Strong signal on 25.2 kHz. From research, this is NML4, a USN transmitter in LaMoure, ND (west of Fargo a little ways) running 500kW.
Signal is coming in very strong, a solid S9.
Also see what looks like other signals centered on 24.0 & 24.8 kHz. According to
http://www.smeter.net/stations/vlf-stations.php24.0 is a USN station in Maine and 24.8 is a USN station in Washington state.
When turning the loop, the bearings of the nulls were consistent with their stated locations.
12/17 0050Z: Picking up a nice strong signal for NAU on 40.75 kHz down in Puerto Rico and possibly a very weak NPM from Hawaii on 21.4 kHz.
Perseus SDR with Wellbrook ALA1530S+ loop