UNDERGROUND COMMUNICATIONS
By Anselmo Stiffan, IZ3BGJ
anselmo.s@inwind.it

INTRODUCTION
Communication to locations deep underground is seldom discussed, probably because there are not many activities going on down there. As an Amateur Radio Operator, I never had the curiosity to investigate communication to the inside of the earth; I was too busy trying to communicate through the air!

One day I happened to receive a short e-mail from the geology section of the National Research Council (the CNR -- or Consiglio Nazionale per la Ricerca) asking me to get in contact with them. How did they find me and what did they want? They just read an article on magnetic loops that I published first in antenneX and then, by permission, in the local Ham Club Internet site for the benefit of Italian Amateur Radio Operators. Their interest was triggered by my sentence stating that a magnetic loop is little affected by the ground.

The study that these geologists were conducting concerned finding a point on the earth’s surface directly above a point deep underground in a cave where a source of water had been found. The objective was to locate the point on the earth’s surface exactly above the source of water to allow precision vertical drilling of a hole, which could accommodate the pipes and cables to be connected to an underground pump.

One would suggest that they carry the pipes through the entrance of the cave, but this solution is often not feasible because of the extreme intricacy and maneuverability of most of the caves. The methods used so far consisted of creating sound waves made by controlled explosions and recording the echoes. This is a normal practice in the oil drilling industry. The accuracy of such a method is insufficient for the needs of the CNR because the area it needs to locate is generally very small compared to the large underground pools of oil.

CAN IT BE DONE?
They wanted to know if radio waves produced by a magnetic loop could penetrate to some depth, and if a direction finding system could be possible. Making a long story short, I had a full day of meeting and “educating” the geologists about the world of the electromagnetic waves and, in turn, getting myself educated in the dark area of geology.

My thoughts on this subject were that the near magnetic field radiated by a magnetic antenna is not greatly attenuated by calcareous rock, and that the radiated field should penetrate into the earth at least the distance of a quarter of a wavelength at the transmitting frequency.

To prove this proposition, it was necessary to transmit from a deep cave and have someone detect the signal on the earth’s surface. My enthusiasm in addressing the issue convinced them that such a test was worth trying. I came back home with plenty of thoughts about the possible solution to the extreme constraints of the environment.

My portable two-turn “magloop” (see Photo 1) has a diameter of 90-cm, which was too large in their opinion. The maximum size needed to be comparable to the average size of human shoulders enabling the antenna to pass where a man could pass. I learned that the way into a cave is not always a large comfortable corridor with steps, lifts etc. The environment is very humid and sometimes there are water falls, big rocks to climb, or descend. In other words there was no easy way to get my 90-cm diameter magnetic loop antenna, transmitter, and batteries through the path of tunnels to reach the intended destiny.

EXPERIMENTS BEGIN
So I decided to build a magnetic antenna of only 20-cm diameter made of 19-turns of thick (6-mm sq.) enameled copper wire with a 300-pF high voltage capacitor.  I used a MOS-FET power transistor made it to self-oscillate and resonate at 800-KHz.  Such a system was now small enough to manipulate inside the caves and also, my theory was that this 375-meter wavelength would be suitable for the test. Unfortunately, the efficiency of such a small antenna was so very low that even transmissions through the air were not detectable beyond a few hundred meters.

Then I considered using a large ferrite body, which, with its high permeability, would have required fewer turns with reduced copper losses. While searching for a large quantity of ferrite material, I received a phone call informing me that a real water search was going on at a site in Sardinia, a large Italian island, where the geologists were already working in a difficult and deep cave up on the mountains. I was asked to rush there with my existing larger magloop in order to test the feasibility of my ideas.

My wife and I were already in the process of departing on a vacation for a couple of weeks, so, after consulting with my wife, we both decided that Sardinia was also a very nice area for a vacation.  So, we accepted the invitation to join the CNR team in Sardinia.

EXPERIMENTS STOPPED
I stopped the experiments with the smaller magloop antenna and decided to have the transmitter located outside on the earth’s surface and send a speleologist (“Speleogist” is a person who scientifically studies and explores caves) inside with a simple receiver equipped with a directional ferrite antenna. Fortunately I found a good receiver for the experiment (at a Ham Fest) that covered the band of 0.5 through 4.0-MHz. It also had a BFO and a small S-meter.

In order to contact the maximum required cave depth of more than 200-meters, the lowest possible frequency had to be used.  Remember that only the near magnetic field penetrates the ground; at least that’s what I believed.

My little Alinco DX70 transceiver goes down to 1.8-MHz. So I equipped my 90-cm magloop with a vacuum capacitor of 1200-pF, packed all the stuff, and after almost two days of traveling by car and ferry, my wife and our two dogs finally made our arrival. The island coast is magnificent and very rocky as shown in Photo 2.

