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Gravimetry Analysis

Since October 2006 the SGF has been running in the facility basement an FG5 Absolute Gravimeter supplied by Micro-g Lacoste. The gravimeter is currently being operated to determine a midweek average value of gravity which can be found to an accuracy of ± 2 µGal.   This routine observational programme will build up a long time series of local gravity at this important co-located site.

The FG5 Absolute Gravimeter works by laser tracking a test mass free falling under gravity in a vacuum.  The base of the dropping chamber is decoupled from vibrations from the outside world by a sophisticated "super-spring".  This provides an absolute measurement of gravity.

An 'experiment' consists of hourly sets of drops that are combined to give an average value of the local gravitational acceleration along with an error estimate.  Both the average value and its RMS can be affected by local weather conditions, high wind in the nearby trees, weather systems over the Atlantic and earthquakes.


A critical factor in the set up of the gravimeter experiment is the verticality of the  tracking laser.  The verticality is checked and adjusted before, during and after each experiment to get the best possible results.  A poorly-set verticality will result in a low gravity reading. 


Preliminary time series of absolute gravity values at SGF, Herstmonceux.
The first few months of measurements show local gravity variations of a few µGal about a mean of approximately 981124000µGal

windsig_060307.jpg
windsig_080307.jpg


(Wind Speed measured in knots)


These four graphs are comparing the sigma of the readings from the Gravimeter with Wind Speed Data from the Met office between 18/02/2007 - 15/03/2007. We are investigating any similarity between the two data sets to see if the wind disrupts our data. The data shows some evidence to support this but further investigation is required. It is possible that the wind moving  the trees creates small seismic tremors due to the roots being moved which are then picked up by our gravimeter.  Another theory is that because we are near the English Channel, the southern winds have a large effect on the Channel and could also disrupt our readings. This information will be used as confirmation of the need of our own personal Wind Monitor. 

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