Dictionary:Weathering (Wx, LVL)

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A near-surface, low-velocity layer, usually the portion where air rather than water fills the pore spaces of rocks and unconsolidated earth. Seismic weathering is usually different from geologic weathering (the result of rock decomposition). The term LVL (low-velocity layer) is often used for the seismic weathering. Frequently the base of the weathering is the water table. Sometimes the weathering velocity is gradational, sometimes it is sharply layered. Weathering velocities are typically 500 to 800 m/s (although weathering velocity may be 150 m/s for the first few cm) compared to subweathering velocities of 1500 m/s or greater. Weathering thickness is calculated from uphole-survey data and from refraction first breaks. See weathering correction.