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overtheedge Offline 708 posts Reply |
seismic determination to bedrock
With geophones being designed for vertical motion and seismic velocity being a variable dependent upon the material, it appears that using any seismic method to determine depth to bedrock only gives a rough estimate at best without prior knowledge of the regolith horizons. I've been approached about the use of seismic testing to determine depth to bedrock with no geologic data available and the miner is somewhat secretive about the exact location. His motivation is purchase of a patented piece of property. However, sampling/bedrock surveys conducted by a large escavator have not provided any data. The area is Not amendable to drilling. The area has been described as a v-shaped valley probably composed of semi-consolidated glacial till and talus. Consequently, I find that the seismic velocity information on hand gives little reliability for depth determination. Now back to the geophones and thier deployment. The idea is to use a strike plate trigger and (please no laughter) deploy a geophone in a pit with a horizontal piston driving a bell crank holding the geophone in its proper vertical orientation. The bell crank would convert horizontal motion to vertical motion needed by the geophone.The geophone deployment would be at a measured distance (100 meters/yards etc.) and the piston would be puddled/packed into the undisturbed sidewall of the pit facing the seismic source with clay. As the strike plate transfers vertical motion, the bell crank could use a short radius for the piston and a long one for the geophone thereby acting as a force multiplier. The fundamental premise behind this concept is that at several feet below the surface, the regolith should be similar in elasticity in both the vertical and horizontal aspect. The time data then derived would be used for the vertical depth determination. I realize that errors will still be present, but any depth determination that is with a few feet error of actual beats the snot out of nothing. The miner has no geologic training beyond experience in the field. Both he and his father have mined the general area for several decades. His experience and real defineable data shows that stripping costs on the front-end and the tail end (reclamation) can make or break the bottom line of profitability. My question is "does this idea have merit for bedrock depth determination within the limits of 90+% reliability or am I just smoking my shorts?" Granted, I'm not in possession of all the needed equipment (A/D recorder), but the question needs to be asked. If not for just me and my miner friend, but for all placer miners. |
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overtheedge Offline 708 posts Reply |
Geo
I'm trying to reduce the problem to its simplest form. The intent is not mapping the substructure (3D), but rather depth determination only (1D). From my experience as a mechanical and electronic fabricator, I've found that each small increment in precision equates to a substantial increase in complexity and cost. Yes, the bedrock will have a seismic velocity at least twice and in some cases five or more times greater than the semi-consolidated I figure it as semi-consolidated due to some voids present in talus deposits and the possiblility of some of it being glacial till material in the overburden. This bedrock will act as a reflector in vertical incidence and then as a line type emitter traveling outward radially from the shot point. In the case presented, the bedrock is deeper than practical digging depth for the escavator. Based upon this premise, I figured that the direct path pressure wave from the shot would reach a 100 or so meters about the same time as the refracted wave from bedrock. This was the reason for using the bell-crank to augment the direct wave energy and the piston, being more sensitive to a horizontal pressure wave and less sensitive to the more vertical vector, acting as an attenuator of the refracted pressure wave re-emitted by the bedrock. Falling back to earthquake seismicity knowledge, I extrapolated this to mean that the pressure wave would arrive faster than the surface wave. Further, the acoustic wave transmitted by the air has a velocity of roughly 1087 fps at standard atmosphere. From this, I figured that the surface wave and acoustic wave could be considered noise if I were willing to sacrifice a bit of accuracy. Further, the geophone would be fairly sheltered from the acoustic wave by being in the pit. From the data from the horizontal test outlined in my first posting, I was going to a near vertical seismic test with the strike plate within a few feet of the geophone. This is one of the reasons I've looked at an A/D recorder rather than a intervalometer reading time from trigger to off event. The idea behind my thoughts on A/D recording is plotting times and amplitude events. With as you saying velocity is distance divided by time: V=D/T or in the vertical mode, D=VT/2. Yes, there is another element to be accounted for and that is what I term the smear factor. The pressure wave arrives not as a discrete pulse, but rather as a rough sine wave with the peak amplitude roughly equivalent to seismic velocity. Just another reason for recording the whole event rather than trigger to time recieved only. If I correctly understand the science, the unconsolidated material acts as secondary pressure emitters and smears the pressure wave, hence my terminology of smear factor. Seismic surveying is relatively old