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APOLLO Gravimometer: April 2, 2009

The gravimometer was installed; below are photos of readings, as well as explanations.

The first shows the measurement (blue), the model (red), and the difference (gray).  The scale factors are important (4, 4, 10, respectively), so that the scale on the left must be divided by this number to be accurate.  The fact that the gray residual is scaled 2.5 times the gravity and model means disturbances are more apparent.

TIME RUNS RIGHT-TO-LEFT (!) ON THESE PLOTS!!  Start and stop times are indicated in the lower corners of the graph.In this first picture, you see the earthquake off New Guinea at 3:55 UTC 2009-04-01 (we saw it 20 minutes later).  The gravimeters at GWR in San Diego also saw it about the same time.

The next picture shows gravity tides (blue) for the last 7 days.  For the early part of this (right side) we were still messing around with installation.  You can see a few glitchy earthquakes, but otherwise all is quiet.  Full scale corresponds to a meter of vertical motion.  The site heaves up and down about 0.5 m in a day.  However, gravity does not directly mean displacement, so some scaling factor has yet to be applied--do not take the 0.5 m literally--it may be closer to 0.3 m.  The green is barometer.

The last plot is the gravity residual, filtered at 60 seconds, which gets rid of microseisms.  The steps may be telescope motion.  The scale (not scale factor of ten) is about 100 nm/s^2, so that an effective millimeter motion is 3% the height of full scale.

Some other info: The earthquake we see in the record is a magnitude 6.3 from New Guinea at 3:55 UTC on 2009-04-01.  We see the first signal at 4:15, then a stronger response at 4:45, taking a while to settle out. A displacement of 1 mm vertically translates to about 3 nm/s^2 change in the strength of gravity (0.3 parts per billion).

Also are two plots :The first is about 1.5 days of gravity measurement last weekend.  We were still on-site, and the noise spikes are me scratching paint off the gravimeter dewar, creating a disturbance.  The scaling is 817 nm/s^2 per volt.  The fuzz throughout would indicate some "noise," but it's really signal. The second shows a blowup around day 87.333, and we now see microseisms: oceanic vibrations carried through the earth crust (dominated by pounding storms) at 4-16 second periods at ~10 nm/s^2 amplitude.  Who knew this stuff vibrates the whole earth this way...  The amplitude is at the 10 nanometer level (angular frequency omega about 1 rad/sec --> ampl = accel_ampl/omega^2 ~ 10 nm), so it's small.
* the APOLLO station needs to know where it is to millimeter accuracy
* the earth heaves by a foot or so twice a day due to solid-earth tides
* the gravimeter is sensitive enough that a 1 mm upward/downward displacement results in measureably weaker/stronger gravity
* the tides stand out like a sore thumb, but subtle shifts will be apparent too
* we see the hum of the oceans, and occasionally earthquakes halfway around the world...

Here is a cleaner version of the 7-day gravity (our installation mayhem is scrolling off the right edge). On the top of the third small hump from the right, you see a glitch associated with a 5.9 earthquake off Kodiak island, Alaska.  Then on the downslope of this (time runs to the left) is another glitch from a 5.8 off the Aleutians.  The big glitch after the fourth big hump is the 6.3 New Guinea quake. The glitches on the right edge are part of installation...