Hagedoorn’s plus/minus method involves the calculation of sums and differences of ﬁrst arrivals in refraction data sets. These quantities can be used to estimate subsurface velocities and the depths of curved refractors. Since seismic interferometry is, in a sense, a formal way of adding and subtracting traveltimes, it follows that the +/- method should be expressible in the form of an interferometric procedure. An algorithm for refractor imaging, expressed in terms of interferometric calculations on forward and reverse shot records, is derived heuristically and tested with a simple synthetic data set.
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