HISTORY OF CAVE RADIO:
Low Frequency Magnetic Induction "Cave Radios" have been used for more
than half a century to precisely Radiolocate "Ground Zero" points on the
earth's surface lying directly above Beacons placed at key spots in dry
cave passages. The main use of Cave Radios is to doublecheck/correct the
in-cave survey and to precisely align the survey with the surface Topographic
map. Specific Ground Zero points have been used to create new cave entrances;
by tourist cave operators to drill elevator shafts; and by farmers to drill
wells to tap the water in underground pools.
HOW MAGNETIC INDUCTION WORKS:
At Wakulla Springs, the antenna of the underwater Beacon consisted of
a ring-shaped loop of wire 2 feet (0.7 mtrs) in diameter lying flat
on the floor of the passage and precisely leveled. A simple battery powered
several amperes of alternating current through the loop at a frequency of about 3500 cycles per second (Hz). The magnetic field looks like the field produced by a classic "bar" magnet standing on one end. The field directly above the Beacon is exactly vertical.
The Receiver uses a similar sized loop to detect the magnetic field which reaches the earth's surface. In use, the Receive loop is held vertically and slowly rotated to find the direction of weakest signal (the null). Ground Zero lies along the Line Of Position (LOP) passing thru the plane of the loop. If the operator moves some distance perpendicular to this LOP, a second LOP will be obtained which crosses the first directly over the underground Beacon. As he approaches this intersection, the operator will soon arrive at a point where the Beacon signal nulls in all directions with the Receive loop held precisely vertical with a bubble level. This is Ground Zero.
The receiver has a calibrated digital readout of signal strength which allows approximate determination of the depth of the Beacon by any of three different methods. This feature is very useful in dry caves with rugged surface terrain but the diver's digital depth gauges were all that were needed at Wakulla.
The electrical conductivity of the limestone affects the strength and shape of the magnetic field generated by the Beacon in the cave, therefore knowledge of conductivity can help in the design of the Radiolocation gear.
The electrical conductivity of the water at Wakulla Springs is
reported to be .028 S/m (mhos/mtr). The Radiolocation gear was used at
Wakulla to obtain a rough estimate of the conductivity of the limestone
at various depths by using a Beacon on the surface. The technique consists
of measuring the relative strengths of the primary and secondary magnetic
fields at measured distances from the Beacon corresponding to the depth
for which one wishes to know the conductivity. Only simple calculations
are needed. The few measurements done so far give about .01 S/m (mhos/mtr)
down to about 10 meters and .014 S/m from 20-100 meters. The passages at
Wakulla are mostly formed in the soft and porous upper Suwannee limestone.
The high measured conductivity should be expected with this soft limestone
saturated with water. For comparison, measurements over typical
dry caves in the US have yielded conductivities of .003-.005 S/m.
An experimental technique was used to derive a crude "bulk" , or average, conductivity for the limestone both above and below the cave passages. Signal strength data was taken with the Beacon on the passage floor at a depth of about 300 feet and reduced with a magnetic field simulation program to yield an average conductivity of .019 S/m. Note that the "measurement includes the limestone up to 300 feet below the cave passage and not just the rock above.
While doing the Ground Zero locations at Wakulla, it was observed that some LOP's gave much sharper nulls than others. That is (in the absence of electrical intereference), there were always two directions (180 degrees apart) which gave a very sharp null as the operator approached ground zero. The same effect was observed while doing the conductivity measurements with both loops on the surface, although conductivity in different directions seemed to be the same. The cause of this assymetric behavior (called anisotropy) is not known, but has been observed elsewhere when doing deep measurements and could be related to vertical jointing in the rock in a particular direction. The deepest null always seems to occur at right angles to the poorest null. This effect was studied during the expedition by recording the magnetic bearing of the sharpest nulls over selected ground zero locations when power line and atmospheric noise were low.
The geophysics (including depth measurements) is a personal interest of the author, and was carried out only on a "not to interfere" basis during the actual expedition.