surveys, specify collected for exploration geophysical surveys, specify the order in which they are applied, and list the names the order in which they are applied, and list the names by which the resulting gravity values go by which the resulting gravity values go

Observed Gravity (Observed Gravity (ggobs)obs) - Gravity readings observed - Gravity readings observed at each gravity station after corrections have been at each gravity station after corrections have been applied for applied for instrument drift and tidesinstrument drift and tides. .

Latitude CorrectionLatitude Correction ( (ggn)n) - Correction subtracted from - Correction subtracted from ggobsobs that accounts for the earth's elliptical shape and that accounts for the earth's elliptical shape and rotation. The gravity value that would be observed if rotation. The gravity value that would be observed if the earth were a perfect (no geologic or topographic the earth were a perfect (no geologic or topographic complexities), rotating ellipsoid is referred to as the complexities), rotating ellipsoid is referred to as the normal gravitynormal gravity. .

Corrected Gravity (Corrected Gravity (ggfa)fa) - The Free-Air correction - The Free-Air correction accounts for gravity variations caused by elevation accounts for gravity variations caused by elevation differences in the observation locations. The form of differences in the observation locations. The form of the Free-Air gravity anomaly, the Free-Air gravity anomaly, ggfafa, is given by;, is given by;

gfa = gobs - gn + 0.3086 hgfa = gobs - gn + 0.3086 h (mgal) (mgal)

where where hh is the elevation at which the gravity station is is the elevation at which the gravity station is above the above the elevation datumelevation datum chosen for the survey (this chosen for the survey (this is usually sea level). is usually sea level).

Corrected Gravity (Corrected Gravity (ggb)b) - The Bouguer correction is a - The Bouguer correction is a first-order correction to account for the excess mass first-order correction to account for the excess mass underlying observation points located at elevations underlying observation points located at elevations higher than the elevation datum. Conversely, it higher than the elevation datum. Conversely, it accounts for a mass deficiency at observations points accounts for a mass deficiency at observations points located below the elevation datum. The form of the located below the elevation datum. The form of the Bouguer gravity anomaly, Bouguer gravity anomaly, GBGB, is given by; , is given by;

GB = gobs - gn + 0.3086 h - 0.04193 rGB = gobs - gn + 0.3086 h - 0.04193 r h (mgal) h (mgal)

where where rr is the average density of the rocks underlying is the average density of the rocks underlying the survey area. the survey area.

Corrected Bouguer Gravity (Corrected Bouguer Gravity (ggt)t) - The Terrain correction - The Terrain correction accounts for variations in the observed gravitational accounts for variations in the observed gravitational acceleration caused by variations in topography near each acceleration caused by variations in topography near each observation point. The terrain correction is positive regardless observation point. The terrain correction is positive regardless of whether the local topography consists of a mountain or a of whether the local topography consists of a mountain or a valley. The form of the Terrain corrected, Bouguer gravity valley. The form of the Terrain corrected, Bouguer gravity anomaly, anomaly, ggtt, is given by; , is given by;

gt = gobs - gn + 0.3086 h - 0.04193 r h + TC gt = gobs - gn + 0.3086 h - 0.04193 r h + TC (mgal)(mgal) where where TCTC is the value of the computed Terrain correction. is the value of the computed Terrain correction.

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