By Morris Hall – Karst Research and Technology Center

A cave without an entrance can be thought of as a closed tank system, which is filled with air (a gas) of nearly constant temperature and composition, at near atmospheric pressure of 14.7 psi. If the cave is totally without leak points and a seal can be established around a borehole that penetrates from the surface into the cave, then a test can be run to estimate the probable volume of the cave. If a passage x-section is assumed, then the length of the probable cave can be estimated.

This idea was developed as an extension of existing, well known engineering concepts that allow the estimation of oil and gas reserves in naturally occurring and man-made reservoirs.

The basic, governing formula for the concept is:

pV/T=nR where R=.831J/mole/K degree (Universal), n= number of moles,T= Temperature Kdegree , p=pressure and V=volume, r=density

The equation can be rearranged to pV=nRT and then rearranged to form

V=nRT/p

In isothermal conditions where the cave and the injected air are at the same temperature, the equation simplifies to:

V=nR/p or using the R constant for air at .286kJ/kg/K degrees yields

(1) V =R/p or p=Rr

Which states that the pressure of air at near surface isothermal conditions is the density of air times the R constant .The volume of air injected into a closed system is directly proportional to the R constant and inversely proportional to the pressure of the tank, meaning that as pressure drops in the tank the remaining volume contained within the tank is less, but the cumulative volume withdrawn from the tank has increased proportionally to the drop in pressure.

If we invert this thinking to ask the question, rather than pulling gas out of a tank(cave), I’m putting gas (air) into a tank(cave), then a pressure rise will occur as the volume of gas injected into the tank increases. As the pressure rises through out the air injection, the volume of the cave or tank can be estimated from the simple equation (1 ).

Procedurally, a cave would be found by drilling, and a source of injection (the air compressors on the rig ) would be found. A packer would be placed to isolate the cave from the surface and a pressure transducer would be lowered into the cave. The instantaneous and cumulative injection volume would be recorded over time with the injection rate held constant. The pressure in the cave/borehole would be monitored and plotted against the injection volume. As an enhancement, commercial smoke producing canisters could be introduced into the injection stream to allow for surface monitoring of the injected air to assure a seal is present.

This technique won’t work if the cave has an entrance or is open to the surface by fractures, faults or joints.

A computer program for the interpretation of the data is being currently developed.