RESOLUTION OF TIME-FREQUENCY ANALYSIS FOR SEISMIC CROSSHOLE MEASUREMENTS

A sensitive processing method to remove the random components of seism ic signals and examine their similarity in the time-frequency domain i s presented. We transform signals to the time-frequency (T-F) domain u sing parametric and nonparametric algorithms. The first method estimat es an autoregressive process (AR) and its spectral density in a shea m oving window, the second computes the instantaneous frequency of the s ignal at each moment of time. The main problems are the trade-off betw een the time and the frequency domain which limits the resolution of t he method and cross terms of the T-F distribution which may lead to mi sinterpretation of the different modes of the signal. Next we present some limitation on the resolution of some popular T-F distributions an d suggest an improved method based on a time-frequency coherency funct ion. We conclude that coherency analysis in the T-F domain makes it po ssible to investigate precisely the repeatability of seismic sources, to control the stacking efficiency and also to estimate the onset of t he same modes (for example P and S components, tube waves).