ERSC 3V93 Lecture 49: Seismic Methods v2 I
6 May 2016
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Plot first break picks for direct and refracted waves: velocities determined from slope of curves, t/ x, direct wave slope is 1/v0, v0 = 1/slope, head wave slope is 1/v1. Time at which back-extrapolated refractor intersects time axis: used to define the linear equation for refractor travel time, t = ti + x/v2. Critical distance: xcrit - distance at which first head wave arrives at surface, xcross - is point at which headwave overtakes direct wave. The travel time of the head wave is obtained by dividing the ray path segments by their respective velocities: distance/velocity = m/ms-1 = s. The travel-time equation can be reformulated into a more useful formusing sin = v1/v2 and trig identities: The travel time equation is straight line with slope 1/v2 and intercept ti. The intercept term is rearranged and solved for depth: valid only for planar, horizontal refractor, depth usually assigned to first shot point.
