Off the top of my head, and with a bit of reference to George Franklin's excellent Britain's Anti-Submarine Capacility, 1919-1939, it would seem that the supposed "area of probably serious damage" for a single depth charge, in the Royal Navy, was forty yards. Given that the charges in question were 300-lb Amatol weapons, that seems likely rather more than any countermeasure designed to trigger a torpedo by concussion would be able to generate; even accounting for improvements in explosives, one imagines that a countermeasure should, ideally, be easy to reload.
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