Monitor plot - Glossary and
explanations
Gravity residual:
This is an ad-hoc residual, observations minus predicted earth
tide minus air pressure effect.
Ad-hoc because the time series involved here are all recorded by
the gravimeter's acquisition system.
Since the plot of the latest 31 days is
updated on the current day, only a reduced range of data types are
available for reduction.
In particular, we don't have the Atmacs pressure effects, so we
work with the local air pressure.
The currently installed prediction table is from a least-squares
analysis of 2015-02-05.
Predicted earth tide:
A preliminary solution, least-squares fit of a luni-solar tide
potential to the observed data, simultaneously with air pressure.
Air pressure effect:
A simple regression model, giving a coefficient in units of nm/s2/hPa
(-3.286 ±0.0034)
Microseismic noise:
Generated either in the deep ocean by short-wavelength standing
waves or on the continental shelves, sometimes as far aways as the
Gulf of Biscay.
Big diagram shows the last one hours, sampling
interval is 1 s.
- Black curve shows gravity residual.
- Gray curve shows the last ten
minutes.
- Yellow curve is low-pass filtered
gravity residual, band corner at 31 mHz, suppressing the
microseismic noise due to ocean waves
- Dark-blue curve shows air pressure
- Light-blue curve shows band-passed
filtered air pressure, with a 40 dB gain factor applied. Big
variance is indicative of strong local winds.
Small diagram shows the latest 31 days.
- Black: Gravity residual with drift
removed and low-pass filtered, 1 smp/min. Low-pass filter is
not narrow enough to suppress seismic waves from big
earthquakes, primarily late-arriving surface waves ("coda")
and early free-oscillations. The drift signal is adjusted on a
weekly schedule (see
the diagram, scroll down to bottom).
- All other series at 1 smp//h
- Red: Predicted earth tide, typically
2000 nm/s2 peak-to-peak.
- Dark-blue: air pressure
- Light-blue: Onsala "bubble mareograph" (previously
Ringhals tide gauge), sign inverted (so the signal tends to
follow air pressure)
- Yellow: Proxy for bottom pressure at
Onsala (previously Ringhals), indicative of mass imbalance in
the local region
MEM - Power spectrum by maximum entropy
Using the Burg algorithm, a long
prediction-error filter is calculated (400 coeffcients) from the 1
smp/s residual. Eleven slices of data each 12 minutes long,
shifted through one hour by 6 minutes' steps.
The curves are colour-coded with
yellowish colours for the oldest slice to blue for the most recent
one. Notice the colour bar along the top of the frame. A baseline
(light-blue) has been determined from a few quiet days, retrieving
the mean-decibel level (i.e. the geometric mean).
The spectra (and the baseline) are affected by the
anti-aliasing filter of the gravimeter's analog-digital converter.
The baseline is used in the spectrograms (follow the link
"spectrograms vs. time") as their 0-dB level.
The oceanic microseisms show up
primarily in the band between 30 and 300 mHz.
Without much careful study, alleged source
region are shown by colour code, seen as vertical pajamas stripes.
.bye
