So, as mentioned in previous posts, I reckon it’s worth trying to use Arduinos as front-end microcontrollers for this project, as shown in the block diagram here. An Arduino Uno has 6 analog inputs, and the ESP8266 WiFi card which I plan to use has one. These are quite limited – 10 bit ADCs with bandwidth that at best may go up into a few kHz. As such, while they should be ok for picking up seismic data, they fall far short for the ELF/VLF radio capture which should really go up to the region of 20kHz.
On the seismic side, I think a first pass worth trying is a home-hacked sensitive, one axis sensor, plus a 3 axis gyro and a 3 axis accelerometer. I’ll come back to this is a while – I need to research & buy the gyro & accelerometers. But I have all the components for an attempt at a useful radio subsystem, provisional design as follows…
Starting at top left, the blue circle represents the actual ELF/VLF radio receiver. This will be some kind of antenna, picking up the electric field with a frequency range from somewhere probably in the 100s of mHz up to around say 200kHz. A good starting point for this seems to be the BBB-4 VLF Receiver. It’s a relatively simple 2-transistor design, with a high impedance FET input followed by a bit of further amplification provided by a regular BJT.
A major problem, as mentioned here before is mains hum interference. It seems that as well as the 50Hz fundamental, there’s also a significant amount of the 3rd harmonic at 150Hz. So I propose using notch filters at these frequencies (also in that earlier post). Given what will follow in the circuit, I don’t think these need to be very high Q/narrow, just enough to prevent these parts of the input swamping everything else, saturating what comes next. These filters are shown as the yellow block in the diagram.
Next comes a bank of bandpass filters. The Arduino+ESP8266 offer 7 channels, so I propose having the first being relatively broadband, pretty much just a buffer for everything coming from the receiver (post notches). After each of these will be a simple peak level detector, shown above as a diode & capacitor. The level on these will be passed onto the Arduino/ESP8266 analogue inputs.
(The diagram is simplified a bit. The gain of the different stages will need to be figured out, additional gain/buffering/level-shifting/limiting stages will be needed).
The key references on ELF/VLF radio precursors to earthquakes are vlf.it (note especially the OPERA project) and a chapter in Roberto Romero’s Radio Nature book. Alas, it seems that research is fairly inconclusive (and in places contradictory). Radio frequencies from the milliHertz right up to microwave are mentioned, may contain useful information. But keeping things simple is a major consideration here, so I’ll stick to somewhere a bit below audio up to a bit above. Yes, this project is experimental…
I intend to do a bit more examination of the signals that appear in VLF before going further, though whatever, the choice of frequency bands at this point has to be fairly arbitrary. Pretty much decades in the audio range seem a reasonable starting point. So on top of 0. broadband, here goes:
- 0.01 … 10Hz
- 40kHz … 200kHz
The question of how narrow/broad to make the filters for best results is another question that I reckon can only be answered with the help of experimentation. But it is possible to make pragmatic educated guesses. I intend using general-purpose op amps for implementation.
At the bottom end of (1.), I suspect it’ll be more effort that it’s worth to worry too much about LF roll off, a simple buffered CR filter, should be adequate. Effectively just DC blocking. For the top end of (1.), a straightforward two op amp LP filter should be fine. For 2. – 5. bandpass filters made from 2 op amps should make a fair starting point. Regarding the steepness of their curves, Butterworth configurations (maximally flat in passband) keep design straightforward.
You may notice that 3. + are at multiples of 50Hz. But I’m hoping that using standard value/tolerance components will make enough offset to alleviate the hum harmonics. E.g. using the Sallen-Key circuit (this is a low pass, but shows what I’m talking about):
This gives fc = 15.9 kHz and Q = 0.5, subject to component tolerances (typical inexpensive capacitors are +/-10%). The kind of values that are probably close enough to the decades above to usefully split ranges, but (hopefully) offcentre for the 50Hz harmonics.
I don’t know if I’ve mentioned it before, but as the radio receiver needs to be as far away as possible from power lines (which will likely be determined by my WiFi range), I’m intending using little solar panels feeding rechargeable batteries for power.
While on the subject, I reckon it’ll also be worthwhile adding data from other environmental sensors, notably for temperature and acoustic noise (a mic). Pretty straightforward for Arduinos. Variations in this data may be unlikely to be useful as earthquake precursors, but they will almost certainly play a part in environmental noise picked up by the radio & seismic sensors. My hope is to get a Deep Learning configuration together that will in effect subtract this from the signals of interest.