 Borehole
Extensometers
The first of thirteen (13)
deep borehole extensometers
(designed and installed by the
United States Geological Survey
– USGS – in the early 1960s)
were used in preparation for the
soon-to-be-built manned
spacecraft center. Of the
thirteen in operation today, six
(6) of those are “subsidence” or
“total depth” monitors (meaning
their bottom is below the
aquifers from which we draw
water), and the other seven (7)
are less than total depth, or
“compaction” monitors.
What are they and how do they
work? Borehole extensometers are
deeply anchored benchmarks. To
construct each, a hole is
drilled to a depth at which the
strata are stable. The hole is
then lined with a steel casing
with slip-joints to prevent
crumpling as subsidence occurs.
An inner pipe rests on a
concrete plug at the bottom of
the borehole and extends to the
top. This inner pipe then
transfers the stable elevation
below to the surface. A
measurement of the distance from
the inner pipe to the
surrounding land surface
provides us with the amount of
compaction that has occurred.
(See picture for visual
reference)
Although the accuracy of this
measurement method is
impressive, there is one
drawback. The high cost to
construct and install the
equipment prohibits their use in
sufficient numbers, resulting in
a lack of adequate information
for the entire Harris-Galveston
and Fort Bend areas. And, as was
stated in the overview, the
sufficient amount of and wide
distribution of data is
extremely important. Over time,
as technologies have evolved, we
have moved toward more
cost-efficient and equally
accurate forms of
measurement…but borehole
extensometers are playing an
important role in this new era.
Three of our existing
extensometers have been
outfitted with GPS (Global
Positioning System) antennas,
and are now the only stable GPS
points within the greater
Houston area.
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