Precise measurement of seismic travel times from an air gun source

Abstract

Summary. Seismic travel times for extrema, zero-crossings, or entire body waves need to be determined precisely to one part in lo3 or better in several varieties of seismic studies employing an impulsive artificial source. Examples are crosshole surveys which delineate rock crack distribution separating the holes and monitoring of crustal seismic travel times in earthquake precursor studies. A timing resolution of one part in lo3 has been achieved previously using digitally recorded seismic data. These methods, however, do not use interpolation between digitized data points as a method to increase the timing resolution. We report travel-time determinations based on interpolation between digitized points which achieve a precision of two parts in lo4, a five-fold improvement over the existing methods. In addition, the effects of seismic noise on travel-time measurement have been compared for the extremum location, the unnormalized correlation, and the normalized correlation method, The following conclusions are drawn from this comparison: (1) the normalized correlation method provides an 18-55 per cent improvement in the standard deviation of the mean over the extremum location method, and (2) results as accurate as those by the normalized correlation can be obtained by the unnormalized correlation if a complete up-and-down swing of the waveform is used as the master trace and if the master trace is close to being sinusoidal. The advantage of the unnormalized correlation over the normalized correlation is speed; the unnormalized correlation is faster by a factor of 28 in computing time.