37 2. Refraction surveying To obtain the true velocity of each marker, its dip and thickness, time-distance curves in both directions are required, i.e. up-dip shooting and down-dip shooting (direct and reverse shots), as shown in Figure 2.1. This requires recordings where geophones are aligned with shot points. Such an implementation makes it possible to measure two apparent velocities Va1 and Va2 and two intercept times I1 and I2 on time-distance curves associated with the refractor. For shot 1 (up-dip), the following relationships apply: t(x) = x/Va1 + I1 with V1/Va1 = sin(ic + j) and I1= 2H1.cos(ic)/V1 For shot 2 (down-dip), the following relationships apply: t(x) = x/Va2 + I2 with V1/Va2 = sin(ic - j) and I2= 2H2.cos(ic)/V1 The measurements of the two apparent velocities Va1 and Va2 of the refraction time-distance curves, and the velocity V1 of the direct wave in the first layer, enable the determination of the critical angle ic, the refractor velocity VR (V1/VR = sin(ic)), its dip j, and the thicknesses (H1 and H2) of the layer at the vertical of the two shot points. The method, known as the intercept-time method, is illustrated in Figure 2.1 for a single layer over a substratum. It can be extended to multilayer models (Palmer, 1986). Figure 2.1 Intercept-time method.
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