technology and I suppose I'm trying to go backwards in technology for the purposes of simplicity and costs. Most of my reference material (mostly dealing with structure conducive to oil traps) dates from the 50's and 60's and rates unconsolidated materials at less than 3000 fps and glacial till usually less than 5000 fps with occasional deposits running as high as 7000 fps. Hmmm, might be time to contact the Gephysical Institute at UAF and see if they have any data tables on seismic velocity tests conducted in Ak. The miner who asked me about the possibility was most interested in the benches, so I tossed out the effects of water saturation that might have increased the velocity to roughly a bit less than 5000 fps. I figure error up to 10% in depth determination is far better than no data. Perhaps my idea has flaws that would give results far exceeding 90% reliability. I just tend to think that older methodologies still have merit. Accuracy costs money. With digital storage and analog to digital conversion being cheap nowadays, I see no reason to go back to 50's/60's era film or storage oscilloscopes. Is the concept practical (within limits) or should I just drop it. I do agree that geophysical methods are the future for increasing a miner's bottom line. Admittedly, trying to explain a concept by typing it out might tend to make it not very understandable. For any errors or oversights, I apologize. |
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overtheedge Offline 708 posts Reply |
The only intent was cheap and dirty depth to bedrock determination. 10% error was fine. The thought was 10% was close enough; kinda the case of 50 feet vs. 45-55 ft. Any difference at this range of depth is within the variance typical of bedrock over the length of a placer deposit.
I'm not familiar with VES. If you have time, how about a brief overview? |
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kringle_mining Offline 2563 posts  Reply |
Over,
For a shallow shot you will be using "reflective" seismisity not refractive which is for a deeper depth profiling. your first interface velocity is a speed of vibration passing through unconsolidated material (overburden), and bedrock. On a reflective profile this is very distinct. Your problem is what Peluk mention to me. The density of permafrost hardened overburden might be the same as the bedrock, so compression waves might not immediately reflect to a geophone, so the first interface on your seismic profile will be skewed or masked. |
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kringle_mining Offline 2563 posts Reply |
Wiz
Reflectivity and Refractivity are two differing seismic profiles both using geophone arrays planted at the surface. Review your geophysics 101 text. Your over-thinking this. |
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vortxrex Offline 180 posts  Reply |
OTE Wrote:" The fundamental premise behind this concept is that at several feet below the surface, the regolith should be similar in elasticity in both the vertical and horizontal aspect. The time data then derived would be used for the vertical depth determination."
You can blank out/disregard the first portion of the recieved signal. Frequencies that will travel through the ground are very low & the wavelength is long. Low resolution, but still plenty to just see where the bedrock is. Elephants use subsonic sound to communicate over long distances. The sounds travel through the ground. If you experimented with frequency & amplitude over known depth locations & different types of overburden (known) then  Dave Frank |
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overtheedge Offline 708 posts Reply |
Thanks everyone.
I'm trying to use the sonar equations and an easy way to determine rough pressure wave propogation velocities by accepting greater errors than geophysicists would. I didn't intend to start a sh** storm or drag people into trying to figure out D/T curves. I have Slotnick's, Meissner's and Rockwell's articles from the 50's and 60's on hand. Lots of folks have a problem with the math and the science behind thier work. Uh, guess that includes me too. I'll pursue this problem on my own. I do have several PICs and a couple of programmers on hand as well as PIC Basic programming. Reckon I will have to spend more time studying on sampling rate and A/D conversion with the PICs. Neither my Brush or HP chart recorders are fast enough. I figure thier error rate will be about ±4mS in material with the same propagation velocity as water and 5000 ft range (based upon chart speed of 125mm/Sec). Have you priced chart paper for these old rigs? At distances less than 5000 ft and any hope of resolution goes away. And here I am looking at max distances of <50 ft. Oh well, I got them for radio astronomy anyway. So back to full circle and the PIC 16F series. Mayhaps it is my desire to simplify and accept errors in ranging trying to over-rule the science that is creating the problem. Sometimes (okay fairly often) that approach fails. Perhaps the earlier thread on fish finders needs further field testing. I see that HumminBird has a line called Fishin'Buddy. Just not enough time, money and not enough of me to get'r'done. Again my thanks and anytime you are thinking inside the box, you just might find you are in a mental institution. New ideas win out not by force of argument, but rather by outliving the detractors. |
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kringle_mining Offline 2563 posts Reply |
No reflection is for the shallow profile. Refraction is for deeper profiles.
