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//)3(/pdpdpBut )4(dpdpddpdrdpUse (2) and (3) in (4) )5(gdrdBut sorGMg,2)6(2rGMdrdEquation (6) is applicable to region of uniform composition within the earth, which is, starting from mantle down. It shows uniform density increase (Fig.1) Where = adiabatic incompressibility (i.e. bulk modulus) M = Mass of the region of the earth with the earth less by the mass of the crust r = radius of the earth’s region with uniform composition G = Gravitational constant
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- 45 -3.2 Variation of Pressure P with Depth This has to do with the pressure distribution and variation of acceleration due to gravity. )1(Re2rGmgthatcall)2(grPandSo pressure gradient )3(2rGmpdrdpEquation 3 shows that pressure decreases with depth, i.e. as r decreases (Fig.1) Also; it shows that pressure has direct relation with the density. 3.3Variation of Acceleration due to gravity with depthRecall that .2rGMgSo, variation of g with depth can be calculated using the relation above. Its value does not differ by more than 1% from 990 km until a depth of 2400 km is reached where it decreases to zero (Fig.1) 3.4Variation of Pressure P and S waves with Depth In seismology P and S waves are used to study the interior of the solid earth. P- Wave is a longitudinal wave that can pass through liquid, solid and gas. S- Wave is a transverse wave that can travel vertically and horizontally and can not pass through fluids. P and S- waves are called body waves because they can pass through the earth’s interior. They however possess different velocities while passing through an earth