A computer method for determining seismic first arrival times

Abstract

Although the seismic first event is most often defined as the first kick on a trace, it is not commonly appreciated that the visual emergence of the kick is dependent upon the signal‐to‐noise ratio and the amplifier gain. Consistent picking can only be achieved by identifying constant phases on the pulse, such as the inflection points or points of peak amplitude. This computer technique identifies both the first kick and the first inflection point on each trace to determine accurate and consistent first arrival times. This is achieved for each shot by determining the mean difference between the times to the first inflection points and the corresponding first kick points and then subtracting this difference from the times to the first inflection points. The remaining constant errors in the arrival times caused by factors such as inaccurate uphole corrections or delays in the detonator or shot‐firing system are removed using a least‐squares procedure which minimizes the difference in the forward and reverse traveltimes between shotpoints.