Photo 2

IT WORKS!
On the first day we tested the system at an easy cave to prove the theory. The area was really wild, only a few pigs, cows, goats and lots of rocks around. Luckily, my theory was correct and the 10-Watt signal at 1.8-MHz was heard below the surface through 50-meters of rock!

Encouraged by this result, we made plans for the big test at the deeper cave. It is more than 2,000-meters long, has two vertical drops (each more than 20-meters straight down), and only skinny people can access its various entrances. Throughout most of the cave, its depth is about 200-meters below the surface.

The next morning, two local speleologists were patiently instructed on how to use the receiver and how to find the direction to the transmitter. It had not been easy to explain how to detect the null and it was even more difficult to communicate the coordinates, via a telephone cable, to the surface. In this case the coordinates involved not only the Azimuth, but also the Elevation. The null had to be found to satisfy both of them!

The two courageous men were familiar with finding Azimuth, but finding Elevation was new to them. Imagine a Radio Direction Finding hunt using bi-directional antenna where the transmitter is held invisibly in a spot up in the sky! This was not easy for non-radio people to understand.

During the process of teaching, we realized that the magnetic compass, to be used for coordinate measurements, was largely affected by the magnet of the radio receiver loudspeaker. So the position of the ferrite antenna had to be determined in some way with a non-magnetic body and the compass reading made only when the receiver was put at few meters away.   (NOTE: Had the problem of the magnet of the radio receiver loudspeaker affecting measurements been foreseen, the magnetic loudspeaker could have been replaced with a speaker or headphones using piezo-electric crystals.)

DESCENT INTO THE CAVES
The task was not an easy one in the hostile environment of the cave. After an hour of discussion and simulation, we believed that the speleologists were knowledgeable enough to start the descent. We installed the transmitter and the magloop in a niche in the rocks (see Photo 3) where a cow with its baby had to be vacated.

It took four hours for the speleologists to get to a 200-meter deep point and the 10-Watt CW signal was still being detected!!   (In Photo 4, see Antonio’s big smile while listening to this good news on the telephone line!)

Photo 4

THE BIG TEST
Once we got the coordinates, we moved the transmitting station to a new point that we supposed to be directly above the receiver. Moving the station with the heavy car batteries several hundred meters away across the rocks was very tiring under the burning sun. At about 2:00 PM, a volunteer brought us some sandwiches (made of local raw ham), a bottle of excellent Sardinian wine, and few bottles of water.

The speleologists, after having a small lunch with chocolate tablets, moved along a long narrow tunnel zigzagging at about a constant depth to reach a waterfall that was the point of interest. It was described to me as a large room about 10-meters on each side. The waterfall was reported to be very noisy in the dark depth and communications became difficult. The telephone cable was not long enough to reach the new transmitter position, so a relay was organized. A “telephonist” was relaying the communication to me via a VHF link. This may have contributed to receiving an end message somewhat different from the original.  

While moving the transmitting system around to various locations, numerous receiving coordinate records were made. Many of them were unrealistic, raising the possibility that the speleologists were getting confused or that the communication linkage setup was tired. It was already 7 PM and the speleologists had been inside the cave for some 9 hours, eating only some chocolate.

With the transmitting antenna located close to the vertical-axis of the speleologist’s position, the coordinates had a high elevation angle that complicated the task for the exhausted explorers working in the dark. The signal was not detected at all times, despite the power being increased to 100-Watts and further complicated by having the instructions of how to orient the ferrite antenna being repeated via the telephone VHF line.

RUNNING OUT WATER, POWER & TIME
We moved the station to several different points and requested the new coordinates each time. The task of moving the station around was not easy because of the wild rocky area, the heavy batteries having to be hand-carried with the fragile antenna. The batteries were getting low and eventually we had to connect to the car battery of a Range Rover (and keep the engine running). The entire team was now exhausted and very thirsty. We had run out of water at 3 PM.

The environment was not favorable for making repeated and accurate tests.  My original intention was to place the receiving antenna inside the cave, oriented for maximum signal, and then orient the magloop to its deepest null position. That would have been a more proper direction finding method. But it was getting late, the batteries were dead and the poor tired speleologists were strongly insisting on getting out of there. Actually, they did not arrive back at the surface until past midnight.

SUCCESS WAS CONCLUDED
Did we achieve our goal? All the team said YES! Just look at the face of Roberto, “the boss”, standing next to me on the right side in photo 5.

Photo 5

As a bonus, we now know that two-way communication below the surface of the earth is possible provided a very small efficient transmitting antenna is made available for those descending into the earth.

The challenge is now for antenneX readers to see who will be the first one to make a very small magnetic antenna capable of operating at about 1-MHz with a transmitting efficiency of at least 0.1%. –30-

~ antenneX ~ August 2002 Online Issue #64 ~

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