Is there anyway to determine weither it's a large cave by the amount of airflow? Assuming the cave only has one entrance (which is possible) how can I determine weither the cave is large (5 miles or more) only using airflow?
If there was a second entrance wouldn't it be possible to determine how far away the other entrance is by measuring the windspeed and refering to isobars on a weather map of that day?[/quote]
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Hello WVCaver,
Right now I know of no way one can determine whether one has found a large cave or not - just using cave air flow. What you get are generalities...possibilities.
Example: Brooks Cave in Pennington County, S.D. has a barometric wind. It has a barometric volume of 35-million to 50-million cubic feet - yet it has an explored volume of only about 250,000 cubic feet, and a surveyed cave length of only about 3,300 feet.
Thus the cave surveyors have discovered only about 1/2 of 1 percent of the total volume of this cave! Yet the geologic footprint of rock the cave wind resides within is 1-1/2 square miles. Thus, using the maze density of 50 miles of cave/1-sq mile at Jewel Cave, S.D. - as a representative example - Brooks Cave could be as long as 75 miles in length. ( I know where the cave wind leaves the known cave - but it would requie mining out 25 feet of almost solid rock to get into the room beyond.)
Another example is Nameless Cave, also in Pennington County, S.D. It has a documented barometric cave wind. Here again this cave also has a cave volume of 35 - 50 million cubic feet. The explored/surveyed cave volume is only about 10,000 cubic feet - for the cave is only about 200 feet long when you include all known side passages.
On average thought, caves that have a barometric cave wind tend to be much longer than normal.
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Second question: on Chimney Effect
Its possible to calculate the "geologic height" of a second cave entrance - that is above or below the first entrance using the velocity of the wind. You do this by back calculating the pressure differential needed to generate the wind in the first place - then calculate the difference in height needed to generate this pressure differential. (Its a lot like a scuba diver watching his depth gauge increase in depth as he slowly drops lower in the water column.)
You must also realize - that a geologic contact - such as your sandstone formation above your cave also will work as the upper entrance of a chimney effect cave. This is what happens at Mammoth - Flint Ridge. This cave has some 23 known entrances (or so)), yet I've been told that all the entrances respond as if they were the "low entrance" of a chimney effect wind! This was explained to me. Water flowing of f the top of the sandstone caprock hits the underlying limestone. This water finds a crack and enlarges it. This water eventually finds the cave and joins it - in doing so it completes an air flow loop alowing air flowing thru the low entrances to exit at or near the base of the sandstone contact.
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I don't know of a way to calculate how far away a second barometric entrance would be from the first one - with caves having a barometric wind. Theoretically, each cave will be responding as if it were the only cave (within its "sphere" of influence).
An example would be Lechuguilla Cave. Its huge! It's barometric. Big Manhole Cave is only one mile as the crow flies away, and it too is barometric. - but its barometric volume is X15 smaller. Then you have the big cave - Carlsbad Cavern 4-12 miles away. It strongly sounds like its barometric for it has a too-tight cave passage that blows winds upwards to 35-mph. deep inside, near the back of the cave.
Enought for now.
CaveAir