1201Gravity BING.ppt

8/10/2019 1201Gravity BING.ppt

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Gravity

For Earths gravity field:

To make measurements of gravity useful, the data mustbe reduced to remove effects of the earth and just dealwith anomalies.

The measurement of g at any specific location is afunction of (1) latitude, (2) elevation, and (3) the massdistribution around the observation point.


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Gravity



Step #1 : Remove latitude effect

glat.cor = g obs - g t

Where (all in mGal)glat.cor = the latitude-corrected gravity valuegobs = your observed (measured) gravity readinggt = the theoretical gravity for the latitude of the

observation point


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Gravity



...gt = g e (1+0.005278895sin 2f +0.000023462sin 4f )

wheregt = the theoretical gravity for the latitude of theobservation pointge = theoretical gravity at the equator (978031.85 mGal)f = latitude of the observation point (degrees)


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Gravity



Step #2

Make the free air correction (FAC), which accounts forthe local change in gravity due to elevation.

gFAC = g lat.cor + FAC

where

FAC = elevation x (0.308 mGal/m)


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Terrain correctionBouguer correction assumes subdued topography. Additional

terrain corrections must be applied where measurements near tomountains or valleys.

If station is next to valley,

there is an absence of thedownward force ongravimeter assumed inBouguer correction, whichreduces free-air anomalytoo much.

If station next to mountain,there is an upward force on

gravimeter from mountain thatreduces reading.

In both cases, terrain correction isadded to Bouguer Anomaly


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Hammer chartsTerrain corrections can be computed using transparent template,called a Hammer Chart, which is placed over a topgraphic map


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Hammer charts

Chart is centred on gravity station andtopography read off at centre of eachsegment.

Contribution to terrain correction is obtainedfrom tabulated values for each segement andthen summed to obtain total correction.

Based on formula for gravitational attractionof cylindrical segment.

Considered an additional part of Bouguercorrection, i.e. results in Bouguer Anomaly.


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Density of geological materials


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Density of geological materials


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1201Gravity BING.ppt

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