Acoustic hole depth tester?

[fastline]

We need to come up with a way to test the depth of a bore hole. The old string method works but takes WAY too long in some instances. I am looking for ideas or help me along with a couple ideas I have

1. Sonar or reflection wave. Problem is possibly if sound wave is not carefully aimed, it might distort and null the return. I am also not sure what electronics are needed to send and receive the signal.

2. Reverberation or standing wave frequency. Sort of like the water glass deal. As the hole gets longer, the waves would get longer, thus a lower frequency. I was thinking of sending an audible (or maybe inaudible) sound wave down the hole and monitor changes is frequency. or maybe initiate a frequency sweep and determine where the resonance is which could then determine the length or depth.

Ideas?

 

[davenn]

We need to come up with a way to test the depth of a bore hole. The old string method works but takes WAY too long in some instances. I am looking for ideas or help me along with a couple ideas I have

1. Sonar or reflection wave. Problem is possibly if sound wave is not carefully aimed, it might distort and null the return. I am also not sure what electronics are needed to send and receive the signal.

Ideas?

what sort of holes are we talking about here ? in the ground ? are they straight? horizontal or vertical ?

or are they smaller drill holes in other materials...wood etc ?

Ultrasonic transducers may be worth experimenting with for larger holes in ground or maybe laser units, I have seen commercial examples of both

Dave

 

[fastline]

These are straight vertical bore holes in earth. I was thinking of laser or the "range finder" but figure there could be some deviation if there is an obstruction.

 

[jpanhalt]

How would ultrasonic avoid inaccuracies introduced by obstructions? I suspect such inaccuracies would be as bad or worse than with a narrow laser T.O.F. detector.

John

 

[ericgibbs]

These are straight vertical bore holes in earth. I was thinking of laser or the "range finder" but figure there could be some deviation if there is an obstruction.

hi,
Whats the range of hole diameters and depths.?? and what measurement accuracy do you need.??

Could the bottom of the hole be water filled.?

 

[fastline]

holes can be 4-12" diam and most will have water at the bottom. Accuracy of +/- 1ft would be fine.

 

[Mikebits]

Maybe I am to old school, but string with weight on one end is best way. Tie a knot every meter, then count the knots as the sting descends the hole. That's how old sailors use to measure speed of the vessel. They would throw a rope with knots in it overboard and count the number of knots as it was being pulled by the water behind in a certain time period. Hence the term knots as used in nautical speed. That was a little trivia for ya.

 

[ericgibbs]

holes can be 4-12" diam and most will have water at the bottom. Accuracy of +/- 1ft would be fine.

and depths.???

 

[fastline]

Depths will vary from probably 20ft to up to 400ft.

 

[jpanhalt]

At 400 ft, ultrasound is out of the question, particularly given a hole diameter of 4" to 12."

How much water at the bottom? Clear or muddy water?

John

 

[canadaelk]

"Time Domain Reflectometer" Electronic Design, Oct 1/1998 issue had a circuit and description "Build your own Cable Radar". If you cant find it I will email you a copy. I adopted this circuit for a similar application as described by the OP. E

 

[KMoffett]

Geologists use a device (one sitting on my desk right now) that basically consists of a tape measure that has two insulated wires running down the edges. The wires are connected to a comparator circuit (with adjustable sensitivity) that drives a buzzer and LED. The tape is lowered down wells. When the exposed ends of the wires at the end of the tape touch the water, it activates the buzzer. Like these: https://www.geotechenv.com/water_level_meters.html

Ken

 

[Mikebits]

That is useful if one is just looking for the water table, but what if the depth goes much deeper? I still think a long rope is best solution, by the time you design, build and test the thing you could be done...

 

[Mr RB]

Ultrasound and radar both spread out at an angle, and will be no good on a 4foot diam hole any deeper than 15 feet or so. Especially if there are lumps on the walls as they will give a strong reflection once the beam starts to spread.

Laser will be difficult but might work, if you can get it reflecting from the bottom. Normally if more than a few yards you need a reflector disk to get a good laser bounce signal.

As one possibly solution, drop something down the hole and measure the time until the "plop" sound is heard. You could drop something biodegradable like a sugar cube etc that won't hurt well water. If the device releases the object you should be able to measure time to a mS or better, and then compute distance.

 

[alec_t]

I think method 2 (frequency sweep and check for resonances of the air column) would be best. You will need to be able to generate powerful pressure waves down to ~1Hz.

 

[Mikebits]

Ya wanna get high tech, use rope, with rope counter.

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