The movable tables of machine tools like lathes and milling machines, and of mechanical balances, can replace a shake table for the absolute calibration of seismometers. The idea is to place the seismometer on the table, let it come to equilibrium, then move the table manually by a known amount and let it rest again. The apparent ``ground'' motion can then be calculated from the seismic signal and compared to the known mechanical displacement. Since the calculation involves triple integrations, offset and drift must be carefully removed from the seismic trace. The main contribution to drift in the apparent horizontal "ground" velocity comes from tilt associated with the motion of the table. With the method subsequently described, it is possible to separate the contributions of displacement and tilt from each other so that the displacement can be reconstructed with good accuracy. This method of calibration is most convenient because it uses only normal workshop equipment; the inherent precision of machine tools and the use of relatively large displacements eliminate the problem of measuring small mechanical displacements. A FORTRAN program named DISPCAL is available for the evaluation.
The precision of the method depends on avoiding two main sources of error:
Static tilt before and after the motion produces linear trends in the velocity which are easily removed before the integration. The effect of tilt during the motion can however only approximately be removed by interpolating the trends before and after the motion. The computational evaluation consists in the following major steps (Fig. 29):
In principle a single steplike displacement is all that is needed. However, the experiment takes so little time that it is convenient to produce a dozen or more equal steps, average the results, and do some error statistics. On a milling machine or lathe, it is recommended to install a mechanical device that stops the motion after each full turn of the spindle. On a balance, the table is repeatedly moved from stop to stop. The displacement may be measured with a micrometer dial or determined from the motion of the beam (Fig. 30).
From the mutual agreement between a number of different experiments, and from the comparison with shake-table calibrations, we estimate the absolute accuracy of the method to be better than 1%.