Geology of Vermont
Geological Surveying Methods Used in the
George Springston, 2000
Geologists use many different techniques to explore the Earth. For
studying sediments that have not been hardened into rock (unconsolidated
deposits), much useful work can be done with shovels and other hand
tools, especially if there are natural stream valleys or human excavations
which expose the materials. However, if there isn't a convenient stream
valley or building site nearby, the geologist has to use other methods.
Two methods used in the recent surficial geologic mapping of the Newbury
quadrangle by the Vermont Geological Survey are geotechnical borings
and seismic refraction surveying.
Geotechnical borings in unconsolidated material are usually done with
a hollow-stem auger. The hollow-stem auger is a heavy steel auger
which is screwed down into the Earth by a truck-mounted drilling rig.
Each section of the auger is five feet long and a series of sections
can be attached together to go down 75 feet or more. The auger is
hollow in the middle to allow a two-foot-long steel sampling tube
on the end of a long rod to be dropped down in the hole. When the
sampling tube reaches the bottom of the hole, the end of the sampling
rod which sticks up out of the ground is pounded down about two feet
so as to force the sampler out below the bottom of the auger. The
sampling tube is made so that sediment can be pushed up into the bottom
but will not fall out when the sampler is pulled back up. The sampling
rod and tube are then pulled up with a winch mounted on the tower
of the drill rig. The sampling tube is removed and opened lengthwise,
revealing the sediment sample inside.
Photo 1. Drill rig augering down into old river sediments and
Lake Hitchcock sediments in the Connecticut River valley,
Photo 2. Close-up of the auger. Note the extra lengths of auger
stem to the left of the truck.
Photo 3. Pounding down the sampling tube.
Photo 4. Pulling up the sampling rod from about 60 feet down.
|Photo 5. Two halves of a freshly
opened sampling tube. The back half contains the sample, which in
this case is thin layers silt and silty clay formed in glacial Lake
Hitchcock after the retreat of the last glaciers.
Seismic Refraction Surveying
Seismic refraction surveying uses an energy source such as a sledgehammer
blow or a small explosion to set up a series of vibrations in the upper
layers of the Earth. For this project, a sledgehammer was used. The
seismic waves from the hammer blow move outward in all directions through
the Earth. Because different materials can transmit these waves at different
speeds, waves are refracted when they reach boundaries between geological
layers with different seismic velocities. A few of these come back up
to the surface of the Earth and can be detected with special microphones
called geophones. The geophones are spaced out along a cable which is
several hundred feet long. The signals from all of the geophones are
fed into an electronic seismograph which records and displays the results.
The signals can then be analyzed to determine how fast sound waves move
in the layers which the waves passed through. From the speed with which
the waves move, the geologist can estimate what types of materials are
present. This method is particularly useful for determining the depth
to ledge, since the seismic waves normally travel much faster in hard
bedrock than in any of the common unconsolidated sediments.
||Photo 9. The sledgehammer and metal plate.
Striking the sledgehammer against the plate supplies the shock waves
which will (hopefully) bounce off layers in the Earth below. The
cable on the hammer is connected to a trigger switch, which sets
the seismograph running when the hammer strikes the plate.
Photo 10. Swinging the sledgehammer. Note
the seismograph to the left of the geologist and the long line
of geophones stretching out past the car.
Photo 11. Striking the plate. Once the
plate is struck, the seismograph will start running so that it
can record any vibrations bouncing off layers in the Earth below.
Using the sledgehammer as a source it is sometimes possible to
receive signals from depths as great as 130 feet or more.
Many thanks to the Geology Department of Norwich
University for the loan of the seismograph equipment used in this study.