The effects of a strongly refracting summer thermocline on the initial pressure pulse from an underwater explosion were accurately measured in a flooded quarry. The familiar exponentially decaying pulse shape was observed to be drastically altered in highly divergent regions of the field. Closely spaced measurements permitted these shape changes, as well as the increased amplitude and decreased time constant in a caustic region, to be followed continuously from the source. It is presently impossible to compute the observed pulse shapes although measured shock pressures agreed well with ray‐theory predictions except near the caustic. Measured travel time differences between initial arrivals and both second direct arrivals and surface reflections agreed quite exactly with predictions.

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