1/5/11 1 - Cal Poly Pomona

1/5/11 GSC334 Winter 2011Exploration GeophysicsGSC334 Winter 2011Exploration GeophysicsSurface Waves: Rayleigh wavesRayleigh Waves• Backward-rotating (retrograde), elliptical motion• Vertical as well as horizontal motions• Travel through both ground and water• Amplitude decays exponentially with depth (greatest atsurface)01/06/11 Cal Poly Pomona31• All particle motion is contained in vertical plane• Strong signal on exploration seismic records, alsoreferred to as ground roll01/06/11 Cal Poly Pomona 32GSC334 Winter 2011Exploration GeophysicsSeismic Wave VelocitiesP- and S-wave velocities depend on elastic properties anddensity of material through which they travelK = bulk modulusG = shear modulusρ = densityV P=K + 4 3 GBecause K and G are both positive, V P > V SG=0 for liquids,€€so shear waves cannot propagate in liquidsρV S=G ρGSC334 Winter 2011Exploration GeophysicsSeismic Waves in Exploration• Routinely used in seismic exploration: P-waves• General rules for velocities:– unsaturated sediments: V P lower than saturated– unconsolidated sediments: V P lower than consolidated– weathered rocks: lower V P than similar rocks that areunweathered– fractured rocks: lower V P than similar rocks that areunfractured• Reasonable values:– 500 m/s for dry unconsolidated materials– 1500 m/s for wet unconsolidated materials– 4000 m/s for sedimentary rocks– 6000 m/s for unweathered igneous and metamorphicrocks01/06/11 Cal Poly Pomona3301/06/11 Cal Poly Pomona 34GSC334 Winter 2011Exploration GeophysicsGSC334 Winter 2011Exploration GeophysicsAir WaveTypical Wavelengths• With hammer blows or explosives near surface,compressional wave will be created that travelsthrough air: air wave.• This wave travels at 331 m/sec, which may beslightly faster than some common surfacematerials.• May be significant signal, that you should beaware of in your interpretation!• Energy sources: f~10-100 Hz• V P ~1500 m/s• Typical wavelength?01/06/11 Cal Poly Pomona 3501/06/11 Cal Poly Pomona 366