Ali Onceloncel@kfupm.edu.saDepartment of Earth SciencesKFUPMGravity Methods 4Introduction to GeophysicsIntroduction to Geophysics-KFUPMPrevious LectureSummary of Equations for Free Air and Bouguer Gravity Anomalies Terrain orrection Gravity MeasurementA!solute Gravity "elative GravityMeasurement of relative gravity Free Fall Methods#eight dro$ Pendulums Gravity %nits Introduction to Geophysics-KFUPMUsing the metric system, we usually think of g as being !" meters#sec$ !" meters#sec$!%ome unit names you will hear when gra&ity applications are discussed include' !" m#sec$"( Gals )or cm#sec$*"(((( milligals )i!e! +(((th of a Gal*,e e-perience similar problems in geological applications, because changes of g associated with subsurface density contrasts can be .uite small!/ecall' Gra&ity Unitshttp://www.esa.int/SPECIALS/Mars_Epress/SEM!"#$%D"E_&.htm'Introduction to Geophysics-KFUPMMars0s twin moons!" )m#sec$* is an easy number to recall but is not uni&ersal! If, howe&er, we were on the Martian moon Phobos, gp is only about (!((12meters#sec$ 3m#sec$4 might not be the most useful units to use on Phobos!htt$&''((()(hfreeman)com'environmentalgeology'E*M+,-.'F-./0)1TMPho!os ,eimosIntroduction to Geophysics-KFUPMMore ,etail& htt$&''history)nasa)gov'SP24.3'ch3)htm+ mGal 5 +( microns#sec$ + milligal e.uals +(-1 m#sec$ or con&ersely + m#sec$ 5 +(1 milligals!6he gra&ity on Phobos is (!((12m#s$ or 12( milligals!7re such small accelerations worth contemplating8 9an they e&en be measured8/ecall' GalGal 5 + cm#sec$5 (!(+ m#sec$+ mGal 5 +(-: GalIstituto e Museo di %toria della %cien;a Introduction to Geophysics-KFUPM"()*+")*! 6he g ranges in scale from appro-imately +-1 milligals!9alculated and obser&ed gra&itational accelerations are plotted across a ma