Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Wheeler Reese
Core Sampling: What and Why?
“Therefore everyone who hears these words of mine and puts them into practice
is like a wise man who built his house on the rock. The rain came down, the
streams rose, and the winds blew and beat against that house; yet it did not
fall, because it had its foundation on the rock. But everyone who hears these
words of mine and does not put them into practice is like a foolish man who
built his house on sand. The rain came down, the streams rose, and the
winds blew and beat against that house, and it fell with a great crash.” Matthew
wants to be that guy, the guy who wakes up one morning to find that his
foundation has been seeing other people, so you ask yourself “How do I know
where to build my house then? How do I know where the rock is and where the sand
is? How does anyone know?” That is where Geology comes in. Of course, one
of the biggest problems in any geological endeavor is that it has a lot to do
with things underground, thus you cannot see any of it. However, as any
scientist can tell you, the easiest way to determine the makeup of something you
can never actually observe in full is to observe several small parts and
extrapolate the rest. This is called sampling, and is just one way to figure out
exactly what is lurking beneath. We’re going to primarily be discussing
commercial coring in respect to engineering and mineral scouting applications.
Core sampling is, simply put, the act of obtaining a sample of material by
“coring” it out of the ground. Much like you would “core” an apple by
removing the core with a hollow metal tube. For most common purposes, this
consists of a machine (or “rig”) with a large hollow auger, and a smaller,
diamond “core bit” drilling into the ground to produce a vertical representation
of what is underneath the surface.
A typical drilling core rig, photo courtesy Branden Sanders.
So how exactly is this accomplished? Well it is almost as simple as it sounds.
The rig uses an auger or larger bit to drill down through the sand, similar to
how water wells are drilled, until it reaches what is referred to as “auger
refusal” or bedrock. Loose material does not stick together, and thus does not
core well, and so it is essential that the driller not start his diamond core
bit until he can get a sample of the rock that is underneath the sand. Of
course, it could simply be a boulder that is being drilled into, in which case
they will have to either figure out a way to drill through it or start a new
hole. If it is bedrock, then most contractors will simply drill down about
5 to 20 feet to get a solid idea of what the layer is composed of. When
the drilling begins, the core moves up the hollow shaft and comes out the top in
a cylindrical shape. Out of those 5 to 20 feet of drilling, most drillers
get a core sample of around 5 feet, however, sometimes only 20% may be readable
material whereas other times it may be as much as 80-90% solid. It depends
on the material being drilled into and what type of rock it is composed of.
Core that has broken into a patch of crystalized material, photo courtesy
Some softer rocks may require a carbide bit rather than a diamond one to
excavate the material without damaging it. There is another method that is
gaining popularity lately known as “Sonic drilling” that allows for a
continuous, unbroken core that can even be cut through metal. Of course,
all of these measurements and figures vary with every single job. Most of
the time, the engineer or mineralogist knows what they are looking for and have
special needs and requirements for each hole, not to mention how many they
typically will need to get a solid grasp of a large area. Some train
tracks and bridges that require pilings will need 60-80 cores per 100 ft. just
to be completely sure of what they are laying the tracks on and where the
pilings should be placed for bridges. After the core has been removed and
the bit extricated, the holes are typically filled up with concrete to prevent
sudden sinking feelings in the local wildlife.
of an excavated core that is ready to be delivered to the customer, photos
courtesy Branden Sanders.
So what exactly are the applications of this process? These
particular cores were used for an engineering project, where the engineer needed
to know how deep the rock was and what it was made out of, but this is just one
of the most common uses of sampling. Other commercial uses include mineral
and oil scouting (It’s easier to tell if there is something valuable if you can
just pull it out of the ground) and soil sampling which is done with a bit known
as a split spoon, also called a Standard Penetration Test. This is also
sometimes done by “grab sampling” or grabbing handfuls of samples that are
coming out of the hole and stuffing them in a bag. All of these methods provide
geologists, mineralogists, surveyors, and engineers valuable information about
the porosity, fluid content, age of the soil, and possible contents underneath
the landscape, sometimes even being stored in “sample libraries” for
preservation, research, and archival use.
Coring doesn’t stop there though. There is also ice coring, which is used
for determining the age of the ice and other bits of history. The thickness of
the layers and the coloring can tell geologists many things about the years they
were laid down in. There is deep sea coring, used for surveying and
mineral exploration. There is even the practice of coring trees to
determine their year-by-year health.
In short, core samples are a valuable way to see what is, and has, been going on where we can’t see.
More information and videos
Geologist Jonathan Moore discussing what happens to a core after it’s out of the ground and how they examine it. Colorado Goldfields.
SPT coring with a split spoon
Piston Coring in Deep Water:
Ice Coring methods in Alaska
Info about Tree coring
Organizations devoted to Scientific Core sampling
Integrate Ocean Drilling Program: http://www.iodp.org/
International Continental Scientific Drilling Program: http://www.icdp-online.org/front_content.php
Examples of state Core Libraries
Kansas Core Library:
North Dakota Core Library: https://www.dmr.nd.gov/ndgs/Offices/Core_Library/clib.asp
Special thanks to Branden Sanders,
Operations Manager for Geomechanics, Southwest Inc.’s Albequerque operations for the pictures and interview.