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Author Topic: depth to bedrock - in my dreams  (Read 634 times)

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Offline tonofsteel

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Re: depth to bedrock - in my dreams
« Reply #10 on: November 08, 2017, 08:37:04 AM »
basically I wonder where to buy a simple, inexpensive electronic device (geophysical method, not drill) to have on hand whenever I find a need to do a few quick and simple depth estimates (depth to bedrock).  A very handy tool to have in any placer miner's toolbox.   My thought is that someone would have made one (DIY) by now and would share their knowledge. 
 

From what I understand the reason why this is not available is a couple of reasons:
- Simple - the circuits, theory and hardware required are a step up from the weekend engineer warrior DIY projects.  This requires some custom engineering and software which is related to:
- Inexpensive - the addressable market for this sort of product is small, it is a concentrated niche area that is a fraction of other hobby markets.  Products available out there reflect this in their price point.  High NRE with low volumes = expensive products.

That is not to mention that with all geophysical methods you need to interpret the results.  Typically this is done by a geophysicist that knows a lot about geology and the electromagnetic signatures that could be given off.  When I see inverted survey data with notes I so far have seen the bedrock layer be labelled: "interpreted bedrock layer" so it is still not a guarantee that even after analysis that is 100% going to be where the bedrock is.

The resistivities of many materials overlap, for example you could see a conductive layer and it could end up being water saturated clay or a layer with a high concentration from magnetite or some other conductive mineral.

The most promising method (cost and complication standpoint) that I have found is using Electrical Resistivity Tomography and Induced Polarization.  The downside is that it is more work than other types of surveys due to the requirement of either setting up many probes or moving them many times for a VES survey.

I think it is more realistic for a cheap device to be able to tell you where the layers are without any ID of what that layer is.  So you would be able to set up your survey over a spot and roughly be able to identify a high/low resistivity layer X feet down.  Experience with these readings in your given area would allow you to interpret what this means.  (truthing)  AFAIK a device that you walk to a spot and hit a button and it pops out a message "Bedrock 35 ft" would be very expensive and possibly not even possible (with decent accuracy) as the bedrock signature can be different in different locations/conditions.

As pascalfortier said:
 Some work good if you correlate your data with actual drilling

I see this commonly referred to with most methods that ground truthing is required many times due to complexity and masking that can occur.  Once you drill a hole you can match up what you see in the survey with what is actually there and have an idea of what resisitivities/layers match up with what physical conditions.

 If there is someone here with more technical knowledge in this area that can confirm/deny simpler ways / shortcuts that would be interesting to read......  I want to see what is under the ground without digging and so far all I can find indicates this is not a simple problem in practice.

Offline pascalfortier

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Re: depth to bedrock - in my dreams
« Reply #11 on: November 08, 2017, 03:38:38 PM »
Great reply!

Offline Oro

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Re: depth to bedrock - in my dreams
« Reply #12 on: November 08, 2017, 04:41:24 PM »
tonofstell - thanks for your input.  You make some interesting points.   

One does not need to be a geophysicist to deploy a simple device in the field and interpret the result.   I see hobby scale seismometers and seismographs, hobby scale metal detecting of all shapes and sizes, hobby scale treasure hunting devices that accurately calculate depth to buried objects and anomalies.  Simple resistivity methods and seismic wave detection is not rocket science nor expensive.  You mention overlap of strata and varying ground conditions so yes, I would assume a learning curve to know your ground, but again, not brain surgery.   Maybe there is no big market for a device that simply measures depth to bedrock but as a commercial scale placer miner I would buy one if it was a reasonable cost.   Heck, I paid $1,000 for my Nokta metal detector.   

To keep it simple a single point measurement would be good enough.  For a seismic device there's no need for a daisy chain of geophones and multi channel 2D-3D output.  Why not just one geophone?  I see them on eBay for $60 - connect to a signal amplifier and  a sledge hammer trigger,  then convert the output to something a human can read, a graph or digital display, probably not complicated software, may even be free open source.  Again, my imagination at work here.   I dream on. 





Offline geezir

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Re: depth to bedrock - in my dreams
« Reply #13 on: November 13, 2017, 08:44:21 PM »
Some one with advanced electronic, math and programing skills could make a workable seismic setup.  Thats not me.

In the mid-80’s and early 90’s I had a geophysical seismic survey done on a property to prove or disprove the existence of a buried channel. One line proved the channel and justified a larger survey. 
A seismic survey is an excellent tool to yield a bedrock profile.  The refraction wave(s) profile is used for shallow depth analyse. A string of geophones is connected to a multisensory unit. 12 and 24 channel surveys are common. (Maybe “were common” as I see that 48 to 120 channels are used today)
A seismic wave is created at one end of the line then at the other end and then in the middle. The wave may be created by hammer, explosive device (shotgun shell or propane)  or dynamite. 
The depth to bedrock is calculated by using what is called an intercept-delay time technique.
The length of the line and the spread of the geophones can be adjusted to increase the ability to reach greater depths.
The intensity of wave created also limits the depth.

(More or less how it works)
Multiple geophones at measured distances and slopes are used to calculate the sound travels through the ground. The exact time of the “seismic shot” and the arrival of the compression (P) waves at each geophone is recorded. The first geophone a few meters away from the shot will receive the “ground roll” and it will be traveling some 500 – 900 meters per second (MPS) Underling that layer is compacted clay and the increase in density increases the waves velocity to 1500 to 2000 MPS, that refracted ray will soon overtake the ground wave and arrive at subsequent geophones prior to it. Underling the clay is bedrock with speeds of 2500 – 6000 MPS, that refracted ray will soon overtake the other wave and arrive at subsequent geophones prior to it. The point the refracted wave overtakes the direct wave is called the “crossover distance” and is used for layer depth calculations. If the layers are sloping the time interval can change depending on the slope, so by using three shots the sloping depths can be calculated. The formulas involved are complicated and time interval calculations are complicated and the geophone frequency needs to match the shot frequency, 5 – 10 for blast 15 – 20 for hammer
A rough “rule of thumb” is that line length should be 5 times the depth to bedrock or use a 250-meter line to reach depths of 50 meters,
A single geophone could be used by hammering multiple times at set distances from the unit. Some way to record times and wave factors would be needed.
To reduce costs, one could do their own line layout. Line layout costs can be 50% or more of the cost of a survey. Your time (and or costs) for the line are allowable expenses as part of a technical report.
Laying a line to a fixed bearing with 5-meter visibility in heavy bush on a steep slope take time. Line direction and slope profile with geophone spacing, and elevation are all critical factors in report precision. Ground staking and line work are incommon skills today.

Good reading:  (1970 so some what dated)
A portable refraction seismograph survey of gold placer areas near Nome, Alaska
USGS Bulletin 1312-B      By: H. Gary Greene