Ali Onceloncel@kfupm.edu.saDepartment of Earth SciencesKFUPMGravity Modeling 2Introduction to GeophysicsIntroduction to Geophysics-KFUPMHighest peaks on the planetPrevious Lecture2D Gravity ModelingGravitational effect of a buried spherei!e "ffect Gravity #ontrast Depth "ffect Introduction to Geophysics-KFUPMIntroduction to Geophysics-KFUPMRed Sea$rabian GulfDSF$rabian hieldGulf of Aden%agros

utureGOASMSOM&&' " ()' "*)' +&,'+-o.e/or0 tatusD"Mhttp122///3sgs3org3sa2gis2i.ages2.nt3gifIntroduction to Geophysics-KFUPMhttp122///3sgs3org3sa2gis2i.ages2peninsul3gifafter #ollenette and Grainger4 *556Gravity Modeling Introduction to Geophysics-KFUPMIn the given e7a.ple4 the ore body is spherical in shape and is buried in sedi.entary roc0s having a unifor. density3 In addition to the ore body4 the sedi.entary roc0s in /hich the ore body resides are underlain by a denser Granitic base.ent that dips to the right3 8his geologic .odel and the gravity profile that /ould be observed over it are sho/n in the figure belo/3ource1 +otes of 8ho.as M3 9oyd- #olorado chool of Minehu.pource1 +otes of 8ho.as M3 9oyd- #olorado chool of Mine:egional Gravity due to dipping LayerIntroduction to Geophysics-KFUPMDecreaseIntroduction to Geophysics-KFUPM$9Local $no.aly; $-9From this simple example you can see that there are two contributions to our observed gravitational acceleration !"he first#caused by large$scale geologic structure that is not of interest "he gravitational acceleration produced by these large$scale features is referred to as the Regional Gravity Anomaly !"he second#caused by smaller$scale structure for which the survey was designed to detect "hat portion of the observed gravitational acceleration associated with these structures is referred to as the Local or the Residual Gravity Anomaly%ecause the &egional 'ravity (nomaly is often much larger in si)e than the *ocal 'ravity (nomaly+ as in the example shown above+ it is very importantthat we develop a means to effectively remove this effect from our gravity observations before attempting to interpret the gravity observations for local geologic structureource1 +otes of 8ho.as M3 9oyd- #olorado chool of MineIntroduction to Geophysics-KFUPM+otice that the :egional Gravity $no.aly is a slo/ly varying function of position along the profile line3 8his feature is a characteristic of all large-scale sources3 8hat is4 sources of gravity ano.alies large in spatial e7tent tart /ith a si.ple4 constrained .odel> Increase sophisticationas .odeling proceeds> o.e e7a.ples:ecall1 Dip Layering "ffectIntroduction to Geophysics-KFUPMy..etryIntroduction to Geophysics-KFUPMLocal Gravity8he ano.aly is attenuated < s.aller lAg!l= as the sphere is buried .ore deeply /ithin the "arth8he /idth of gravity ano.aly increases as the sphere is buried .ore deeply3:ecall1 Gravity ano.aly profile assu.e that .ass is essentially a Gpoint sourceC - as if .ass /ere concentrated in the centre> 7 is distance fro. centre of sphere> : is radius bet/een centre of sphere and .easure.ent point> ! depth to centre of sphereIntroduction to Geophysics-KFUPM$s you re.e.ber fro. the previous lecture4 /e can obtain vertical co.ponent of gravitational attraction of e7cess .ass in .Gal as1:ecall1 Gravity "ffect of phereIntroduction to Geophysics-KFUPM% $% $ '()*+ . '( (,z xzR gz+ =ee pp3 26, of Lillie-alf Didth Depth "sti.ates> 8he depth to the top of a gravity source can be deter.ined appro7i.ately fro. half-/idth 7H of ano.aly

! is half distance fro. the centre of ano.aly at /hich a.plitude has decreased to half its pea0 value " is depth for spherical obFect ! ; *3&)I7H ) 0 1/,2x3Introduction to Geophysics-KFUPM