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Author Topic: depth to bedrock - in my dreams  (Read 1649 times)

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Offline PascalGold

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Re: depth to bedrock - in my dreams
« Reply #10 on: November 08, 2017, 03:38:38 PM »
Great reply!

Offline Oro

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Re: depth to bedrock - in my dreams
« Reply #11 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 #12 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

Offline geezir

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Re: depth to bedrock - in my dreams
« Reply #13 on: November 27, 2017, 12:24:04 PM »
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.

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

Some very good information guys.
The older method (1D) did require multiple sets to gain depth because the depth and distance of the rods are related.

Most resistivity imaging surveys today use "Multi-Electrode" systems.
  In a multi-electrode system a number of rods are connected to the ground and connected by wire to a multi-channel sensitive meter that reads ground rho.(ohms)  In this method of vertical electric soundings (VES) the depth of sounding is about 0.2 the length of the line. A 500 meter line should reach a depth of about 100m. As the distance from the source increases so will the depth. The available detail depends on distance between each rod.
In most geological situation current is applied at both ends giving a triangle shape with the greatest depth in the center. If one wishes to reach a consistent depth over a distance staggered lines may be used.
This produces overlapping triangles producing a consistent depth reading.
When a line is used it is called 2D. If a grid of rods are used with multiple, multiple channel boxes they call it 3D,

Electrical- magnetic) EM also includes Induced Polarization (IP) Measures the voltage transmitted and a host of similar system and interpretation methods including using submarine radio broadcasts.  Very-Low-Frequency (VLF)

Seismic uses low frequency sound wave and is a different methodology.