Sound reflects off thermoclines. If there's a thermocline at say 1000ft and you have a hydrophone at 900ft, a sound originating at 1500ft will reflect off the t

Sound reflects off thermoclines. If there's a thermocline at say 1000ft and you have a hydrophone at 900ft, a sound originating at 1500ft will reflect off the thermocline and won't be detected by the 900ft hydrophone above the thermocline (frictionless pulleys and spherical cow assumptions).