Electric Logs 1

[pigdog]

Has anyone used down-hole logging instruments to map sand and clay layers in a shallow (100') environment? The oil boys do this all the time for deep holes. What parameters would you use and what would it cost in the southeast U.S.?
Thanks,
roger

 

[coneboy]

Hello Pigdog

The simple answer to your question is "Yes". Logging the subsurface soil to depths of greater than 100 feet is easy with the electronic cone. A very detailed log of the interpreted soil type, SPT values, undrained shear strength, water table depth, detection of upward/downward hydraulic gradients can all be determined from a single CPT test. Add on a resistivity module to the electric cone and you can also recorded the soil+groundwater resistivity/conductivity.

I am not sure if you are looking for something more complex, or different, or maybe soil conditions are not favorable, but CPT would be the easy method.

Hope this helps
Coneboy

Hope this helps

 

[geom]

Hi pigdog!

I came across your question dated Jun 17. I don't know if somebody has answered you that what you are looking for is an electric logger (also called borehole logger). It is used by hydrogeologists and well drillers. The instrument measures the resistivity values and specific potential of the underlying layers. The plotted data produces 2 curves where the contrasting sand and clay layers can be easily interpreted.

Actually, i am looking for such an instrument, preferably second-hand. Did you come across such during the posting of your query? I'd appreciate your reply. Thanks.

geom

 

[pigdog]

Hey Geom
I got a rental and a purchase price quote back from Layne Christensen, (Mike Culig 303-279-0171) for rental, and a rental or purchase quote from Robertson Geologging, (John Mims,281-807-6165. Websites @

http://www.laynechristensen.com

and

http://www.geologging.com

roger

 

[geom]

Thanks pigdog! You're a great help. I'll contact them.

Anybody else know where I can get a second hand electric logger?

geom

 

[TJWATKINSPG]

I DONT KNOW IF ANYONE CARES ABOUT THIS ANY MORE BUT AS I JUST JOINED I WILL ANSWER THIS AS A PRACTICE. I AM A GEOLOGIST WHO WAS A "OIL GUY" ONCE THE ELECTRIC LOG WAS MOST INSTRUMENTAL IN DETERMINING ROCK TYPE BASED ON THE MINERALOGY AND PHYSICAL PROPERTIES OF THE BOREHOLE. THE RESULTS WERE VERY REPEATABLE. THE ANALYSIS WAS SIMPLE. THE STRAT THAT WE WERE PLAYING WITH WERE OFTEN FOUR (4) INCHES ACROSS. THE LOGS COULD FIND THEM. THE TWO CURVES THAT I WOULD SUGGEST YOU TRY TO USE ARE SP (SPONTANEOUS POTENTIAL) AND RESISTIVITY (CALCULATED FROM THE INVERSE OF THE MEASURED CONDUCTIVITY. SCHLUMBERGER WAS VERY GOOD ABOUT TEACHING ME THE LATEST IN WHAT THEY HAD FOR INSTRUMENTS IN HAND. THERE WERE OTHER EQUALLY GOOD "WIRELINE" COMPANIES OUT THERE.
I HAVE SINCE WORKED WITH SOME GOOD "CONE" TRUCKS THE CPT IS PROBABLY A BETTER TOOL WHERE NO DRILLING IS REQUIRED. WHEN YOU START TO ADD A HAMMER TO THE TOP END OF A CONE PENETROMETER I WONDER ABOUT THE QUALITY OF DATA. CONES ARE GREAT TO THE TOP OF THE FIRST REFUSAL LAYER. IF YOU WANT TO HAVE A COMPLETE PICTURE OF THE SITE CONDITIONS UNDER THE FIST 50 BLOW COUNT MATERIAL THE SMALL ELECTRIC LOG WOULD BE A FANTASTIC TOOL.
HOPE THIS HELPS SOMEONE.

 

[toolpusher]

Pigdog

If you are drilling a borehole you can always run downhole tools inside the hole, if you install plastic casing after the hole is bottomed. Doesn't take much extra time and you benefit by being certain of where key horizons are, as downhole logs don't lie (they can mislead sometimes, but with care you gain a tremendous amount of info).

I find that 2.5 inch PVC threaded casing works much better than 3.0 inch in 4 inch-diamater boreholes, as the wider stuff tends to bind up in any soft squeezing zones. It has to be filled with water and pushed in by hand as it has hardly any compressive strength. You will need to plug the bottom of the casing and be sure to use a screen, or make one with a handrill. This is allow you to run quickset grout to fill the annulus between the casing and the soil; any air or water voids will negatively affect some downhole logs (conductivity especially).

You could check with local geophysical contractors for downhole conductivity tooling. The test is very fast to run. It would easily distinguish between sand and clay layers so long as the porewater conductivity is constant with depth (if you are near the ocean, watch out as conductivity is much more sensitive to salt content than clay mineral content).

Given a choice I would usually run resistivity CPT soundings, as the CPT data can be more useful than what you get from samples and is better for groundwater investigations / contaminated sites.

Keep in mind that an RCPT has a lower maximum pushing capacity than a standard piezocone due to the reduced steel section in the R section of the tool.