The British Caving Association

Dear sirs:

I am a geologist and subscriber to the creg journal residing in the United States, and I have some thoughts on underground communication for your group that I would like to present.

Cave communications have focused on radio which does indeed penetrate rocks however there is a alternative approach that apparently has not been considered.
As a geologist, I am familiar with seismic equipment. In seismic prospecting a vibration of 1 to 8 cycles per second is sent into the earth and the reflections from rock layers at depth are detected. Depth penetration is greater than 20,000 feet! This suggests that a system using vibrations sent through rock could be a viable system for cave communication.

The reason for the low frequency used in seismic exploration is that as the frequency increases, the attenuation of the signal by the rock increases rapidly.

Some ham radio software uses audio tones to transmit text at slow rates. These text messages can be read by the computer when the signal is completely buried by the noise level. Signals buried under noise 20 db louder than the signal can be detected!

One system for transmitting text on ham radio is Jason. This system uses 16 tones to transmit text, but the changes in frequency are so slight that the ear only hears a single tone when listening to the transmitter. Software for ham radio transmitting text is commonly labeled qrss software. Jason qrss software appears to be possibly the best for adaptation to cave communications since it uses a series of audio tones at 40 hz to transmit text.

The important point about Jason is that the program allows the transmitted signal to work at 40 cycles per second. This is close enough to the frequencies used in seismic exploration that it is likely these waves could penetrate substantial amounts of rock. Modification of the software to allow changing the transmit frequency to lower frequencies would allow substantial increases in depth penetration!

Injecting vibrations into the rock might be accomplished by using a stack of piezo electric disks wegded into a fracture or snugged up against the rock with a small hydraulic jack. When the audio signal is injected into the piezo disks they change their size and produce vibrations which will be transmitted into the rock. The same sensor would also serve as a signal pickup as well since piezo disk transducers are used in seismic wave detection and when connected to a suitable amplifier, can pick up vehicular traffic at about a mile. The redwood seismic net in the usa ( http://psn.quake.net/) has members using piezo disks to monitor for earthquakes.

A suitable recieve amplifer can be seen in the article titled a easy vlf loop: http://www.vlf.it/easyloop/_easyloop.htm,
The amplifier will provide a lot of gain however a higher gain was found by me to be necessary to override fan noise in my computer sound card. In this instance a second stage of amplification was used with a gain of 5 was added. Your mileage may vary!

A transmitter would be as simple as a 10 watt audio amplifier attached to a computer running jason, and connected to a piezo transducer.

I am unable to pursue this project and would be pleased if someone would try this approach to determine the usefulness of seismic waves as a communications method.
Best Regards

Jim
kd6iwd

sounds interesting, Jim. A couple of questions...

1. Do you have a feel for the attenuation in the system? For a magnetic system, we would relate the transmitter's magnetic moment to the received magnetic field strength. What is the equivalent in seismic terms? Would it be to relate the pressure level at the transmitter to the displacement at the receiver? Do you have a feel for what those figures are? For example, a pressure of 1MPa at the transmitter gives rise to 1um displacement 1000 metres away ... or whatever?

2. Do you think a piezo system is necessary? I would think it would be expensive to relay enough power through a piezo-electric system. I think that perhaps an electro-mechanical transducer would be more efficient and cheaper. 40Hz is only 2400rpm, so a d.c. motor with a cam attached might make quite a good hammer.

Aside: I suggested a seismic system for mines rescue some time ago, but it was never taken up - and I never did the calculations to check its feasibility. The concept was to use a compressed air cylinder to drive a rotating cam that would send a 'buzz' through the rock. The point being that such a system is 'intrinsically safe'' (i.e. explosion proof) so it could be used in a coal mine. Also, you can pack a lot more power into an air cylinder than into the type of battery that would be allowed in coal mines

The attenuation at seismic frequencies is quite low. The reflections from a 20,000 foot deep reflector are detected by a seismic sensor at the surface- so the signal had to transit 40,000 feet of rock to both get to 20,000 feet and then the weak reflection then returns to the surface. So, At seismic frequencies there is very little attenuation. There are maths which can indicate what sort of attenuation can be expected, but at 40 hz you should be able to traverse several thousand feet of rock since there is a receiver at depth to detect the signal- and you are not looking for the much weaker reflection of the signal. Jason is the software I think will work the best, and a conversation with the author of the software might get a modification of it to work at lower frequencies. The software transmits data at relatively slow rates but with computer processing the text is read at incredibly weak signal levels.

The piezo disks were suggested because they cost about a dollar apiece on the surplus market, and a stack of them could be purchased relatively inexpensively. They are limited in how much power they can stand so a fair number of disks would need to be used to handle 10 watts. A point- when you put piezo disks under pressure a very high voltage will develop across the terminals. This voltage must be bled off with a resistor to avoid blowing up your amplifier.

Best Regards

Jim

A thought on a signal source, perhaps a electric solenoid could be used as a tapper controlled by the computer. There are many auto solenoids that could be used or you could roll your own. This might be a interesting approach fot generating a signal.

Best Regards

Jim

i just checked a page on sea floor mapping. The lowest frequency used for a ship towed array is 50 Hz. At that frequency depth penetration is in excess of 10,000 feet so in practical terms where the receiver is at depth rather than detecting a reflection, the depth penetration could easily be 10,000 feet.

a good reference to seismic exploration and frequency is:

https://woodshole.er.usgs.gov/operation ... eismic.htm

Best Regards

Jim

Here is a reference using 70-100 Hz to image a oil field at a depth of 3000' and the reflection of layers at 12,000' were measured all with frequencies higher than 40 Hz.
See pages 55-58 in the following reference: https://www.slb.com/~/media/Files/resou ... 955260.pdf

This reference shows that it is possible to measure the reflection from rock layers at a depth of 12,000 feet from the surface using seismic waves at a frequency of up to 100 Hz, which means that at a minimum, the seismic method of communications might work to a depth of 24, 000 feet from the surface! Clearly, this penetration depth would be a major advance in cave communications.

Best Regards

Jim