The Theory and Design of Two Types of Portable Seismograph

The sensitivity of optical recorders is derived by considering the power required to maintain a critically damped, suspended mirror in angular oscillations of sufficient amplitude to displace the reflected beam through an angle comparable with the width of the diffraction maximum. For a thin mirror, this power is simply related to its dimensions, and to the natural frequency of the suspension. The power which a seismometer can develop from ground motion of given amplitude is related to the suspended mass and its natural frequency. Combining these results, it is found that a seismometer mass of a few hundred grams is sufficient to detect ground motion of 10-7 cm. without using an excessively small mirror in the recorder. Modern movingcoil galvanometers have sensitivities approaching the limit imposed by the inertia of the mirror, so that small electromagnetic seismographs are theoretically capable of providing the highest sensitivity required in the field. Moving-coil seismometers are compared with variable-reluctance instruments, and it is decided that the former type would be more convenient in the field. Descriptions are included of seismometers having natural periods of 0. 5 sec. and of I sec. respectively. Both instruments can be adapted for horizontal or vertical operation, and are fitted into watertight cases. Tolerance to changes of level is sufficient to enable the seismometers to be left buried in soft ground, and a method of calibrating the buried instruments is described.