[Radio location information provided by expedition member Brian Pease prior to the expedition.]

Radiolocation will be used at Wakulla Springs to establish a network of precisely known points in the underwater cave system which will be used to correct for errors caused by drift in the Mapper's inertial navigation system. Low Frequency Magnetic Induction Beacons placed in the cave during exploration will be precisely located on the surface using special receivers. These new points will be tied into the growing cave map by precise overland survey back to a reference marker at the Spring Pool. The Radiolocation equipment has been successfully tested at a depth of 300 feet.

History

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 consists of a ring-shaped loop of wire lying flat on the floor of the passage and precisely leveled. A simple battery powered circuit forces 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 are all that is needed at Wakulla.

Geophysics

The electrical conductivity of the limestone affects the strength and shape of the magnetic field generated by the Beacon, 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 has been 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 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 240 feet and reduced with a magnetic field simulation program to yield an average conductivity of .0036 S/m. This lower conductivity was expected given the denser lower Suwannee and Ocala limestones that underlie the passages.

While doing the Ground Zero locations at Wakulla, it was observed that some LOP's gave much sharper nulls than others. 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 and could be related to vertical jointing in the rock. The deepest null always seems to occur at right angles to the poorest null. This effect will be studied during the expedition by recording the magnetic bearing of the sharpest nulls over each ground zero location.

The geophysics (including depth measurements) is a personal interest of the author, and will be carried out only on a "not-to-interfere" basis during the actual expedition in the fall.

Read about dive lights ->