Downhole seismic wave velocity uncertainty evaluation by theoretical simulation and experimental data acquired during the seismic microzonation of Central Italy

This paper examines downhole measurement inaccuracy with respect to the sampling interval by utilizing theoretical simulation and experimental data collected during the Seismic Microzonation study in Central Italy. Numerical simulation concerning the expected experimental data shows that the percent uncertainty relative to measured P-wave velocity (Vp) values increases with increasing sampling intervals of seismometric measure and Vp values and decreases with depth. In particular, setting Vp = 4000 m/s and by varying the sampling interval in the range (0.125, 0.250, 0.500 ms), the uncertainty reaches 40% at 2 m of depth for dt = 0.50 ms. When dt = 0.125 ms, the uncertainty decreases to 10%. A general equation relating uncertainty on Vp measurements with the actual layer velocity, allowed to derive ambiguity on the first break time, the source-sensor distance and the depth. Experimental data confirmed validity of this equation and put in evidence the effect of measurements sampling interval on the resolving power of DH measurements.