Measuring Deep Pits

[Scott McCrea]

Since we were talking about deep pits (1600' in China) over here, it makes me wonder how they are measured.

Fiberglass tapes are 100' at the most. Distos only do about 300' (the expensive models will do 600'). I've heard of surveyors lowing a thin copper wire, marking the length, then measuring it later. Did they carry a big spool of copper wire all the way to China?

I suppose you could get a rough measurement from the rope if you calculate for suspected stretch. But what if there are rebelays?

I only ask because I plan on finding a 2000'+ pit in WV soon and want to be ready to measure it.

[wyandottecaver]

Drop a rock and use a stopwatch. Math will do the rest

[Scott McCrea]

Oh yea, the old drop a rock and count the seconds trick. I forgot that one. Great for estimating, but not accurate enough for a survey.

I have seen a chart for converting the time into feet for estimating the depth, but I can't find it now. Anyone have it?

[mgmills]

Oh yea, the old drop a rock and count the seconds trick. I forgot that one. Great for estimating, but not accurate enough for a survey.

I have seen a chart for converting the time into feet for estimating the depth, but I can't find it now. Anyone have it?

Try ON ROPE (the book). In the first edition (the one I have) on page 86 there is a chart for up to 1000 feet (sorry - won't help when you find that 2000-footer).

The book gives 2 formulas
(note in the formulas t2 is for t squared. I couldn't figure out how on here I could make the 2 come out right for the squared indication)

Depth in feet = 16t2, where t = time in seconds (book states this formula was designed to work in a vacuum)

Distance in feet = 11t2 + 8.9t - 7.7, where t = time in seconds

The chart I mentioned shows these two formulas plotted on the same chart.

[Scott McCrea]

I was looking for my copy of On Station. Didn't even think to look in On Rope. Oh well, thanks, Martha!

In On Rope, the revised edition, it's on page 92.

For a wildly generalized rule-of-thumb chart:
1 sec = 40'
2 sec = 100'
3 sec = 200'
4 sec = 300'
5 sec = 400'
6 sec = 550'
7 sec = 700'
8 sec = 800'
9.5 sec = 1000'
13.5 sec = 2000'

[Caverdale]

I worked briefly with a fellow who measured the depth of mine shafts for the Utah State Abandoned Mine Reclamation Program. He used a deep sea fishing pole and reel with about 1,500 feet of line with a weight on the end. He said it worked every time. He never bothered to compensate for stretching of the line, but I would think that problem could be measured on the surface.

By the way, 300 foot fiberglass name-brand measuring tapes can be bought at most good hardware stores. Mine cost about $30 several years ago. I've seen cheaper 100 meter tapes (~330 feet) at Harbor Freight.

[Jeff Bartlett]

Oh yea, the old drop a rock and count the seconds trick. I forgot that one. Great for estimating, but not accurate enough for a survey.

If you have an Auriga-enabled palm, there is a "pit sounder" feature that does the math for you. But you're right, NOT accurate enough for a survey.
How well would you be able to hear a rock hit bottom on a 2000' pit anyway? You'd have to roll your truck in there!

[ian mckenzie]

There probably isn't an accurate way to measure a really deep pit in one go. EDFs might work, but it's hard to hit the target card from so far away. A rebelayed pit can be surveyed bolt-to-bolt in multiple stations.

The metric formula for timed distances is easier to remember than the Imperial one. It's D=5t2 (distance in metres is equal to five times the time in seconds squared). Apparently it's most accurate between 2 and 4 seconds. Our deepest pit is seven seconds.

[Stelios Zacharias]

US cavers may not be familiar with the Topofil or Vulcain survey equipment used by cavers in Europe (see google images for some examples).

It is essentially a spool of cotton or other thread which activates a counter as it is spooled out. Cotton is preferable as it biodegrades if not taken out of a cave. There is a calibration necessary as it will sometimes slip as the wheels turn, but in a predictable way which can then be corrected for. The bookkeeper writes both the start and finish numbers off the counter into the survey table instead of trying to do the subtraction in his head and then the correction (if necessary) is applied back home or automaticlly by the survey software.

You are limited only by the length of the cotton bobbin. Some topofils are made commercially and include a compass and sometimes a clino as well (eg. Vulcain). There are also many home-made ones around.

Cheers,
Stelios

[cavedoc]

How well would you be able to hear a rock hit bottom on a 2000' pit anyway? You'd have to roll your truck in there!

I suspect after it falls 2000 feet that most rocks will make a BIG noise. Haven't had a chance to check it out though (sigh).

[Jeff Bartlett]

I suspect after it falls 2000 feet that most rocks will make a BIG noise.

depends, i suppose. if the floor is leaves or mud it might not make much of anything. isn't that how we ended up with all those "bottomless" pits?

[Ralph E. Powers]

I suspect after it falls 2000 feet that most rocks will make a BIG noise.

depends, i suppose. if the floor is leaves or mud it might not make much of anything. isn't that how we ended up with all those "bottomless" pits?

I've dropped some BIG rocks down DEEP mine shafts before (with help of course) ... mud or not... they make a noise alright. Problem is making out the "final" noise from all the others as the rock hits the walls on the way down and the sound echoes back up to you.
One mine we estimated the shaft to be well over 1600 feet, it got scary because the booming noise reverberated back up and caused the walls and timbers to groan, didn't drop any more after that and got the heck outta there.
This video has one mine shaft which has been stated at 2200 feet as told to me by a Highway Patrolman out in Utah. You can see it here http://www.youtube.com/watch?v=SZXioMAoyx0 and it'll be at the 7:52 mark if you want to jump to that (or see the whole thing)... you'll see fireballs being dropped down this sucka and it's cool, though not as cool as actually being there. It'll give you an idea of how deep is deep. We've managed one that got all the way to the bottom... a very, very long way down.

[Bob Thrun]

Richard Schreiber measured the depth of the pits in Ellisons and Fern by lowering a hammer on an iron wire. He put a twist in the wire at the top and taped the wire outside. The old depth of Fern was 426 feet, measured between knots in the rope. The new depth is 437 feet. He timed a rock drop at Fern at 6.0 seconds. He used a very round rock that he brought from Mexico. I have played softball with balls that were less spherical than the rock he used.

[Martin Sluka]

US cavers may not be familiar with the Topofil or Vulcain survey equipment used by cavers in Europe
It is essentially a spool of cotton or other thread which activates a counter as it is spooled out. Cotton is preferable as it biodegrades if not taken out of a cave.
Cheers,
Stelios

Just a note - much better than cotton thread it is the linen thread because it is not so elastic. I bought several boxes of old surgery linen threads many years ago and I used them. I use Disto now, but for deep pits topofil was the best tool. One was able to measure depth of unexplored pit too. And if the thread was connected to small balloon with helium it was possible to measure the height of chimneys too.

Martin

[Aaron Addison]

The LaserTec 200XL hanheld impluse laser is good for distances of ~2000 feet off of a rock face.

More at -- > http://www.lasertech.com/Impulse200XLSpec.pdf

Aaron