Inertial Seismograph Design-Limitations in Principle and Practice (Or How Not to Build a Sensitive Seismograph)

Seismograph design is discussed in the general terms of band pass, control of response characteristics through mechanical and electrical modifications of the system, and the limitations imposed on sensitivity when some seemingly innocuous numbers are ignored. Earth vibration amplitudes of one Angstrom or less, and power-sensing levels as low as 10-20 watts are considered, implying that instrumental noise levels should be even lower under some circumstances. A discussion of transducer elements leads to the conclusion that the incorporation of a galvanometer into the motion-amplifying system offers at least as good a system as can be devised by other means. The concept of a platform stabilized against seismic disturbances is outlined as an interesting and probably important sidelight on seismograph design. Remarks on mechanical construction are included as guidance to designers because certain details, such as spring proportions, lever systems, crossed flexure plate hinges and suspension members with controlled regions of bending have been found extremely effective while other details of design have produced elusive troubles. Some design features are illustrated in the construction of certain seismographs in whose design the authors have an interest. The problem of level sensitivity for horizontal component seismometers is outlined to show its basic nature, and some possible solutions.