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It is well known that sensitive vertical seismometers require a protective case to shield against the affects of atmospheric air pressure changes. The case for my Inyo vertical is built with 3/8" polycarbonate sheet plastic and is very strong.
 Figure 1, Inyo Protective Case The top edge seal consists of 1/16" rubber sheet between the top cover. The bottom seal consisted of automobile windshield sealant tape. The sealant is a soft rubber material that is easily formed in place. At times when the weather is windy, the vertical seismometer traces show increased levels of background noise. The question I had, "is my case leaking?"
 Figure 2, Typical Inyo High Gain Trace with Digital Filter, 0.08 Hz Low Pass and 0.02 Hz High Pass
 Figure 3, Infra-Sonic Atmospheric Air Pressure Trace with Digital Filter, High Pass 0.003 Hz
The atmospheric air pressure changes slowly with time. When the weather is very calm you can see a straight line. However when the weather is changing you will see much more change with time.
 Figure 4, Typical Inyo High Gain Trace with Digital Filter, 0.08 Hz Low Pass and 0.003 Hz High Pass
Figure 4 is the same data as Figure 2, but now the digital high pass filter is now 0.003 Hz.
 Figure 5, Infra-Sonic Atmospheric Air Pressure Trace with Digital Filter, High Pass 0.003 Hz
Now compare Figures 4 and 5. You will notice that Figure 4, the high gain Inyo trace follows the same general direction as the infra-sonic trace, but is just delayed by about 60 seconds. It is almost as if the case is breathing in and out as the outside pressure changes with time. Is the case leaking?
I then removed the case bottom seal of rubber sealant and replaced it with a polyurethane window and door sealant. The inside corners of the case were also sealed with the sealant just to make sure the case was really air tight. I also used silicone grease on the cover rubber seal. With the grease you can visually see that the cover makes a good seal as the thumb screws are tightened in place.
 Figure 6, Typical Inyo High Gain Trace with Digital Filter, 0.08 Hz Low Pass and 0.02 Hz High Pass
It took several days to get the vertical seismometer going again and Figure 6 is the normal trace with the 0.02 Hz high pass filter.
 Figure 7, Typical Inyo High Gain Trace with Digital Filter, 0.08 Hz Low Pass and 0.003 Hz High Pass
Figure 7 is the same as Figure 6 except that the high pass filter is now 0.003 Hz.
 Figure 8, Infra-Sonic Atmospheric Air Pressure Trace with Digital Filter, High Pass 0.003 Hz
Compare Figure 8 and Figure 7 and you will see that just as before the seal was changed, the high gain Inyo vertical seismometer trace follows the same general flow as the infra-sonic air pressure trace and just as before the Inyo trace is 60 seconds behind the air pressure changes. I know the case is well sealed, there must be something else that explains why the high gain trace seems to follow the atmospheric pressure changes. The seismometer is installed in a basement wine cellar. The floor is clay tile on concrete that is 4" thick. The concrete is over crushed rock on undisturbed soil. Could the concrete be moving up and down with air pressure changes? Any ideas? |