18 Seismic Imaging • Propagation velocity of the shear S-waves in the rock: VS (expressed in m/s), • Density ρ (expressed in g/cm3 or kg/m3), • Quality factor Q which characterizes the ability of the rock to absorb seismic energy: a higher value indicates lower absorption of seismic energy. Sedimentary rocks have a Q value ranging from about 10 to several hundred. 3. An elastic deformation of the medium after the initial shaking caused by the source. A deformation is considered elastic when the medium returns to its original state after the causes of deformation have disappeared, i.e. when the medium has not been damaged by the wave passing through it. 4. A receiver spread. Which must be capable of recording the deformations generated by the source after propagation in the geological medium: • Either by variations in the displacement, velocity or acceleration of particles (geophones, accelerometers), • Or by pressure variations (hydrophones). Table 1.1 Seismic velocities and densities, and mechanical moduli (after Lavergne, 1989). Type of rock or medium P-velocity VP (m/s) S-velocity VS (m/s) Density ρ (g/cm3) Weathered rocks 300 – 700 100 – 300 1.7 – 2.4 Dry sands 400 – 1200 100 – 500 1.5 – 1.7 Wet sands 1500 – 4000 400 – 1200 1.9 – 2.1 Clay 1100 – 2500 200 – 800 2.0 – 2.4 Marl/shale 2000 – 3000 750 – 1500 2.1 – 2.6 Sandstone 3000 – 4500 1200 – 2800 2.1 – 2.4 Limestone 3500 – 6000 2000 – 3300 2.4 – 2.7 Chalk 2300 – 2600 1100 – 1300 1.8 – 2.3 Salt 4500 – 5500 2500 – 3100 2.1 – 2.3 Anhydrite 4000 – 5500 2200 – 3100 2.9 – 3.0 Dolomite 3500 – 6500 1900 – 3600 2.5 – 2.9 Granite 4500 – 6000 2500 – 3300 2.5 – 2.7 Basalt 5000 – 6000 2800 – 3400 2.7 – 3.1 Coal 2200 – 2700 1000 – 1400 1.3 – 1.8 Water 1450 – 1500 – 1 Ice 3400 – 3800 1700 – 1900 0.9 Oil 1200 – 1250 – 0.6 – 0.9 First Lamé parameter λ=ρ − ( ) V V P S 2 2 2 Shear modulus (or second Lamé parameter) µ=ρVS 2 Poisson’s coefficient σ γ γ = − − ( ) 2 2 2 2 1 where γ = V V P S Young’s modulus E VP = − ( ) + ( ) − ρ σ σ σ 2 1 2 1 1 Bulk modulus K V V P S = − ( ) ρ 2 2 4 3
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