First-break timing; arrival onset times by direct correlation

Abstract In attenuating media, pulse characteristics evolve with propagation distance and saturation or pressure-dependent changes in rock properties. This nonstationarity of the waveform complicates determination of meaningful traveltimes. As a result, depending on the time-picking criteria used, substantially different values of interval velocity can be obtained. This problem is particularly severe in high-frequency laboratory time-of-flight measurements on porous rock. A potentially less ambiguous measure of wave speed is the signal velocity that is calculated using the pulse onset time. Here, a semiautomated method is developed to determine this onset time in high-fidelity, pressure-dependent core measurements. The greatest value of Pearson's correlation coefficient between segments of observed waveforms near the pulse onset and at an appropriate reference serves as the time determination criterion. Tests of the method on artificial data suggest the signal velocity may be determined to better than 0.3% for -60 dB noise or 1.2% for -37 dB noise. A real data set is tested, comprised of a series of ultrasonic (1 MHz) velocity measurements in microcracked rock to confining pressures of 300 MPa ( approximately 45,000 psi). At the lowest confining pressure, where attenuation is greatest, signal onset and more conventionally derived traveltimes differ by more than 4%. This large discrepancy illustrates that care should be exercised when determining velocity in such attenuating materials. Conversely, the consistency of waveform attributes, such as the difference between the onset time and the first peak time or the apparent quality factor, is useful when estimating intrinsic material velocities in low-porosity, microcracked carbonate and metamorphic rocks at high confining pressures.