Dam Monitoring with 3D Tracker GPS - Jewell Instruments
Dam Monitoring with 3D Tracker GPS - Jewell Instruments
Dam Monitoring with 3D Tracker GPS - Jewell Instruments
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RELATIVE DISPLACEMENT (m)<br />
RELATIVE DISPLACEMENT (m)<br />
0.015<br />
0.010<br />
<strong>GPS</strong> Upstream/Downstream − Left Base to Mono 35<br />
Suspended Plumb Line − Mono 35<br />
<strong>GPS</strong> 7-Day Running Average<br />
0.010<br />
0.005<br />
<strong>GPS</strong> Upstream/Downstream − Left Base to Mono 23<br />
Suspended Plumb Line − Mono 23<br />
<strong>GPS</strong> 7-Day Running Average<br />
0.005 0.000<br />
0.000 -0.005<br />
-0.005 -0.010<br />
-0.010 -0.015<br />
-0.015<br />
APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR<br />
2002-2003<br />
-0.020<br />
APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR<br />
2002-2003<br />
Figure 3. Comparison of Plumb Line and <strong>GPS</strong> data over a 1-year period.<br />
value for each displacement vector roughly equivalent to<br />
one standard deviation. The Army Corps designed the <strong>GPS</strong><br />
monitoring system <strong>with</strong> two <strong>GPS</strong> reference stations to<br />
ensure that at least one is running at all times and to<br />
provide integrity monitoring. The integrity monitoring comes<br />
in the form of two independent solutions for each point on<br />
the dam computed from the separate reference stations. To<br />
further monitor system integrity, plumb line measurements<br />
collocated <strong>with</strong> <strong>GPS</strong> installation locations were collected<br />
and analyzed.<br />
MULTIPLE REFERENCE SYSTEMS<br />
A byproduct of using two reference stations is the ability to<br />
gauge system accuracy and precision. The two reference<br />
stations allow for two independent solutions to be derived<br />
for each remote station at any given time. Agreement in the<br />
real-time computed solutions for the two independent<br />
processes serve as an immediate indicator of the overall<br />
system precision and accuracy. Disagreement between<br />
independent solutions, however, will alert an operator<br />
to pursue an immediate investigation of the cause for the<br />
deviating solutions. Installing reference stations on<br />
opposing slopes of the Kootenai River canyon has<br />
introduced confirmation processes from each respective<br />
receiver station.<br />
An extremely close agreement between independent processes<br />
for each remote station on the dam reinforces<br />
confidence in accuracy and precision of the computed<br />
remote positions. The difference between the computed<br />
solutions roughly equates to<br />
one standard deviation for 24-hour data sets. These<br />
independent processes accurately represent the definitive<br />
position of each remote station <strong>with</strong>in the operating error<br />
envelopes for the monitoring system.<br />
PLUMB LINE MEASUREMENTS<br />
Four of the six <strong>GPS</strong> instruments were collocated <strong>with</strong><br />
existing plumb line instruments contained <strong>with</strong>in the dam.<br />
The suspended plumb lines consist of wires suspended in<br />
vertical shafts. Manual readings are taken at the bottom of<br />
the shaft. The collocation of <strong>GPS</strong> and the plumb lines<br />
provides not only a way to compare the two measurement<br />
systems, but also allows the <strong>GPS</strong> system to provide<br />
integrity monitoring for the plumb line system and viceversa.<br />
While <strong>GPS</strong> can measure vertical motion as well as<br />
horizon- tal motion, the plump line system can only<br />
measure horizontal motion. The graphs above show good<br />
agreement between the lateral <strong>GPS</strong> measurements and<br />
the plumb line measurements at the dam crest.<br />
THE RESULTS<br />
An analysis of error envelopes, accuracy/precision of the<br />
computed solutions using multiple baselines <strong>with</strong> multiple<br />
reference stations, and comparisons of the collocated<br />
plumb line instrumentation and <strong>GPS</strong> provides three unique<br />
indicators of system performance. These three indicators<br />
have been defined at Libby <strong>Dam</strong> and are serving the US<br />
Army Corps of Engineers system operators to monitor<br />
motion of the dam at the millimeter level.<br />
Figure 4. <strong>Dam</strong> monolith cross-section.