Reflection uses the rebound of compresional waves Refraction incorporates the use of shear waves Over needs to know the speed/velocity of the placer bedrock being measured. And a velocity of waves through unconsolidated overburden dry Through unconsolidated overburden wet and a velocity for unconsolidated "permafrost overburden. These velocities will be needed to convert his time profile into a depth measurement to located the first interface. Vortex moose also communicate the same way by tremors, low bleeting sounds and keen sense of smell |
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Seden Offline 222 posts Reply |
OTE,
While all this is being sorted out, I bought a nice Geophone from B.G. Micro here:http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=10783 Then download one of these freeware P.C. O'Scopes that are triggered by sound here:http://www.dxzone.com/catalog/Software/Oscilloscope/ Doesn't anyone use RISC 8051's? Randy |
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overtheedge Offline 708 posts Reply |
By RISC 8051, do you mean the 8051Basic that has a basic interpreter built in?
I haven't only because of the price. I do like using basic language for programming; mostly because I'm far better at it than assembly, C, Pascal, etc. The PICs are just soooo cheap and have a wide assortment of capabilities for the money. Then when you figure the forums and books written for the PICs, it is awful hard to consider other MCUs. Been a couple of years since I spent much time at the bench and I know I'll have to upgrade my reference library. I do like the books from Square One. Now if I would just get away from parallel ports and move into this century and USB. BTW, don't overlook the projects from Nuts and Volts magazine and the stuff from Scott Edwards such as the Basic Stamp series of MCUs. As I recall, my PIC Basic compiler came from Scott Edwards or maybe it was microEngineering Labs. One programmer O/H is from MicroChip and my other one is from Andomeda. I picked up my geophones from BG Micro too as I recall. They were part of a kit that included a cheapo bar graph board to measure voltage from the geophones. Thanks for the link on the PC scope software. --------------------------- I contend that the broader a person's skill set and tools to match decreases the probability of an empty stomach in a struggling economy. Of course, I've been wrong before. |
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growler Offline 51 posts Reply |
Would a SmartSeis SE that was rented be a solution? jimmy
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growler Offline 51 posts Reply |
Thanks Geo. I had not seen a rental that affordable. Very cool. Jimmy
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overtheedge Offline 708 posts Reply |
Geo
Count me among those interested in PICs, applications for geophysics and the software. Checked out the programmer I got from MicroChip a few tears ago and it is a PICLite. Not sure if it will program the newer fare from MicroChip. Might have to upgrade. Winter coming on so have to get a plan of action together to keep from going nuts. Usually I have found that my best bet is educating myself. Gotta send off an order to Square-1 Electronics and upgrade my library on PICs and the stepper motor/CNC books look interesting. Thanks for getting me re-interested in PICs. I grew up in an analog world (RF communications) and digital is quite different. Not so much more difficult, but rather a different mindset: on-off, timing and conditionals rather than biasing, feedback and amplification factors. Oh well, I figure if you aren't learning why are you hanging around stealing air? From another of your postings on another thread, I totally agree, teach others what you have learned. I tend to think the old saying about re-inventing the wheel has a counterpart; don't re-invent failure. Share the failures too. Again, my thanks. |
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overtheedge Offline 708 posts Reply |
Figured I'd share a resource from a different angle that includes a bunch of info; okay its parallel port I/O stuff. Granted parallel port is old technology, but it is still quite usable and the theory/application info is good stuff.
Parallel Port Manuals #1 & #2 hxxp://www.phanderson.com/ I got my copies while he was still at Morgan State University. These were the student driven projects with schematics and source code. The rumors that DOS is dead isn't exactly true. It still exists in older versions of Windows® such as 95 - ME. It is stable and pretty small, ie no bloatware. Not sure if it is in Windows 2000®. In newer versions, it is just a compatibility window that may or may not work for your applications. Another resource is hxxp:www.sq-1.com This is the place for a bunch of books on PICs. They are written in a tutorial type format with the schematics and source code. Don't overlook the stuff on the MicroChip site. Check out the forums and application notes. Hope this helps. Just trying to pay folks back and forward for all the info I find interesting and informative that you all have posted. |
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