Phase‐array decomposition of a seismic section

A seismic re fraction/wide‐angle reflection profile is analysed for the presence of correlated events (‘phases’). The correlation problem is formulated in terms of temporally, spatially and frequency‐local complex covariances. For robustness, the method concentrates on phase rather than amplitude information. This allows a computationally efficient algorithm that can make allowance for signal correlation length and can model curved wavefronts. A statistical test based on residual phase misfit across the analysed subarray is used to assess the probability that a detected event represents a real correlated signal. With our chosen analysis parameters and confidence level (over 99.9 per cent). 1222 events were detected in the data. Using simple techniques based on 1‐D earth models, detected events are associated with a small number of particular wave types. In this way, we have succeeded in classifying almost 95 per cent of the detected events. Those that remain describe those components of the data that are inconsistent with our simple ray paths in the 1‐D assumption and with our prescribed tolerance. These include reverberations, near‐surface guided waves and reflected waves from strongly laterally inhomogeneous structures. According to our modelling, about 25 per cent of the detected events are consistent with simple P ‐wave reflected energy, and these are to a very large extent (over 85 per cent) distinct from all the other wave‐type models we have used. A direct mapping of the detected events into the offset‐depth domain reveals dear internal and external consistencies among the detections for the various wave types. Estimated earth structure is consistent with models from previous analyses based on much larger data sets. We have thus succeeded in extracting correlated events from the data and decomposing these, approximately but meaningfully, into distinct classes (ray paths)