weird audio range hit in passive NGB capture, anyone seen this

been doing passive EM surveys at the NGB site in Nunavik on and off for about four years now, building a dataset on background substrate resonance in the cummingtonite amphibolite. boring stuff, nothing unusual.

third day of this last trip the capture array picked up something i cant account for. not in the ELF band im usually watching. audio range. and structured. been doing this long enough to know what geological noise looks like, this isnt it.

signal shows up at about 3:20 into the recording. runs ~40 seconds. its underneath the ambient substrate noise so you have to actually look for it or run it through something. the ambient is real, thats the formation doing what the formation does. whatever is underneath it is something else.

attaching the raw WAV, no filtering, no processing. what came off the array is what youre getting. happy to be told its an equipment artifact but i want someone else to look before i decide that.

setup: modified Geometrics G-858 cesium vapor magnetometer in passive EM capture mode, soundcard bridge at 44.1kHz, 24-bit. array grounded to bedrock. no powered equipment within 400m except the unit itself. same setup at this site for two seasons, never picked up anything like this before.

signal only appears in this one recording. i have 14 other captures from the same trip and none of them show it.

listening now on laptop speakers which is useless. pulling it up on the rig and running audacity, back in a bit.

quick question while im doing that - was the capture lead shielded all the way to the array? at audio range you can get pickup from unshielded runs if theres anything powered even at distance. 400m should be fine but worth noting for the record.

yes, double-shielded coax the whole run, grounded braid, no floating connections. same setup ive been running at NGB for two seasons and its never done this before. if it were a pickup artifact id expect to see it in other recordings from the same trip. i dont.

ran it through my stack. few things.

substrate noise is consistent with what id expect from a crystalline shield formation at that latitude. low freq rumble below 50Hz, periodic telluric current impulses, schumann fundamental in the noise floor around 7-8Hz if you look hard. normal profile. nothing weird there.

the signal at 3:20 is not normal.

im going to say what i think this looks like and you can tell me im wrong. frequency pairing and timing is consistent with DTMF. dual-tone multi-frequency, same as telephone touch-tone signaling. row tones 697/770/852/941 Hz, column tones 1209/1336/1477 Hz. the pairs im seeing in the analysis map to that grid.

not saying it IS dtmf. im saying the signature is consistent with it. which if true would mean a deliberate signal source operating in audio range at or very near your capture point. which raises more questions than it answers given what you said about site conditions.

have you tried running it through a DTMF decoder? something like multimon-ng or even an online decoder if you filter the noise down first. curious what sequence comes out if it is DTMF.

confirming this after seeing K0RFQ's post. ran the 200-215s window through multimon-ng after applying a bandpass to cut some of the substrate noise. getting partial hits, some tones decoding clean, others buried. getting a digit sequence but the noise floor is making it incomplete on my end. not confident enough in what im getting to post the partial decode yet in case im introducing errors.

VE3KWT - any way to get directional data? asking because if this is coming up through the formation vs arriving laterally that would narrow things considerably.

also - is there any old survey infrastructure at that site? previous expeditions leave anything behind? im not saying that explains it at audio range but worth ruling out.

no, single point passive capture, not an array config. been thinking about going back up with a proper spatial setup but the NGB site isnt a day trip from ontario. if this goes somewhere ill look at it.

nothing permanent. seasonal research camp about 3km north, confirmed unoccupied when i was there. no comms relay, no equipment left behind that im aware of. remote access only, no installations.

been thinking about K0RFQ's DTMF read. if thats right the interesting question is whats in the sequence not how it got there. i dont have the tools to do a clean decode with the substrate noise in the way. if someone with better filtering capability wants to take a run at it im curious what comes out.

theres also something ive been sitting on since i got back that i havent mentioned yet. didnt want to muddy the technical question but now that K0RFQ has confirmed the structured signal ill put it on the record.

theres a second anomaly in the recording. very low amplitude, runs through the entire 8 minutes continuously. not in the substrate noise. not geological. doesnt show up in any of my other 14 captures from this trip.

dont know what to make of it. putting it out there in case someone else does.

7934 Hz. noted.

confirmed it on second pass specifically looking for that frequency. stable, low amplitude, no modulation, present throughout. youre right that theres no geological source for a stable tone at that specific value. reads as artificial. but the amplitude is too low for any powered transmitter at reasonable distance. so thats another question without an answer.

DTMF structure plus a persistent 7934 Hz carrier in the same capture is not a coincidence i know how to account for. been monitoring ELF/VLF and geological EM for seventeen years and i dont have a clean explanation for this recording.

somebody needs to do a proper decode on the DTMF sequence. ive got partial hits but the noise floor on my end is making the result unreliable and i dont want to post a partial decode that turns out to be wrong. if anyone has better filtering capability or a cleaner DTMF decoder that can work against noisy audio this is the time.

tagging this unresolved. VE3KWT if you go back up there with a spatial array let me know, id contribute to equipment costs.