Research on the nature of ground tilts in the period range one thousand to ten million seconds

The apparent tidal tilt anomaly first identified at Bedford, Massachusetts is shown to extend to Maynard approximately 17km WSW and to be attributable to ocean tide loading effects. The analysis technique used has been demonstrated to be capable of determining the M sub 2 tide with a standard deviation of 2 nanoradians using a single months data. Evidence is presented that the observed atmospheric pressure effects are site rather than instrumental in nature. While stability of individual deep borehole installations is substantially better than for shallow installations local effects still mask true secular ground movement. The nature of the tilt records is such that simple averaging will not significantly improve the secular tilt estimates. It appears that new techniques capable of recognizing only those effects which act simultaneously on all tiltmeters are required.